NEW MATERIALS FOR OPTICAL SENSING OF...

NEW MATERIALS FOR OPTICAL SENSING OF EXPLOSIVES

COPOLYMERS CONTAINING 2-VINYL-4,6-DIAMINO-1,3,5-TRIAZINE

AND CO-CRYSTALS OF ELECTRON RICH AROMATIC MOLECULES AND

1,3-DINITROBENZENE

by

STEVEN KEITH MCNEIL

DAVID E. NIKLES, COMMITTEE CHAIR

MARTIN G. BAKKER CHRISTOPHER S. BRAZEL

SHANLIN PAN SHANE C. STREET

A DISSERTATION

Submitted in partial fulfillment of the requirements

for the degree of Doctor of Philosophy in the Department of Chemistry

in the Graduate School of The University of Alabama

TUSCALOOSA, ALABAMA

2013

ii

ABSTRACT

This dissertation focuses on the development of electron rich polymers with an affinity

for nitroaromatics. Thin polymer films of the electron rich polymers could be applied in an

optical waveguide sensor to detect nitroaromatics by changes in the optical properties of the

polymer thin films. Charge transfer complexes between electron rich aromatic reagents and

electron deficient nitroaromatics were produced providing an understanding of the

intermolecular interactions between the electron donor and electron acceptor.

Electron rich copolymers were synthesized with 2-vinyl-4,6-diamino-1,3,5-triazine

(VDAT) using a published literature procedure. The polymerization procedure was extended to a

variety of electron rich monomers, resulting in the production of a number of electron rich

copolymers. Thin films of the copolymers were spin coated and their optical properties were

characterized by spectroscopic ellipsometry before and after exposure to a nitroaromatic vapor.

The exposure to the nitroaromatic vapor allowed the formation of complexes with the electron

rich copolymers and the nitroaromatic molecules, creating a change in the optical properties of

the polymer films. This refractive index change after exposure to a nitroaromatic demonstrated

the possibility of these films to be applied in an optical waveguide sensor for explosive detection.

Co-crystals were grown between electron rich donors and the electron deficient 1,3-

dinitrobenzene by the slow evaporation method. When the electron donor solution and electron

acceptor solution were combined in a crystallization dish, significant color changes were

observed. The interaction between the electron donor and electron acceptor were characterized

using analytical techniques.

iii

DEDICATION

To my wife, Jeanna, who gives me support, encouragement, and love.

To my father and mother, Steve and Pam, who give me guidance and encouragement.

To my sister, Megan, who cares and supports me unconditionally.

To my family and friends.

iv

LIST OF ABBREVIATIONS AND SYMBOLS cm/s

centimeters per second

km/s

kilometers per second

cal/g

calories per gram

TNT

2,4,6-trinitrotoluene

NG

nitroglycerine

RDX

tetranitro-triazacyclohexane

HMX

tetranitro-tetracyclooctane

Tetryl

tetranitro-N-methylamine

picric acid

2,4,6-trinitrophenol

NH4NO3

ammonium nitrate

O

oxygen

N

nitrogen

C

carbon

PbN6

lead azide

AgN3

silver azide

NaN3

sodium azide

DMNB

2,3-dimethyl-2,3-dinitrobutane

°C

degrees Celsius

v

o-MNT

2-nitrotoluene

EGDN

ethylene glycol dinitrate

PETN

pentaerythritol tetranitrate

NC

nitrocellulose

TATP

triacetone triperoxide

HMTD

hexamethylene triperoxide diamine

$

dollars

km2

square kilometers

AT

anti-tank landmine

AP

anti-personnel landmine

U.N.

United Nations

mm

millimeters

kg

kilograms

g

grams

Composition B

RDX + TNT

C-4

RDX based explosive

1,3-DNB

1,3-dinitrobenzene

2,4-DNT

2,4-dinitrotoluene

2,6-DNT

2-6-dinitrotoluene

2,4-DNB

2,4-dinitrobenzene

pg/mL

picograms per milliliter

2-ADNT

2-amino-4,6-dinitrotoluene

4-ADNT

4-amino-2,6-dinitrotoluene

vi

%

percent

°

degrees

GPR

Ground Penetrating Radar

GHz

gigahertz

IR

Infrared

EIT

Electrical Impedance Tomography

XBT

X-Ray Backscatter

keV

kilo electron volts

cm

centimeters

min

minutes

m2

square meters

LIDAR

Light Detection and Ranging System

MMDS

Microbial Mine Detection System

NQR

Nuclear Quadrupole Resonance

MHz

megahertz

AM

amplitude modulation

H

hydrogen

ppt

parts per trillion

kV

kilovolts

TNA

thermal neutron analysis

FNA

fast neutron analysis

PFNA

pulsed fast neutron analysis

PFTNA

pulsed fast thermal neutron analysis

vii

NRA

nuclear resonance absorption

Cl

chlorine

P

phosphorus

S

sulfur

Si

silicon

γ

gamma

n

thermal

MeV

mega electron volts

IMS

Ion Mobility Spectrometry

Ni

nickel

V/cm

volts per centimeter

MS

Mass Spectrometry

THz

terahertz

UV

ultraviolet

NIR

near-infrared

LIBS

Laser-Induced Breakdown Spectroscopy

ng

nanogram

pg

picogram

pH

negative log of hydrogen concentration in a solution

DNA

Deoxyribonucleic acid

sec.

seconds

mins.

minutes

hrs.

hours

viii

MZI

Mach-Zehnder interferometer

L

liter

π

pi

Δn

change in refractive index

ppm

parts per million

n

refractive index

L

path length

λ

wavelength

nm

nanometers

He

helium

Ne

neon

VDAT

2-vinyl-4,6-diamino-1,3,5-triazine

Co.

company

HPLC

High-performance liquid chromatography

ACS

American Chemical Society

M.W.

molecular weight

AIBN

2,2'-azobisisobutyronitrile

PVDAT

Poly(2-vinyl-4,6-diamino-1,3,5-triazine)

g

grams

D.I. H2O

deionized water

mL

milliliter

mmols

millimols

Na2S2O8

sodium persulfate

ix

MeOH

methanol

PS-co-PVDAT

Polystyrene-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine)

DMSO

methyl sulfoxide

CaH2

calcium hydride

EtOH

ethanol

PMMA-co-PVDAT

Poly(methyl methacrylate)-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine)

MMA

methyl methacrylate

PMMA

Poly(methyl methacrylate)

PS

Polystyrene

PMA-co-PVDAT

Poly(methyl acrylate)-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine)

PMA

Poly(methyl acrylate)

MA

methyl acrylate

P2VP-co-PVDAT

Poly(2-vinylpyridine)-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine)

2-VP

2-vinylpyridine

(w.t)

weight

NaCl

sodium chloride

P2VP

Poly(2-vinylpyridine)

PAM-co-PVDAT

Poly(acrylamide)-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine)

PAM

Poly(acrylamide)

PVK-co-PVDAT

Poly(N-vinylcarbazole)-co- Poly(2-vinyl-4,6-diamino-1,3,5-triazine)

DMF

dimethylformamide

PVK

Poly(N-vinylcarbazole)

PS-co-PVK

Polystyrene-co-Poly(N-vinylcarbazole)

x

PMMA-co-PVK

Poly(methyl methacrylate)-co-Poly(N-vinylcarbazole)

≈

approximate

PVI-co-PVDAT

Poly(N-vinylimidazole)-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine)

VI

1-vinylimidazole

PS-co-PVI

Polystyrene-co-Poly(N-vinylimidazole)

PMMA-co-PVI

Poly(methyl methacrylate)-co-Poly(N-vinylimidazole)

MDAT

2,4-diamino-6-methyl-1,3,5-triazine

2-NT

2-nitrotoluene

3-NT

3-nitrotoluene

PNT

4-nitrotoluene

NB

nitrobenzene

9-VC

9-vinylcarbazole

9-EC

9-ethylcarbazole

CBZ

carbazole

10-M

10-methylphenothiazine

PHZ

phenothiazine

FTIR

Fourier transform infrared spectroscopy

mg

milligrams

KBr

potassium bromide

1H NMR

proton nuclear magnetic resonance

13C NMR

carbon nuclear magnetic resonance

D1

relaxation delay

TD

time domain

xi

NS

number of scans

CDCl3

deuterated chloroform

DMSO-d6

deuterated methyl sulfoxide

Tg

glass transition temperature

DSC

Differential Scanning Calorimetry

TGA

Thermogravimetric analysis

°C min-1

degrees Celsius per minute

Td

decomposition temperature

SEC

size exclusion chromatography

THF

tetrahydrofuran

mg mL-1

milligram per milliliter

mL min-1

milliliter per minute

RI

refractive index

VASE

variable angle spectroscopic ellipsometry

ψ

psi

Δ

delta

SiO2

silicon dioxide

Å

angstrom

k

extinction coefficient

in.

inch

UV/Vis

ultraviolet/visible

cm-1

wave numbers

NH2

amine functional group

xii

Tm

melting endotherm

GPC

Gel Permeation Chromatography

δ

ppm

s

singlet

d

doublet

dd

double of doublets

td

triplet of doublets

m

multiplet

q

quartet

PS

Polystyrene

Tc

ceiling temperature

Mn

number average molecular weight

Mw

weight average molecular weight

Mz

Z-average molecular weight

PDI

polydispersity index

r

reactivity ratio

α

alpha

M-1cm-1

molar absorptivity

pm

picometers

M

molarity

vas

asymmetric vibration mode

vs

symmetric vibration mode

S.P.

splitting pattern

xiii

Int.

integration values

NO2

nitro functional group

Z

number of formula units

Ñ

complex index of refraction

j

√-1

c

speed of light

ν

velocity

φi

angle between the incidence light and the material

φt

angle of reflection

Ep

electric field vector parallel to the plane of incidence

Es

electric field vector perpendicular to the plane of incidence

rs

perpendicular wave Fresnel reflection coefficient

rp

parallel wave Fresnel reflection coefficient

ts

perpendicular wave Fresnel transmission coefficient

tp

parallel wave Fresnel transmission coefficient

β

film phase thickness

δ1

phase difference before the reflection

δ2

phase difference after the reflection

RP

parallel wave total reflection coefficient

RS

perpendicular wave total reflection coefficient

tan Ψ

ratio of the magnitudes of the total reflection coefficients

ρ

the complex ratio of the total reflection coefficients

MSE

mean square error

xiv

A,B,C

Cauchy parameters

b.p.

boiling point

MEK

methyl ethyl ketone

≤

less than or equal to

rpm

revolutions per minute

PVI

Poly(vinylimidazole)

PVI-co-PVA

Poly(vinylimidazole)-co-Poly(vinylaniline)

P4VP

Poly(4-vinylpyridine)

PTFE

Polytetrafluoroethylene

μm

micrometers

O.K.

Oklahoma

U.S.

United States

Comp. B

Composition B

ng/L

nanograms per liter

pg/L

picograms per liter

Temp.

temperature

VK

9-vinylcarbazole

%T

percent transmittance

d

distance traveled

p-wave

wave parallel to the plane of incidence

s-wave

wave perpendicular to the plane of incidence

λ

wavelength

Lit.

literature value

xv

ACKNOWLEDGEMENTS

First and foremost, I am appreciative of my research advisor, Dr. Nikles. Without your

help, patience, and guidance, I would have never developed into the scientist that I am today.

Through the years, you have taught me how to understand and manage research problems,

encouraging me to think outside of the box. Your guidance and advice has helped me reach this

milestone in my scientific career. It was an honor and a privilege to work with you on this

research project. Thank you for everything you have done for me.

I would also like to thank my Nikles's group colleagues, Dr. Jeremy Pritchett, Clifton,

Amanda, Lei, Greg, Dr. Medhat Farahat, Adam, Todd, and Dr. Jackie Nikles, for your support,

assistance, and friendship throughout my time here at the university. You all were always willing

to lend a helping hand whenever I needed anything, and it was a privilege to work with all of

you. You all were my scientist support net, helping me to collect or interpret data and were

helpful in discussing ideas and possible applications.

To the Nikles's undergraduates and high school students, Cameran, Margaret, Morgan,

Lindsey, Jesse, John, Ben, Kim, Hamilton, Kirsten, and Jerome, thank you for keeping the lab

interesting during the semesters. There was never a dull moment with all of you in the lab.

To my committee members and chemistry faculty, thank you for your guidance and

teaching. Because of you, my knowledge and experience in chemistry reached levels I did not

originally realize were achievable.

Lastly, thank you to the chemistry department and the University of Alabama for giving

me this opportunity to further my education.

xvi

CONTENTS

ABSTRACT .................................................................................................................................... ii

DEDICATION ............................................................................................................................... iii

LIST OF ABBREVIATIONS AND SYMBOLS .......................................................................... iv

ACKNOWLEDGEMENTS ...........................................................................................................xv

LIST OF TABLES ....................................................................................................................... xxi

LIST OF FIGURES ................................................................................................................... xxvi

LIST OF SCHEMES.............................................................................................................. xxxvii

CHAPTER 1: Introduction .............................................................................................................1

1.1 Explosives ................................................................................................................1

1.1.2 Types and Properties of Explosives .........................................................................3

1.1.3 Economic and Human Cost from Explosives ..........................................................6

1.2 Landmine Overview .................................................................................................6

1.2.1 History of Landmines ..............................................................................................7

1.2.2 Landmines: The Problem .........................................................................................7

1.2.3 Landmines: Human and Economic Costs ................................................................8

1.3 Classification of Landmines ..................................................................................10

1.3.1 Anti-Tank Landmines ............................................................................................10

1.3.2 Anti-Personnel Landmines.....................................................................................10

1.3.3 Chemical Signatures of Landmines .......................................................................11

xvii

1.4 Landmine Detection Methods ................................................................................14

1.4.1 Manual Detection Method .....................................................................................14

1.4.2 Electromagnetic Detection Systems ......................................................................15

1.4.3 Acoustic and Seismic Detection Systems ..............................................................19

1.4.4 Biological and Biomimetic Systems ......................................................................20

1.4.5 Bulk Explosive Landmine Detection Systems .......................................................24

1.4.6 Chemical Landmine Vapor Detection Systems .....................................................27

1.5 Explosive Detection Techniques ............................................................................28

1.5.1 Bulk Explosive Detection Systems ........................................................................28

1.5.2 Spectroscopic Explosive Detection Systems .........................................................32

1.5.3 Olfactory Explosive Detection Systems ................................................................35

1.5.4 Chemical Sensors for Explosive Detection ............................................................36

1.5.5 Explosive Sensors Summary..................................................................................38

1.6 Mach-Zehnder Interferometer Optical Waveguide Sensor ....................................39

1.7 Research Objectives ...............................................................................................41

CHAPTER 2: Experimental ...........................................................................................................42

2.1 Sources of All Chemicals .......................................................................................42

2.2 Polymer Syntheses .................................................................................................43

2.2.1 Poly(2-vinyl-4,6-diamino-1,3,5-triazine) (PVDAT) ..............................................43

2.2.2 Polystyrene-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine) (PS-co-PVDAT) .........44

2.2.3 Poly(methyl methacrylate)-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine) (PMMA-co-PVDAT) .............................................................................................45

2.2.4 Poly(methyl acrylate)-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine) (PMA-co-PVDAT) ................................................................................................47

xviii

2.2.5 Poly(2-vinylpyridine)-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine) (P2VP-co-PVDAT) ................................................................................................49

2.2.6 Poly(acrylamide)-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine) (PAM-co-PVDAT) ................................................................................................50

2.2.7 Poly(N-vinylcarbazole)-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine) (PVK-co-PVDAT) .................................................................................................52

2.2.8 Polystyrene-co-Poly(N-vinylcarbazole) (PS-co-PVK) ..........................................54

2.2.9 Poly(methyl methacrylate)-co-Poly(N-vinylcarbazole) (PMMA-co-PVK) ..........56

2.2.10 Poly(N-vinylimidazole)-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine) (PVI-co-PVDAT) ...................................................................................................57

2.2.11 Polystyrene-co-Poly(N-vinylimidazole) (PS-co-PVI) ...........................................59

2.2.12 Poly(methyl methacrylate)-co-Poly(N-vinylimidazole) (PMMA-co-PVI) ...........60

2.3 Co-Crystals with Nitroaromatics ...........................................................................61

2.3.1 General Co-Crystal Procedure with Nitroaromatics ..............................................61

2.3.2 2,4-Diamino-6-methyl-1,3,5-triazine (MDAT) Co-Crystals with Nitroaromatics ................................................................................................................................61

2.3.3 2-Vinyl-4,6-diamino-1,3,5-triazine (VDAT) Co-Crystals with 1,3-Dinitrobenzene ................................................................................................................................62

2.3.4 9-Vinylcarbazole (9-VC) Co-Crystals with 1,3-Dinitrobenzene ...........................62

2.4 Instrumentation ......................................................................................................65

CHAPTER 3: Polymer Characterization .......................................................................................69

3.1 PVDAT Characterization .......................................................................................69

3.2 PS-co-PVDAT Copolymers Characterization .......................................................73

3.3 PMMA-co-PVDAT Copolymers Characterization ................................................82

3.4 PMA-co-PVDAT Copolymers Characterization ...................................................91

xix

3.5 P2VP-co-PVDAT Copolymers Characterization ..................................................95

3.6 PAM-co-PVDAT Copolymers Characterization .................................................100

3.7 PVK-co-PVDAT Copolymers Characterization ..................................................105

3.8 PS-co-PVK Copolymers Characterization ...........................................................107

3.9 PMMA-co-PVK Copolymers Characterization ...................................................109

3.10 PVI-co-PVDAT Copolymer Characterization .....................................................115

3.11 PS-co-PVI Copolymer Characterization ..............................................................119

3.12 PMMA-co-PVI Copolymer Characterization ......................................................122

CHAPTER 4: Polymer Thin Films Characterization by Variable Angle Spectroscopic Ellipsometry after Exposure to a Nitroaromatic Vapor .......................................127

4.1 Ellipsometry Overview ........................................................................................127

4.2 Data Analysis .......................................................................................................133

4.3 Cauchy Model ......................................................................................................134

4.4 PS-co-PVDAT Films ...........................................................................................134

4.5 PMMA-co-PVDAT Films ...................................................................................156

4.6 P2VP Polymer Film .............................................................................................169

4.7 Commercial Polymers Films................................................................................172

4.8 Polystyrene Thin Films Containing 10-Methylphenothiazine .............................182

4.9 Polymer Thin Films Summary .............................................................................198

CHAPTER 5: Co-Crystals Containing Electron Rich Aromatic Molecules and Electron Poor Nitroaromatic Molecules .....................................................................................200

5.1 1,3-Dinitrobenzene Crystals (1,3-DNB) ..............................................................201

5.2 9-Ethylcarbazole (9-EC) Co-Crystals with Nitroaromatics .................................206

xx

5.3 9-Vinylcarbazole (9-VC) Co-crystals with 1,3-DNB .........................................213

5.4 Carbazole (CBZ) Co-Crystals with 1,3-DNB ......................................................218

5.5 Phenothiazine (PHZ) Co-Crystals with 1,3-DNB ................................................226

5.6 10-Methylphenothiazine (10-M) Co-Crystals with 1,3-DNB ..............................234

CHAPTER 6: Conclusions and Future Works.............................................................................247

REFERENCES ............................................................................................................................251

APPENDIX .................................................................................................................................259

xxi

LISTS OF TABLES

Table 1.1.2.1 Common explosives' vapor pressures ......................................................................5

Table 1.2.1 List of countries with estimated unexploded landmines ..........................................9

Table 1.3.2.1 Examples of the variety of AP mines based on their material, color, fuse, explosive charge, and weight .................................................................................12

Table 1.5.1.1 Neutron analysis techniques ..................................................................................31

Table 1.5.5.1 Explosive sensors limit of detection ranges ...........................................................39

Table 2.2.2.1 Experimental amounts and conditions for PS-co-PVDAT polymerizations .........45

Table 2.2.3.1 Experimental amounts for the PMMA-co-PVDAT copolymers and PMMA polymerizations ......................................................................................................47

Table 2.2.5.1 Experimental amounts for P2VP-co-PVDAT copolymers and P2VP polymerizations ......................................................................................................50

Table 2.2.6.1 Experimental amounts for PAM-co-PVDAT copolymers and PAM polymerizations ......................................................................................................52

Table 2.2.7.1 PVK-co-PVDAT copolymers and PVK experimental amounts for free radical polymerizations ......................................................................................................54

Table 2.2.8.1 Experimental amounts for the PS-co-PVK polymerizations .................................55

Table 2.2.9.1 Experimental amounts for PMMA-co-PVK copolymers polymerizations ............57

Table 2.3.1.1 Co-crystals experimental reagents, solvents, and descriptions of crystals ............64

Table 3.2.1 Polystyrene and PS-co-PVDAT 20 mol % VDAT copolymer 13C NMR peaks ...76

Table 3.2.2 Thermal decomposition temperatures, Td (10% weight loss for PS and PS-co- PVDAT copolymers) .............................................................................................79

xxii

Table 3.2.3 Glass transition temperatures for PVDAT, PS-co-PVDAT copolymers, and PS ................................................................................................................................81

Table 3.2.4 PS-co-PVDAT copolymers GPC data ...................................................................82

Table 3.3.2 Thermal decomposition temperatures for PMMA and the copolymers of PMMA and PVDAT ...........................................................................................................88

Table 3.3.3 The glass transition temperatures for PMMA, PMMA-co-PVDAT copolymers, and PVDAT ...........................................................................................................90

Table 3.3.4 Molecular weights for the PMMA-co-PVDAT copolymers determined by GPC ................................................................................................................................90

Table 3.5.1 P2VP and P2VP-co-PVDAT copolymers glass transition temperatures (°C) .....100

Table 3.6.1 PAM and PAM-co-PVDAT copolymers glass transition temperatures (°C) ......104

Table 3.7.1 PVK and PVK-co-PVDAT copolymers glass transition temperatures (°C) ........106

Table 4.4.1 The Cauchy parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a polystyrene film exposed to PNT vapors for ten seconds .................................................................................140

Table 4.4.2 The ellipsometry MSE, film thickness, refractive index, average change in refractive index (Δn), optical constants MSE, profilometer thickness, and spin coating parameters for a PS-co-PVDAT 20 mol % VDAT copolymer film produced from a 1% (w.t.) 1,4-dioxane solution .................................................142

Table 4.4.3 The ellipsometry Cauchy model MSE, thickness, refractive index, and optical constants MSE, profilometer measured thickness, and spin coating parameters for a PS-co-PVDAT 10 mol % VDAT copolymer film ............................................144

Table 4.4.4 The ellipsometry Cauchy model MSE, thickness, refractive index, optical constants MSE, profilometer measured thickness, and spin coating parameters for a PS-co-PVDAT 5 mol % VDAT copolymer film ..............................................146

Table 4.4.5 The ellipsometry Cauchy model MSE, thickness, refractive index, optical constants MSE, profilometer measured thickness, and spin coating parameters for the PS-co-PVDAT 1 mol % VDAT copolymer film ...........................................148

Table 4.4.6 The ellipsometry Cauchy model MSE, thickness, refractive index, optical constants MSE, profilometer measured thickness, and spin coating parameters for the PS-co-PVDAT 1 mol % VDAT copolymer film exposed to PNT for five seconds .................................................................................................................151

xxiii

Table 4.4.7 The ellipsometry Cauchy model MSE, thickness, refractive index, optical constants MSE, profilometer measured thickness, and spin coating parameters for the PS-co-PVDAT 1 mol % VDAT copolymer film exposed to PNT for twenty seconds .................................................................................................................153

Table 4.4.8 The ellipsometry Cauchy model MSE, thickness, refractive index, optical constants MSE, profilometer measured thickness, and spin coating parameters for a PS-co-PVDAT 1 mol % VDAT copolymer film exposed to PNT for forty seconds .................................................................................................................155

Table 4.5.1 The before and after Cauchy parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a PMMA film exposed to 1,3-DNB for ten seconds ...................................................................160

Table 4.5.2 The ellipsometry Cauchy model parameters, profilometer thickness, average change in refractive index (Δn), and spin coating parameters for a PMMA-co- PVDAT 1 mol % VDAT copolymer film exposed to 1,3-DNB for sixteen minutes ..............................................................................................................................162

Table 4.5.3 The ellipsometry Cauchy parameters, profilometer measured thickness, average change in refractive index (Δn), and spin coating parameters for a PMMA-co- PVDAT 1 mol % VDAT copolymer film exposed to NB for twenty-five minutes ..............................................................................................................................164

Table 4.5.4 The ellipsometry Cauchy parameters, profilometer measured thickness, average change in refractive index (Δn), and spin coating parameters for a PMMA-co- PVDAT 5 mol % VDAT copolymer film exposed to NB for twenty-five minutes ..............................................................................................................................166

Table 4.5.5 The ellipsometry Cauchy parameters, profilometer measured thickness, average change in refractive index (Δn), and spin coating parameters for a PMMA-co- PVDAT 5 mol % VDAT copolymer film exposed to 1,3-DNB for twenty-five minutes .................................................................................................................168

Table 4.6.1 The Cauchy parameters, average change in refractive index, and spin coating parameters for a spin coated P2VP film exposed to PNT for five seconds. ........171

Table 4.7.1 The ellipsometry Cauchy model MSE, film thickness, average change in refractive index (Δn), optical constants MSE, profilometer measured thickness, and spin coating parameters for the P4VP film exposed to PNT for five seconds ..............................................................................................................................175 Table 4.7.2 The Cauchy model parameters and spin coating parameters for a PVI polymer film spin coated from a 3% (w.t.) EtOH solution exposed to PNT for 5 seconds .............................................................................................................................178

xxiv

Table 4.7.3 The before and after Cauchy parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a PVI-co-PVA polymer film exposed to PNT for five seconds ..................................................181

Table 4.8.1 The before and after Cauchy model parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a polystyrene film containing 10-M exposed to 1,3-DNB for two hours ...............186

Table 4.8.2 The ellipsometry Cauchy parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a film spin coated from a 1% (w.t.) polystyrene solution containing 0.5% (w.t.) 10-M exposed to 1,3-DNB for three hours ....................................................................189

Table 4.8.3 The Cauchy model parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a film spin coated from a 1% (w.t.) polystyrene solution containing 0.1% (w.t.) 10-M exposed to 1,3-DNB for three hours ............................................................................................................192

Table 4.8.4 The Cauchy parameters before and after exposure to 1,3-DNB, average change in refractive index, profilometer measured thickness, and spin coating parameters for a film spin coated from a 1% (w.t.) polystyrene/toluene solution containing 0.1% (w.t.) 10-M exposed to 1,3-DNB for ten seconds ................................................195

Table 4.9.1 Polymer films average change in refractive index after a five-second exposure to a nitroaromatic vapor ..............................................................................................199

Table 5.2.1 1H NMR peak positions, splitting patterns, and integration values of the 1,3-DNB crystals, 9-EC co-crystals, and 9-EC crystals ......................................................208

Table 5.2.2 Comparison of NO2 asymmetric and symmetric stretching vibrations between 1, 3-DNB crystals and 9-EC + 1,3-DNB co-crystals ...............................................209

Table 5.2.3 Melting points of 1,3-DNB crystals, 9-EC crystals, and 9-EC co-crystals with 1,3- DNB .....................................................................................................................213

Table 5.3.1 1H NMR peak positions, splitting patterns, and integration values of 1,3-DNB crystals, 9-VC co-crystals, and 9-VC crystals .....................................................215

Table 5.3.2 NO2 asymmetric and symmetric stretching vibrations for 1,3-DNB crystals and 9- VC co-crystals ......................................................................................................217

Table 5.3.3 1,3-DNB crystals, 9-VC crystals, and 9-VC co-crystals melting points .............217

Table 5.4.1 1H NMR peak positions, splitting patterns, and integration values of 1,3-DNB crystals, CBZ co-crystals, and CBZ crystals .......................................................220

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Table 5.4.2 NO2 asymmetric and symmetric stretching vibrations for 1,3-DNB crystals and CBZ co-crystals ...................................................................................................221

Table 5.4.3 1,3-DNB crystals, CBZ crystals, and CBZ co-crystals melting points ................225

Table 5.5.1 1H NMR peak positions, splitting patterns, and integration values of 1,3-DNB crystals, PHZ co-crystals, and PHZ crystals ........................................................227

Table 5.5.2 NO2 asymmetric and symmetric stretching modes for the PHZ co-crystals and 1,3-DNB crystals..................................................................................................229

Table 5.5.3 Melting points of the 1,3-DNB crystals, PHZ crystals, and PHZ co-crystals .....233

Table 5.6.1 1H NMR peak positions, splitting patterns, and integration values of 1,3-DNB crystals, 10-M co-crystals, and 10-M crystals .....................................................235

Table 5.6.2 NO2 asymmetric and symmetric stretching vibrations for the 1,3-DNB crystals and 10-M co-crystals............................................................................................237

Table 5.6.3 Melting points of the 1,3-DNB crystals, 10-M crystals, and 10-M co-crystals ...241

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LIST OF FIGURES

Figure 1.3.3.1 Chemical structures of common explosives and TNT impurities found and used in landmines .........................................................................................................13

Figure 1.6.1 MZI consisting of a polymer waveguide with two optical paths ...........................40

Figure 3.1.1 The FTIR spectrum of PVDAT recorded in KBr ..................................................70

Figure 3.1.2 PVDAT TGA curve ...............................................................................................71

Figure 3.1.3 PVDAT DSC curve ...............................................................................................72

Figure 3.2.1 FTIR spectra for the PS-co-PVDAT copolymers ..................................................73

Figure 3.2.2 PS-co-PVDAT 20 mol % 1H NMR (360 MHz, DMSO-d6) spectrum ...................75

Figure 3.2.3 PS-co-PVDAT 20 mol % VDAT 13C NMR spectrum (500 MHz, DMSO-d6) .....76

Figure 3.2.4 TGA curves for PS and PS-co-PVDAT 20 mol % VDAT copolymer ..................79

Figure 3.2.5 PVDAT, PS-co-PVDAT copolymers, and PS DSC curves ...................................80

Figure 3.3.1 FTIR spectra for the PMMA-co-PVDAT copolymers ..........................................83

Figure 3.3.2 1H NMR spectrum in DMSO-d6 for PMMA-co-PVDAT (20 mol %) ..................84

Figure 3.3.3 13C NMR spectrum in DMSO-d6 for the PMMA (80%)-co-PVDAT (20%) copolymer ..............................................................................................................86

Figure 3.3.4 TGA curves for PMMA and the copolymer containing 10 mol % VDAT ............88

Figure 3.3.5 PMMA and PMMA-co-PVDAT copolymers DSC curves ....................................89

Figure 3.4.1 FTIR spectrum for the PMA-co-PVDAT 20 mol % VDAT copolymer ...............91

Figure 3.4.2 1H NMR spectrum for the PMA-co-PVDAT 20 mol % VDAT copolymer ..........92

Figure 3.4.3 The 13C NMR spectrum (500 MHz, DMSO-d6) for the PMA-co-PVDAT 20 mol % VDAT copolymer ..............................................................................................93

xxvii

Figure 3.4.4 DSC curve for the PMA-co-PVDAT 20 mol % VDAT copolymer ......................94 Figure 3.5.1 FTIR spectra for P2VP and P2VP-co-PVDAT copolymers ..................................95

Figure 3.5.2 1H NMR spectrum for the P2VP-co-PVDAT 20 mol % VDAT copolymer in DMSO-d6 using the 360 MHz spectrometer ..........................................................97

Figure 3.5.3 The 13C NMR spectrum (500 MHz, DMSO-d6) for the P2VP-co-PVDAT 1 mol % VDAT copolymer ..............................................................................................98

Figure 3.5.4 P2VP and P2VP-co-PVDAT copolymers DSC curves .........................................99

Figure 3.6.1 PAM and PAM-co-PVDAT copolymers FTIR spectra .......................................101

Figure 3.6.2 The 1H NMR spectrum (360 MHz, D2O) for the PAM-co-PVDAT 20 mol % VDAT copolymer ................................................................................................102

Figure 3.6.3 The 13C NMR spectrum (500 MHz) recorded in D2O for the PAM-co-PVDAT 20 mol % VDAT copolymer .....................................................................................103

Figure 3.6.4 DSC curves for the PAM and PAM-co-PVDAT copolymers .............................104

Figure 3.7.1 The FTIR spectra for PVK and PVK-co-PVDAT copolymers recorded in KBr pellets ...................................................................................................................105

Figure 3.7.2 DSC curves for PVK and PVK-co-PVDAT copolymers ....................................107

Figure 3.8.1 The 1H NMR spectra for the PVK and PS-co-PVK copolymers recorded in CDCl3 (360 MHz) ............................................................................................................108

Figure 3.9.1 The FTIR spectra for KBr pellets containing PVK and PMMA-co-PVK copolymers ...........................................................................................................109

Figure 3.9.2 The 1H NMR spectrum for the PVK homopolymer recorded in CDCl3 using the 360 MHz spectrometer .........................................................................................110

Figure 3.9.3 The 1H NMR spectra for PMMA-co-PVK 50 and 20 mol % copolymers recorded in CDCl3 (360 MHz) ............................................................................................111

Figure 3.9.4 The 13C NMR spectrum for PVK recorded in CDCl3 (500 MHz) .......................113

Figure 3.9.5 The 13C NMR spectrum for the (50:50) PMMA-co-PVK copolymer ( 500 MHz, CDCl3)..................................................................................................................114

Figure 3.9.6 The DSC curves for the PMMA-co-PVK copolymers ........................................115

xxviii

Figure 3.10.1 The FTIR spectra for PVI homopolymer (black) and PVI-co-PVDAT 20 mol % VDAT copolymer (red) ........................................................................................116

Figure 3.10.2 The 1H NMR spectra for the PVI homopolymer and PVI-co-PVDAT copolymer ..............................................................................................................................117

Figure 3.10.3 The 13C NMR spectrum for the PVI-co-PVDAT copolymer recorded in DMSO-d6 (500 MHz) ............................................................................................................118

Figure 3.11.1 The FTIR spectra for the PVI homopolymer (red curve) and the PS-co-PVI 20 mol % copolymer (black curve) recorded in KBr pellets at room temperature ..............................................................................................................................119

Figure 3.11.2 The 1H NMR spectra for the PS-co-PVI copolymer and polystyrene homopolymer recorded in CDCl3 (360 MHz) ............................................................................121

Figure 3.11.3 The 13C NMR spectra for the PS-co-PVI copolymer and the homopolymer (PVI) ..............................................................................................................................122

Figure 3.12.1 The FTIR spectra for the PMMA-co-PVI copolymer and the PVI homopolymer recorded in KBr pellets ........................................................................................123

Figure 3.12.2 The 1H NMR spectra for the homopolymers, PVI and PMMA, and the PMMA- co-PVI copolymer containing 20 mol % vinylimidazole ....................................124

Figure 3.12.3 The 13C NMR spectrum for the PMMA-co-PVI copolymer containing 20 mol % VI recorded in DMSO-d6 (500 MHz) ..................................................................125

Figure 3.12.4 The DSC curves shown from 80 °C to 160 °C for the PMMA-co-PVI 20 mol % VI copolymer and PMMA homopolymer ............................................................126

Figure 4.1.1 Two linearly polarized waves combined out of phase producing elliptically polarized light. Modified from http://www.jawoollam.com/tutorial_2.html (accessed Feb. 15, 2013) ......................................................................................128

Figure 4.1.2 Light reflecting and refracting at the interface between air and the surface of a material. Modified from http://www.jawoollam.com/tutorial_3.html (accessed Feb. 15, 2013) ......................................................................................................129

Figure 4.1.3 Schematic representation for a typical ellipsometry measurement showing a polarization state change when linearly polarized light is reflected from a sample's surface. Modified from http://www.jawoollam.com/tutorial_4.html (accessed Feb. 15, 2013) ..............................................................................................................131

Figure 4.1.4 Schematic representation of a wave propagating through a film, producing multiple reflections and transmissions .................................................................131

xxix

Figure 4.4.1 The before and after refractive index curves for a polystyrene film spin coated from a 3% (w.t.) toluene solution exposed to PNT for ten seconds ....................139

Figure 4.4.2 The change in refractive index for a PS-co-PVDAT 20 mol % VDAT copolymer film spin coated from a 1% (w.t.) 1,4-dioxane solution exposed to NB for five seconds ................................................................................................................141

Figure 4.4.3 The change in refractive index for a PS-co-PVDAT 10 mol % VDAT copolymer film produced from a 1% (w.t.) MEK solution exposed to NB for five seconds .................................................................................................................143

Figure 4.4.4 The change in refractive index for a PS-co-PVDAT 5 mol % VDAT copolymer film produced from a 3% (w.t.) toluene solution exposed to PNT for sixty seconds ..............................................................................................................................145

Figure 4.4.5 The change in refractive index for a PS-co-PVDAT 1 mol % VDAT copolymer film produced from a 3% (w.t.) toluene solution exposed to NB for five seconds ..............................................................................................................................147

Figure 4.4.6 The change in refractive index for a PS-co-PVDAT 1 mol % VDAT copolymer film produced from a 3% (w.t.) toluene solution exposed to PNT for five seconds ..............................................................................................................................150

Figure 4.4.7 The change in refractive index for a PS-co-PVDAT 1 mol % VDAT copolymer film produced from a 3% (w.t.) toluene solution exposed to PNT for twenty seconds .................................................................................................................152

Figure 4.4.8 The change in refractive index for the PS-co-PVDAT 1 mol % VDAT copolymer film produced from a 3% (w.t.) toluene solution exposed to PNT for forty seconds ..............................................................................................................................154

Figure 4.5.1 The before and after refractive index curves for a PMMA film spin coated from a 3% (w.t.) toluene solution exposed to 1,3-DNB for ten seconds .........................159

Figure 4.5.2 The refractive index curves for a PMMA-co-PVDAT 1 mol % VDAT copolymer thin film spin coated from a 3% (w.t.) toluene polymer solution exposed to 1,3- DNB for sixteen minutes .....................................................................................161

Figure 4.5.3 The refractive index curves for a PMMA-co-PVDAT 1 mol % VDAT copolymer thin film spin coated from a 3% toluene solution exposed to NB for twenty-five minutes .................................................................................................................163

Figure 4.5.4 The refractive index curves for a PMMA-co-PVDAT 5 mol % VDAT copolymer film spin coated from a 1% (w.t.) toluene solution before and after exposure to NB for twenty-five minutes .................................................................................165

xxx

Figure 4.5.5 The ellipsometry curves for a PMMA-co-PVDAT 5 mol % VDAT copolymer film spin coated from a 1% (w.t.) toluene solution before and after exposure to 1,3-DNB for twenty-five minutes .......................................................................167

Figure 4.6.1 The before and after ellipsometry curves for a P2VP polymer film spin coated from a 3% (w.t.) toluene solution exposed to PNT for five seconds ..................170

Figure 4.7.1 The before and after refractive index curves for a P4VP film spin coated from a 3% (w.t.) ethanol solution exposed to a concentrated PNT vapor for five seconds ..............................................................................................................................174

Figure 4.7.2 The PVI thin film spectroscopic ellipsometry curves showing a change in refractive index after a five second exposure to PNT .........................................177

Figure 4.7.3 The before and after refractive index curves for a thin PVI-co-PVA polymer film exposed to PNT for five seconds .........................................................................180

Figure 4.8.1 The ellipsometry curves for a spin coated polystyrene/10-M film from a 3% (w.t.) polystyrene solution containing 1% (w.t.) 10-M exposed to 1,3-DNB for two hours ....................................................................................................................185

Figure 4.8.2 Refractive index curves for a polystyrene/10-M film spin coated from 1% (w.t.) polystyrene/toluene polymer solution containing 0.5% (w.t.) 10-M exposed to 1,3-DNB for three hours .....................................................................................188

Figure 4.8.3 The refractive index curves for a polystyrene/10-M film spin coated from a 1% (w.t.) polystyrene toluene solution containing 0.1% (w.t.) 10-M exposed to 1,3- DNB for three hours ............................................................................................191

Figure 4.8.4 The refractive index curves for a polystyrene/10-M film spin coated from a 1% (w.t.) polystyrene/toluene solution containing 0.1% (w.t.) 10-M exposed to 1,3- DNB for ten seconds ...........................................................................................194

Figure 5.1.1 Image of 1,3-DNB crystals ..................................................................................201

Figure 5.1.2 1H NMR spectrum of 1,3-DNB crystals (360 MHz, CDCl3) ...............................202

Figure 5.1.3 The FTIR spectrum of the 1,3-dinitrobenzene crystals .......................................203

Figure 5.1.4 Electronic absorption spectrum of the 1,3-DNB crystals in acetonitrile .............204

Figure 5.1.5 Diffuse reflectance spectrum of the 1,3-DNB crystals ........................................205

Figure 5.2.1 9-EC + 1,3-DNB crystals after drying for two days, producing yellow-orange tint crystals .................................................................................................................206

xxxi

Figure 5.2.2 1H NMR (360 MHz, CDCl3) spectra for the 9-ethylcarbazole crystals (9-EC), the co-crystals (9-EC co-crystals with 1,3-DNB), and 1,3-dinitrobenzene crystals (1,3-DNB) ............................................................................................................207

Figure 5.2.3 FTIR spectra of KBr pellets containing either 9-EC crystals (black curve) or the co-crystals containing 9-EC and 1,3-DNB (red curve) ........................................209

Figure 5.2.4 Electronic absorption spectra of 1,3-DNB crystals (black), 9-EC crystals (red), and 9-EC co-crystals with 1,3-DNB (blue) in acetonitrile ..................................210

Figure 5.2.5 Electronic absorption spectra in acetonitrile for the 9-EC co-crystals (red) and the sum of the spectra for 9-EC and 1,3-DNB crystals (black) .................................211

Figure 5.2.6 Diffuse reflectance spectra for the 9-EC crystals (black) and the co-crystals of 9- EC and 1,3-DNB (red) .........................................................................................212

Figure 5.3.1 Co-crystals of 9-VC and 1,3-DNB.......................................................................214

Figure 5.3.2 1H NMR spectra of the 1,3-DNB crystals, 9-VC crystals, and 9-VC co-crystals with 1,3-DNB (360 MHz, CDCl3) .......................................................................215

Figure 5.3.3 FTIR spectra of KBr pellets containing either 9-VC crystals (red) or the co- crystals of 9-VC and 1,3-DNB (black) ................................................................216

Figure 5.4.1 Crystals of carbazole (A) and co-crystals of carbazole and 1,3-DNB (B) ..........218

Figure 5.4.2 1H NMR spectra of CBZ crystals, CBZ co-crystals with 1,3-DNB, and 1,3-DNB crystals (360 MHz, CDCl3) ..................................................................................219

Figure 5.4.3 FTIR spectra for KBr pellets containing CBZ crystals (black) and the CBZ co- crystals with 1,3-DNB (red) .................................................................................221

Figure 5.4.4 Electronic absorption spectra in acetonitrile for 1,3-DNB crystals (black), CBZ crystals (red), and CBZ co-crystals containing 1,3-DNB and CBZ (blue) ..........223

Figure 5.4.5 Comparison of the electronic absorption spectrum in acetonitrile for the co- crystals containing 1,3-DNB and CBZ (red) and the sum of the spectrum for 1,3- DNB crystals and the spectrum for the CBZ crystals (black) ..............................224

Figure 5.5.1 Images of PHZ co-crystals (A) and PHZ crystals (B) .........................................226

Figure 5.5.2 1H NMR spectra (360 MHz, CDCl3) of the 1,3-DNB crystals, PHZ crystals, and the co-crystals made from PHZ and 1,3-DNB .....................................................227

Figure 5.5.3 FTIR spectra for KBr pellets containing either PHZ (black) or the co-crystal of PHZ and 1,3-DNB (blue) .....................................................................................228

xxxii

Figure 5.5.4 Electronic absorption spectra recorded in acetonitrile for 1,3-DNB crystals (black), PHZ crystals (red), and the co-crystals containing PHZ and 1,3-DNB (blue) ....................................................................................................................230

Figure 5.5.5 Electronic absorption spectra in acetonitrile for the PHZ co-crystals (red) and the sum of the spectra for 1,3-DNB crystals and PHZ crystals (black) ...............231

Figure 5.5.6 Diffuse reflectance spectra for PHZ crystals (black) and co-crystals containing PHZ and 1,3-DNB (red) .......................................................................................232

Figure 5.6.1 Images of 10-M co-crystals (A) and 10-M co-crystals with 1,3-DNB (B) ..........234

Figure 5.6.2 1H NMR spectra of 1,3-DNB crystals, 10-M crystals, and co-crystals containing 10-M and 1,3-DNB (360 MHz, CDCl3) ...............................................................235

Figure 5.6.3 Infrared spectra of KBr pellets containing either 10-M crystals (black) or the co- crystals containing 10-M and 1,3-DNB (red) ......................................................236

Figure 5.6.4 Electronic absorption spectra for 1,3-DNB crystals (black), 10-M crystals (red), and the co-crystals (blue) .....................................................................................238

Figure 5.6.5 Electronic absorption spectra for the co-crystals (red) and the sum of the spectra for 1,3-DNB crystals and 10-M crystals (black) ..................................................239

Figure 5.6.6 Diffuse reflectance spectra for the 10-M crystals (black) and the co-crystals containing 10-M and 1,3-DNB (red) ...................................................................240

Figure 5.6.7 50% probability ellipsoid plot of the asymmetric unit of the co-crystal. The dashed lines indicate distances that were less than the sum of the van der Waals radii ......................................................................................................................242

Figure 5.6.8 Short contact environment around 1,3-DNB A (left) and B (right). The green lines indicate distances that were less than the sum of the van der Waals distance .....243

Figure 5.6.9 Short contact environment around 10-M A (left) and B (right). The green lines indicate distances that were less than the sum of the van der Waals contacts ....244

Figure 5.6.10 The 1,3-DNB-10-M H-bonded chain along b. The green lines indicate the distances that were less than the sum of the van der Waals contacts ..................244

Figure 5.6.11 View along b axis of π- π stacking interactions between A chains. The green lines indicate the distances that were less than the sum of the van der Waals contacts ..............................................................................................................................245

xxxiii

Figure 5.6.12 Packing down b axis showing only A chains (left) and all atoms (right). A chains are colored blue in both pictures. B chains are colored red. Crystallographic axes are color coded as a = red, b = green, c = blue ....................................................246

Appendix Figure 1 VDAT 1H NMR (360 MHz, DMSO-d6) ..................................................259

Appendix Figure 2 PS-co-PVDAT 10 mol % VDAT copolymer 1H NMR spectrum (360 MHz, CDCl3) ...........................................................................................260

Appendix Figure 3 PS-co-PVDAT 5 mol % VDAT copolymer 1H NMR spectrum (360 MHz, CDCl3)......................................................................................................261

Appendix Figure 4 PS-co-PVDAT 1 mol % VDAT copolymer 1H NMR spectrum (360 MHz, CDCl3)......................................................................................................262

Appendix Figure 5 Polystyrene 1H NMR (360 MHz, CDCl3) spectrum ................................263

Appendix Figure 6 Polystyrene 13C NMR (500 MHz, CDCl3) spectrum ...............................264

Appendix Figure 7 PS-co-PVDAT 1 mol % VDAT copolymer 13C NMR spectrum (500 MHz, CDCl3)......................................................................................................265

Appendix Figure 8 PS-co-PVDAT 5 mol % VDAT copolymer 13C NMR spectrum (500 MHz, CDCl3)......................................................................................................266

Appendix Figure 9 PS-co-PVDAT 10 mol % VDAT 13C NMR spectrum (500 MHz, CDCl3) ..................................................................................................................267

Appendix Figure 10 VDAT 13C NMR spectrum (500 MHz, DMSO-d6) .................................268

Appendix Figure 11 Polystyrene TGA curve ............................................................................269

Appendix Figure 12 PS-co-PVDAT 1 mol % VDAT copolymer TGA curve .........................270

Appendix Figure 13 PS-co-PVDAT 5 mol % VDAT copolymer TGA curve .........................271

Appendix Figure 14 The PS-co-PVDAT 10 mol % VDAT copolymer TGA curve ................272

Appendix Figure 15 The PS-co-PVDAT 1 mol % VDAT copolymer GPC data .....................273

Appendix Figure 16 The PS-co-PVDAT 5 mol % VDAT copolymer GPC data .....................273

Appendix Figure 17 The PS-co-PVDAT 10 mol % VDAT copolymer GPC data ...................274

xxxiv

Appendix Figure 18 The PS-co-PVDAT 20 mol % VDAT copolymer GPC data ...................274

Appendix Figure 19 FTIR spectrum of the PMMA-co-PVDAT 20 mol % VDAT copolymer ..................................................................................................................275

Appendix Figure 20 FTIR spectrum of the PMMA-co-PVDAT 10 mol % VDAT copolymer .................................................................................................................276



Appendix Figure 23 The 1H NMR spectrum of PMMA in DMSO-d6 using the 500 MHz spectrometer ............................................................................................279

Appendix Figure 24 The 1H NMR spectrum for the PMMA-co-PVDAT 1 mol % VDAT copolymer in CDCl3 using the 500 MHz spectrometer ..........................280

Appendix Figure 25 The 1H NMR spectrum for the PMMA-co-PVDAT 1 mol % VDAT copolymer with the spectrum intensity increased showing the vinyl protons for either MMA or VDAT (6.18, 5.48, and 5.45 ppm) suggesting unreacted monomer present within the polymer matrix ..........................281

Appendix Figure 26 The 1H NMR spectrum of the PMMA-co-PVDAT 5 mol % VDAT in CDCl3 using the 500 MHz spectrometer .................................................282

Appendix Figure 27 The 1H NMR spectrum of the PMMA-co-PVDAT 10 mol % VDAT copolymer in CDCl3 using the 500 MHz spectrometer ..........................283

Appendix Figure 28 The 13C NMR spectrum of PMMA in DMSO-d6 using the 500 MHz spectrometer .............................................................................................284

Appendix Figure 29 The 13C NMR spectrum of the PMMA-co-PVDAT 10 mol % VDAT copolymer in CDCl3 using the 500 MHz spectrometer ...........................285

Appendix Figure 30 The 13C NMR spectrum of the PMMA-co-PVDAT 5 mol % VDAT copolymer in CDCl3 using the 500 MHz spectrometer ...........................286

Appendix Figure 31 The 13C NMR spectrum of the PMMA-co-PVDAT 1 mol % VDAT copolymer in DMSO-d6 using the 500 MHz spectrometer ......................287

Appendix Figure 32 TGA curve for the PMMA-co-PVDAT 1 mol % VDAT copolymer ......288

xxxv

Appendix Figure 33 The TGA curve for the PMMA-co-PVDAT 5 mol % VDAT copolymer ..................................................................................................................289

Appendix Figure 34 The TGA curve for the PMMA-co-PVDAT 20 mol % VDAT copolymer ..................................................................................................................290

Appendix Figure 35 GPC curve and data for the PMMA-co-PVDAT 20 mol % VDAT copolymer ...............................................................................................291

Appendix Figure 36 The GPC curve and data for the PMMA-co-PVDAT 10 mol % VDAT copolymer ...............................................................................................292

Appendix Figure 37 The GPC curve and data for the PMMA-co-PVDAT 5 mol % VDAT copolymer ...............................................................................................293

Appendix Figure 38 The PMA 1H NMR spectrum (360 MHz, CDCl3) ...................................294

Appendix Figure 39 The PMA 13C NMR spectrum (500 MHz, DMSO-d6) ............................295

Appendix Figure 40 The P2VP 1H NMR spectrum (360 MHz, CDCl3) ..................................296

Appendix Figure 41 The P2VP-co-PVDAT 5 mol % VDAT copolymer 1H NMR spectrum (360 MHz, DMSO-d6) .............................................................................297

Appendix Figure 42 The P2VP-co-PVDAT 1 mol % VDAT copolymer 1H NMR spectrum (360 MHz, DMSO-d6) .............................................................................298

Appendix Figure 43 The P2VP 13C NMR spectrum (500 MHz, DMSO-d6) ............................299

Appendix Figure 44 The P2VP-co-PVDAT 5 mol % VDAT copolymer 13C NMR spectrum (500 MHz, DMSO-d6) from 180 - 110 ppm ............................................300

Appendix Figure 45 The P2VP-co-PVDAT 20 mol % VDAT copolymer 13C NMR spectrum (500 MHz, DMSO-d6) from 190 - 110 ppm ............................................301

Appendix Figure 46 The PAM polymer 1H NMR spectrum (360 MHz, D2O) ........................302

Appendix Figure 47 The PAM-co-PVDAT 1 mol % VDAT copolymer 1H NMR spectrum recorded in D2O (360 MHz) ....................................................................303



xxxvi

Appendix Figure 50 The Poly(acrylamide) (PAM) 13C NMR spectrum (500 MHz, D2O) ......306

Appendix Figure 51 The PAM-co-PVDAT 1 mol % VDAT copolymer 13C NMR spectrum (500 MHz, D2O).......................................................................................307

Appendix Figure 52 The PAM-co-PVDAT 5 mol % VDAT copolymer 13C NMR spectrum (500 MHz, D2O).......................................................................................308

Appendix Figure 53 The PAM-co-PVDAT 10 mol % VDAT copolymer 13C NMR spectrum (500 MHZ, D2O) from 190 - 150 ppm showing the PMA carbonyl carbon signal (C3) and the PVDAT carbon signal (C7) confirming the presence of PVDAT in the copolymer ........................................................................309

Appendix Figure 54 The 13C NMR spectrum for the PMMA-co-PVK 20 mol % vinylcarbazole recorded in CDCl3 (500 MHz) .................................................................310

Appendix Figure 55 The DSC curve for the PVI homopolymer ..............................................311

xxxvii

LIST OF SCHEMES

Scheme 1.1 Classification of explosives .........................................................................2

Scheme 2.2.1.1 Free radical polymerization of PVDAT .....................................................43

Scheme 2.2.2.1 Free radical polymerization for the synthesis of PS-co-PVDAT random copolymers .................................................................................................45

Scheme 2.2.3.1 Free radical polymerization for the synthesis of PMMA-co-PVDAT random copolymers ....................................................................................47

Scheme 2.2.4.1 Free radical polymerization for the synthesis of a PMA-co-PVDAT random copolymer .....................................................................................48

Scheme 2.2.5.1 Free radical polymerization for the synthesis of P2VP-co-PVDAT random copolymers .................................................................................................50

Scheme 2.2.6.1 Free radical polymerization for the synthesis of PAM-co-PVDAT random copolymers .................................................................................................52

Scheme 2.2.7.1. Free radical polymerization for the synthesis of PVK-co-PVDAT random copolymers .................................................................................................54

Scheme 2.2.8.1. Free radical polymerization for the synthesis of PS-co-PVK random copolymers .................................................................................................56

Scheme 2.2.9.1 Free radical polymerization for the synthesis of PMMA-co-PVK random copolymers .................................................................................................57

Scheme 2.2.10.1 Free radical polymerization for the synthesis of a PVI-co-PVDAT random copolymer ..................................................................................................58

Scheme 2.2.11.1 Free radical polymerization for the synthesis of a PS-co-PVI random copolymer ..................................................................................................60

Scheme 2.2.12.1 Free radical polymerization for the synthesis of a PMMA-co-PVI random copolymer ..................................................................................................61

1

Chapter 1

Introduction

1.1 Explosives

Explosives are defined as energetic materials which react to produce rapid oxidation of

products accompanied by the generation of heat, light, or gas.1 Explosives are classified by

structure and performance, and fall into two broad categories, low and high explosives, with high

explosives being further classified into additional categories. Scheme 1.1.1 shows the

classification for high and low explosives. Low explosives, which include propellants and

pyrotechnics, burn at relatively low rates (cm/s) and are capable of producing heat, light, smoke,

gas, or sound with propellants.1 High explosives are capable of detonating at high velocities (1 to

9 km/s) and produce vast amounts of energy (400 to 1,200 cal/g).1 High explosives are further

classified into primary and secondary explosives based on their ability to detonate. Primary

explosives are very susceptible to initiation and are used as a source for igniting secondary

explosives. Secondary explosives, which consist of nitroaromatics and nitro-amines, are

prevalent for military and industry uses, since they are designed to detonate under specific

conditions.

2

Scheme 1.1.1. Classification of explosives.2

Explosives

High Explosives Low Explosives

Pyrotechnics Propellants Primary Explosives

Secondary Explosives

Military Explosives

Industrial Explosives

3

1.1.2 Types and Properties of Explosives

There are several different types of manufactured explosive materials designed to

detonate, but the most common are organic based compounds. The organic based secondary

explosives fall into two categories based on their structure: aromatic and aliphatic. Aromatic

explosives contain a benzene ring, and the aliphatic explosives do not. A variety of aromatic

explosives exists due to the fact one or more molecular subgroups can be substituted for a

hydrogen atom on the benzene ring. Aliphatic explosive materials primarily consist of aliphatic

nitrate esters (─ONO2), aliphatic nitramines (─N─NO2), and nitro-aliphatics (─NO2). The most

common aliphatic explosives are the nitrate esters. Examples of aromatic and aliphatic

explosives are: 2,4,6-trinitrotoluene (TNT), nitroglycerine (NG), tetranitro-triazacyclohexane

(RDX), tetranitro-tetrazacyclooctane (HMX), tetranitro-N-methylamine (Tetryl), and 2,4,6-

trinitrophenol (picric acid).

Even though most explosives are organic based materials, inorganic explosive materials

also exist and are composed of fuels and oxidizers. A common inorganic explosive is ammonium

nitrate (NH4NO3), which is commercially produced as a fertilizer. A common inexpensive

explosive is created when NH4NO3 is combined with other explosives or fuels. Fulminates

(Metal─ONC) are pure inorganic primary or initiating explosives, but are seldom used due to

their susceptible detonation and poor storage characteristics. Azides are another type of inorganic

explosives, which are the salts of hydrazoic acids. The most common explosive in the azide

family is lead azide (PbN6), which can be used for initiating explosives. Other azides exist such

as silver azide (AgN3) and sodium azide (NaN3), which are classified as low explosives since the

combustion reaction is less reactive than that of high explosives.

4

Most explosives contain carbon, hydrogen, oxygen, and nitrogen, many of which are

richer in oxygen and nitrogen than carbon and hydrogen. The oxygen:nitrogen ratio is an

important characteristic that many anomaly detectors rely on for detecting explosives. Explosives

have a wide range of vapor pressures at ambient temperatures shown in Table 1.1.2.1. The low

vapor pressures of explosives make detection by vapor methods difficult. Detecting explosives

with low vapor pressures require vapor sensors to either sample large volumes of air or have low

limits of detection. To improve the detection of low vapor pressure explosives, 2,3-dimethyl-2,3-

dinitrobutane (DMNB) is incorporated in some explosives as a marker. DMNB contributes to

locating explosives either by vapor pressure or by odor contamination due to its high vapor

pressure, high permeability, and no known industrial applications.3

5

Table 1.1.2.1. Common explosives' vapor pressures.3

Class Explosive Vapor pressure at 25 °C (Torr)

Acid salt Ammonium nitrate NH4NO3 5.0 x 10-6

Aliphatic nitro Nitromethane 2.8 x 101 2,3-Dimethyl-2,3-dinitrobutane DMNB 2.1 x 10-3

Aromatic nitro

2-Nitrotoluene o-MNT 1.5 x 10-1 4-Nitrotoluene PNT 4.1 x 10-2

2,4-Dinitrotoluene 2,4-DNT 2.1 x 10-4

2,4,6-Trinitrotoluene TNT 3.0 x 10-6

2,4,6-Trinitrophenol Picric acid 5.8 x 10-9

Nitrate ester

Ethylene glycol dinitrate EGDN 2.8 x 10-2

Nitroglycerin NG 2.4 x 10-5

Pentaerythritol tetranitrate PETN 3.8 x 10-10

Nitrocellulose NC N/A

Nitramine Tetranitro-N-methylamine Tetryl 5.7 x 10-9

Tetranitro-triazacyclohexane RDX 1.4 x 10-9

Tetranitro-tetrazacyclooctane HMX 1.6 x 10-13

Peroxide Triacetone triperoxide TATP 3.7 x 10-1 Hexamethylene triperoxide diamine HMTD N/A

6

1.1.3 Economic and Human Cost from Explosives

The development and use of explosive materials has been considered beneficial both

socially and economically, but these materials have conversely been used to create terror, chaos,

destruction, and human fatalities. Between 1999 and 2009, there were approximately 74,000

causalities in 119 countries due to landmines, explosive remnants from previous wars, and

improvised explosive devices.4 The Unabomber, Ted Kaczynski, mailed packages containing

small explosives. On April 19, 1995, the Alfred P. Murrah federal office building in Oklahoma

City, O.K. was bombed, killing 167 and causing property destruction totaling $652 million.5 On

December 25, 2009, a Nigerian man on a flight from Amsterdam to Detroit attempted to ignite a

hidden explosive device consisting of a mixture of powder and liquid that passed airport security.

On May 1, 2010, a car bomb was discovered near Times Square, in New York City, after smoke

was observed coming from the vehicle. The bomb ignited, but did not detonate, preventing any

harm to the surrounding environment. These few examples listed above illustrate how easily

explosives can be utilized and how potentially detrimental they can be to innocent civilians.

1.2 Landmine Overview

A landmine can be generally described as an explosive device that is placed under, on, or

near the surface of an area and is designed to detonate by the contact of a person or vehicle.6

Landmines are constructed such that when detonated, they are capable of causing either

immediate injury or death to individuals, while also having the ability to disable military

vehicles. Landmines are indiscriminate weapons primarily used to target opposing enemies, but

in other instances affect innocent civilians. A significant problem with landmines is that once

laid, the mines remain in place long after conflicts and remain a danger.

7

1.2.1 History of Landmines

Anti-tank (AT) landmines first originated in the First World War as a means of disabling

advancing tanks. These mines were effective against military vehicles, but were relatively large,

easily noticeable, and easily removed. To combat the removal of these mines, Germans and

Italians developed anti-personnel (AP) landmines during the Second World War which consisted

of grenades and fuses to prevent allied forces from removing AT landmines.6 These mines were

much smaller than the AT landmines and were primarily designed to injure or kill military

soldiers, as opposed to merely disabling military vehicles. After the war, the employment of AP

landmines became more common and is still used in modern warfare. Today, landmines are often

deployed by terrorist organizations within civil conflicts and guerilla warfare, frequently creating

excessive human causalities.

1.2.2 Landmines: The Problem

The International Committee for the Red Cross estimated that 120 million unexploded

mines are buried in 70 countries around the world.7 In 1995, A United Nations (UN) estimate

indicated that at that time, only about 80,000 of the 120 million landmines were cleared and in

the same time period an additional 2.5 million mines were buried.7 From that estimate, with the

current technology at that time, it would take approximately 1,100 years to clear all mined areas

from the 1995 estimate.7 A more recent estimate indicated that there are 50 to 100 million

landmines buried in more than 80 countries with only 100,000 mines deactivated per year; this is

compared to the estimated two million landmines that are additionally laid annually.8 Table 1.2.1

shows countries that have ongoing issues with unexploded landmines. Due to the lifetime of

these mines and placement in public areas, civilians who were not associated with the original

conflict are often affected. One reason that many undetonated mines are still in place is the lack

8

of resources needed to remove them. The production of a landmine can cost as little as $3 U.S.,

but the removal cost can be as high as $1,000 U.S.8

1.2.3 Landmines: Human and Economic Costs

Landmines have affected both social and economic aspects of civilians’ lives in many

ways; from human causalities and injuries, to the limitation of the use of lands for farming or

schools, and general concerns of security are just a few of the problems faced due to hidden

mines. There are estimates that mines kill or injure a person every 20 minutes, 70 people a day,

or more than 20,000 people a year.8 Most of those killed or injured from these landmines are

noncombatants. Many countries dealing with landmine problems do not have adequate medical

resources to provide proper treatment and rehabilitation necessary for the injured victims.

Landmines are most detrimental to the economic development of countries by limiting

the use of lands for agriculture. It is estimated that landmines have limited access or degraded

some 900,000 km2 of land globally.9 The loss of productivity from agricultural lands due to

landmines has been linked to four factors: access denial, loss of biodiversity, chemical

contamination, and micro-relief disruption.9 This in turn affects the local economy, since many

of the countries depend on agriculture as a means of income, food, and goods for export.

9

Table 1.2.1. List of countries with estimated unexploded landmines.7

Countries

Unexploded Landmine Estimates

Afghanistan

10 million

Angola

15 million

Bosnia and Herzegovina

3 - 6 million

Cambodia

10 million

Croatia

3 million

Egypt

23 million

Iran

16 million

Iraq

10 million

Mozambique

2 million

10

1.3 Classification of Landmines

Landmines are classified into two categories: anti-personnel mines (AP) and anti-tank

mines (AT). These mines are encased in different types of materials; these include metal, plastic,

wood, or in some instances, contain no encasement at all. Modern landmines are comprised of

very few metal components, thereby preventing detection by metal detectors. The detonating

mechanism for landmines can be constructed from simple trip wires, pressure triggers, tilt rods,

acoustic or seismic fuses, and light or magnetic fuses. Landmines are utilized in a variety of

arrangements, some of which include, burial in fields with a mixture of other objects and clutter,

burial at various depths, scattered on the surface, planted in buildings, hidden using plant

vegetation, and placed near or under important roads.

1.3.1 Anti-Tank Landmines

Anti-tank (AT) mines have truncated square or cylinder shapes with lengths ranging from

150 to 300 mm and a thickness of 50 to 90 mm.10, 11 These mines are designed to be buried near

the surface or to depths greater than 150 mm below the surface. The AT mines contain

approximately 2 to 10 kg of explosives, and are activated by pressures of hundreds of kg.10, 11

These mines are typically buried under roads of critical importance for military use. The

limitations of roads affects transportation and economic development, but are considered less

damaging and destructive than AP mines.

1.3.2 Anti-Personnel Landmines

Anti-personnel (AP) mines are shaped in the form of disks or cylinders with diameters

from 20 to 150 mm and lengths from 50 to 100 mm.10, 11 These mines contain 10 to 250 g of

explosives, and are buried near the surface to a maximum depth of 50 mm, detonating under

pressures of 0.5 to 50 kg.10, 11 Locating and removing AP mines provides a challenge due to a

11

wide variety of mines; with more than 700 different types in existence, these mines are

constructed in different shapes, sizes, and materials. Table 1.3.2.1 provides an example of the

variety of AP mines based on their material, color, fuse, explosive charge, and weight. Many of

the injuries and causalities mentioned previously are caused by AP mines used in previous or

ongoing conflicts.

1.3.3 Chemical Signatures of Landmines

Common explosives used for the main charge in landmines shown in Figure 1.3.3.1 are

RDX, Composition B, tetryl, C-4, with the most common explosive material being TNT. Nearly

80% of the manufactured landmines in the world contain TNT.7 Even though military grade TNT

is the majority of the explosive material, other impurities were also found with higher vapor

concentrations than TNT itself, including 1,3-DNB, 2,4-DNT, 2,6-DNT, and 2,4-DNB (Figure

1.3.3.1).7, 12 The headspace above a military grade TNT filled landmine contained 0.35 to 9.7

pg/mL 1,3-DNB, 0.28 to 1.4 pg/ml of 2,4-DNT, and 0.070 to 0.078 pg/ml of TNT.12 Jenkins et

al. discovered that in many cases the most prevalent chemical signatures in the surface soil were

2,4-DNT, 2-ADNT, and 4-ADNT.12 The mines including plastic casings or those that were not

well-sealed allowed greater concentrations of the chemical signatures to escape, compared with

mines containing metal cases and those that were adequately sealed. Flux measurements were

recorded of 1,3-DNB, 2,4-DNT, and TNT from a few landmines at various temperatures.7 The

impurities, which accompanied TNT, showed an increase in vapor concentrations higher than

TNT itself. Since the impurities found with TNT exhibited higher vapor concentrations,

landmine and explosive detection systems could be directed to detect 1,3-DNB and 2,4-DNT

vapors as signatures for TNT based explosives.

12

Table 1.3.2.1. Examples of the variety of AP mines based on their material, color, fuse, explosive charge, and weight.13

Type Weight (kg)

Case Material Case Color Fuse Explosive

charge

Explosive Weight

(g)

Type 69 1.35 Cast Iron Olive drab Pressure or trip wire TNT 105

Type 72 0.125 Plastic Green Pressure TNT/RDX 75-100

M14 0.158 Plastic Olive drab Pressure Tetryl 29

M16A1 3.57 Steel Green Pressure or trip wire TNT 513

M18A1 1.58 Plastic Olive drab Command detonation C-4 682

Valmara 69 3.3 Plastic Green/Sand Trip wire or pressure Comp. B 597

VS-50 0.185 Plastic Olive drab /Sand Pressure RDX 43

PP-MI-SR 3.2 Steel/Plastic Olive drab Trip wire or pressure TNT 362

MON-200 25 Metal Olive drab Trip wire / Command detonation

TNT 12 kg

PMN 0.55 Bakelite Black Delay-armed/ pressure TNT/Tetryl 200

POMZ-2 2.3 Metal Olive drab Trip wire TNT 75

PMD-6 0.4 Wood Wood Pressure TNT 200

13

N+

O

-O

N+ O

-O

N+ O

-O

Tri-nitro-toluene(TNT)

HN

N+

O

-O

N+

O-O

N+

O

O-N+

O

-O

2,4,6-Trinitrophenylmethylnitramine(Tetryl)

N

N

N

N+ O

-O

N+

O

O-

N+

O

-O

1,3,5-Trinitro-1,3,5-triazacyclohexane(RDX)

C-4 (RDX Based)

Comp. B (RDX + TNT)N+

O

-ON+

O

O-

1,3-dinitrobenzene(1,3-DNB)

N+

O

-ON+

O

O-

2,4-dinitrotoluene(2,4-DNT)

N+

O

O-

N+

O

-O

2,6-dinitrotoluene(2,6-DNT)

N+

O

-O

2,4-dinitrobenzene(2,4-DNB)

NO O

Figure 1.3.3.1. Chemical structures of common explosives and TNT impurities found and used in landmines.

14

1.4 Landmine Detection Methods

The detection and removal of buried landmines is a significant concern in many countries

throughout the world. The development of AP mines has made detection very challenging, as

modern mines contain few metal components. They are commonly buried in areas containing

metallic debris to avoid detection, thus producing false alarms. A typical rate for locating and

eliminating mines is 80%, but for humanitarian demining, the U.N. requires a rate approaching

perfection, 99.6%.14 An optimal landmine detection system should have the ability to detect

landmines without reference to the type of explosive material the mine contains, shape, size, or

casing material. Additionally, the detection system should virtually produce no false alarms, be

operator friendly, exhibit reasonable operation speed, and be cost efficient. The following

sections will review currently employed and developing landmine detection systems used to

locate buried landmines.

1.4.1 Manual Detection Method

Prodders

The most common approach to humanitarian landmine detection is prodding. Typically, a

deminer will use a prod to enter the soil and scan a small grid. When the prod encounters an

object, the deminer will assess the contour of the object and cautiously remove the surrounding

soil until the object can be identified. This technique has a very successful detection rate, but is

very laborious and hazardous. The success of the deminer is achieved through experience

learning to recognize the physical characteristics of certain objects, and perceiving the difference

between a mine's casing and other various objects by sound. Even though this technique has a

high detection rate, the deminer is always at a constant risk of either stepping on a mine or

applying too much force during the prodding process, triggering detonation of the mine. Even

15

though this technique is applicable for humanitarian demining, alternative methods are being

investigated to lower the risk associated with demining.

1.4.2 Electromagnetic Detection Systems

Metal Detectors

Another common landmine detection system for humanitarian demining is hand held

metal detectors. The metal detector works by emitting a magnetic field to produce an eddy

current in a metallic object to generate a detectable magnetic field.6, 8, 11, 14 One disadvantage

associated with this detection system is that most modern mines contain very little metal

(primarily the firing pin) or contain no metal at all. The sensitivity of the metal detector can be

increased to detect smaller quantities of metal, but in return increases the possibility of false

alarms triggered by other metallic objects. Metal detectors primarily indicate that the buried

object contains metal, but cannot identify whether or not the object contains any explosive

materials. This detection system is relatively inexpensive when compared to other detection

systems, but is not time efficient and is a hazardous process for the operator.

Ground Penetrating Radar (GPR)

Ground penetrating radar (GPR) is used in many applications including civil engineering,

geology, and archeology for studying soil and detecting buried objects. GPR is capable of

detecting buried landmines by emitting radio waves into the ground, creating changes in the

reflected signal that arise from variations in the dielectric constants for objects in the soil.6, 8, 14, 15

The reflected signals create an image of a vertical slice of soil, which is then analyzed. Shorter

wavelengths provide greater soil penetration depth and better image resolution, but wide-band

frequencies provide better details and improve the signal to noise ratio.8, 15 GPR provides the

advantage of being able to detect landmines with a wide variety of casing materials due to the

16

different dielectric constants (compared to the soil) and is capable of generating an image of the

landmine.

A disadvantage of this detection system is that small objects require higher frequencies

(GHz), which have limited penetration depth.6, 8, 14 Higher frequencies also have increased image

clutter. Also, inhomogeneous subsoil can create false alarms. The system is also sensitive to

complex interactions produced by metal content, soil moisture, soil surface smoothness, and

radar frequencies.6 The high cost for this detection system precludes its application for

humanitarian demining.

Infrared (IR) and Hyper Spectral Methods

Infrared (IR) and hyper spectral detection systems have the ability to detect landmines by

observing variations in the electromagnetic radiation reflected or emitted by the mine or the soil

and vegetation above the mine compared to the surrounding environment.6, 8, 16 These detection

systems operate by two methods: thermal and non-thermal. Thermal detection systems observe

variations in temperatures between the soil and vegetation located near the mine compared to the

surrounding environment. During the day, mines absorb and release heat at a different rate than

their surrounding environment. Laser illumination or high-powered microwaves have been used

to produce these temperature profiles. Non-thermal detection systems observe the difference of

the reflected light (either natural or artificial) from areas surrounding a landmine. This system

can also detect the presence of a mine from the burying process, disrupting the soil's particle

distribution, and affecting the way the soil scatters light.6

These detection systems are advantageous, as they do not require physical contact and

can be used at a distance. The equipment is also lightweight, capable of analyzing a large area

(employed from an airplane), and offers fast image acquisition.

17

Although these systems possess some ideal characteristics for detecting landmines, there

are limitations that prevent this technology from being an ideal application for landmine

detection. The primary disadvantage is that the systems are dependent on environmental

conditions. The performance variability cannot always accurately locate and identify buried

landmines, which also prevent these methods from being a reliable detection system. The

wavelength frequencies employed by these system cannot penetrate the soil's surface and the

hyper spectral signatures produced from the landmine burying process can be eliminated by

weathering.6

Electrical Impedance Tomography (EIT)

Electrical impedance tomography (EIT) utilizes electrical currents to image the

conductivity distribution of the medium under investigation.6, 17 An array of electrodes is placed

on the ground to detect signals from the conductivity distribution, providing evidence of an

existing mine. This system is capable of detecting both metal and non-metal mines due to

anomalies produced in the conductivity distribution. EIT is well suited for detecting landmines

buried in wet soil environments, since moisture enhances the conductivity. The equipment is also

lightweight and relatively inexpensive compared to other systems.

A major disadvantage of this detection system is that it requires direct contact with the

area under investigation, thus increasing the possibility of detonating a mine. EIT does not work

in dry non-conductive or rocky surfaces, as the existence of conductivity potential would be

decreased. EIT is sensitive to electrical noise and performance deteriorates as the depth of the

object increases. This system could possibly be considered for the detection of shallow buried

mines located in wet environments.

18

X-Ray Backscatter (XBT)

X-Ray backscatter (XBT) employs the direct transmission of X-rays into the ground

which are backscattered to a detector from an irradiated object, producing an image of the object

due to X-rays passing through matter with an attenuation (absorbed or scattered).6, 8 This system

is advantageous, since landmines and soil have different mass densities and atomic numbers.

Since pass-through X-ray imaging is not achievable, the XBT system exploits the Compton

Principle due to the photons being captured from the irradiated object, allowing the detector and

emitter to be placed above the surface.6, 8 This system includes two techniques for capturing

images of buried landmines: collimate and un-collimated methods. Collimated methods utilize

focused beams and collimated detectors to produce an image, but this process increase the size

and weight of the instrument, while reducing the number of photons for imaging. Collimated

systems also require high power X-ray generators as sources, which in turn limit the systems for

humanitarian demining due to the power requirements, increase in size, and weight. Un-

collimated systems can be constructed at a smaller scale, made more lightweight, and illuminate

a large area with X-rays, making this method ideal for portable detection.

XBT systems are capable of distinguishing mines from soils using low energy photons in

the energy range of 60 to 200 keV.6, 8 The photons allow for larger cross sections compared to

other systems that employ nuclear reactions for sensing. Un-collimated systems can be made

smaller and more lightweight due to the reduced shielding thickness required to impede the low

energy photons.

A disadvantage of these systems is that the required energy range for XBT devices has

poor soil penetration depth, limiting the detection to shallow mines (less than 10 cm).6, 8 If source

strengths remain low for more secure operation, longer times are required for obtaining images.

19

These systems are sensitive to source/detector standoff variations and ground surface

fluctuations. In order to obtain images of small AP mines, high-resolution cameras

(approximately 1 cm) are needed, but image acquisition is difficult in the field.6, 8 Additionally,

this technology emits radiation, which would limit field use due to public concerns.

1.4.3 Acoustic and Seismic Detection Systems

Acoustic and seismic systems are capable of detecting landmines by exploiting the

vibrations of the mechanical properties of the mine's casing and components rather than the

electromagnetic properties.6, 8, 15 Acoustic and seismic systems work by emitting sound/seismic

waves from loud speakers above the ground. Some of the waves are reflected at the surface, but

the remaining waves propagate through the soil and are reflected (upward) toward the surface by

a buried mine; causing vibrations at the ground surface. Sensors located above the ground detect

these surface vibrations by differences in amplitude and frequency.

These systems are capable of detecting mechanical differences between soil and mines

and could complement GPR and EMI detection systems. One field study indicated that these

systems are proficient methods for the detection of AT mines due to a 95% detection rate with

lower occurrences of false alarms.6 These systems have the potential for low false alarm rates

from natural occurring clutter (rocks or scrap metal), but hollow items such as cans or bottles

may produce false alarms as the resonance signals are very similar to those of mines. A

disadvantage of these systems is the inability to detect deeply buried mines, as the resonant

signal decreases significantly with the depth. The scan speeds for these systems are also

considerably slow (in the range of 2 to 15 min per m2).6 These systems are not susceptible to

environmental conditions or weathering, but frozen soil and vegetation may affect the sensor's

capabilities.

20

1.4.4 Biological and Biomimetic Systems

These systems employ the use of mammals, insects, vegetation, and microorganisms for

the detection of landmines by sensing trace vapor chemical signatures released by landmines into

the soil and above the ground surface. Unlike the systems previously discussed which rely on

mechanical or electromagnetic properties for detection, these systems are classified by the ability

to locate buried landmines by the explosive materials incorporated in landmines. By focusing on

the explosive compounds, these methods have the potential to reduce false alarms. This section

will assess the main principles for the detection of landmines citing both the advantages and

disadvantages associated with each system.

Canines, Rats, and Pigs

Canines are commonly employed for humanitarian demining because of their keen sense

of smell and low false alarm rate. Their unique sense of smell can detect a wide range of

explosive materials and explosive vapors at very low concentrations. Canines are known to be

able to detect explosive vapor concentrations comparatively lower than any other chemical

sensor available. Canines are trained to sit when they sense explosive residues escaping from

buried landmines and are rewarded when they correctly identify the explosive vapor. After the

canine indicates the presence of a mine, a deminer will probe the area of interest. Another

technique known as remote explosive scent tracing mode allows the canines to smell samples

collected near suspected mine areas. If the canine indicates the presence of explosive vapors in a

sample from a certain area, the deminer will return to that area to locate the mine.6, 18 Canines

offer the ability to search a large area in a reasonable amount of time, work under many

environmental conditions, are easy to transport, and are highly reliable. The limitations

associated with using canines include cost and time of training, performance variations

21

depending on the canine, and the possibility distractions. The canines are also at a constant risk,

since they work directly in areas where the mines are located. Thus far, canines are expected to

be a mainstay in the detection of landmines for humanitarian demining.

African giant pouch rats provide an alternative option for the detection of buried

landmines, as they offer certain advantages over canines. These rats rely heavily on their sense of

smell due to poor eyesight, which makes them ideal for detection of explosive vapors. Like the

canines the rats are trained to associate the smell of explosives with a food reward.18 In the field,

rats indicate the presence of an explosive by stopping and scratching the area, allowing their

handler to mark the identified mine. Advantages of employing rats compared with canines are

lower cost and less training time. Training time and costs are much less compared with that of

canines, since fewer resources are needed to raise and maintain them. These rats are lightweight

and rarely detonate landmines when walking over them. A rat and its handler can search a

relatively large area (150 m2) in about half an hour and a larger number of them could be used in

an assigned area, further reducing the time to search the area.18 Some limitations associated with

the use of rats for landmine detection include their inability to indicate the type of explosive

material and the breed of rats must be considered with respect to climate and disease.18

Pigs are another source for landmine detection, as they are considered to have an

enhanced sense of smell.18 Pigs are considered very tranquil animals, and are not as easily

distracted when compared to canines. The pigs are trained by a four-stage process by which they

learn to locate mines by receiving food rewards, as with the training of dogs and rats. In the final

training stage, the pigs are taught to sit in order to indicate the presence of a mine; but this can

prove to be a difficult process, as sitting is not a natural response for pigs. The limitations

associated with pigs for detecting landmines include the challenging and time consuming

22

training process, difficulty of transporting, and effects of local climate and disease. Although

there are very few successful documented trials of pigs detecting landmines, they do provide an

additional option for landmine detection.

Insects

Honeybees offer a unique option for humanitarian landmine detection, as they are

capable of covering large areas in a short amount of time, have an acute sense of smell, and their

bodies act as portable samplers with the ability to collect contaminants in the gaseous, liquid, and

particle forms. Honey bees were trained to locate mines by teaching the bees to associate the

smell of a nitroaromatic with a possible food source.6, 18, 19 When trained, the bees would hover

over the explosive plume for a few seconds, indicating the presence of a mine. Researchers

developed an light detection and ranging system (LIDAR) capable of detecting flying bees that

were trained to locate buried mines.19 This remote standoff system utilizes a laser light emitted

over the area the where the bees fly; the light then strikes the bees and is scattered back and

collected by a receiver. The time between the outgoing laser pulse and the return signal is used to

measure the distance from the bees to the LIDAR, which provides both the range and coordinates

of the bees over the landmine. One advantage bees offer as landmine detectors is their ability to

detect buried landmines by explosive vapor plumes at low concentrations, limiting false alarms.

Bees are also capable of scanning large areas, require short training time, and are a means of a

more remote standoff detection system. While bees offer some advantages, there are also

challenges associated with using them. Bees are influenced by environmental conditions (climate

and weather) which limit their use based on the season. The main limitation with the LIDAR

system is distinguishing signals between bees and vegetation or other interfering objects. Bees

are also difficult to track and specialized equipment may be required to improve locating the

23

bees. Although bees provide a unique alternative for landmine detection, it appears that they

could be considered a less reliable and accurate detection system than other options.

Bacterial Biosensors

The general description for biosensors defines them as sensing devices that combine a

biological recognition element to a physiochemical transducer. When a specific interaction

occurs between the recognition element and targeted analyte, it produces a physiochemical

change, which is detected by the transducer. This provides a signal which is proportional to the

concentration of the targeted analyte.18 A reporter gene is incorporated into the biosensor, and

the biosensor will then fluoresce when the biosensor encounters the explosive material. Scientists

have engineered a strain of bacteria that fluoresces under laser light when it encounters TNT.

This system is known as the Microbial Mine Detection System (MMDS).6, 18 Generally, this

method requires spraying the engineered bacteria over a mine-affected area, likely by airplane.

The bacteria are allowed to grow for a few hours, allowing them the opportunity to absorb any

explosive residues present. A survey is then conducted in order to review the mine-affected area

by searching for fluorescent signals, indicating the location of the landmines.

The bacterial biosensors offer a system that can be engineered to detect specific explosive

materials such as TNT and other similar structure analogs, therefore minimizing the likelihood of

false alarms. The sensors can be applied to large areas in a short amount of time and allow

remote standoff detection. The cost of this method could be considered moderate, and even less

expensive depending on the search area.

The main limitation with the MMDS is that the bacteria are extremely sensitive to

environmental conditions. The bacteria cannot survive in extreme temperatures, and also cannot

be applied to areas with dry soil, since the soil would absorb the bacteria and eliminate a

24

detectable signal. The performance of the system is dependent upon the transport of explosive

residues in the soil, which could provide inaccurate locations of the mines. Public concerns

regarding the application of engineered bacteria could also limit this application.

1.4.5 Bulk Explosive Landmine Detection Systems

Bulk detection systems emphasize finding the bulk explosive contained within the mine

rather than trace explosive residues or vapors. The ability to detect bulk explosives helps reduce

the rate of false positives from clutter and other factors. A variety of these systems uses neutrons

to exploit their interaction with the 14N nuclei present in explosives.

Nuclear Quadrupole Resonance (NQR)

Nuclear quadrupole resonance (NQR) is a radio frequency based technique that exploits

the resonant response from the 14N nuclei present in explosive materials, therefore locating and

providing an estimated quantity and/or depth of the mine.6, 8, 14 NQR induces a radio frequency

pulse of an appropriate frequency in the subsurface by a coil suspended above the ground. The

radio frequency pulse causes the 14N nuclei in explosives to resonate and induce an electrical

potential in the receiver coil. The resonant signals' frequencies oscillate between 0.5 to 6 MHz

and are characteristic of a specific explosive.6, 8, 14

A significant aspect of NQR is its specificity to landmines due to signals only produced

when specific bulk explosive materials are present. NQR has a high detection rate and low

occurrences of false alarms; its alarm rate is driven by signal to noise ratio rather than ground

clutter, which affects other detection systems. Another progressive feature of NQR is that by

allowing sufficient inspection time, the detection rate is increased with a false alarm rate

approaching zero. NQR is also capable of success in diverse soil conditions and only detects the

presence of bulk explosive materials rather than simply detecting explosive residues.

25

One limitation of this system, however, is that the nuclear properties of TNT, the major

explosive material in many mines, has a substantially weak signal that may pose a considerable

signal to noise problem. NQR is also susceptible to radio frequency interferences, as the

frequencies required to detect TNT are in the AM frequency range.6, 14 Additionally, NQR is not

capable of detecting landmines with metal casings because the radio frequency is unable to

penetrate them or detect liquid explosives within a landmine.6 These restrictions are not

considered significant factors, since most modern mines are encased with plastics and are not

composed of liquid explosives. The detection rate is also susceptible to the distance between the

coil and bulk explosive; the coil is suspended very close to the surface, which may be difficult in

rough terrain or in areas where vegetation is present. Stationary detection is ideal for achieving

the best results, since motion detection reduces the signal to noise ratio and increases the time to

inspect an area. NQR has certain advantages over other detection systems, as it is capable of

detecting bulk explosives and is not affected by ground clutter or explosive residues. Conversely,

due to interferences from AM radio frequencies, this system's performance may be considered

unreliable for detecting landmines containing TNT.

Neutron Detection systems

Neutrons have the ability to pass easily through matter, interacting with atomic nuclei.

Neutron detection systems are capable of detecting mines by using gamma rays or neutrons. The

interactions between the neutrons and matter produce gamma rays or charged particles, which

provide a unique signature regarding the nucleus and chemical element with which they are

interacting. These provide elemental information to distinguish the mine from its surrounding

environment, since explosives are rich in hydrogen and nitrogen.6, 10 The observation of

variances in intensity, energy, and returning radiation allow identification of the bulk explosive

26

contained within the mine. There are many possible reactions involving neutrons or gamma rays,

but only three have shown potential to be used for landmine detection: thermal neutron analysis,

fast neutron analysis, and neutron moderation.6, 10 Thermal neutron analysis relies on the

emission of specific gamma rays from the nitrogen nuclei when thermal neutrons are captured.6,

10 This technique was used by the Canadian military as a means of detecting AT mines, but was

not as effective in detecting AP mines due to low amounts of explosive materials.20 Fast neutron

analysis employs the use of fast neutrons to excite the nuclei of the soil and mine by inelastic

scattering, causing the backscattered slow neutrons to be detected.6, 10 The potential advantage of

this system is that it could be used to detect explosives from soil by measuring the carbon,

hydrogen, and oxygen concentration ratios. Lastly, neutron moderation involves scanning an area

with neutrons from a low strength radiation source, and detecting the returning moderate and

slow neutrons.6, 10 This method observes anomalies produced from hydrogen nuclei, as

explosives contain two to three percent hydrogen by weight, compared to the soil which can

contain as little as zero percent to more than fifty percent hydrogen. The hydrogen density

anomaly could thereby be used to indicate the presence of a mine.

Neutron detection systems show promising potential to aid in the uncovering of

landmines by exploiting interactions between neutrons and nuclei present in the bulk explosive

material contained within the mine. Neutron moderation uses a low strength radiation source,

which reduces the shield requirements to protect deminers from radiation exposure. This allows

the potential to develop a hand held system in the future. By focusing on detecting the bulk

explosive material, a low false alarm rate can be achieved, since the system would not be

affected by ground clutter that affects other detection systems.

27

The main limiting factors for these systems are the large/heavy shielding requirements

and the possibility of public exposure to radiation. These systems are not capable of providing

information regarding the molecular structure present and are affected by ground surface

fluctuations and sensor height variations, increasing the potential of false alarms. These systems

are also sensitive to the nuclei of interest (C, N, O, and H), which are found in both the mine and

soil, making detection difficult and increasing the possibility of false alarms. Although these

systems do possess some ideal detection characteristics, the cost and concerns with working with

nuclear systems limit them from being applied for humanitarian demining.

1.4.6 Chemical Landmine Vapor Detection Systems

Chemical vapor detection systems are currently being researched and developed for

detecting landmines by identifying low concentrations of explosives in the air and soil. These

systems exploit the trace escaping vapors and residues from the bulk materials within the mine.

The development of these systems is based on the capability to distinguish a specific explosive

signature related to the particular nitroaromatic compounds of the explosive material. A variety

of vapor detection systems and techniques have been studied for landmine detection including:

electrochemical methods, fluorescence techniques, ion mobility spectrophotometers, polymer

coated sensors, and electronic nose systems.7, 13 The potential advantage these systems offer is

that they could be engineered to be lightweight, portable, small, inexpensive, and easy to operate.

Important criteria for these systems include low limits of detection (ppt), short response time,

and minimal false alarms. For many of these systems, the detection limit is not adequately

sensitive for trace vapor detection. Another limitation includes the ability to develop a sensor

with a low false alarm rate, as nitroaromatic residues other than explosives could potentially

28

produce a false alarm. These systems appear to be a promising option for humanitarian

demining, but further work is required to improve the detection limits and eliminate false alarms.

1.5 Explosive Detection Techniques

Similar to the detection of explosive materials in landmines, there is an ever-growing

concern for the detection of explosive materials being used by terrorist organizations in other

applications. Terrorists have been able to utilize explosives to attack civilian transportation

(airplanes, buses, etc.), federal buildings, and large public areas. As a result, security agencies

are focusing on establishing preventative measures to detect hidden explosives efficiently. Some

systems are already in use at airports and federal buildings, but like all systems, include

limitations that prevent the detection of all hidden explosives. The following section will review

current and developing explosive detection systems.

1.5.1 Bulk Explosive Detection Systems

These detection systems emphasize the identification of bulk explosive materials rather

than trace explosive residues or vapors. Bulk detection systems employ the use of penetrating

radiation that interacts with certain nuclei present in the explosive, which produces a specific

signal characteristic of the explosive. These systems provide a way to screen for hidden

explosives in an effective and non-intrusive manner. Even though these systems are considerably

large, many are already in use today at airports, docks, and government buildings.

X-Ray Detection Methods

X-ray detection systems are primarily used for identifying hidden explosives and

weapons, namely in luggage for airport security. X-rays offer the ability to provide information

about an object's density and effective atomic number.21 From the density and effective atomic

number, the materials of an object under investigation can be further identified. High-density

29

materials absorb more energy and appear darker in the image compared to low density materials,

which appear lighter at high energy levels. Low-density materials, such as explosives, rich in

nitrogen and oxygen, appear darker in the image at lower energy levels allowing low-density

materials to be identified by their effective atomic number. There is a variety of X-ray methods

used for airport security screening, with the most common being conventional transmission

imaging, dual energy X-ray imaging, scatter imaging, and 3-D imaging.13, 21-23

Dual Energy X-rays

Dual energy X-ray screening systems are an effective, non-intrusive system used to

screen for weapons and explosive materials.21 These systems employ both high energy and low

energy X-rays to image objects. Using high energy x-rays (>100 kV), high-density materials

appear darker in the image, since denser objects absorb more energy.22, 23 Conversely, low-

density materials appear lighter in the image. This allows for clearer image screening for metal

objects and potential weapons. An obstacle associated with X-rays is that an explosive material

could be concealed behind a high-density object, preventing the explosive from being identified.

To address this issue, objects are scanned with low X-ray energies (< 80 kV), as the absorption is

dependent on the effective atomic number and thickness of the material.22, 23 Since most

chemical explosives are rich in nitrogen and oxygen, explosive materials concealed behind high-

density materials can be identified by appearing darker in the image and analysis of the material's

atomic number. Advantages associated with dual energy X-ray systems include the ability to

distinguish a material based on shape, whether or not it contains metal properties, its ability to

identify materials by effective atomic number, and cost efficiency. Limitations associated with

X-ray systems include the difficulty to distinguish objects from each other in an image, and they

30

cannot accurately determine an object's density in order to produce an estimated effective atomic

number.

Neutron Detection Systems

Neutron detection systems are similar to other methods used for landmine detection in

that they can determine elemental composition, have greater penetration depths, difficult to

shield materials from the probing radiation, and are capable of detecting nuclear materials.21

Common techniques for neutron analysis are thermal neutron analysis (TNA), fast neutron

analysis (FNA), pulsed fast neutron analysis (PFNA), pulsed fast thermal neutron analysis

(PFTNA), and nuclear resonance absorption (NRA).24 Table 1.5.1.1 provides a brief summary of

these techniques that includes the probing radiation, nuclear reactions, detected radiation, and

detected elements.

31

Table 1.5.1.1. Neutron analysis techniques.24

# Technique Probing Radiation Nuclear Reaction

Detected Radiation

Detected Elements

1 TNA Thermalized neutrons n, ɣ Neutron capture

ɣ- rays Cl, N, H, P, S,

nuclear materials

2 FNA Fast neutrons (14 MeV) n, n' ɣ

ɣ-rays produced from

inelastically scattered neutrons

O, C, N, H, Cl, P

3 PFNA Nanosecond pulses of fast neutrons n, n' ɣ

ɣ-rays produced from

inelastically scattered neutrons

O, C, N, Cl, H, Metals, Si, P, S, nuclear materials

4 PFTNA

Pulsed neutron source: fast neutrons

during the pulse, thermal neutrons between pulses

(n, n' ɣ) + (n, ɣ)

During pulse #2 + after pulse -

#1

N, Cl, H, C, O, P, S, nuclear

materials

5 NRA Nanosecond pulsed

fast neutron (0.5 - 4 MeV)

n, n

Elastically and resonantly scattered neutrons

H, O, C, N

32

1.5.2 Spectroscopic Explosive Detection Systems

Ion Mobility Spectrometry (IMS)

Ion Mobility Spectrometry (IMS) is used extensively in airports as a means of detecting

trace level concentrations of explosives on luggage. IMS is efficient in explosive detection

because it provides quantitative chemical information, structural information, low detection

limits, short analysis time, and low false alarm rates.21, 22, 25 IMS identifies explosives by their

mass/charge ratio and mobility. IMS instruments consist of an ion source (63Ni), ion gate, a drift

region, and a detector.21, 22, 25 IMS ionizes sample vapors at atmospheric pressure in the ion

source region. The ions are then injected into the drift tube where an electric field (≈ 200 V/cm)

is applied by the ion gate. Ions then travel in the electric field toward the detector, producing a

signal by collision neutralization. From the magnitude and position in time of the peak signals,

the sample vapor can be identified. Previous experiments have shown that IMS is able to detect

low concentrations of high vapor pressure explosives like TNT, but has difficultly detecting low

vapor pressure explosives such as RDX and HMX.25 False alarms can be produced in IMS from

nitrated compounds that have similar structures to explosive compounds, since IMS is not able to

differentiate nitrated compounds.26

Mass Spectrometry (MS)

Mass spectrometry (MS) has been extensively studied as an explosive detection system

due to its sensitivity and selectivity. MS separates ions based on their mass-to-charge ratio.27 The

sample vapor enters a high vacuum chamber where it is ionized by a variety of methods. Next,

the ions are accelerated into the spectrometer where they are separated by two approaches: time

separation or geometric separation. A variety of MS techniques has been successful in detecting

explosives with low limits of detection.21, 28-30 In recent years, issues with size, portability, and

33

power requirements for MS have been addressed and have led to the development of miniature

and mobile spectrometers.21 MS spectrometers have also been incorporated into personnel

screening portals at airports and federal buildings, allowing a wide variety of explosives to be

detected in a short amount of time.21

Terahertz Spectroscopy (THz)

Terahertz spectroscopy and imaging are emerging detection techniques that have the

ability to distinguish and identify hidden explosives, metallic weapons, and illegal drugs from

other materials. Three aspects generating interest in these systems are: (1) terahertz radiation is

able to transmit through non-metallic and non-polar mediums (2) hazardous materials

(explosives, biological agents, and chemical agents) have unique fingerprint characteristic THz

spectra that can be used to identify the materials, and (3) Terahertz radiation does not pose a

health risk to the individual being scanned or the system's operator.21 Terahertz radiation falls

between the microwave and infrared regions in the electromagnetic spectrum (0.1 to 10 THz).

THz spectroscopy exploits crystal lattice vibrations, hydrogen bond stretches, and intermolecular

vibrations of molecules in explosives. These compounds produce unique fingerprint spectral

signatures that allow these systems to identify explosives in both the pure, crystalline, and plastic

forms. THz spectroscopy and imaging have been able to identify common explosives (TNT,

RDX, HMX, and PETN) in various states using different techniques.31, 32 There are a few topics

that must be addressed regarding THz systems which include increasing the frame rate for real

time imaging, portability, hand held size detectors, cost, and power requirements.

Raman Spectroscopy

Raman spectroscopy is an analytical technique that examines molecular motions and

fingerprinting species through vibrational transitions after a molecule has experienced a laser

34

excitation.30, 33 During the instantaneous Raman process, some of the energy is lost to or gained

from the molecule under investigation. The returning scattered light is at a different wavelength

due to inelastic scattering. The energy difference correlates to the vibrational or rotational energy

of the molecule. By probing these vibrational modes and analyzing the resulting spectra, the

vibrational modes provide a fingerprint that allows identification of individual components of the

molecule. Raman spectroscopy offers several advantages as a detection system such as:

fingerprint identification, application to a variety of optically accessible samples, the ability of

solid, liquid, or gaseous samples to be analyzed, no sample preparation, non-invasive, detection

can be performed over a wide region of the spectrum (UV-NIR), detection can be performed

during the day or night, and construction of a portable detection system.34 A variety of Raman

spectroscopy techniques have shown the ability to detect explosive materials and the

development of portable Raman explosive detectors can detect explosives from a standoff

distance.21, 28, 33-35 Two principle concerns with Raman spectroscopy detection are that the

Raman signal has a weak intensity and fluorescence occurs when using near UV/Vis

wavelengths.

Laser-Induced Breakdown Spectroscopy (LIBS)

Laser-Induced Breakdown Spectroscopy (LIBS) is a relatively new technique that has

presented the ability to detect explosive materials optically.21, 28, 33, 36 LIBS is a spectroscopic

technique that relies on light emitted from a focused laser pulse to generate a micro plasma. The

plasma emits light with frequencies characteristic of the atomic, ionic, and molecular fragments

in the plasma plume. The line emissions collected by a spectrophotometer generate a spectrum

that allows elemental composition identification. LIBS as an explosive detection technique

presents several advantages, which include: no sample preparation, ideal sensitivity (ng to pg),

35

real time response, field portability, miniaturized components, and either point or standoff

detection.36 LIBS is capable of identifying energetic materials based on C:H:N:O elemental

ratios, since explosive materials contain more oxygen and nitrogen relative to carbon and

hydrogen. An obstacle associated with standoff explosive detection using LIBS is interference

from atmospheric oxygen and nitrogen. The atmospheric oxygen and nitrogen affect the N:O

ratio, complicating the identification of the explosive materials. This issue has been addressed

using pulsing techniques to reduce the effects from atmospheric oxygen and nitrogen. Even

though the focused laser pulses used to generate the micro plasma has not been reported to ignite

secondary explosives in laboratory experiments, there are concerns that the laser pulses might

possibly ignite bulk amounts of extremely sensitive explosives.

1.5.3 Olfactory Explosive Detection Systems

Animals

In section 1.4.4 Biological and Biomimetic Systems for Landmine Detection, the use of

animals, primarily canines, to detect explosive materials by their sense of smell was reviewed.3,

13, 21, 28, 37 Canines are renowned as the most commonly deployed detector for sensing explosives.

Several government and private agencies rely on the canines' sense of smell for detecting illegal

drugs, explosives, human remains, and human scent. The use of canines as a reliable explosive

detector is based on the sensitivity and specificity of the dog's sense of smell. A trained canine

has the capability of identifying a variety of explosive odor signatures. While canines represent

the fastest and most dependable explosive detector to date, the cost and time associated with

training and maintaining canines, behavioral variations, and workload performance are a few

disadvantages associated with the employment of canines.

36

Electronic Noses

The advantages associated with a canine's sense of smell created interest for researchers

to investigate the possibility of developing an artificial sensor that could mimic the canine's

selectivity and sensitivity without the obstacles presented by actually using canines. An

electronic nose typically consists of an array of sensing elements that are capable of interacting

with a vapor in a variety ways, coupled with a pattern recognition system.38 The interaction

between the vapor and sensing elements produces a signature response "fingerprint" that allows

the pattern recognition system to analyze the response, identifying the analyte. The array of

sensors provides sensitivity and selectivity to a wide range of analytes, component analysis, and

analyte identification. Electronic nose explosive detection sensors promote the development of

relatively inexpensive miniature sensors, which are ideal for the use for government and private

agencies. A variety of electronic nose sensors have been used for explosive detection such as:

fluorescent polymers, surface acoustic wave detectors, fiber optics and beads, polymeric thin

films, nanoparticle nanoclusters, microelectrochemical systems, and quartz crystal

microbalances.21, 29, 38-41 Many of these sensors have shown great potential for explosive

detection under laboratory conditions, but more work is needed for the development of these

sensors to be used in the field.

1.5.4 Chemical Sensors for Explosive Detection

Electrochemical Sensors

Electrochemical sensors are able to detect explosives by monitoring a signal that is

produced by changes in the electric current between the electrodes interacting with an explosive.

Electrochemical sensors are categorized into three groups: (1) potentiometric (measurement of

potential difference or voltage), (2) amperometric (measurement of current), and (3)

37

conductometric (measurement of conductivity).2, 28 Due to the redox properties of nitroaromatic

explosives, the ease of reducing the nitro groups are ideal for electrochemical detection. The

reduction processes of the nitro groups are dependent on pH, the number of nitro groups present,

position of the nitro groups, and the presence of substituents. Typically, the trinitroaromatic

species (i.e. TNT) are more easily reduced compared to dinitro or mononitroaromatic

compounds, nitramines, polynitrate esters, or peroxide base explosives. Advantages of using

electrochemical sensors include fast response times, cost efficiency, excellent sensitivity, low

detection limits, and miniature components for hand held devices. A wide variety of

electrochemical sensors and techniques have been used for detecting explosives such as:

electrode strips, on-line flow analysis, remote monitoring, gas phase detection, and lab on chip

detection.2, 10, 21, 28, 29, 42

Biosensors for Explosive Detection

Biosensors are analytical sensing devices that incorporate a biological recognition

element capable of producing a specific biological interaction with the targeted analyte and a

signal transducer. A variety of engineered biological recognition elements such as enzymes,

antibodies, peptides, single stranded DNA, etc. have presented the ability to detect explosives.2,

10, 28, 29, 43 Beneficial aspects of using biosensors include high specificity, mass production, long-

term storage, and commercially availability.43 Typically, biosensor methods for explosive

detection are based on solution phase detection from water or soil, but there are some examples

that have shown the ability to detect high vapor pressure explosives in the gaseous state. This

presents a limitation for biosensors for standoff detection for low vapor pressure explosives, as

there must be a direct interaction between receptor and explosive molecules. The time required

for the interaction between the recognition element and explosive molecule to produce a signal

38

may require an extended amount of time (on the order of sec., min., hrs., or days). This

uncertainty of the amount of time required for producing a signal presents problems for

expedited real time detection.

Polymer Sensors for Explosive Detection

Varieties of polymers have been synthesized for use in detecting explosives. Polymers

have demonstrated the ability to act as sensors, and be incorporated in detection systems as the

sensing elements for explosive detection. Polymers have been incorporated into such systems as

electronic noses, surface acoustic wave sensors, micro-cantilever sensors, fiber optic sensors,

fluorescence systems, luminescence sensors, etc.2, 21, 28, 29, 39-41 Polymers are capable of

interacting with explosive molecules by a wide range of interactions. Polymer sensors are very

advantageous components used in explosive detectors due to ease of synthesis, variety of

interactions with explosive vapors or particles, application to numerous systems, feasible

fabrication techniques, and cost efficiency. Polymer sensors can also achieve high specificity by

developing molecularly imprinted polymers with recognition sites for a specific target analyte.44

Many polymer sensors have shown adequate limits of detection in laboratory experiments, but

elimination of false positives in field experiments are still necessary to accurately determine the

reliability of polymer sensors for explosive detection.

1.5.5 Explosive Sensors Summary

The explosive sensors previously discussed focus on detecting trace vapors or residues

from explosive compounds. Table 1.5.5.1 provides information comparing the limit of detection

ranges for some explosive sensors currently in use and being developed. These sensors have

demonstrated adequate detection limits (in the ppt - ppm range), which could potentially detect

high explosives with low vapor pressures. These sensors provide the ability to detect explosives

39

in real time and do not require samples to be analyzed in the lab. Explosive sensors are

vulnerable to false positives, but intense efforts are being investigated to eliminate false

positives, increasing their reliability. Skepticism should be considered for the reported detection

limits, since there is not a universal testing standard for determining limits of detection, making

comparisons between sensors' sensitivity difficult. The cost, size, and operation associated with

these sensors should be considered, since these three factors would preclude some systems from

being utilized at security checkpoints.

Table 1.5.5.1. Explosive sensors limit of detection ranges.

1.6 Mach-Zehnder Interferometer Optical Waveguide Sensor

Mach-Zehnder interferometers (MZI) are extremely sensitive sensors of optical phase and

are capable of detecting very small changes in refractive index. A waveguide Mach-Zehnder

interferometer was designed to detect triazine at concentrations as low as 100 ng/L.45 A

waveguide sensor in the form of a Mach-Zehnder interferometer was capable of detecting optical

phase changes less than 2.2 π milliradians and was sensitive to refractive index changes of 10-6.46

The sensor could detect the difference in refractive index between pure water and a solution

containing 0.007% glucose (Δn ~ 10-5). A Mach-Zehnder interferometer gas sensor was capable

of detecting perchloroethylene with the limit of detection being 100 ppm using polysiloxane

Explosive Sensor Limit of Detection Range

Electrochemical ppb - ppm

IMS pg – ng

Electronic Nose ppt – ppm

Polymer ppt – ppm

Biosensor ng/L - pg/L

Canines ppt – ppm

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40

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41

1.7 Research Objectives

The main objective of this research project was to synthesize polymers, which could be

used as sensing materials for a MZI explosive detection sensor. Our objective was to synthesize

polymers containing electron rich aromatic monomers. Our interest particularly focused on

synthesizing random electron rich copolymers with the electron rich monomer, 2-vinyl-4,6-

diamino-1,3,5-triazine (VDAT). The VDAT electron rich copolymers could potentially have a

strong affinity for nitroaromatics due to hydrogen bonding between the amino group on the

polymer chain and the nitro groups or an electrostatic affinity toward the electron deficient

nitroaromatics by a complex formation.

To determine if the random electron rich copolymers had an affinity for nitroaromatics,

spin coated polymer films' refractive indices would be determined before and after exposure to a

nitroaromatic vapor using spectroscopic ellipsometry. An increase in the polymer film's

refractive index after exposure to a nitroaromatic vapor would confirm the polymer's affinity

toward the nitroaromatic. No change in the refractive index after exposure to a nitroaromatic

vapor would suggest that the polymer did not have an affinity for the nitroaromatic.

The last objective was to investigate new materials for sensing nitroaromatics by growing

co-crystals between electron rich donors and nitroaromatics. If a strong interaction was observed

between the electron rich donor and nitroaromatic, this would suggest a reagent to be used for

synthesizing an electron rich copolymer. Characterizing this strong interaction would also

provide some insight on the possible interaction between the random electron rich copolymers

and the nitroaromatics.

42

Chapter 2

Experimental

2.1 Sources of All Chemicals

2-Vinyl-4,6-diamino-1,3,5-triazine (VDAT) was purchased from Tokyo Kasei Kogyo Co.

Sodium persulfate, 1,4-dioxane 99+%, 1,3-dinitrobenzene 97%, methyl acrylate 99%,

2-vinylpyridine 97%, 2,2'- azobisisobutyronitrile 98%, acrylamide 97%, 1-vinylimidazole, 2,4-

diamino-6-methyl-1,3,5-triazine 98%, 10-methylphenothiazine 98%, 9-ethylcarbazole 97%, 9-

vinylcarbazole, phenothiazine 98+%, and styrene reagent plus ≥ 99%, were purchased from

Sigma Aldrich. Toluene HPLC grade and methyl ethyl ketone were purchased from Fisher

Scientific. Nitrobenzene ACS reagent grade, carbazole 96%, and methyl methacrylate were

purchased from ACROS. 2-Nitrotoluene 99+%, 3-nitrotoluene 99+%, benzoguanamine 99%,

Chloroform-d 99.8% were purchased from Alfa Aesar. Poly(4-Vinylpyridine) M.W. 300,000

was purchased from Polysciences Inc. Polyvinylimidazole M.W. 3,500 and polyvinylimidazole-

co-polyvinylaniline copolymer, M.W. 7,000, 5 mol % aniline were purchased from Selective

Technologies Inc. Acetonitrile was purchased from BDH. Methyl sulfoxide was purchased from

EMD. Dimethyl sulfoxide-d6 was purchased from Cambridge Isotope Laboratories Inc. Ethanol

and methanol was purchased from the chemistry stockroom. 2,2'-azobisisobutyronitrile 98%,

(AIBN) was recrystallized from methanol prior to using as a free radical initiator.

43

2.2 Polymer Syntheses

2.2.1 Poly(2-vinyl-4,6-diamino-1,3,5-triazine) (PVDAT)

PVDAT was synthesized according to the method of Chen and Sun.49 For the PVDAT

synthesis, 4 mmols (0.55 g) of VDAT and 25 mL of D.I. H2O were transferred to a 250 mL three

neck round bottom flask. The flask was placed in an oil bath on a hot plate, fitted with a

condenser under a nitrogen atmosphere, equipped with a stir bar, and a thermometer. When the

reaction temperature inside the round bottom flask reached approximately 75 °C, 1.39 mmols

(0.33 g) of Na2S2O8 dissolved in 5 mL of D.I. H2O was added to initiate the free radical

polymerization shown in Scheme 2.2.1.1. The reaction mixture was heated to 95 °C and held for

two hours, dissolving the monomer and producing a colorless solution. After two hours, the free

radical polymerization was terminated by allowing the polymer solution to cool to room

temperature. After the solution cooled to room temperature, the polymer solution was transferred

to a 600 mL beaker. Excess MeOH (≈ 250 mL) was added, precipitating a white polymer, and

was stirred for several minutes. The polymer was collected by filtration and washed with copious

amounts of MeOH and D.I. H2O. The polymer was then dried under vacuum at 50 °C overnight.

The free radical polymerization produced an 87% yield (0.48 g) of PVDAT.

N

N

N

NH2H2N

N

N

N

NH2H2N

n

D.I. H2O at 95°CNa2S2O8

Scheme 2.2.1.1. Free radical polymerization of PVDAT.

44

2.2.2 Polystyrene-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine) (PS-co-PVDAT)

PS-co-PVDAT copolymers were synthesized according to the method of Chen and Sun.49

Scheme 2.2.2.1 shows the free radical polymerization for the PS-co-PVDAT polymerization.

Styrene was purified by distillation under reduced pressure and VDAT was used as received.

DMSO was purified by distillation under reduced pressure from CaH2. For the PS-co-PVDAT

free radical polymerizations, the VDAT mol % concentrations were varied from 1, 5, 10, and 20

with styrene to equal 0.1 total mols. In the synthesis for the 20 mol % PS-co-PVDAT copolymer,

0.08 mols (8.33 g) of styrene and 0.02 mols (2.74 g) of VDAT were added to a 250 mL three

neck round bottom flask. The flask was placed in an oil bath on a hot plate fitted with a

condenser containing 100 mL of freshly distilled DMSO under a nitrogen atmosphere, equipped

with a stir bar, and a thermometer. Once the reaction temperature inside the round bottom flask

reached 60 °C, 2.0 mmols (0.33 g) of recrystallized AIBN was added to the solution to initiate

the free radical polymerization. The reaction temperature was increased to approximately 80 °C

and stirred for five hours. Once the monomers and initiator dissolved in DMSO, the reaction

solution appeared colorless. During the reaction, the solution gradually changed from colorless to

yellow, indicating the reaction had come to completion. The polymer solution was allowed to

cool to room temperature and was then transferred to a 600 mL beaker. The copolymer was

precipitated in excess EtOH (approximately 250 mL), producing a white polymer, and was

stirred for several minutes. The polymer was collected by filtration and washed successively with

copious amounts of EtOH and D.I. H2O. The copolymer was then dried under vacuum at 50 °C

overnight. Washing the copolymer in excess EtOH typically removed all the unreacted

monomer. In some instances, Soxhlet extraction with EtOH or stirring the polymer in EtOH

under reflux in a one neck round bottom flask fitted with a condenser was performed to remove

45

any unreacted monomer. Polystyrene was synthesized by the same experimental procedure.

Table 2.2.2.1 lists the experimental amounts of VDAT, styrene, AIBN, DMSO, yields, and

temperature for PS-co-PVDAT copolymers and polystyrene polymerizations.

Table 2.2.2.1. Experimental amounts and conditions for PS-co-PVDAT polymerizations.

Polymer VDAT (mol %)

VDAT (mols)

VDAT (g)

Styrene (mol %)

Styrene (mols)

Styrene (g)

Yield (%)

Yield (g)

Temp. (°C)

PS-co-PVDAT 20 0.02 2.74 80 0.08 8.33 49 5.42 ≈ 80 °C

PS-co-PVDAT 10 0.01 1.37 90 0.09 9.37 34 3.64 ≈ 80 °C

PS-co-PVDAT 5 0.005 0.69 95 0.095 9.89 17 1.81 ≈ 80 °C

PS-co-PVDAT 1 0.001 0.14 99 0.099 10.31 11 1.17 ≈ 80 °C

PS 0 0 0 100 0.1 10.41 22 2.29 ≈ 80 °C *All polymers were synthesized using 2 mmols (0.33 g) of AIBN and 100 mL of DMSO.

Scheme 2.2.2.1. Free radical polymerization for the synthesis of PS-co-PVDAT random copolymers.

2.2.3 Poly(methyl methacrylate)-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine)

(PMMA-co-PVDAT)

PMMA-co-PVDAT copolymers were synthesized according to the method of Chen and

Sun.49 Scheme 2.2.3.1 displays the free radical polymerization for the PMMA-co-PVDAT

copolymers. MMA was purified by distillation under reduced pressure and VDAT was used as

46

received. DMSO was purified by distillation under reduced pressure from CaH2. For the PMMA-

co-PVDAT free radical polymerizations, the VDAT mol % concentrations were varied from 1, 5,

10, and 20 with MMA to equal 0.1 total mols. In the synthesis for the 20 mol % PMMA-co-

PVDAT copolymer, 0.08 mols (8.01 g) of MMA and 0.02 mols (2.74 g) of VDAT were added to

a 250 mL three neck round bottom flask. The flask was placed in an oil bath on a hot plate, fitted

with a condenser containing 100 mL of freshly distilled DMSO under a nitrogen atmosphere, and

equipped with a stir bar and thermometer. Once the reaction temperature inside the round bottom

flask reached 60 °C, 2.0 mmols (0.33 g) of recrystallized AIBN was added to the solution to

initiate the free radical polymerization. The reaction temperature was increased to approximately

80 °C and was stirred for five hours. Once the monomers and initiator dissolved in DMSO, the

reaction solution appeared a white-brown color. During the reaction, the solution gradually

changed from white-brown to yellow in color, indicating the reaction had come to completion.

The polymer solution was allowed to cool to room temperature and was then transferred to a 600

mL beaker. The copolymer was precipitated in excess MeOH (approximately 250 mL),

producing a white polymer with a faint white-brown tint and was stirred for several minutes. The

polymer was collected by filtration and washed successively with copious amounts of MeOH

and D.I. H2O. The copolymer was then dried under vacuum at 50 °C overnight. To remove

unreacted monomer from the copolymers, the copolymers were washed in MeOH under reflux

while being stirred in a one neck round bottom flask fitted with a condenser. PMMA was

synthesized by the same experimental procedure. Table 2.2.3.1 lists the experimental amounts of

VDAT, MMA, AIBN, DMSO, yields, and temperatures for the PMMA-co-PVDAT copolymers

and PMMA polymerizations.

47

Table 2.2.3.1. Experimental amounts for the PMMA-co-PVDAT copolymers and PMMA polymerizations.


VDAT (mols)

VDAT (g)

MMA (mol %)

MMA (mols)

MMA (g)

Yield (%)

Yield (g)

Temp. (°C)

PMMA-co-PVDAT 20 0.02 2.74 80 0.08 8.01 65 6.96 ≈ 80 °C

PMMA-co-PVDAT 10 0.01 1.37 90 0.09 9.01 40 4.18 ≈ 80 °C

PMMA-co-PVDAT 5 0.005 0.69 95 0.095 9.51 34 3.43 ≈ 80 °C

PMMA-co-PVDAT 1 0.001 0.14 99 0.099 9.91 47 4.72 ≈ 80 °C

PMMA 0 0 0 100 0.1 10.01 54 5.37 ≈ 80 °C * All polymers were synthesized using 2 mmols of AIBN (0.33 g) and 100 mL of DMSO.

Scheme 2.2.3.1. Free radical polymerization for the synthesis of PMMA-co-PVDAT random copolymers.

2.2.4 Poly(methyl acrylate)-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine) (PMA-co-PVDAT)

The PMA-co-PVDAT copolymer was synthesized according to the method of Chen and

Sun.49 Scheme 2.2.4.1 shows the free radical polymerization for the PMA-co-PVDAT

copolymer. MA was purified by distillation under reduced pressure and VDAT was used as

received. DMSO was purified by distillation under reduced pressure from CaH2. For the PMA-

co-PVDAT free radical polymerization, the VDAT concentration was 20 mol %. For the

synthesis of the 20 mol % PMA-co-PVDAT copolymer, 0.08 mols (6.89 g) of MA and 0.02 mols

(2.74 g) of VDAT were added to a 250 mL three neck round bottom flask. The flask was placed

48

in an oil bath on a hot plate, fitted with a condenser containing 100 mL of freshly distilled

DMSO under a nitrogen atmosphere, and equipped with a stir bar and thermometer. Once the

reaction temperature inside the round bottom flask reached 60 °C, 2 mmols (0.33 g) of

recrystallized AIBN was added to the solution to initiate the free radical polymerization. The

reaction temperature was increased to approximately 80 °C and was stirred for five hours. Once

the monomers and initiator dissolved in DMSO, the reaction solution appeared a white-brown

color. During the reaction, the solution gradually changed from white-brown to a bright yellow

in color, indicating the reaction had come to completion. The polymer solution was allowed to

cool to room temperature and was transferred to a 600 mL beaker. The copolymer was

precipitated in excess MeOH (approximately 500 mL), producing a white-yellow tinted soft

polymer and was stirred for several minutes. The polymer was collected by filtration and washed

successively with copious amounts of MeOH and D.I. H2O. The copolymer was dried under

vacuum at 50 °C overnight. The copolymerization of PMA-co-PVDAT (20 mol % VDAT)

produced a 45% yield (4.33 g). PMA was synthesized by the same experimental procedure,

producing a soft transparent polymer with a 53% yield (4.55 g). The precipitated homopolymer

was allowed to sit overnight to allow the polymer to collect in the bottom of the beaker. After

sitting over night, the DMSO was decanted, and the polymer was washed with MeOH and D.I.

H2O. The polymer was dried overnight under vacuum at 50 °C.

Scheme 2.2.4.1. Free radical polymerization for the synthesis of a PMA-co-PVDAT random copolymer.

49

2.2.5 Poly(2-vinylpyridine)-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine) (P2VP-co-PVDAT)

P2VP-co-PVDAT copolymers were synthesized according to the method of Chen and

Sun.49 Scheme 2.2.5.1 shows the free radical polymerization for the P2VP-co-PVDAT

copolymers. 2-VP was purified by distillation under reduced pressure and VDAT was used as

received. DMSO was purified by distillation under reduced pressure from CaH2. For the P2VP-

co-PVDAT free radical polymerizations, the VDAT mol % concentrations were varied from 20,

5, and 1 mol %. In the synthesis for the 20 mol % P2VP-co-PVDAT copolymer, 0.04 mols (4.21

g) of 2-VP and 10 mmols (1.37 g) of VDAT were added to a 250 mL three neck round bottom

flask. The flask was placed in an oil bath on a hot plate, fitted with a condenser containing 50

mL of freshly distilled DMSO under a nitrogen atmosphere, and equipped with a stir bar and

thermometer. Once the reaction temperature reached 60 °C, 1.04 mmols (0.17 g) of recrystallized

AIBN was added to the solution to initiate the free radical polymerization. The reaction

temperature was increased to approximately 80 °C and was stirred for five hours. Once the

monomers and initiator dissolved in DMSO, the reaction solution appeared colorless. During the

reaction, the solution gradually changed from colorless to a red-orange color, indicating the

reaction had come to completion. The polymer solution was allowed to cool to room temperature

and was transferred to a 600 mL beaker. The copolymer was precipitated in 10% (w.t.) NaCl D.I.

H2O solution (250 mL), producing an orange polymer. The polymer was collected by filtration

and washed successively with copious amounts of D.I. H2O. The copolymer was dried overnight

at 50 °C, producing a brittle polymer. P2VP was synthesized by the same experimental

procedure using hexane to precipitate a white-orange powder polymer. Table 2.2.5.1 lists the

experimental amounts of 2-VP, VDAT, yields, AIBN, DMSO, and temperature for the

polymerizations.

50

Table 2.2.5.1. Experimental amounts for P2VP-co-PVDAT copolymers and P2VP polymerizations.


VDAT (mmols)

VDAT (g)

2-VP (mol %)

2-VP (mols)

2-VP (g)

Yield (%)

Yield (g)

DMSO (mL)

P2VP-co-PVDATa 20 10 1.37 80 0.04 4.21 31 1.71 50

P2VP-co-PVDATb 5 1.25 0.1714 95 0.0237 2.4971 50 1.33 25

P2VP-co-PVDATb 1 0.025 0.0343 99 0.0247 2.6022 74 1.96 25

Poly(2-VP)c 0 0 0 100 0.1 10.51 65 6.86 100

a 1.04 mmols (0.17 g) of AIBN was used as the free radical initiator b 0.05 mmols (0.0821 g) of AIBN was used as the free radical initiator c 2 mmols (0.33 g) of AIBN was used as the free radical initiator * All polymerizations were performed at approximately 80 °C

Scheme 2.2.5.1. Free radical polymerization for the synthesis of P2VP-co-PVDAT random copolymers.

2.2.6 Poly(acrylamide)-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine) (PAM-co-PVDAT)

PAM-co-PVDAT copolymers were synthesized according to the method of Chen and

Sun.49 Scheme 2.2.6.1 shows the free radical polymerization for PAM-co-PVDAT copolymers.

Acrylamide and VDAT were used as received. DMSO was purified by distillation under reduced

pressure from CaH2. For the PAM-co-PVDAT free radical polymerizations, the VDAT mol %

concentrations were varied from 1, 5, 10, and 20 with acrylamide. In the synthesis for the 20 mol

51

% PAM-co-PVDAT copolymer, 0.1 mols (7.18 g) of acrylamide and 0.025 mols (3.43 g) of

VDAT were added to a 250 mL three neck round bottom flask. The flask was placed in an oil

bath on a hot plate, fitted with a condenser containing 100 mL of freshly distilled DMSO under a

nitrogen atmosphere, and equipped with a stir bar and thermometer. Once the reaction

temperature inside the round bottom flask reached 60 °C, 2.0 mmols (0.33 g) of recrystallized

AIBN was added to the solution to initiate the free radical polymerization. The reaction

temperature was increased to approximately 80 °C and stirred for five hours. Once the monomers

and initiator dissolved in DMSO, the reaction solution appeared an opaque white color. During

the reaction, the solution gradually changed from an opaque white to a light yellow tint,

indicating the reaction had come to completion. The polymer solution was allowed to cool to

room temperature and was transferred to a 600 mL beaker. The copolymers were precipitated in

excess MeOH (approximately 200 mL), producing a white polymer with a slight yellow tint and

was stirred for several minutes. The copolymer consisted of a fine powder with large clumps.

The polymer was collected by filtration and washed successively with copious amounts of

MeOH. The copolymer was dried under vacuum at 50 °C overnight. The copolymers were

purified by washing in MeOH under reflux by stirring in a one neck round bottom flask fitted

with a condenser. Table 2.2.6.1 lists the experimental amounts of VDAT, acrylamide, AIBN,

DMSO, yields, and temperature for the PAM-co-PVDAT copolymers and PAM polymerizations.

The homopolymer, polyacrylamide, produced a percent yield greater than 100% (149%). This

percent yield may be attributed to polyacrylamide absorbing water from the atmosphere or

water/DMSO trapped within the polymer matrix.

52

Table 2.2.6.1. Experimental amounts for PAM-co-PVDAT copolymers and PAM polymerizations.


VDAT (mols)

VDAT (g)

AM (mol %)

AM (mols)

AM (g)

Yield (%)

Yield (g)

Temp. (°C)

PAM-co-PVDAT 20 0.025 3.43 80 0.11 7.82 96 10.81 ≈ 80 °C

PAM-co-PVDAT 10 0.011 1.50 90 0.11 7.82 97 9.04 ≈ 80 °C

PAM-co-PVDAT 5 0.0055 0.75 95 0.11 7.82 95 8.14 ≈ 80 °C

PAM-co-PVDATa 1 2.5 E-4 0.0343 99 2.48 E-2 1.76 96 1.72 ≈ 80 °C

PAM 0 0 0 100 0.1 7.11 149 10.60 ≈ 80 °C a Polymer was synthesized using 0.05 mmols (0.0821 g) of AIBN and 25 mL of DMSO Polymers were synthesized using 2 mmols (0.33 g) of AIBN and 100 mL of DMSO

Scheme 2.2.6.1. Free radical polymerization for the synthesis of PAM-co-PVDAT random copolymers.

2.2.7 Poly(N-vinylcarbazole)-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine)

(PVK-co-PVDAT)

PVK-co-PVDAT copolymers were synthesized according to the method of Chen and

Sun.49 Scheme 2.2.7.1 shows the free radical polymerization for the PVK-co-PVDAT

copolymers. N-vinylcarbazole and VDAT were used as received. DMSO was purified by

distillation under reduced pressure from CaH2. For the PVK-co-PVDAT free radical

polymerizations, the VDAT mol % concentrations were varied from 5, 10, and 20 with N-

vinylcarbazole. In the synthesis for the 20 mol % PVK-co-PVDAT copolymer, 0.08 mols (15.46

53

g) of N-vinylcarbazole and 0.02 mols (2.74 g) of VDAT were added to a 250 mL three neck

round bottom flask. The flask was placed in an oil bath on a hot plate, fitted with a condenser

containing 100 mL of freshly distilled DMSO under a nitrogen atmosphere, and equipped with a

stir bar and thermometer. Once the reaction temperature inside the round bottom flask reached

60 °C, 2 mmols (0.33 g) of recrystallized AIBN was added to the solution to initiate the free

radical polymerization. The reaction temperature was increased to approximately 80 °C and was

stirred for five hours. Once the monomers and initiator dissolved in DMSO, the reaction solution

appeared colorless. During the reaction, the solution gradually changed from colorless to yellow

in color, indicating the reaction had come to completion. The polymer solution was allowed to

cool to room temperature and was then transferred to a 600 mL beaker. The copolymer was

precipitated in excess MeOH (approximately 200 mL), producing a white-brown polymer and

was stirred for several minutes. The polymer was collected by filtration and washed successively

with copious amounts of MeOH and D.I. H2O. The copolymer was dried under vacuum at 50 °C

overnight. The copolymer consisted of a fine powder with large clumps present. The copolymers

were purified by the previously stated purification step using MeOH under reflux while being

stirred in a one neck round bottom flask fitted with a condenser. The purification step did not

improve the percent yields. All of the copolymers' percent yields were greater than 100%. The

percent yields greater than 100% were most likely due to residual DMSO trapped within the

polymer matrix. The residual DMSO could not be removed due to its high boiling point. Table

2.2.7.1 lists the experimental amounts of VDAT, N-vinylcarbazole, AIBN, DMSO, DMF, yields,

and temperature for the PVK-co-PVDAT copolymers and PVK polymerizations.

54

Table 2.2.7.1. PVK-co-PVDAT copolymers and PVK experimental amounts for free radical polymerizations.


VDAT (mols)

VDAT (g)

VK (mol %)

VK (mols)

VK (g)

Yield (%)

Yield (g)

Temp. (°C)

PVK-co-PVDAT* 20 0.02 2.74 80 0.08 15.46 114 20.68 ≈ 80 °C

PVK-co-PVDAT* 10 0.01 1.37 90 0.09 17.39 158 29.71 ≈ 80 °C

PVK-co-PVDATa 5 0.0025 0.34 95 0.0475 9.18 103 9.77 ≈ 80 °C

PVKb 0 0 0 100 0.1 19.33 93 17.89 ≈ 80 °C

Scheme 2.2.7.1. Free radical polymerization for the synthesis of PVK-co-PVDAT random copolymers.

2.2.8 Polystyrene-co-Poly(N-vinylcarbazole) (PS-co-PVK)

PS-co-PVK copolymers were attempted to be synthesized according to the method of

Chen and Sun.49 Scheme 2.2.8.1 shows the free radical polymerization for the PS-co-PVK

copolymers. Styrene was purified by distillation under reduced pressure and N-vinylcarbazole

was used as received. DMF was purified by distillation under reduced pressure from CaH2. For

the PS-co-PVK free radical polymerizations, the N-vinylcarbazole mol % concentrations were

varied from 10, 15, and 20 with styrene. In the synthesis for the 20 mol % PS-co-PVK

copolymer, 0.08 mols (8.33 g) of styrene and 0.02 mols (3.87 g) of vinylcarbazole were added to


a 1 mmols (0.165 g) of AIBN was used as the free radical initiator b 100 mL of DMF was used as the reaction solvent *Polymers were synthesized using 2 mmols (0.33 g) of AIBN and 100 mL of DMSO

55

with a condenser containing 100 mL of freshly distilled DMF under a nitrogen atmosphere, and




80 °C and was stirred for five hours. Once the monomers and initiator dissolved in DMF, the

reaction solution appeared colorless. During the reaction, the solution gradually changed from

colorless to a light yellow color, indicating the reaction had come to completion. The polymer

solution was allowed to cool to room temperature and then was transferred to a 600 mL beaker.

The copolymer was precipitated in excess MeOH (approximately 250 mL), producing a white

polymer, and was stirred for several minutes. The polymer was collected by filtration and

washed successively with copious amounts of MeOH and D.I. H2O. The copolymer was dried

under vacuum at 50 °C overnight. Table 2.2.8.1 lists the experimental amounts of styrene, N-

vinylcarbazole, AIBN, DMF, yields, and temperatures for the PVK-co-PVDAT copolymers and

PVK polymerizations.

Table 2.2.8.1. Experimental amounts for the PS-co-PVK polymerizations.

Polymer VK (mol %)

VK (mols)

VK (g)

Styrene (mol %)

Styrene (mols)

Styrene (g)

Yield (%)

Yield (g)

Temp. (°C)

PS-co-PVK 20 0.02 3.87 80 0.08 8.33 5 0.62 ≈ 80 °C

PS-co-PVK 15 0.015 2.90 85 0.09 8.85 21 2.52 ≈ 80 °C

PS-co-PVK 10 0.01 1.93 90 0.09 9.37 22 2.54 ≈ 80 °C

*Polymers were synthesized using 2 mmols (0.33 g) of AIBN and 100 mL of DMF

56

Scheme 2.2.8.1. Free radical polymerization for the synthesis of PS-co-PVK random copolymers.

2.2.9 Poly(methyl methacrylate)-co-Poly(N-vinylcarbazole) (PMMA-co-PVK)

PMMA-co-PVK copolymers were synthesized according to the method of Chen and

Sun.49 Scheme 2.2.9.1 shows the free radical polymerization for the PMMA-co-PVK

copolymers. MMA was purified by distillation under reduced pressure and N-vinylcarbazole was

used as received. DMF was purified by distillation under reduced pressure from CaH2. For the

PMMA-co-PVK free radical polymerizations, the N-vinylcarbazole mol % concentrations were

varied from 50 and 20 with MMA. In the synthesis for the 50 mol % PMMA-co-PVK

copolymer, 0.05 mols (5.01 g) of MMA and 0.05 mols (9.66 g) of vinylcarbazole were added to


with a condenser containing 100 mL of freshly distilled DMF under a nitrogen atmosphere, and




80 °C and was stirred for five hours. Once the monomers and initiator dissolved in DMF, the

reaction solution appeared colorless. During the reaction, the solution gradually changed from

colorless to a light brown-yellow color, indicating the reaction had come to completion. The

polymer solution was allowed to cool to room temperature and then was transferred to a 600 mL

57

beaker. The copolymer was precipitated in excess MeOH (approximately 200 mL), producing a

white-brown polymer and was stirred for several minutes. The polymer was collected by

filtration and washed successively with copious amounts of MeOH and D.I. H2O. The copolymer

was dried under vacuum at 50 °C overnight. Table 2.2.9.1 lists the experimental amounts of

MMA, yields, N-vinylcarbazole, DMF, AIBN, and temperatures for the PMMA-co-PVK

copolymers.

Table 2.2.9.1. Experimental amounts for PMMA-co-PVK copolymer polymerizations.

Polymer VK (mol %)

VK (mols)

VK (g)

MMA (mol %)

MMA (mols)

MMA (g)

Yield (%)

Yield (g)

Temp. (°C)

PMMA-co-PVK 50 0.05 9.66 50 0.05 5.01 12 1.82 ≈ 80 °C

PMMA-co-PVK 20 0.02 3.87 80 0.08 8.01 80 9.49 ≈ 80 °C

*Polymers were synthesized using 2 mmols (0.33 g) of AIBN and 100 mL of DMF

Scheme 2.2.9.1. Free radical polymerization for the synthesis of PMMA-co-PVK random copolymers.

2.2.10 Poly(N-vinylimidazole)-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine) (PVI-co-PVDAT)

The PVI-co-PVDAT copolymer was synthesized according to the method of Chen and

Sun.49 Scheme 2.2.10.1 shows the free radical polymerization for the PVI-co-PVDAT

copolymer. VI was purified by distillation under reduced pressure and VDAT was used as

received. DMSO was purified by distillation under reduced pressure from CaH2. For the PVI-co-

58

PVDAT free radical polymerization, the VDAT mol % concentration was 20 with VI. In the

synthesis for the 20 mol % PVI-co-PVDAT copolymer, 0.08 mols (7.53 g) of VI and 0.02 mols

(2.74 g) of VDAT were added to a 250 mL three neck round bottom flask. The flask was placed

in an oil bath on a hot plate, fitted with a condenser containing 100 mL of freshly distilled

DMSO under a nitrogen atmosphere, and equipped with a stir bar and thermometer. Once the

reaction temperature inside the round bottom flask reached 60 °C, 0.002 mols (0.33 g) of

recrystallized AIBN was added to the solution to initiate the free radical polymerization. The

reaction temperature was increased to approximately 70 °C and was stirred for five hours. Once

the monomers and initiator dissolved in DMSO, the reaction solution appeared colorless. During

the reaction, the solution gradually changed from colorless to a light orange color, indicating the

reaction had come to completion. The polymer solution was allowed to cool to room temperature

and then was transferred to a 600 mL beaker. The copolymer was precipitated in excess MEK

(approximately 300 mL) producing a brittle orange polymer and was stirred for several minutes.

The polymer was collected by filtration and washed successively with copious amounts of

MeOH and D.I. H2O. The copolymer was dried under vacuum at 50 °C overnight. The

copolymerization produced a 71% yield (7.27 g) for PVI-co-PVDAT. Poly(N-vinylimidazole)

was synthesized by the same experimental procedure and conditions producing a 17% yield (1.62

g) light orange brittle polymer.

N

N

N

NH2 NH2

CH2 CH2

N

N

N

NH2 NH2

+ AIBN

DMSO, 70 °C

m n

N

N

N

N

Scheme 2.2.10.1. Free radical polymerization for the synthesis of a PVI-co-PVDAT random copolymer.

59

2.2.11 Polystyrene-co-Poly(N-vinylimidazole) (PS-co-PVI)

The PS-co-PVI copolymer was synthesized according to the method of Chen and Sun.49

Scheme 2.2.11.1 shows the free radical polymerization for the PS-co-PVI copolymer. Styrene

and VI were purified by distillation under reduced pressure. DMSO was purified by distillation

under reduced pressure from CaH2. For the PS-co-PVI free radical polymerization, the VI mol %

concentration was 20 with styrene. In the synthesis for the 20 mol % PS-co-PVI copolymer, 0.08

mols (8.33 g) of styrene and 0.02 mols (1.88 g) of VI were added to a 250 mL three neck round

bottom flask. The flask was placed in an oil bath on a hot plate, fitted with a condenser

containing 100 mL of freshly distilled DMSO under a nitrogen atmosphere, and equipped with a

stir bar and thermometer. Once the reaction temperature inside the round bottom flask reached

60 °C, 2.0 mmols (0.33 g) of recrystallized AIBN was added to the solution to initiate the free

radical polymerization. The reaction temperature was increased to approximately 70 °C and was

stirred for five hours. Once the monomers and initiator dissolved in DMSO, the reaction solution

appeared colorless. During the reaction, the solution gradually changed from colorless to a slight

orange color, indicating the reaction had come to completion. The polymer solution was allowed

to cool to room temperature and then was transferred to a 600 mL beaker. The copolymer was

precipitated in excess MeOH (approximately 250 mL) producing a brittle white-orange polymer

and was stirred for several minutes. The polymer was collected by filtration and washed

successively with copious amounts of MeOH. The copolymer was dried under vacuum at 50 °C

overnight. The copolymerization produced an 11% yield (1.13 g).

60

Scheme 2.2.11.1. Free radical polymerization for the synthesis of a PS-co-PVI random copolymer.

2.2.12 Poly(methyl methacrylate)-co-Poly(N-vinylimidazole) (PMMA-co-PVI)

The PMMA-co-PVI copolymer was synthesized according to the method of Chen and

Sun.49 Scheme 2.2.12.1 shows the free radical polymerization for the PMMA-co-PVI copolymer.

MMA and VI were purified by distillation under reduced pressure. DMSO was purified by

distillation under reduced pressure from CaH2. For the PMMA-co-PVI free radical

polymerization, the VI mol % concentration was 20 with MMA. In the synthesis for the 20 mol

% PMMA-co-PVI copolymer, 0.08 mols (8.01 g) of MMA and 0.02 mols (1.88 g) of VI were

added to a 250 mL three neck round bottom flask. The flask was placed in an oil bath on a hot

plate, fitted with a condenser containing 100 mL of freshly distilled DMSO under a nitrogen

atmosphere, and equipped with a stir bar and thermometer. Once the reaction temperature inside

the round bottom flask reached 60 °C, 2.0 mmols (0.33 g) of recrystallized AIBN was added to

the solution to initiate the free radical polymerization. The reaction temperature was increased to

approximately 70 °C and was stirred for five hours. Once the monomers and initiator dissolved

in DMSO, the reaction solution appeared colorless. During the reaction, the solution gradually

changed from colorless to a light orange color, indicating the reaction had come to completion.

The polymer solution was allowed to cool to room temperature and transferred to a 600 mL

beaker. The copolymer was precipitated in excess MeOH (approximately 300 mL), producing a

brittle white-orange polymer and was stirred for several minutes. The polymer was collected by

CH2 CH2

+ AIBN

DMSO, 70 °C

m n

N

N

N

N

61

filtration and washed successively with copious amounts of MeOH. The copolymer was dried

under vacuum at 50 °C overnight. The copolymerization produced a 64% yield (6.57 g).

CH2 CH2

+ AIBN

DMSO, 70 °C

m n

N

N

C

N

N

O

O

CH3

O

CH3

O

Scheme 2.2.12.1. Free radical polymerization for the synthesis of a PMMA-co-PVI random copolymer.

2.3 Co-Crystals with Nitroaromatics

2.3.1 General Co-Crystal Procedure with Nitroaromatics

1.0 mmol of the electron donor and electron acceptor were dissolved in separate test

tubes in the appropriate solvent by sonication or wrist action shaking. After the reagents

completely dissolved in the separate test tubes, they were transferred to a crystallization dish to

allow the solvent to evaporate at room temperature. After the solvent completely evaporated, the

crystals were collected from the crystallization dish. Table 2.3.1.1 lists the electron donors,

electron acceptors, and solvents used for growing co-crystals.

2.3.2 2,4-Diamino-6-methyl-1,3,5-triazine (MDAT) Co-Crystals with Nitroaromatics

MDAT co-crystals were attempted by the method of Xiao.50 5.0 mmols of MDAT and

5.0 mmols of a nitroaromatic reagent (2-NT, 3-NT, or 1,3-DNB) were added to a 250 mL three

neck round bottom flask in an oil bath on a hot plate. The flask was fitted with a condenser

containing 100 mL of EtOH and equipped with a stir bar and thermometer. The reaction

temperature was increased to approximately 50 °C and was stirred for three hours. During the

62

reaction, the nitroaromatic reagent dissolved, but MDAT was partially soluble in EtOH. After

three hours, the reaction solution was allowed to cool to room temperature and was then filtered.

The filtrate appeared colorless, but included small white particles. The filtrate was set aside for

one week and obtained white crystals with a faint yellow tint.

2.3.3 2-Vinyl-4,6-diamino-1,3,5-triazine (VDAT) Co-Crystals with 1,3-Dinitrobenzene (1,3-DNB)

1.0 mmol of VDAT (0.14 g) was dissolved in 5 mL of DMSO by gently heating in a test

tube. 1.0 mmol (0.17 g) of 1,3-DNB was dissolved in 15 mL of EtOH in a test tube by wrist

action shaking. The solutions were transferred to crystallization dish, allowing the formation of

crystals over six days. Light orange, needle like crystals formed and were removed from the

remaining DMSO.

2.3.4 9-Vinylcarbazole (9-VC) Co-Crystals with 1,3-Dinitrobenzene (1,3-DNB)

Three different ratios between 9-VC and 1,3-DNB were used to grow co-crystals. For the

1:1 ratio, 1.0 mmol (0.19 g) of 9-VC was dissolved in 10 mL of EtOH in a test tube by

sonication, producing a colorless solution. 1.0 mmol (0.17 g) of 1,3-DNB was dissolved in 10

mL of EtOH in a test tube by sonication, producing a light yellow color solution. When the two

solutions were combined in a crystallization dish, a bright yellow color solution rapidly formed.

The EtOH was allowed to evaporate at room temperature for two weeks. Yellow-orange clumps

formed in the crystallization dish after the EtOH evaporated.

For the 1:2 ratio, 1.0 mmol (0.19 g) of 9-VC was dissolved in 10 mL of EtOH in a test

tube by sonication, producing a colorless solution. 2.0 mmols (0.34 g) of 1,3-DNB was dissolved

in 15 mL of EtOH in a test tube by sonication, producing a light yellow colored solution. When

the two solutions were combined in a crystallization dish, a bright yellow color solution rapidly

63

formed. The EtOH was allowed to evaporate at room temperature for 3 weeks. Yellow-orange

needle like crystals formed in the crystallization dish after the EtOH evaporated.

For the 2:1 ratio, 2.0 mmols (0.38 g) of 9-VC was dissolved in 15 mL of EtOH in a test

tube by sonication, producing a colorless solution. 1.0 mmol (0.17 g) of 1,3-DNB was dissolved

in 10 mL of EtOH in a test tube by sonication, producing a light yellow color solution. When the

two solutions were combined in a crystallization dish, a bright yellow color solution rapidly

formed. The EtOH was allowed to evaporate at room temperature for four weeks. Yellow-orange

clumps formed in the crystallization after the EtOH evaporated.

64

Table 2.3.1.1. Co-crystals experimental reagents, solvents, and descriptions of crystals.

Electron Donor Electron Acceptor Solvent Description

Color Complex

Interaction 9-EC 2-NT EtOH White NO 9-EC 3-NT EtOH White NO 9-EC PNT EtOH White-Yellow NO 9-EC 1,3-DNB EtOH/Toluene Yellow-Orange YES 9-VC 2-NT EtOH White NO 9-VC NB EtOH White NO 9-VC 1,3-DNB EtOH/Toluene Yellow YES

Carbazole 2-NT EtOH Brown NO Carbazole NB EtOH Brown NO Carbazole 1,3-DNB EtOH/Toluene Brown YES

10-M 2-NT EtOH White NO 10-M 3-NT EtOH White NO 10-M PNT EtOH White NO 10-M 1,3-DNB EtOH Red-Purple YES

Phenothiazine 2-NT EtOH Brown NO Phenothiazine 3-NT EtOH Brown NO Phenothiazine PNT EtOH Brown NO Phenothiazine 1,3-DNB EtOH Brown YES

VDAT 2-NT H2O/EtOH (50:50) 65 °C White NO VDAT 3-NT H2O/EtOH (50:50) 65 °C White NO VDAT PNT H2O/EtOH (50:50) 65 °C White NO VDAT 1,3-DNB H2O/EtOH (50:50) 65 °C White NO MDAT 2-NT EtOH White NO MDAT 3-NT EtOH White NO MDAT PNT EtOH White NO MDAT 1,3-DNB EtOH White NO

Benzoguanamine 2-NT EtOH White NO Benzoguanamine 3-NT EtOH White NO Benzoguanamine PNT EtOH White NO Benzoguanamine 1,3-DNB EtOH White NO

2-VP 1,3-DNB EtOH White NO Vinylimidazole 1,3-DNB EtOH White NO

Acrylamide 1,3-DNB EtOH White NO

65

2.4 Instrumentation

FTIR spectra were recorded using a Jasco FT-IR 410 with the following parameters: 32

scans, 4 cm-1 resolution, % T, and a single background. 1 to 5 mg of a sample combined with

approximately 100 mg of dry KBr was ground into a fine powder. A pellet press was used to

produce a KBr pellet of the sample.

1H and 13C NMR spectra were recorded in CDCl3 or DMSO-d6 using a Bruker 500 or 360

MHz spectrometers. For recording 13C NMR spectra, the following parameters were used: D1

(relaxation delay) was set to zero, TD (time domain) was set to 16,000 points, and NS (number

of scans) was set to 60,000 scans. For processing the 13C NMR spectra, a line broadening value

of 20 was used as a smoothing function.

The glass transition temperatures (Tg) of the copolymers were collected by using a TA Q-

200 DSC. A small polymer sample was ground into a fine powder. 5 to 10 mg of the polymer

sample was heated with an initial heating ramp rate of 20 °C min-1 to the appropriate temperature

for DSC use and was held for three minutes. The polymer was then cooled to -40 °C with a

cooling ramp rate of 10 °C min-1, and was re-heated with a ramp rate of 10 °C min-1. The Tg was

determined as the inflection point between the upper and lower points of the heat capacity

transition.

Thermogravimetric analysis was performed with a TA 2950 TGA. A small polymer

sample was ground into a fine powder. The heating ramp rate was 5 °C min-1, with the

temperature being held at 75 °C for one hour in order to remove any residual water. The

temperature was increased to 600 °C with a heating ramp rate of 5 °C min-1. The decomposition

temperature (Td) was determined when 10% weight loss occurred.

66

Size exclusion chromatography (SEC) was performed on a Tosoh EcoSEC system with a

refractive index detector and a TSKgel Super HZ4000 column. PS-co-PVDAT copolymers were

dissolved in HPLC grade tetrahydrofuran (THF) at a concentration of 1.0 mg mL-1 and a flow

rate of 0.35 mL min-1 was utilized for the system. The injection port, column, and RI detector

were all set at 40 ˚C and the system was calibrated with polystyrene standards of narrow

polydispersity.

Silicon wafers were etched to produce 1 in. x 1 in. wafers for spin coating. The wafers'

surfaces were cleaned with D.I. H2O by spin coating and a small amount of acetone was applied

to a kimwipe to remove any dust from the etching process. The wafers' surfaces were dried using

filtered nitrogen. The wafers were stored in petri dishes.

Thin polymer films were spin coated using a Laurell (Model WS-400B-6NPP/LITE). The

polymer solutions were spin coated by two techniques: static and dynamic. The static spin

coating technique involved flooding the surface of the silicon wafer before spin coating. The

dynamic technique required the polymer solution to be dispensed during the spin coating process

at low speeds. A silicon wafer (1 in. x 1 in.) surface was flooded with a polymer solution. The

rotation speeds and time were varied in order to control film thicknesses. After spin coating, the

films were dried in an oven at 60 °C for 2 hours to remove any residual solvent.

Refractive Index measurements were performed on a J.A. Woollam Co. variable angle

spectroscopic (VASE) ellipsometer. Each of the films ψ and Δ were determined from 300 to

1000 nm at angles of 60°, 65°, 70°, 75°, and 80°. The data was fit and constrained to a Cauchy

Model with a polymer layer on a silicon wafer. An estimated SiO2 thickness of 20 Å was used

for the Cauchy model. The optical constants (n and k) were fitted to the Cauchy model. The

67

refractive index (n) and extinction coefficient (k) of the polymer films were recorded before and

after exposure to a concentrated nitroaromatic vapor.

The polymer films were exposed to concentrated nitroaromatic vapors using a simple

exposure apparatus. A large jar approximately 4 in. x 3 in. contained a large amount of a

nitroaromatic compound. A smaller jar approximately 2 in. x 2 in. was placed inverted inside the

larger jar with the nitroaromatic compound, so the bottom of the smaller jar could be used as a

sample holder allowing the film to rest above the nitroaromatic compound. Once the film was

placed inside the exposure apparatus and the lid tighten, the exposure time would be recorded.

After the film was exposed to a nitroaromatic vapor for a determined amount of time, the film

would then be removed and the refractive index would be measured using ellipsometry. The

exposure experiments were performed at temperatures between 22 - 25 °C.

A DekTak II A profilometer was used to measure film thickness to confirm the film

thickness measurements of the ellipsometer. A 1 mm scan was performed using a slow scan

method to measure the thickness of the films from an etched made into the films and observe

surface roughness.

A Siemens CCD Smart (Area Detector) and Enraf-Nonius CAD-4 computer controlled

X-ray diffractometer was used to measure the crystal structure. Steven Kelley determined the

crystal structure.

The melting points of the co-crystals with nitroaromatics were determined using an

Electrothermal IA9100. Approximately 2 mg of the co-crystals were ground into a fine powder

and transferred to a capillary tube. The instrument was dried by increasing the temperature to

300 °C with the lens removed to allow water vapor to escape the instrument. The instrument was

allowed to cool and equilibrate at 30 °C for 24 hours. After equilibrating for 24 hours, three

68

samples were placed in the capillary tube holder and heated at a ramp rate of 1 °C/min. The

melting point was determined from the first sign of liquid formation until the formation of a

meniscus after the sample completely melted.

A Shimadzu UV-3600 (UV/Vis - NIR) spectrophotometer was used to record the diffuse

reflectance spectra of the nitroaromatic co-crystals. A crystal sample of 200 mg was ground into

a fine powder and transferred to the holder with two Teflon spacers. A microscope slide was

used to make the sample flush with the top of the holder. The scan range for diffuse reflectance

measurements were from 900 - 200 nm by 0.5 nm.

A Cary 5G UV-Vis-NIR spectrophotometer was used to record the electronic absorption

spectra of the nitroaromatic co-crystals. Scan Mode was used to record the electronic absorption

spectra with the following parameters: Ave. time (s) - 0.100, Data interval (nm) - 1.00, Scan rate

(nm/min) - 200, SBW - 2.00, Beam mode - Double, Slit height - Full. The electronic absorption

spectra were recorded from 800 to 200 nm using a Zero/Baseline correction. A stock solution

was prepared and dilutions were made until the solution's absorbance was below two.

69

Chapter 3

Polymer Characterization

A series of random copolymers were prepared containing 2-vinyl-4,6-diamino-1,3,5-

triazine (VDAT). The VDAT content was varied from 0 to 20 mole percent. Polymers with

higher VDAT content were insoluble and could not be used to cast films by spin coating. In this

dissertation, the VDAT content reported is the one based on the mole ratio of VDAT added to

the polymerization reaction.

3.1 PVDAT Characterization

The FTIR spectrum for PVDAT was recorded in KBr, as shown in Figure 3.1.1. The

broad peaks located at 3465, 3343, and 3215 cm-1 were assigned to the NH2 asymmetric and

symmetric stretching vibration modes. The broad peak's shoulder located at 2987 cm-1 was

assigned to the CH stretching vibrations. The peak located at 1635 cm-1 was assigned to the NH2

internal deformation vibrational mode. The peaks located at 1544 and 1449 cm-1 were assigned

as the triazine ring in-plane vibrations.51 The peak located at 1113 cm-1 was assigned as the (C-

N) bond of the amine group bound to the triazine ring carbon. The peak appearing at 822 cm-1

represented the out-of-plane triazine ring bending vibration.51 The triazine ring, in plane and out

of plane stretching vibrations, and amine stretching vibrations were used as a reference for

determining the presence of PVDAT within the copolymers.

70

Figure 3.1.1. The FTIR spectrum of PVDAT recorded in KBr.

4000 3500 3000 2500 2000 1500 1000 5000

20

40

60

80

100

% T

rans

mitt

ance

Wavenumbers (cm-1)

3343

3215

1635 1544

14493465

1113

822

71

DSC and TGA were used to characterize PVDAT's thermal properties. Figure 3.1.2

shows the decomposition temperature (Td) of PVDAT by TGA under an air atmosphere. From

the graph, there was observed weight loss from 75 °C to ≈ 200 °C due to the presence of residual

water. The decomposition temperature was determined when 10% weight loss occurred from 201

°C (75.41%, 7.662 mg). The region near approximately 200 °C was chosen for calculating the Td

after residual water was removed from the sample. PVDAT exhibited a 10% weight loss at 371

°C (65.42%, 6.646 mg). The polymer completely decomposed at approximately 594 °C (0 mg).

The VDAT moieties improve the thermal stability of the polymer, since they are capable of

acting as free radical scavengers limiting chain scissions and depolymerization. 49

100 200 300 400 500 6000

20

40

60

80

100

Wei

ght L

oss

(%)

Temperature (οC)

Figure 3.1.2. PVDAT TGA curve.

72

Figure 3.1.3 shows the DSC curve for PVDAT between 60 °C and 180 °C. The PVDAT

DSC curve did not exhibit a glass transition (Tg) or melting endotherm (Tm) due to strong

intermolecular interactions between the polymer's VDAT repeating units. The Tg may be at a

temperature above the thermal decomposition temperature. Since no Tg or Tm was observed, the

polymer was amorphous or possessed very little crystallinity.

60 80 100 120 140 160 180

-2.4

-2.2

-2.0

-1.8

-1.6

Hea

t Flo

w (W

/g)

E

ndot

herm

Temperature (oC)

Figure 3.1.3. PVDAT DSC curve.

PVDAT was not soluble in any common solvents, due to strong intermolecular

interactions. This prevented the acquisition of 1H NMR, 13C NMR, and GPC data (Molecular

Weight).

73

3.2 PS-co-PVDAT Copolymers Characterization

The PS-co-PVDAT copolymers FTIR spectra were recorded in KBr, shown in Figure

3.2.1. The peaks located at 1601 and 1492 cm-1 were assigned the benzene ring (C=C) vibrations

for polystyrene. The NH2 asymmetric and symmetric stretching vibrational modes of PVDAT

appeared at 3470, 3403, and 3319 cm-1.52 The triazine ring in-plane vibrational modes were

observed at 1546 cm-1 and 1451 cm-1 with the out of plane bending vibrational mode appearing

at 822 cm-1. The appearance of the referenced PVDAT peaks provided evidence that PVDAT

was incorporated into the copolymers. As the VDAT concentration increased, a decrease in

transmittance was observed for the NH2 and the triazine ring vibrational modes.

4000 3500 3000 2500 2000 1500 1000 500

% T

rans

mitt

ance

(A.U

.)

Wavenumbers (cm-1)

16011546

1492

1451

3470

3403 3319828

1 mol % VDAT

5 mol % VDAT

10 mol % VDAT

20 mol % VDAT

Figure 3.2.1. FTIR spectra for the PS-co-PVDAT copolymers.

74

1H and 13C NMR experiments were performed for copolymer structure characterization.

The 1H and 13C NMR spectra were recorded in CDCl3 or DMSO-d6 using a Bruker 500 or 360

MHz spectrometer. To confirm the presence of VDAT incorporated into the copolymers, the

VDAT 1H NMR spectrum (Appendix Figure 1) was recorded and shown in the appendix to

determine the location of the vinyl and amine protons.

VDAT: 1H NMR (360 MHz, DMSO-d6, δ) 6.67 (br., 3.98 H, NH2), 6.42-6.26 (m, 2.02 H, CH),

5.62 (dd, 1.00 H, CH).

Figure 3.2.2 shows the PS-co-PVDAT 20 mol % VDAT 1H NMR spectrum. For the PS-co-

PVDAT 20 mol % VDAT 1H NMR spectrum, the VDAT vinyl protons were absent, indicating

that the starting monomer was completely incorporated into the copolymer. The two broad peaks

located between 2.0 - 0.8 ppm were assigned to the polymer's backbone methine and methylene

protons. The polystyrene phenyl protons appeared in the region from 7.50 - 6.50 ppm with the

PVDAT amino protons overlapping with the polystyrene phenyl protons. A trend observed in the

PS-co-PVDAT copolymers 1H NMR spectra was that as the VDAT concentration decreased, the

spectra appeared similar to the polystyrene 1H NMR spectrum. Less broadening was observed in

the polystyrene phenyl protons region with an increase in the number of peaks present. Some of

the PS-co-PVDAT copolymers spectra do show small peaks located in the vinyl protons region

for styrene and VDAT. These peaks were likely unreacted monomer from the free radical

polymerization, which were unable to be removed even after repeated washings with EtOH or

soxhlet extraction. The methine and methylene polymer backbone protons for polystyrene and

PS-co-PVDAT copolymers could not be differentiated due to both peaks being very broad and

overlapping.

75

Figure 3.2.2. PS-co-PVDAT 20 mol % 1H NMR (360 MHz, DMSO-d6) spectrum.

13C NMR experiments provided further confirmation that VDAT was incorporated into

the copolymers. Figure 3.2.3 shows the PS-co-PVDAT 20 mol % VDAT 13C NMR spectrum

recorded in DMSO-d6. The methine and methylene carbons were not observed in the spectrum

due to the overlapping DMSO-d6 solvent peak. The peaks located between 145 and 127 ppm

were assigned to the polystyrene phenyl carbons. The peak located at 166.91 ppm (C7) was

assigned to the two equivalent triazine carbons attached to the amino groups.49 A new peak

observed at 180.61 ppm was assigned as the triazine quaternary carbon (C8). The (C8) carbon

signal was only observed in the 20 mol % copolymer. The triazine (C7) carbon signal was

observed only in the 10 and 20 mol % copolymers. The weak intensities of the triazine carbons

peaks were due to the slow relaxation process and small VDAT concentrations.

0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.0ppm

H2 H1

TMS

DMSO-d6

H2O

H3, H6

H4, H5

76

Figure 3.2.3. PS-co-PVDAT 20 mol % VDAT 13C NMR spectrum (500 MHz, DMSO-d6).

Table 3.2.1. Polystyrene and PS-co-PVDAT 20 mol % VDAT copolymer 13C NMR peaks.

Carbon # Polystyrene (ppm) PS-co-PVDAT 20 mol % (ppm)1 40.62 N/A

2 46.06, 44.09 N/A

3 145.33 144.97

4 128.19 127.84, 127.27

5 128.19 127.84, 127.27

6 125.87 125.51

7 N/A 166.91

8 N/A 180.61

120125130135140145150155160165170175180185ppm

C8 C7 C3

C4, C5

C6

77

To confirm the triazine (C8) peak located at 180.61 ppm, the VDAT 13C NMR (Appendix

Figure 10) spectrum was recorded in DMSO-d6 and shown in the appendix.

VDAT: 13C NMR (500 MHz, DMSO-d6, δ) 169.90 (s, 1C), 167.72 (s, 2C), 136.75 (s, 1C),

123.75 (s, 1C).

The PS-co-PVDAT peak at 166.91 ppm (C7) was in agreement with the peak observed at 167.72

ppm for the VDAT two equivalent triazine carbons (C4) attached to the amino groups. The

VDAT triazine quaternary carbon (C3) attached to the vinyl group was observed at 169.90 ppm.

The VDAT triazine carbon (C3) signal confirmed the presence of the peak located at 180.61

ppm. The peak was shifted downfield to 180.61 ppm for the PS-co-PVDAT 20 mol % VDAT

copolymer due to polymer backbone shielding effects. The PS-co-PVDAT copolymers 13C NMR

spectra trend observed was that the triazine carbon signals were concentration dependent. One

triazine carbon signal (C7) was observed only in the 10 mol % copolymer and two triazine

carbon signals (C7 and C8) were observed in the 20 mol % VDAT copolymer. The 1 and 5 mol

% VDAT copolymers did not exhibit these peaks and appeared similar to the polystyrene 13C

NMR spectrum.

DSC and TGA characterized the PS-co-PVDAT copolymers and PS thermal properties.

TGA experiments were performed, characterizing the PS-co-PVDAT copolymers decomposition

temperatures (Td). The Td temperatures when 10% weight loss occurred are shown in Table

3.2.2. For calculating Td when 10% weight loss occurred, the starting temperature chosen was

100 °C to account for the removal of residual solvent or water present in the polymer matrix.

Figure 3.2.4 displays the TGA curves for PS and PS-co-PVDAT 20 mol % VDAT copolymer.

Polystyrene began to decompose near 300 °C compared to the copolymer that began showing

weight loss above 300 °C. Polystyrene completely decomposed at approximately 430 °C

78

compared to the copolymer that decomposed at approximately 594 °C. The noticeable curve

features in the copolymer's TGA curve resulted from temperatures above polystyrene's ceiling

temperature (Tc), leading to chain scissions and polystyrene depolymerization. Polystyrene has a

Tc at 310 °C.53 For vinyl polymers, there is an equilibrium between the free monomers and the

copolymer. The Tc is the temperature at which the propagation and depropagation rates are equal.

As the temperature increased above the Tc, the depropagation rate constant increased, leading to

depolymerization of the polymer back to the monomers. A temperature below the Tc, the

equilibrium favors the polymer formation. The curve features were present in the 10 and 20 mol

% VDAT copolymers. The lower VDAT concentration copolymers exhibited a TGA curve

similar to polystyrene with the exception of weight loss occurring at slightly higher temperatures

and complete decomposition at elevated temperatures. The VDAT moieties played an important

role in the thermal stability of the copolymers. The VDAT moieties were capable of acting as

free radical scavengers at high temperatures, slowing chain scission and depolymerization of the

styrene moieties and improving the thermal stability of the copolymers.49 The Td at 10% weight

loss began showing an increasing trend in the copolymers as the VDAT concentration increased,

except for the 10 and 20 mol % VDAT copolymers. The change in the trend for the two

copolymers may be attributed to the amount of polymer sample used during the experiment, the

depolymerization of the copolymer, and/or the loss of residual solvent or water trapped within

the polymer matrix increased weight loss. Even though the two copolymers do not follow the Td

trend, the thermal stability increase was observed in the TGA curves with complete

decomposition occurring at higher temperatures than polystyrene.

79

100 200 300 400 500 6000

20

40

60

80

100W

eigh

t Los

s (%

)

Temperature (οC)

PS 20 mol % VDAT

Figure 3.2.4. TGA curves for PS and PS-co-PVDAT 20 mol % VDAT copolymer.

Table 3.2.2. Thermal decomposition temperatures, Td (10% weight loss for PS and PS-co-PVDAT copolymers).

Polymer Td (°C)

PS 316

1 mol % 323

5 mol % 344

10 mol % 305

20 mol % 325

80

Figure 3.2.5 shows the DSC curves for PVDAT, PS-co-PVDAT copolymers, and PS. The

Tg temperatures for the PS-co-PVDAT copolymers and PS are listed in Table 3.2.3. In Figure

3.2.5, all polymers demonstrated a Tg, but not a Tm, which would indicate that the polymers were

amorphous, or possessed little crystallinity. A general trend observed was that as the VDAT

concentration increased, there was a gradual increase in Tg. To demonstrate this trend, PS

showed a Tg at 80 °C compared to the PS-co-PVDAT 20 mol % copolymer, which showed a Tg

at 144 °C. The Tg observed for polystyrene was lower than the literature reported Tg (100 °C).54

This decrease in Tg may be attributed to a low molecular weight homopolymer or unreacted

monomer trapped within the polymer matrix. The trend observed in the DSC curves

60 80 100 120 140 160 180

(W/g

)

End

othe

rm

Temperature (oC)

PS

1 mol%

5 mol %

10 mol %

20 mol %

PVDAT

Figure 3.2.5. PVDAT, PS-co-PVDAT copolymers, and PS DSC curves.

81

Table 3.2.3. Glass transition temperatures for PVDAT, PS-co-PVDAT copolymers, and PS.

Polymer Tg (°C) PVDAT N/A

20 mol % 144

10 mol % 121

5 mol % 107

1 mol % 86

PS 80

was attributed to an increase in strong intermolecular forces. A similar trend was observed by

Keo et al. for PS-co-PVDAT copolymers' DSC curves.52 By increasing the VDAT concentration,

more hydrogen bonding and dipole - dipole interactions were present, resulting in decreased

chain mobility and higher glass transition temperatures.

Dr. Todd Saylor performed size exclusion chromatography (SEC) on a Tosoh EcoSEC

system with a refractive index detector and a TSKgel Super HZ4000 column. The negative peak

at approximately eleven minutes was from the THF solvent, while the three peaks from

approximately nine to eleven minutes were from the THF leaching a plasticizer or a contaminant

present in the syringe filters. Table 3.2.4 lists the number average molecular weight (Mn), the

weight average molecular weight (Mw), the Z-average molecular weight (Mz), and the

polydispersity (Mw/ Mn). As the VDAT concentration increased in the copolymers, the number

average molecular weight (Mn) values showed a slight decrease. Mw and PDI followed a similar

trend as Mn, with the exception of the 5 mol % copolymer.

82

Table 3.2.4. PS-co-PVDAT copolymers GPC data.

Polymer Mn Mw Mz PDI (Mw/ Mn)

20 mol % 5379 7318 14703 1.36

10 mol % 5383 7552 11466 1.40

5 mol % 5453 10557 69666 1.94

1 mol % 6670 10279 15279 1.54

3.3 PMMA-co-PVDAT Copolymers Characterization

The FTIR spectra for the PMMA-co-PVDAT copolymers are shown in Figure 3.3.1. The

PVDAT NH2 asymmetric and symmetric stretching modes appeared at 3425 (broad) and 3228

(shoulder) cm-1. The CH stretching modes appeared between 2998 - 2843 cm-1. The methyl

methacrylate carbonyl peak was located at 1729 cm-1. The triazine ring in plane vibrations were

observed at 1638 and 1570 cm-1 which were in agreement with the reported literature

assignments.55 The triazine ring out of plane stretching vibration signal was located at 829 cm-1.

The triazine in plane stretching modes and NH2 stretching vibrations decreased in transmittance

as the VDAT concentration was increased. The methyl methacrylate carbonyl signal appearing at

1729 cm-1 showed little variation, indicating that the carbonyl did not undergo a chemical shift as

VDAT was incorporated into the copolymer. The observed PVDAT vibrational modes confirmed

the presence of PVDAT incorporated in the copolymers.

83

4000 3500 3000 2500 2000 1500 1000 500

% T

rans

mitt

ance

(A.U

.)

Wavenumbers (cm-1)

1 mol %

5 mol %

10 mol %

20 mol %

Figure 3.3.1. FTIR spectra for the PMMA-co-PVDAT copolymers.

To confirm the presence of VDAT in the copolymers, 1H and 13C NMR experiments were

performed on a Bruker 500 or 360 MHz NMR for copolymer structure characterization. Figure

3.3.2 shows the 1H NMR spectrum for the PMMA-co-PVDAT 20 mol % VDAT copolymer

recorded in DMSO-d6. The PMMA alpha methyl proton signals appeared between 1.25 to 0.72

ppm overlapping with the methylene PVDAT (H1V) proton signal.56 The PMMA polymer

backbone methylene proton signal (H1M) appeared between 2.04 - 1.31 ppm, overlapping with

the PVDAT methine proton (H2V) signal.56 The OCH3 (H4) proton signal was observed at 3.54

ppm, producing an intense peak. The VDAT vinyl protons (6.38 - 6.26 ppm and 5.63 - 5.60 ppm)

were absent in the PMMA-co-PVDAT 20 mol % VDAT copolymer spectrum, indicating

84

Figure 3.3.2. 1H NMR spectrum in DMSO-d6 for PMMA-co-PVDAT (20 mol %).

that the monomer was completely incorporated into the copolymer. The appearance of the broad

amine peak (6.55 ppm) in the copolymer spectrum was another indication that VDAT was

included in the copolymer. The broad amine peak was only present in the 20 mol % and only

slightly present in the 10 mol % copolymer spectra. The amine peak was observed in the 1 and 5

mol % copolymer spectra, but could only be observed when the spectra's intensities were

increased significantly. The PVDAT methylene and methine proton signals could not be

accurately identified. As the VDAT concentration increased in the copolymers, the PMMA alpha

methyl and methylene proton resonance signals became broader, suggesting that the PVDAT

-0.0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.5ppm

TMS

H5

H4

H1M, H2V H3, H1V

H3

85

polymer backbone proton signals were present. An observed trend in the copolymer 1H NMR

spectra was that as the VDAT concentration decreased in the copolymers, the 1H NMR spectrum

appeared more similar to the PMMA 1H NMR spectrum.

To provide further copolymer characterization, 13C NMR (500 MHz) experiments were

performed. The 13C NMR spectrum for the PMMA-co-PVDAT 20 mol % VDAT copolymer is

shown in Figure 3.3.3. In the copolymer's spectrum, two new peaks observed at 181.06 (C8) and

167.01 (C9) ppm provided evidence that VDAT was incorporated into the copolymer. The

PVDAT quaternary carbon (C8) signal was only observed in the 20 mol % copolymer. The

PVDAT (C9) carbon signal was observed in the 20, 10, and 5 mol % copolymers. PMMA carbon

signals in the copolymer spectrum did not exhibit any chemical shifts after the incorporation of

VDAT, compared with that of the PMMA 13C NMR spectrum. PVDAT's methine and methylene

carbon signals were not observed due to overlapping with the PMMA quaternary carbon (C3)

signal, PMMA methylene carbon signals (C1), and the DMSO-d6 solvent peak.56 An observed

13C NMR copolymer trend was that as the VDAT concentration decreased, the spectra appeared

similar to the PMMA 13C NMR spectrum.

86

Figure 3.3.3. 13C NMR spectrum in DMSO-d6 for the PMMA (80%)-co-PVDAT (20%) copolymer.

PMMA-co-PVDAT 20 mol % 13C NMR (500 MHz, DMSO-d6, δ): 181.06 (C8), 177.36-176.48

(C4), 167.01 (C9), 53.57-48.02 (C1), 51.60 (C5), 44.19-43.86 (C3), 20.68-16.29 (C2).

The thermal decomposition temperatures (Td) were determined for each copolymer using

TGA. The temperature at which the polymers' mass decreased to 10% of its original value was

reported as the Td. The Td for each polymer is listed in Table 3.3.1. The TGA curves for PMMA

and PMMA-co-PVDAT 10 mol % VDAT copolymer are shown in Figure 3.3.4. PMMA began

decomposing at approximately 225 °C and completely decomposed at approximately 385 °C.

The 10 mol % copolymer, on the other hand, began decomposing at approximately 280 °C and

completely decomposed at approximately 600 °C. The features observed in the 10 mol %

102030405060708090110130150170190ppm

C8C4

C9C2

C5

C3

C1

87

copolymer TGA curve occurred at temperatures above 350 °C. These features were the result of

chain scissions and PMMA depropagation to the free monomer due to temperatures higher than

PMMA's Tc (220 °C).53 These features were only observed in the 10 and 20 mol % copolymers.

For the four copolymers, as the VDAT concentration increased, Td increased, indicating a higher

thermal stability. The TGA curves for the 1 and 5 mol % copolymers appeared similar to the

PMMA TGA curve with the exception of weight loss and complete decomposition occurring at

elevated temperatures. The VDAT moieties played an important role in the thermal stability of

the copolymers. The VDAT moieties acted as free radical scavengers at high temperatures,

slowing chain scissions and depolymerization of the PMMA moieties improving the thermal

stability of the copolymers.49 The initial temperature used for calculating 10% weight loss was

76 °C, as the TGA curves did not exhibit significant weight loss after being heated for 1 hour at

75 °C.

88

100 200 300 400 500 6000

20

40

60

80

100W

eigh

t Los

s (%

)

Temperature (oC)

PMMA 10 mol %

Figure 3.3.4. TGA curves for PMMA and the copolymer containing 10 mol % VDAT.

Table 3.3.2. Thermal decomposition temperatures for PMMA and the copolymers of PMMA and PVDAT.

Polymer Td (°C)

PMMA 236

1 mol % 263

5 mol % 254

10 mol % 286

20 mol % 285

89

The DSC curves for the PMMA-co-PVDAT copolymers are shown in Figure 3.3.5, and

Table 3.3.3 lists the glass transition temperatures. The Tg for PMMA was observed at 103 °C,

which was lower than the reported literature Tg (105 °C).54 The 1 and 5 mol % copolymers, glass

transition temperatures were observed at 90 °C and 86 °C lower than the homopolymer. The

lower glass transition temperatures may be attributed to unreacted monomer within the

copolymer matrix. The 1H NMR spectrum with an increased intensity for the 1 mol % copolymer

(Appendix Figure 25) showed unreacted vinyl protons from either VDAT or MMA. The 10 and

20 mol % copolymers' glass transition temperatures were observed at 122 and 154 °C.

80 100 120 140 160

Hea

t Flo

w (W

/g)

End

othe

rm

Temperature (οC)

20 mol %

10 mol %

5 mol %

1 mol %

PMMA

Figure 3.3.5. PMMA and PMMA-co-PVDAT copolymers DSC curves.

90

Table 3.3.3. The glass transition temperatures for PMMA, PMMA-co-PVDAT copolymers, and PVDAT.

Polymer Tg (°C) PVDAT N/A

20 mol % 154

10 mol % 122

5 mol % 86

1 mol % 90

PMMA 105

The higher mol % copolymers follow a similar trend as the PS-co-PVDAT copolymers. As the

VDAT concentration increased, the Tg increased and became higher than the homopolymer's Tg.

Increasing the VDAT concentration increased hydrogen bonding and dipole - dipole interactions

within the copolymer matrix, limiting chain mobility and resulted in higher glass transition

temperatures.

GPC experiments were performed by Dr. Medhat Farahat on three PMMA-co-PVDAT

copolymer samples (20, 10, and 5 mol %) and Table 3.3.4 lists the Mn, Mw, Mz, and PDI. The

lower VDAT concentration copolymers (10 and 5 mol %) exhibited higher Mn, Mw, and Mz

compared to the 20 mol % copolymer. The PDI decreased as the VDAT concentration increased

in the copolymers.

Table 3.3.4. Molecular weights for the PMMA-co-PVDAT copolymers determined by GPC.

Polymer Mn Mw Mz PDI (Mw/ Mn)

20 mol % 6,100 7,870 9,350 1.27

10 mol % 16,100 24,100 32,600 1.49

5 mol % 14,100 23,100 31,100 1.64

91

3.4 PMA-co-PVDAT Copolymer Characterization

The FTIR spectrum for the PMA-co-PVDAT 20 mol % VDAT copolymer was recorded

in KBr shown in Figure 3.4.1. The broad peak and shoulder located at 3436 and 3224 cm-1 were

assigned to the NH2 asymmetric and symmetric vibrations overlapping with an OH stretching

vibration. The methyl acrylate carbonyl peak appeared at 1734 cm-1. The position of the carbonyl

(1734 cm-1) did not exhibit a chemical shift, indicating no interaction with VDAT during the

polymerization. The PVDAT in plane vibrations could not be accurately identified, but the out of

plane stretching vibration was observed at 829 cm-1. The copolymer spectrum was

4000 3500 3000 2500 2000 1500 1000 500

60

70

80

90

100

% T

rans

mitt

ance

Wavenumbers (cm-1)

1734C=O

CH

NH2

829

Figure 3.4.1. FTIR spectrum for the PMA-co-PVDAT 20 mol % VDAT copolymer.

92

compared to the FTIR spectrum for Poly(methyl acrylate) recorded by Haken.57

The PMA and PMA-co-PVDAT copolymer structures were characterized by 1H and 13C

NMR. Figure 3.4.2 shows the PMA-co-PVDAT 20 mol % VDAT copolymer 1H NMR spectrum

(360 MHz, DMSO-d6). The PMA and PVDAT methine and methylene peaks overlapped

forming broad peaks located between 2.29 - 1.98 ppm (CH) and 1.83 - 1.15 ppm (CH2). The (O-

CH3) proton signal appeared at 3.51 ppm. The PVDAT NH2 proton signal was assigned to the

broad peak observed at 6.53 ppm. The VDAT and methyl acrylate vinyl proton signals (6.40 -

4.00 ppm) were not present in the spectrum, indicating that the monomers were completely

incorporated in the copolymer.

Figure 3.4.2. 1H NMR spectrum for the PMA-co-PVDAT 20 mol % VDAT copolymer.

0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.0ppm

H4

H3

H2 H1

93

Figure 3.4.3 shows the 13C NMR spectrum (500 MHz, DMSO-d6) for the PMA-co-

PVDAT 20 mol % VDAT copolymer. First, the PMA 13C NMR spectrum (Appendix Figure 39)

was recorded and used as a reference for comparison. The PMA carbon signals were in

agreement with the reported literature assignments.58

PMA: 13C NMR (500 MHz, DMSO-d6, δ) 174.36 (C=O), 51.52 (O-CH3), 40.76 (CH), 34.27

(CH2).

Figure 3.4.3. The 13C NMR spectrum (500 MHz, DMSO-d6) for the PMA-co-PVDAT 20 mol % VDAT copolymer.

2030405060708090100110120130140150160170180ppm

C5C3

C6C4

C2 C1

94

The PMA carbon signals appeared at 174.72, 174.30 (C=O), 51.37 (O-CH3), 43.36 (CH), and

34.23 (CH2) ppm. The PVDAT carbon signals were observed at 178.77 (C5) and 166.94 (C6).

The PVDAT methine and methylene carbon signals could not be identified due to overlapping

with the PMA polymer backbone carbon signals and the DMSO-d6 solvent peak. The observed

broad peaks in the methine and methylene region suggested that the PVDAT methine and

methylene carbons were present in the polymer backbone. The appearance of the C5 and C6

carbon signals provided evidence that PVDAT was incorporated within the polymer matrix.

A DSC experiment was performed on the PMA-co-PVDAT 20 mol % VDAT copolymer

for characterizing the Tg (Figure 3.4.4). A DSC experiment was not performed on PMA due to

the soft properties of the homopolymer and low Tg. PMA has a reported Tg below room

Figure 3.4.4. DSC curve for the PMA-co-PVDAT 20 mol % VDAT copolymer.

95

temperature, at approximately 12.5 °C.59 The PMA-co-PVDAT copolymer exhibited a Tg at 60

°C. Introducing intermolecular forces (hydrogen bonding and dipole - dipole interactions)

increased the copolymer's Tg significantly, limiting the polymer chains' mobility. The increase in

Tg follows the trend observed in other PVDAT copolymers.

3.5 P2VP-co-PVDAT Copolymers Characterization

FTIR experiments were performed on the copolymers to determine if VDAT was

incorporated into the copolymers. The FTIR spectra for the P2VP-co-PVDAT copolymers were

recorded in KBr, shown in Figure 3.5.1. In the copolymers spectra, the PVDAT amine groups'

4000 3500 3000 2500 2000 1500 1000 500

% T

rans

mitt

ance

(A.U

.)

Wavenumbers (cm-1)

P2VP

1 mol %

5 mol %

20 mol %

Figure 3.5.1. FTIR spectra for P2VP and P2VP-co-PVDAT copolymers.

96

symmetric and asymmetric stretching vibrations appeared at 3198 and 3387 cm-1, overlapping

with a OH stretching vibration due to the presence of water in the sample. The amine vibrational

modes decreased in transmittance as the VDAT concentration increased. The copolymers' CH

stretching vibrations appeared between 2800 and 3000 cm-1. The triazine ring in plane vibrations

could not be accurately identified due to the peaks' broadness. The triazine ring out of plane

vibration was observed at 830 cm-1. The observed PVDAT amine and out of plane vibration

modes confirmed the presence of PVDAT in the copolymers.

1H and 13C NMR experiments were performed to provide structural characterization of

the copolymers. The P2VP 1H NMR spectrum (Appendix Figure 40) was recorded with the peak

positions used as a reference.60 Figure 3.5.2 shows the 1H NMR spectrum for the P2VP-co-

PVDAT 20 mol % VDAT copolymer. The copolymer's methine and methylene proton signals

were located between 2.4 - 0.7 ppm. The methine and methylene protons could not be

differentiated due to overlapping resonance signals. The PVDAT amine proton signal was

located between 6.75 - 6.05 ppm, overlapping with the P2VP H4 proton signal. The P2VP

aromatic proton signals were observed at 8.64 - 8.07 (H1), 7.80 - 7.21 (H3), and 6.98 - 6.56 (H2)

ppm. The VDAT vinyl protons were absent in the spectrum which indicated that the monomer

was completely incorporated in the copolymer. When compared to the P2VP 1H NMR spectrum,

peak intensities decreased and peak broadness increased as the VDAT concentration increased.

An observed 1H NMR spectra trend was that as the VDAT concentration decreased, the

copolymers' spectra appeared similar to the P2VP 1H NMR spectrum.

97

Figure 3.5.2. 1H NMR spectrum for the P2VP-co-PVDAT 20 mol % VDAT copolymer in DMSO-d6 using the 360 MHz spectrometer.

13C NMR experiments were performed to provide structure characterization. The 13C

NMR spectrum for the P2VP homopolymer (Appendix Figure 43) was recorded with the peak

assignments used as a reference.61 Figure 3.5.3 shows the P2VP-co-PVDAT 1 mol % VDAT

copolymer 13C NMR spectrum. The methine and methylene carbon signals appeared in the

region from 42 - 39 ppm, overlapping with the DMSO-d6 solvent peak. The peak located at

42.07 could not be assigned due to strong resonances from the P2VP methine and methylene

carbons.61 The P2VP aromatic carbon signals appeared at 164.10, 163.43 (C2), 148.79, 148.69

(C6), 135.74, 135.45 (C4), 122.52 (C3), and 120.79 (C5). The PVDAT quaternary carbon (C7)

0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.5ppm

H1 H3H2

H4, H5

98

signal was absent in the spectrum, but the PVDAT (C8) carbon signal was observed at 166.91

ppm. The PVDAT carbon (C8) signal was also observed in the 20 mol % copolymer, but not

observed in the 5 mol % VDAT copolymer.

Figure 3.5.3. The 13C NMR spectrum (500 MHz, DMSO-d6) for the P2VP-co-PVDAT 1 mol % VDAT copolymer.

DSC experiments were performed to determine the values of Tg for the copolymers.

Table 3.5.1 lists the homopolymer and copolymers' glass transition temperatures. The observed

Tg for P2VP was at 92 °C. The 20 mol % copolymer produced by the same experimental amounts

showed a Tg at 156 °C, significantly higher than the homopolymer. The 1 mol % copolymer

synthesized with smaller experimental amounts showed a slightly higher Tg than the

405060708090100110120130140150160170180ppm

C8

C2

C4C6

C3

C5

99

homopolymer (105 °C). The 1 mol % copolymer curve produced two endotherm features, but the

second feature at a higher temperature was chosen to be interpreted as the Tg. The 5 mol %

copolymer synthesized by smaller experimental amounts displayed a Tg much lower than the

homopolymer. This change in the Tg trend was likely due to experimental conditions and

amounts producing a copolymer with shorter polymer chains and low molecular weight. The 1

and 20 mol % copolymers follow the Tg trend observed in other PVDAT copolymers. By

increasing the VDAT concentration, strong intermolecular forces (hydrogen bonding and dipole -

dipole interactions) decreased the polymer's chains mobility, increasing the Tg.

20 40 60 80 100 120 140 160

(W/g

)

End

othe

rm

Temperature (οC)

P2VP

1 mol %

5 mol %

20 mol %

Figure 3.5.4. P2VP and P2VP-co-PVDAT copolymers DSC curves.

100

Table 3.5.1. P2VP and P2VP-co-PVDAT copolymers glass transition temperatures (°C).

Polymer Tg (°C) P2VP 92

1 mol % 105

5 mol % 60

20 mol % 156

3.6 PAM-co-PVDAT Copolymers Characterization

The FTIR spectra for the PAM and PAM-co-PVDAT copolymers are shown in Figure

3.6.1. The PVDAT and acrylamide amine asymmetric and symmetric stretching vibrations

appeared at 3350 and 3195 cm-1. The CH stretching vibration modes produced a broad peak at

2934 cm-1. The polyacrylamide carbonyl peak was observed at 1663 cm-1. The carbonyl peak's

position indicated that the copolymers possessed low average molecular weight, since ʋC=O

frequency increases as average molecular weight increases.62 One of the PVDAT triazine in

plane vibration modes was observed at 1545 cm-1 for the 20, 10, and 5 mol % copolymers. The

PVDAT triazine out of plane bending mode appeared at 829 cm-1 observed in the 20, 10, and 5

mol % copolymers. As the VDAT concentration increased in the copolymers, the triazine ring in

plane and out of plane bending modes became more apparent, suggesting that PVDAT was

incorporated into the copolymer.

101

4000 3500 3000 2500 2000 1500 1000 500

% T

rans

mitt

ance

(A.U

.)

Wavenumbers (cm-1)

20 mol %

10 mol %

5 mol %

1 mol %

PAM

Figure 3.6.1. PAM and PAM-co-PVDAT copolymers FTIR spectra.

1H and 13C NMR experiments were performed on the homopolymer and copolymers for

structure characterization. Figure 3.6.2 shows the 1H NMR spectrum for the PAM-co-PVDAT 20

mol % VDAT copolymer recorded in D2O. The copolymer's methine and methylene protons

signals appeared between 2.46 - 2.02 ppm and 1.92 - 1.03 ppm. The PAM methylene and

methine peak positions were in agreement with the peak positions reported in the literature.63

The PVDAT and PAM amine proton signals were not observed due to proton exchange with the

NMR solvent.

102

Figure 3.6.2. The 1H NMR spectrum (360 MHz, D2O) for the PAM-co-PVDAT 20 mol % VDAT copolymer.

The 13C NMR spectrum of the 20 mol % copolymer is shown in Figure 3.6.3. The

spectrum was referenced according to the carbonyl carbon signal observed at 180.00 ppm.63 The

polymer backbone methine and methylene carbon signals appeared at 42.28 ppm and 36.34 -

35.22 ppm. The triazine equivalent carbon signal (C5) was observed at 166.63 ppm. These

signals were observed in the 13C NMR spectra for the 20, 10, and 5 mol % copolymers. The

triazine quaternary carbon signal (C4) was not observed due to overlap with the acrylamide

carbonyl carbon signal (C3) located at 180.00 ppm.

0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.5ppm

103

Figure 3.6.3. The 13C NMR spectrum (500 MHz) recorded in D2O for the PAM-co-PVDAT 20 mol % VDAT copolymer.

The DSC curves for the copolymers were recorded to characterize the glass transition

temperatures shown in Figure 3.6.4. Table 3.6.1 lists the glass transition temperatures for the

polymers. The Tg for the PAM homopolymer was 146 °C. All of the values of Tg for the

copolymers were higher than the homopolymer, following the trend observed for other PVDAT

copolymers. The 5, 10, and 20 mol % copolymers exhibited the trend that as the VDAT

concentration increased, the Tg increased due to strong intermolecular forces. The 1 mol % Tg

was observed at a higher temperature than the homopolymer and 5 mol % copolymer.

2030405060708090100110120130140150160170180190ppm

C3

C5

104

100 120 140 160 180 200 220 240 260

Hea

t Flo

w (W

/g)

End

othe

rm

Temperature (οC)

20 mol %

10 mol %

1 mol %

5 mol %

PAM

Figure 3.6.4. DSC curves for the PAM and PAM-co-PVDAT copolymers.

Table 3.6.1. PAM and PAM-co-PVDAT copolymers glass transition temperatures (°C).

Polymer Tg (°C) PAM 146

1 mol % 192

5 mol % 160

10 mol % 193

20 mol % 236

105

3.7 PVK-co-PVDAT Copolymers Characterization

The FTIR spectra for the PVK and PVK-co-PVDAT copolymers were recorded in KBr

shown in Figure 3.7.1. The discontinuity in the spectra eliminated the featureless region from

2800 to 2000 cm-1, thereby allowing a better display of the region from 500 to 2000 cm-1. The

VDAT amino groups asymmetric and symmetric stretching vibration modes appeared at 3396

and 3213 cm-1. The PVDAT triazine ring out of plane vibration mode was observed at 825 cm-1

and only one of the triazine ring in plane vibration modes was observed at 1543 cm-1. The CH

stretching vibration modes appeared between 3079 - 2919 cm-1.

4000 3500 3000 2000 1500 1000 500

% T

rans

mitt

ance

(A.U

.)

Wavenumbers (cm-1)

PVK

5 mol %

10 mol %

20 mol %

Figure 3.7.1. The FTIR spectra for PVK and PVK-co-PVDAT copolymers recorded in KBr pellets.

106

The PVK-co-PVDAT copolymers were insoluble in CDCl3 or DMSO-d6, preventing the

acquisition of 1H and 13C NMR spectra. The copolymers' insolubility suggested possible cross-

linking within the copolymers, since PVK was soluble in both the appropriate NMR solvents.

DSC experiments were performed to characterize the values of Tg for the homopolymer

and copolymers. The PVK and PVK-co-PVDAT DSC curves are shown in Figure 3.7.2 and

Table 3.7.1 lists the glass transition temperatures. A wide range of glass transition temperatures

from 150 to 248 °C have been reported for high molecular weight PVK.63 This range was

attributed to impurities, degradation products, or stereoregularity of the cationic polymerized

carbazole.64 The copolymers did not show an increasing trend in Tg observed by other PVDAT

copolymers. The Tg for the 20 and 5 mol % copolymers were slightly higher than the

homopolymer, which may be attributed to the molecular weight of the copolymers or non-

covalent interactions introduced by PVDAT. The 10 mol % copolymer Tg (188 °C) was

considerably lower than the other copolymers and homopolymer, suggesting a copolymer of

lower molecular weight or impurities present within the copolymer matrix.

Table 3.7.1. PVK and PVK-co-PVDAT copolymers glass transition temperatures (°C).

Polymer Tg (°C) PVK 216

5 mol % 223

10 mol % 188

20 mol % 218

107

100 120 140 160 180 200 220 240 260

Hea

t Flo

w (W

/g)

E

ndot

herm

Temperature (οC)

PVK

5 mol %

10 mol %

20 mol %

Figure 3.7.2. DSC curves for PVK and PVK-co-PVDAT copolymers.

3.8 PS-co-PVK Copolymers Characterization

The 1H NMR spectra for the PS-co-PVK copolymers (Figure 3.8.1) were recorded in

CDCl3 for structure characterization. The PVK aromatic proton signals were not observed in the

copolymer spectra. The copolymers' spectra were identical to the polystyrene 1H NMR spectrum.

The reported calculated reactivity ratios found in the literature for styrene and vinylcarbazole in

DMF were rstyrene = 5.83 and rvinylcarbazole = 0.17.65 The reactivity ratio is defined as the ratio of

the rate constant of the propagating species, adding its own type of monomer to the rate constant

for the addition of the other monomer.53 The tendency for two monomers to copolymerize is

determined by r values between zero and unity. Since styrene's reactivity ratio is greater than

108

unity compared to vinylcarbazole reactivity ratio in DMF, the styrene monomer preferentially

adds the styrene monomer instead of the vinylcarbazole monomer. The 1H NMR spectra for the

copolymers confirmed that vinylcarbazole was not incorporated into the polymers.

Figure 3.8.1. The 1H NMR spectra for the PVK and PS-co-PVK copolymers recorded in CDCl3 (360 MHz).

0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.59.0ppm

10 mol %

15 mol %

20 mol %

PVK

109

3.9 PMMA-co-PVK Copolymers Characterization

The FTIR spectra for the PVK homopolymer and the PMMA-co-PVK copolymers were

recorded by the KBr pellet method. The CH stretching modes appeared at 3051 - 2938 cm-1. The

PMMA carbonyl peak (C=O) was observed at 1730 cm-1. The PVK C=C vibrational bands were

assigned at 1618 and 1452 cm-1.

4000 3500 3000 2500 2000 1500 1000 500

% T

rans

mitt

ance

(A.U

.)

Wavenumbers (cm-1)

PVK

20 mol %

50 mol %

Figure 3.9.1. The FTIR spectra for KBr pellets containing PVK and PMMA-co-PVK copolymers.

110

The 1H NMR spectrum for the PVK homopolymer was recorded in CDCl3, shown in

Figure 3.9.2, and assignments were made to the aromatic proton signals to be used as a reference.

Figure 3.9.2. The 1H NMR spectrum for the PVK homopolymer recorded in CDCl3 using the 360 MHz spectrometer.

The aromatic proton signals were assigned according to the reported literature assignments by

Karali et al.66 The polymer's backbone methine and methylene proton signals were located

between 3.60 - 2.47 ppm (H9) and 2.40 - 0.90 ppm (H10). The aromatic proton signals were

assigned as follows: (H1) 4.91 ppm, (H2) 6.19 ppm, (H3) 6.49 ppm, (H4) 7.55 ppm, (H5) 7.69

ppm, (H6) 7.02 ppm, (H7) 6.91 ppm, and (H8) 6.39 ppm. The 9-vinylcarbazole vinyl proton

0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.59.0ppm

DMF

DMF

H5 H4 H6H7

H3 H2H8

H1

H9 H10

111

signals (5.50 - 5.00 ppm) were not present, which indicated complete polymerization of the

monomer to the homopolymer.

After assigning the PVK proton signals, the 1H NMR spectra for the PMMA-co-PVK

copolymers were recorded and compared to the PVK spectrum. The spectra for the PMMA-co-

PVK 50 and 20 mol % copolymers are shown Figure 3.9.3. The vinylcarbazole vinyl proton

signals (5.51- 5.10 ppm) and the MMA vinyl proton signals (6.10 - 5.55 ppm) were not observed

in both spectra, indicating complete polymerization for both monomers. A noticeable difference

between the two spectra was that increasing the vinylcarbazole concentration led to

Figure 3.9.3. The 1H NMR spectra for PMMA-co-PVK 50 and 20 mol % copolymers recorded in CDCl3 (360 MHz).

0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.59.0ppm

20 mol %

50 mol %

112

an increase in peak broadness, especially in the region for the methylene, methine, and α-methyl

group proton signals. This increase in peak broadness was attributed to the restricted rotation of

the bulky carbazole. For the PMMA-co-PVK 20 mol % spectrum, the methyl methacrylate

proton signals α-CH3 (1.18, 0.98, and 0.81 ppm), methylene (1.96 - 1.33 ppm), and OCH3 (3.57

and 3.55 ppm) were clearly observed, with some overlapping with the PVK methine and

methylene proton signals. The PVK aromatic proton signals were observed in the spectrum,

overlapping with the CDCl3 NMR solvent signal (7.24 ppm) and DMF solvent signal (8.02

ppm). On the other hand, the 50:50 (PMMA:PVK) copolymer spectrum's region between 3.00 -

0.00 ppm showed several broad peaks which could not be differentiated between the PVK's

methylene and methine protons signals or the PMMA α-methyl and methylene proton signals.

The PMMA OCH3 proton signal was observed at 3.52 and 3.34 ppm with the PVK aromatic

proton signals.

The 13C NMR spectra for PVK and the PMMA-co-PVK copolymers were recorded for

further characterization. The PVK homopolymer peak positions were used as a reference when

analyzing the copolymers' spectra. The PVK carbon assignments were in agreement with the

reported literature assignments.66 The 13C NMR spectrum for PVK and PMMA-co-PVK 50 mol

% vinylcarbazole are shown in Figure 3.9.4 and Figure 3.9.5. The PVK-co-PMMA copolymer

carbon assignments were in agreement with the reported carbon assignments found in the

literature.67 The PMMA carbon signals were observed at 177.18 - 175.50 ppm (C=O), 51.93 -

50.84 ppm (OCH3), 44.55 ppm (methyl methacrylate quaternary carbon (C (M)), and 19.13 -

16.53 ppm (α-CH3). The PVK aromatic carbon signals appeared between 140.34 - 109.79 ppm.

The PVK methine peak was observed at 48.20 ppm (CH (V)), overlapping with the methylene

carbon signal in the same region of the spectrum. An observable difference between the two 13C

113

NMR spectra was that as the vinylcarbazole concentration decreased, the PMMA carbon signals

were much clearer with less peak broadness.

Figure 3.9.4. The 13C NMR spectrum for PVK recorded in CDCl3 (500 MHz).

30405060708090100110120130140ppm

1a 8a

7,2

5

4, 6

3

5a 4a

8 19 10

114

Figure 3.9.5. The 13C NMR spectrum for the (50:50) PMMA-co-PVK copolymer (500 MHz, CDCl3).

The PMMA-co-PVK copolymers Tg values were characterized by DSC. The copolymers'

DSC curves are shown in Figure 3.9.6 from 80 °C to 150 °C. The glass transition temperatures

for the synthesized homopolymers PMMA (103 °C) and PVK (216 °C) have already been

reported. The PMMA-co-PVK copolymers containing 50 and 20 mol % vinylcarbazole glass

transition temperatures (Tg) were observed at 143 °C (50 mol %) and 132 °C (20 mol %). The

copolymers' glass transition temperatures increased, compared to the Tg for PMMA, but were

significantly less than the Tg for the PVK homopolymer. Increasing the vinylcarbazole

concentration increased the Tg to higher temperatures, which was attributed to non-covalent

interactions (hydrogen bonding), and the bulky carbazole units which restricted chain mobility.

102030405060708090100110120130140150160170180ppm

C=O 1a 8a

7,25

4,6,3

8 1

5a,4a

CH (V)

C (M)

(V+M)CH2

CH3

OCH3

115

80 90 100 110 120 130 140 150

Hea

t Flo

w (W

/g)

End

othe

rm

Temperature (οC)

50 mol %

20 mol %

Figure 3.9.6. The DSC curves for the PMMA-co-PVK copolymers.

3.10 PVI-co-PVDAT Copolymer Characterization

The FTIR spectra for the PVI homopolymer and PVI-co-PVDAT copolymer were

recorded in KBr shown in Figure 3.10.1. The assignments for the vibrational modes for the PVI

homopolymer were in agreement with the structure assignments reported by Lippert and co-

workers.68 For the 20 mol % VDAT copolymer, the asymmetric and symmetric vibrational

modes for the primary amine appeared at 3310 and 3100 cm-1, overlapping with a broad OH

vibrational mode from the presence of water. Only one of the triazine ring in plane stretching

modes was observed at 1546 cm-1.

116

4000 3500 3000 2500 2000 1500 1000 500

% T

rans

mitt

ance

(A.U

.)

Wavenumbers (cm-1)

PVI

PVI-co-PVDAT

Figure 3.10.1. The FTIR spectra for PVI homopolymer (black) and PVI-co-PVDAT 20 mol % VDAT copolymer (red).

The 1H NMR spectra for the PVI homopolymer and PVI-co-PVDAT 20 mol % VDAT

copolymer were recorded shown in Figure 3.10.2. The spectrum for the homopolymer showed

aromatic proton peaks located between 7.19 - 6.52 ppm. The PVI methine and methylene peaks

were observed between 3.83 - 2.51 ppm and 2.34 - 1.82 ppm. The PVI proton assignments were

in agreement with the reported literature assignments.69 The copolymer 1H NMR spectrum

showed very broad peak signals due to overlapping with the PVDAT proton signals. The

vinylimidazole and VDAT vinyl proton signals were not observed, suggesting complete

copolymerization. The PVI aromatic peaks were located between 7.55 - 6.38 ppm, overlapping

117

Figure 3.10.2. The 1H NMR spectra for the PVI homopolymer and PVI-co-PVDAT copolymer.

with PVDAT's amine proton signal. The PVI methylene and methine proton signals were located

in similar regions compared to the PVI copolymer spectrum, but overlap from PVDAT's

methylene and methine proton signals lead to broad peaks observed between 3.92 - 1.25 ppm.

For a more thorough structure characterization, 13C NMR experiments were performed.

The PVI aromatic and methine carbon signals were observed at 136.26 (C1), 129.21 (C2),

116.59 (C3), and 50.70 ppm (PVI methine carbon signal). The observed PVI carbon peaks were

in agreement with reported literature assignments.70 The copolymer's methylene carbon signal

-0.0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.0ppm

PVI

PVI-co-PVDAT

ETOH

ETOH

CH

CH2

118

Figure 3.10.3. The 13C NMR spectrum for the PVI-co-PVDAT copolymer recorded in DMSO-d6 (500 MHz).

was not observed due to the DMSO-d6 solvent signal overlapping with the methylene carbon

signal. The PVDAT aromatic carbon signals were observed at 166.91 (C4) and 178.82 (C5). The

observed PVDAT peaks indicated the copolymer was synthesized.

DSC experiments characterized the values of Tg for the homopolymer and copolymer.

The DSC curve for PVI (Appendix Figure 55) showed a Tg at 159 °C. The reported Tg value for

the PVI homopolymer is 182 °C.70 The synthesized homopolymer's Tg was lower than the

reported literature Tg. This decrease in Tg was attributed to impurities within the polymer matrix.

The copolymer Tg was not observed below 250 °C. This result suggested that the copolymer has

5060708090100110120130140150160170180ppm

119

a high thermal stability temperature. Introducing VDAT into the copolymer increased the Tg

significantly, which was attributed to an increase in hydrogen bonding and dipole - dipole

interactions, limiting chain mobility and resulting in a higher Tg.

3.11 PS-co-PVI Copolymer Characterization

The FTIR spectra for the PVI homopolymer and the PS-co-PVI 20 mol % vinylimidazole

copolymer were recorded in KBr shown in Figure 3.11.1. The copolymer spectrum did not

contain any strong PVI vibrational modes, which suggested that the copolymer was not

synthesized. The polystyrene vibrational modes were observed in the copolymer spectrum. The

4000 3500 3000 2500 2000 1500 1000 500

% T

rans

mitt

ance

(A.U

.)

Wavenumbers (cm-1)

PVI

PS-co-PVI

Figure 3.11.1. The FTIR spectra for the PVI homopolymer (red curve) and the PS-co-PVI 20 mol % copolymer (black curve) recorded in KBr pellets at room temperature.

120

polystyrene C=C overtones were located between 1937 - 1637 cm-1. The polystyrene C=C

aromatic vibrational modes were observed at 1600 and 1492 cm-1.

To confirm synthesis of the copolymer, 1H and 13C NMR experiments were performed to

characterize the copolymer's structure. The 1H NMR spectrum for the PS-co-PVI 20 mol %

vinylimidazole copolymer is shown in Figure 3.11.2 and compared to the polystyrene 1H NMR

spectrum. The two spectra were identical; suggesting the copolymer between styrene and

vinylimidazole was not synthesized. The 13C NMR spectra (Figure 3.11.3) also confirmed that

the copolymer was not synthesized. The polystyrene aromatic carbon peaks were observed

between 145.00 - 115.00 ppm and the polymer backbone carbon peaks (methine and methylene

carbons) were observed between 30.00 - 20.00 ppm. None of the PVI carbon signals were

observed in the copolymer's 13C NMR spectrum. A possible explanation for the un-synthesized

copolymer may be attributed the N-vinyl monomer. The N-vinylimidazole monomer produces a

highly reactive radical due to a lack of resonance stabilization in the propagation step of the

polymerization.71 The reactive propagating radical increases the possibility for chain transfer and

chain termination events, resulting in polymers with low molecular weights. In the literature, N-

vinylimidazole copolymers with styrene were synthesized by a controlled free radical

polymerizations.70, 71

121

Figure 3.11.2. The 1H NMR spectra for the PS-co-PVI copolymer and polystyrene homopolymer recorded in CDCl3 (360 MHz).

0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.0ppm

PS-co-PVI

Polystyrene

122

Figure 3.11.3. The 13C NMR spectra for the PS-co-PVI copolymer and the homopolymer (PVI).

3.12 PMMA-co-PVI Copolymer Characterization

The FTIR spectra for the PMMA-co-PVI 20 mol % vinylimidazole copolymer and the

PVI homopolymer are shown in Figure 3.12.1. The PMMA carbonyl (C=O) vibrational mode

was observed at 1728 cm-1. A broad OH vibrational mode was observed at 3435 cm-1 that

resulted from the presence of water in the FTIR sample. The peaks located at 1485, 1238, and

665 cm-1 were the observed imidazole ring stretching modes, which confirmed the presence of

PVI in the copolymer with methyl methacrylate.

2030405060708090100110120130140150160ppm

PS-co-PVI

PVI

123

4000 3500 3000 2500 2000 1500 1000 500

% T

rans

mitt

ance

(A.U

.)

Wavenumbers (cm-1)

PVI

PMMA-co-PVI

Figure 3.12.1. The FTIR spectra for the PMMA-co-PVI copolymer and the PVI homopolymer recorded in KBr pellets.

The 1H NMR spectra for the PMMA-co-PVI copolymer, the PMMA homopolymer, and

the PVI homopolymer were recorded and compared (Figure 3.12.2). For the copolymer

spectrum, the α-CH3 proton signals were observed at 1.26 - 0.44 ppm. The PMMA methylene

proton signals were located at 2.24 - 1.29 ppm, overlapping with the PVI methylene proton

signal. The PVI methine proton signal was observed at 3.38 - 3.15 ppm. The PMMA O-CH3

proton signals were located at 3.76 - 3.43 ppm. The PVI aromatic proton signals appeared

between 7.08 - 6.60 ppm. The vinyl proton signals for methyl methacrylate and 1-vinylimidazole

124

were not observed in the copolymer spectrum, suggesting that the monomers were

copolymerized.

Figure 3.12.2. The 1H NMR spectra for the homopolymers, PVI and PMMA, and the PMMA-co-PVI copolymer containing 20 mol % vinylimidazole.

The 13C NMR spectrum for the PMMA-co-PVI 20 mol % VI copolymer is shown in

Figure 3.12.3. The copolymer's carbon signals were in agreement with the assignments made by

Chiu and co-workers.72 The carbonyl carbon signals for PMMA were observed at 177.71 and

176.23 ppm. The PVI aromatic carbon signals were located at 136.91, 128.91, and 117.64 ppm.

The spectrum region between 55.00 - 48.30 ppm consisted of the following carbon signals: the

0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.0ppm

PVI

PMMA-co-PVI

PMMA

125

PVI methine, the PVI methylene, the PMMA OCH3, the PMMA quaternary carbon, and the

PMMA methylene carbon. The carbon signal located at 51.52 ppm corresponded to the PMMA

OCH3, overlapping with the PVI methine and PMMA methylene carbon signals. The PMMA

quaternary carbon signal was observed at 44.46 - 43.87 ppm, overlapping with the PVI

methylene carbon signals. The PMMA α-CH3 carbon signals were observed between 20.56 -

16.11 ppm.

Figure 3.12.3. The 13C NMR spectrum for the PMMA-co-PVI copolymer containing 20 mol % VI recorded in DMSO-d6 (500 MHz).

102030405060708090110130150170ppm

126

The DSC curves from 80 °C to 160 °C for the PMMA-co-PVI copolymer and PMMA

homopolymer are shown in Figure 3.12.4. The observed glass transition temperatures for the

copolymer and homopolymer were 132 °C (PMMA-co-PVI) and 103 °C (PMMA). The

copolymer exhibited a sharp endotherm transition compared to the homopolymer's gradual

endotherm transition. Incorporating PVI into the copolymer increased the Tg, compared to the

PMMA's Tg. This increase in Tg was attributed to an increase in hydrogen bonding, dipole -

dipole interactions, or the bulky vinylimidazole units limiting the polymer's chains mobility.

80 90 100 110 120 130 140 150

Hea

t Flo

w (W

/g)

E

ndot

herm

Temperature (οC)

PMMA-co-PVI

PMMA

Figure 3.12.4. The DSC curves shown from 80 °C to 160 °C for the PMMA-co-PVI 20 mol % VI copolymer and PMMA homopolymer.

127

Chapter 4

Polymer Thin Films Characterization by Variable Angle Spectroscopic Ellipsometry after Exposure to a Nitroaromatic Vapor In this chapter, a brief introduction to the fundamentals of ellipsometry will be discussed,

and the thin films' optical constants measured by ellipsometry will be presented. The first section

will discuss how ellipsometry is used to characterize the thickness and optical properties of thin

films. The next section will discuss the Cauchy model and data analysis. Lastly, the extracted

optical constants and thicknesses of the thin films after exposure to a nitroaromatic vapor will be

shown, providing supporting evidence that these films have the potential to be applied in the

development of a waveguide explosive sensor.

The polymers synthesized in Chapter 2 were used to make polymer solutions for spin

coating. Some of the polymers were not suitable for spin coating due to either insolubility in an

appropriate spin coating solvent or produced heterogeneous films (haziness or phase separation).

The data presented in this chapter is from the polymers that were used in spin coating of

homogeneous films without defects.

4.1 Ellipsometry Overview

Ellipsometry is a non-destructive technique that measures the change in the state of

polarization as light is reflected from the surface of a material.73 Ellipsometry is primarily used

to determine a material's thickness and optical constants, but also offers the ability to extract

128

properties such as composition, crystallinity, surface roughness, and other factors that are

dependent on the material's optical properties.

Ellipsometry utilizes elliptical polarized light, hence the technique's name. Light is an

electromagnetic wave traveling through space, consisting of an electric field vector and a

magnetic field vector. These vectors are mutually perpendicular to each other and perpendicular

to the propagation direction, allowing the wave to be described by its x and y components

traveling along the z-axis. Ellipsometry is concerned with the electric field vector (polarized

light). Elliptically polarized light is produced when two linearly polarized waves with the same

frequency are combined out of phase.73 If viewed (end-on) of the z-axis, the tips of the arrows

would appear to be moving on an ellipse shown in Figure 4.1.1.

Figure 4.1.1. Two linearly polarized waves combined out of phase producing elliptically polarized light. Modified from http://www.jawoollam.com/tutorial_2.html (accessed Feb. 15, 2013).

When an electromagnetic wave arrives at the interface between the air and film, the wave

can begin to slow, change direction, or be transmitted into the material (Figure 4.1.2). Not all of

the light enters the material, but some is reflected at the interface back into the air. The reflected

light from the material's surface allows the optical properties (n and k) to be characterized by the

complex index of refraction (Eq. 1), which describes how the light interacts with the material.

129

Figure 4.1.2. Light reflecting and refracting at the interface between air and the surface of a material. Modified from http://www.jawoollam.com/tutorial_3.html (accessed Feb. 15, 2013).

The complex index of refraction can be described by a real and an imaginary number:

(Equation 1)

where n is the index of refraction, k is the extinction coefficient, and j is the √ 1. The index of

refraction (n) describes the inverse measure of the phase of velocity for light as it enters a

dielectric material compared to the speed of light expressed as:

(Equation 2)

where c is the speed of light and ʋ is the phase velocity. The extinction coefficient (k) related to

the absorption coefficient (α) describes the light's loss of intensity as it travels through the

material expressed as:

(Equation 3)

(Equation 4)

where d is the distance traveled into the material.

The incident light is reflected or transmitted at the air/material interface (Figure 4.1.2). It

is known that the angle between the incidence light and the material ( ) is equal to the angle of

130

reflection ( ). The refracted angle ( ) as light is transmitted into a dielectric material (k = 0)

can be described by Snell's law where all terms are real numbers:

(Equation 5)

As previously mentioned, ellipsometry measures the change in the state of polarization

as light is reflected or transmitted at the air/surface interface. The electric field vectors of linear

polarized light are projected in two orthogonal components shown in Figure 4.1.3. The electric

field vector parallel to the plane of incidence is referred to as Ep and the electric field vector that

is perpendicular to the plane of incidence as referred to as Es. Both the Ep and Es are independent

components and can interact differently when reflected from the material's surface. The Fresnel

reflection coefficients describe the ratio of the amplitude of the incidence wave compared to the

reflected wave denoted as rs (perpendicular wave to the plane of incidence) and rp (parallel wave

to the plane of incidence). The Fresnel reflection and transmission coefficients provide

information about the phase and amplitude ratio between the p-wave and s-wave given by:

(Equation 6.1)

(Equation 6.2)

(Equation 6.3)

(Equation 6.4)

131

Figure 4.1.3. Schematic representation for a typical ellipsometry measurement showing a polarization state change when linearly polarized light is reflected from a sample's surface. Modified from http://www.jawoollam.com/tutorial_4.html (accessed Feb. 15, 2013).

When a film is present on a substrate, the incident light will be reflected or transmitted at

the film/air interface. The resulting transmitted wave will propagate through the film, producing

multiple reflections and transmissions between the film/air interface and the film/substrate

interface, shown in Figure 4.1.4. The presence of multiple waves in the film introduces

interference, which is dependent on the amplitude and phase of the electric fields.

Figure 4.1.4. Schematic representation of a wave propagating through a film, producing multiple reflections and transmissions.74

132

From the total reflection coefficients comparable to the Fresnel reflection coefficients, the phase

change in the wave as it propagates from the top to bottom through the film can be determined.

The film thickness can be determined by , film phase thickness, expressed as:

(Equation 7)

The p-waves and s-waves are not always in phase; after a reflection, there is a possibility of a

phase shift being produced, which can be different for both waves. The phase difference between

the p-wave and s-wave before the reflection and after the reflection can be described by the

parameter Δ, given by:

(Equation 8)

where is the phase difference before the reflection and is the phase difference after the

reflection. Similar to the phase shift, after the reflection a reduction in amplitude can be induced

for the p-wave and s-wave and the change in amplitude may not be the same for both waves. The

total reflection coefficients (the ratio of the amplitude for the reflected wave to the incidence

wave) for the p-wave (RP) and s-wave (RS) contain the magnitudes of the amplitude changes.

The tan Ψ is defined as the ratio of the magnitudes of the total reflection coefficients and is a real

number given by:

(Equation 9)

where Ψ is the angle whose tangent is the ratio of the magnitudes of the total reflection

coefficients.73 The complex number is defined as the complex ratio of the total reflection

coefficients that describe the change in polarization between the p-wave and s-wave expressed

as:

(Equation 10)

133

Ellipsometry addresses the phase and amplitude ratio between the p-wave and s-wave, which are

independent of each other. Using the fundamental equation of ellipsometry allows the two

independent parameters to be determined:

· (Equation 11)

where and Δ are measured quantities which characterize polarization effects of the surface

after the incident light has undergone a phase and amplitude change for the p-wave and s-wave.

These measured quantities are dependent on the wavelength, angle of incidence, optical

constants, and film morphology.

4.2 Data Analysis

Ellipsometry is able to determine the optical constants and film thickness by directly

measuring Δ and Ψ. After measuring ∆ and Ψ, a model is constructed to describe the sample's

response. The model includes layers with each optical constants and thickness of each layer

defined. If the optical constants and thickness are not known, an approximate value is given to

allow preliminary data calculations. The model and the Fresnel's equations provide the predicted

calculated response for ∆ and Ψ. The predicted values of ∆ and Ψ are then compared to the

experimental values of ∆ and Ψ. The optical properties and thickness of the unknown layer are

varied until the generated values of ∆ and Ψ are close to the experimental values. A fitting step is

performed to find the best fit between the generated data and experimental data. The fit between

the generated data and experimental data is evaluated by the mean square error (MSE). The MSE

quantifies the difference between the data curves, allowing parameters for the unknown material

layer to be adjusted until a minimum MSE is reached. When fitting the experimental and

generated data, the best fit is the assessment with the lowest value of MSE.

134

4.3 Cauchy Model

For many materials, there are electronic absorptions deep in the UV region of the

spectrum. This would lend to large values of k in this region. In the near infrared and through the

visible into the near UV there are no electronic absorptions and the value of k is zero. Through

the region from the near infrared to the near UV, the refractive index increases as the wavelength

decreases. This is optical dispersion. As the wavelength decreases into the UV region and

approaches the region where there are strong electronic absorptions, the refractive index

increases greatly. This is called anomalous dispersion.75

The Cauchy Dispersion model is an empirical model that is capable of describing the

wavelength dependence index of refraction for dielectric materials with little or no optical

absorption.76 The Cauchy model describes the relationship between the index of refraction and

wavelength given by:

(Equation 12)

where is the index of refraction, is the wavelength, and A, B, and C are Cauchy

parameters. The three parameters describe the index of refraction over a range of wavelengths.

The Cauchy model typically shows a decrease in refractive indices (n) as the wavelength

increases.

4.4 PS-co-PVDAT Films

Before spin coating thin films of the different copolymers, the copolymers solubility in an

ideal spin coating solvent (boiling point (b.p.) ≈ 100 to 130 °C) was determined. The ideal spin

coating solvent should possess suitable substrate wetting abilities, a b.p. that allows film

formation and does not quench the film in place (producing phase separation or haziness), and

also allows the films to be dried, removing the solvent leaving only the film. The lower VDAT

135

mol % copolymers (1 and 5 mol % VDAT) were soluble in toluene (b.p. 110 °C54), similar to

polystyrene. The 10 and 20 mol % VDAT copolymers were insoluble in toluene, but were

soluble in other polar organic solvents such as MEK (80 °C54), THF (66 °C54), DMF (153 °C54),

DMSO (192 °C54), pyridine (115 °C54), and 1,4-dioxane (101 °C54). The difference in solubility

between the lower mol % VDAT copolymers and higher VDAT mol % copolymers was

attributed to an increase in non-covalent interactions (hydrogen bonding and dipole - dipole

interactions). As the VDAT concentration increased, the solvents required a higher polar

solubility parameter. Copolymers containing concentrations greater than 20 mol % VDAT would

not likely be soluble in an appropriate spin coating solvents due to the insolubility of PVDAT.

To lower the b.p. of the higher b.p. polar organic solvents, attempts were made to include

lower b.p. solvents, which were miscible with the polar organic solvents such as EtOH, MeOH,

toluene, H2O, THF, or isopropanol. The amounts of the lower b.p. solvents were adjusted to a

maximum concentration in the higher b.p. solvents so that the copolymers remained in the

solution phase and did not precipitate. To increase the b.p. of the lower b.p. solvents, a similar

approach was used to include higher b.p. solvents. The adjusted b.p. attempts resulted in

heterogeneous films, which were not ideal for ellipsometry characterization.

Attempts were made to spin coat films of the PS-co-PVDAT 20 mol % VDAT copolymer

using THF, DMF, and 1,4-dioxane. The films spin coated from THF and DMF were very thin (≤

10 nm) and exhibited extreme surface roughness. It appeared that the solvents did not produce

viscous solutions, therefore creating thin films. Viscosity is a critical factor for spin coating thick

films. In addition, it was assumed that it would not be possible to remove DMF from the polymer

films due to the solvent's high b.p. and hygroscopic nature. The presence of DMF in the polymer

films may be potentially beneficial, acting as a plasticizing agent, allowing the films to become

136

more porous and polymer chains to be more mobile. 1,4-dioxane with low heat was capable of

dissolving the 20 mol % copolymer. After twenty-four hours, the 20 mol % VDAT copolymer

began to precipitate out of the 1,4-dioxane solution, but introducing heat allowed the copolymer

to dissolve back into the solution. Over time, the copolymer would precipitate out of 1,4-dioxane

and would not dissolve in the solvent.

The PS-co-PVDAT 10 mol % VDAT copolymer was soluble in MEK, 1,4-dioxane, THF,

and DMF. Similar to the 20 mol % VDAT copolymer, THF and DMF produced thin films with

rough surfaces. MEK produced films with transparent centers and haziness near the edges of the

silicon wafer. 1,4-dioxane produced homogenous films with no defects. The copolymer

precipitated out of the solvent over time.

For the PS-co-PVDAT copolymers, different film thicknesses were spin coated by

adjusting spin coating speeds and polymer solution concentrations. 0.3%, 1%, and 3% (w.t.)

copolymer solutions were prepared for spin coating, allowing variances in film thickness by

concentration. The 3% (w.t.) solutions produced films with a blue tint from the reflecting light,

indicating thick polymer films. The 1% and 0.3% (w.t.) solutions produced transparent films,

indicating thin copolymer films. The copolymer solutions were spin coated by both the static and

dynamic techniques. The static technique produced homogenous films with minimal surface

roughness, compared to the dynamic technique that produced homogeneous films with rough

surfaces. After determining appropriate spin coating parameters and concentrations for

producing quality films, the copolymer films were characterized by ellipsometry to determine

film thickness and optical constants (n and k) before and after exposure to a concentrated

nitroaromatic vapor.

137

The expected refractive index for the PS-co-PVDAT films were assumed to be similar to

the refractive index of polystyrene (n = 1.55 - 1.5977). The refractive index of PVDAT is not

known, but was estimated to possess a refractive index similar or higher than polystyrene. The

films were exposed to the concentrated high refractive index nitroaromatic vapors of PNT (n =

1.538), NB (n = 1.55654), and 1,3-DNB (n = 1.612) for a determined amount of time. The change

in film refractive index was attributed to the nitroaromatic vapor molecules with a higher

refractive index interacting with the PVDAT, by hydrogen bonding or electro-static interactions.

Figures 4.4.1 - 4.4.5 show a plot of the refractive index (n) as a function of wavelength for

polystyrene and the PS-co-PVDAT copolymers exposed to a concentrated nitroaromatic vapor.

Tables 4.4.1 - 4.4.5 provide the copolymers' Cauchy parameters fitted for the experimental data.

The ellipsometry curves shown in Figures 4.4.1 - 4.4.5 display expected refractive indices

for the Cauchy dispersion model. A decrease in refractive indices was observed as the

wavelength increased from the UV, through the visible, and into the near infrared. All of the

curves showed reasonable refractive indices that would be expected for polystyrene before

exposure to the nitroaromatic vapor. The extinction coefficient (k), describing the films' optical

absorption property, showed very little optical absorption (primarily in the UV region of the

spectrum from 380 - 300 nm) and did not affect the fit between the experimental and generated

data. The features observed between 700 - 1,000 nm were not expected, since the curves

typically lay flat in this region of the spectrum, due to no optical absorption. It was hypothesized

that these features may be attributed to surface roughness or a film defect, scattering or reflecting

the polarized light in a different manner than that of the bulk of the film. There was an observed

difference between the thicknesses measured by the profilometer and thicknesses determined by

the Cauchy model. The films' thicknesses measured by the profilometer were between 3 - 6 nm

138

less than the film thicknesses determined by the Cauchy model. These differences in thickness

occurred because of the etching process. It was likely that the etch made in the films did not

penetrate through the entire film to the substrate's surface. This would explain the slight

differences between the two measurements. The profilometer measurements were used to

confirm the ellipsometer's film thicknesses measurements, which were accurate. The low MSE

values for the spectra represented an effective comparison between the generated and

experimental data. The changes in refractive index for these spectra may not appear significant,

but MZI have shown the capability of detecting small changes in refractive index (10-6).

The refractive index curves for the polystyrene film exposed to PNT did not exhibit a

significant change in refractive index after exposure to the nitroaromatic. The PS-co-PVDAT

copolymers refractive index curves showed changes in the refractive index curves after the films

were exposed to a nitroaromatic vapor. This change in refractive index described the copolymer

films' affinity toward the nitroaromatics. The addition of PVDAT allowed nitroaromatic

molecules to form molecular complexes with the electron rich VDAT aromatic structure or the

nitro groups to form hydrogen bonds with the PVDAT amine groups.

The films exposed to concentrated nitroaromatic vapors showed varied results producing

large, minimal, or no change in refractive index. These changes may be attributed to the ability

of the nitroaromatic vapor molecules to enter the porous films and interact with PVDAT.

139

Figure 4.4.1. The before and after refractive index curves for a polystyrene film spin coated from a 3% (w.t.) toluene solution exposed to PNT for ten seconds.

1.54

1.56

1.58

1.60

1.62

1.64

1.66

1.68

1.70

1.72

1.74

300 400 500 600 700 800 900 1000

Ref

ract

ive I

ndex

(n)

Wavelength (nm)

Before

After

140

Table 4.4.1. The Cauchy parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a polystyrene film exposed to PNT vapors for ten seconds.

Polymer: PS, 3% (w.t.) Toluene

Nitroaromatic: PNT 10 sec. exposure

Before After

MSE 5.233 5.327

Thickness (Å) 813.3 ± 0.7 813.3 ± 0.7

A 1.567 ± 2.31 E-3 1.566 ± 2.37 E-3

B 4.359 E-3 ± 8.23 E-4 4.33 E-3 ± 8.35 E-4

C 7.621 E-4 ± 7.61 E-5 7.741 E-4 ± 7.66 E-5

Δn 0.0006

Optical Constants MSE 4.586 4.725

Dektak (Å) 780

Spin Coating 5,000 rpm for 40 sec.

141

Figure 4.4.2. The change in refractive index for a PS-co-PVDAT 20 mol % VDAT copolymer film spin coated from a 1% (w.t.) 1,4-dioxane solution exposed to NB for five seconds.

1.5

1.52

1.54

1.56

1.58

1.6

1.62

1.64

1.66

1.68

1.7

300 400 500 600 700 800 900 1000

Ref

ract

ive I

ndex

(n)

Wavelength (nm)

Before

After

142

Table 4.4.2. The ellipsometry MSE, film thickness, refractive index, average change in refractive index (Δn), optical constants MSE, profilometer thickness, and spin coating parameters for a PS-co-PVDAT 20 mol % VDAT copolymer film produced from a 1% (w.t.) 1,4-dioxane solution.

Polymer: PS-co-PVDAT 20 mol % VDAT, 1% (w.t.) 1,4-dioxane

Nitroaromatic: NB 5 sec. exposure

Before After

MSE 2.96 2.944

Thickness (Å) 357.6 ± 0.7 358.2 ± 0.7

A 1.501 ± 4.00 E-3 1.511 ± 4.01 E-3

Δn 0.009


Dektak (Å) 337


143

Figure 4.4.3. The change in refractive index for a PS-co-PVDAT 10 mol % VDAT copolymer film produced from a 1% (w.t.) MEK solution exposed to NB for five seconds.

1.54

1.56

1.58

1.6

1.62

1.64

1.66

1.68

1.7

1.72

300 400 500 600 700 800 900 1000

Ref

ract

ive I

ndex

(n)

Wavelength (nm)

Before

After

144

Table 4.4.3. The ellipsometry Cauchy model MSE, thickness, refractive index, and optical constants MSE, profilometer measured thickness, and spin coating parameters for a PS-co-PVDAT 10 mol % VDAT copolymer film.

Polymer: PS-co-PVDAT 10 mol % VDAT, 1% (w.t.) MEK


Before After

MSE 2.122 3.035

Thickness (Å) 508.7 ± 0.3 511.3 ± 0.3

A 1.555 ± 1.49 E-3 1.558 ± 2.09 E-3

Δn 0.003


Dektak (Å) 454


145

Figure 4.4.4. The change in refractive index for a PS-co-PVDAT 5 mol % VDAT copolymer film produced from a 3% (w.t.) toluene solution exposed to PNT for sixty seconds.

1.54

1.56

1.58

1.6

1.62

1.64

1.66

1.68

1.7

1.72

300 400 500 600 700 800 900 1000

Ref

ract

ive I

ndex

(n)

Wavelength (nm)

Before

After

146

Table 4.4.4. The ellipsometry Cauchy model MSE, thickness, refractive index, optical constants MSE, profilometer measured thickness, and spin coating parameters for a PS-co-PVDAT 5 mol % VDAT copolymer film.

Polymer: PS-co-PVDAT 5 mol % VDAT, 3% (w.t.) Toluene


Before After

MSE 3.402 4.151

Thickness (Å) 893.5 ± 0.3 894.2 ± 0.3

A 1.561 ± 1.05 E-3 1.562 ± 1.22 E-3

B 4.853 E-3 ± 3.49 E-4 4.967 E-3 ± 4.07 E-3

C 7.257 E-4 ± 2.76 E-5 7.346 E-4 ± 3.25 E-5

Δn .002


Dektak (Å) 861

Spin Coating 6,000 rpm for 1 min.

147

Figure 4.4.5. The change in refractive index for a PS-co-PVDAT 1 mol % VDAT copolymer film produced from a 3% (w.t.) toluene solution exposed to NB for five seconds.

1.54

1.56

1.58

1.6

1.62

1.64

1.66

1.68

1.7

1.72

300 400 500 600 700 800 900 1000

Ref

ract

ive I

ndex

(n)

Wavelength (nm)

Before

After

148

Table 4.4.5. The ellipsometry Cauchy model MSE, thickness, refractive index, optical constants MSE, profilometer measured thickness, and spin coating parameters for the PS-co-PVDAT 1 mol % VDAT copolymer film.



Before After

MSE 2.985 3.285

Thickness (Å) 729.4 ± 0.4 732.5 ± 0.4

A 1.556 ± 1.47 E-3 1.558 ± 1.60 E-3

Δn 0.001


Dektak (Å) 703


149

Time exposure experiments were performed to determine the maximum concentration of

a nitroaromatic vapor that could be absorbed by the VDAT copolymer films to produce a

significant change in refractive index. PS-co-PVDAT 1 mol % VDAT copolymer films were

spin coated and exposed to PNT for five, twenty, and forty seconds to determine the maximum

change in refractive index over an extended exposure time. The ellipsometry curves shown in

Figures 4.4.6 - 4.4.8 show the change in refractive indices as a function of wavelength as the

exposure time to PNT was increased. Tables 4.4.6 - 4.4.8 provide the Cauchy parameters

determined before and after exposure to the PNT vapor.

The spectra's ranges were reduced to the region between 400 - 1,000 nm in order to

eliminate any optical absorption, which may have occurred in the UV region of the spectrum.

The ellipsometry curves displayed expected Cauchy dispersion model curves showing decreases

in refractive indices as the wavelength increased through the visible to the near infrared. The

refractive index curves before exposure to PNT were consistent with the refractive index for

polystyrene. All of the spectra displayed features in the region between 700 - 1,000 nm,

consistent with the previously shown spectra. The PS-co-PVDAT 1 mol % VDAT copolymer

film exposed to PNT for five seconds produced an average change in refractive index (Δn) of

0.002. When the exposure time was increased to twenty seconds, the average change in

refractive index (Δn) increased to 0.012. After exposure to PNT for forty seconds, there was no

observed increase in the average change in refractive index, suggesting the copolymer was

saturated with PNT.

The differences in film thickness measurements between the Cauchy model and

profilometer varied from 6 - 8 nm. These results were consistent with previously results and

confirmed the approximate thicknesses determined by the ellipsometer. The spin coated

150

copolymer film exposed to PNT for five seconds was approximately 12 nm thinner than the other

two copolymer films. The difference in film thickness could not be justified, but the profilometer

did confirm the film's approximate thickness.

Figure 4.4.6. The change in refractive index for a PS-co-PVDAT 1 mol % VDAT copolymer film produced from a 3% (w.t.) toluene solution exposed to PNT for five seconds.

1.55

1.56

1.57

1.58

1.59

1.6

1.61

1.62

1.63

400 500 600 700 800 900 1000

Ref

ract

ive I

ndex

(n)

Wavelength (nm)

Before

After

151

Table 4.4.6. The ellipsometry Cauchy model MSE, thickness, refractive index, optical constants MSE, profilometer measured thickness, and spin coating parameters for the PS-co-PVDAT 1 mol % VDAT copolymer film exposed to PNT for five seconds.



Before After

MSE 2.043 4.136

Thickness (Å) 663.5 ± 0.3 662.5 ± 0.5

A 1.547 ± 1.82 E-3 1.559 ± 2.24 E-3

B 1.413 E-2 ± 9.51 E-4 7.829 E-3 ± 7.18 E-4

C -4.139 E-4 ± 1.16 E-4 3.329 E-4 ± 6.06 E-5

Δn 0.002


Dektak (Å) 580


152

Figure 4.4.7. The change in refractive index for a PS-co-PVDAT 1 mol % VDAT copolymer film produced from a 3% (w.t.) toluene solution exposed to PNT for twenty seconds.

1.55

1.56

1.57

1.58

1.59

1.6

1.61

1.62

400 500 600 700 800 900 1000

Ref

ract

ive

Inde

x (n

)

Wavelength (nm)

Before

After

153

Table 4.4.7. The ellipsometry Cauchy model MSE, thickness, refractive index, optical constants MSE, profilometer measured thickness, and spin coating parameters for the PS-co-PVDAT 1 mol % VDAT copolymer film exposed to PNT for twenty seconds.



Before After

MSE 2.108 5.439

Thickness (Å) 784.4 ± 0.2 782.5 ± 0.5

A 1.550 ± 1.14 E-3 1.557 ± 2.24 E-3

B 1.031 E-2 ± 5.27 E-4 2.865 E-3 ± 7.03 E-4

C -3.689 E-4 ± 6.05 E-5 8.442 E-4 ± 6.17 E-5

Δn 0.012


Dektak (Å) 715


154

Figure 4.4.8. The change in refractive index for the PS-co-PVDAT 1 mol % VDAT copolymer film produced from a 3% (w.t.) toluene solution exposed to PNT for forty seconds.

1.55

1.56

1.57

1.58

1.59

1.6

1.61

1.62

400 500 600 700 800 900 1000

Ref

ract

ive

Inde

x (n

)

Wavelength (nm)

Before

After

155

Table 4.4.8. The ellipsometry Cauchy model MSE, thickness, refractive index, optical constants MSE, profilometer measured thickness, and spin coating parameters for a PS-co-PVDAT 1 mol % VDAT copolymer film exposed to PNT for forty seconds.



Before After

MSE 2.104 3.559

Thickness (Å) 788.0 ± 0.2 788.6 ± 0.3

A 1.550 ± 1.10 E-3 1.566 ± 1.35 E-3

B 1.064 E-2 ± 5.07 E-4 3.208 E-3 ± 4.94 E-4

C -2.906 E-5 ± 5.81 E-5 7.950 E-4 ± 3.921 E-5

Δn 0.012


Dektak (Å) 724


156

4.5 PMMA-co-PVDAT Films

Similar to the PS-co-PVDAT copolymers, the solubility of the PMMA-co-PVDAT

copolymers in spin coating solvents was determined. The PMMA-co-PVDAT copolymers

exhibited the same trend as the PS-co-PVDAT copolymers. As the VDAT mol % increased in

the copolymers, the copolymers required more polar organic solvents. The lower mol % PMMA-

co-PVDAT copolymers were soluble in toluene and MEK, and produced homogeneous films

with no visible defects. Toluene was used as the spin coating due to its ideal b.p. The 10 and 20

mol % copolymers were soluble in polar organic solvents such as DMF, DMSO, and 1,4-

dioxane. 1,4-dioxane was used to prepare spin coated thin films. All of the polymer solutions

were filtered to remove any particles present in the polymer solutions. The polymer solutions'

concentrations and spin coating speeds were varied, allowing a variety of film thicknesses to be

prepared. After the films were spin coated, the polymer thin films were dried in an oven at 60 °C

for two hours to remove any residual solvent.

Initial attempts were performed to expose the PMMA-co-PVDAT copolymers thin films

to nitroaromatic vapors for short amounts of time (five, ten, twenty, and thirty seconds). The

films did not always produce changes in refractive indices after exposure to the nitroaromatic

vapors. In order to produce refractive index changes, the exposure times were increased

significantly to determine whether the films had an affinity for the nitroaromatic vapors.

Polymer films spin coated from the 20 and 10 mol % copolymers were exposed to PNT,

NB, and 1,3-DNB for two minutes. There was no observed change in the refractive indices after

exposure to the nitroaromatics. Significant surface roughness was observed for the 3% and 1%

(w.t.) polymer solutions confirmed by the profilometer. The surface roughness affected the

157

refractive index measurements, which produced curved features in the ellipsometry spectra not

representative of the polymer films' refractive indices.

Polymer films prepared from the 1 mol % and 5 mol % copolymers solutions allowed

spin coated films with less surface roughness that allowed refractive index measurements

representative of the copolymer thin films. Since no change in refractive index for the 10 and 20

mol % copolymer films were observed when exposed to a nitroaromatic for two minutes, the

exposure time was increased to determine whether a change in refractive index would occur.

Spin coated films from the 1 mol % and 5 mol % copolymers were exposed to NB, PNT, and

1,3-DNB for several minutes. Figures 4.5.1 - 4.5.4 show the ellipsometry curves plotted as the

wavelength dependence of the refractive index (n) for the copolymer films. Tables 4.5.1 - 4.5.4

list the Cauchy parameters, profilometer measured thicknesses, average change in refractive

index, exposure times, and spin coating parameters for the copolymer films.

The refractive indices observed for the ellipsometry curves for the PMMA, 1 mol %, and

5 mol % copolymer films were in agreement with the reported PMMA refractive index in the

literature (n = 1.491477). The ellipsometry curves displayed ideal refractive indices for the

Cauchy dispersion model (refractive index decreased as wavelength increased). The features

observed in the ellipsometry curves were attributed to the copolymer films' surface roughness,

reflecting light in a different manner compared to the bulk of the film. Surface roughness was not

accounted for during the refractive indices measurements. The PMMA film exposed to 1,3-DNB

for ten seconds showed no change in refractive index after exposure to the nitroaromatic vapor.

However, the PMMA-co-PVDAT copolymer films produced a change in refractive index after

exposure to a nitroaromatic vapor, similar to the PS-co-PVDAT. Again, the addition of PVDAT

in the copolymer films showed an affinity for nitroaromatics vapor molecules. It was observed

158

that the copolymer films exposed to NB produced larger refractive index changes compared to

the films exposed to 1,3-DNB. These results correlated to the vapor pressures of the

nitroaromatics. 1,3-DNB has a low vapor pressure (0.027 Pa at 20 °C) compared to the vapor

pressure of NB (24 Pa at 20 °C).78 The higher vapor pressure of NB allowed the vapor phase

molecules to enter the amorphous films more readily and interact with the PVDAT units, which

produced a large change in refractive index. There were observed differences for the film

thickness measurements between the Cauchy model and profilometer. These small differences

between the profilometer and Cauchy model were negligible, but did confirm the approximate

film thicknesses. The PMMA-co-PVDAT films demonstrated mixed results after being exposed

to a nitroaromatic vapor by producing small, large, or no change in refractive index. These mixed

results may be linked to the ability of the nitroaromatic vapor molecules to enter the amorphous

film and interact with the PVDAT moieties giving rise to a change in refractive index.

159

Figure 4.5.1. The before and after refractive index curves for a PMMA film spin coated from a 3% (w.t.) toluene solution exposed to 1,3-DNB for ten seconds.

1.46

1.47

1.48

1.49

1.5

1.51

1.52

1.53

1.54

300 400 500 600 700 800 900 1000

Ref

ract

ive I

ndex

(n)

Wavelength (nm)

Before

After

160

Table 4.5.1. The before and after Cauchy parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a PMMA film exposed to 1,3-DNB for ten seconds.

Polymer: PMMA, 3% (w.t.) Toluene

Nitroaromatic: 1,3-DNB 10 sec. exposure

Before After

MSE 3.143 3.098

Thickness (Å) 1,205.2 ± 0.4 1,205.5 ± 0.4

A 1.474 ± 7.60 E-4 1.474 ± 7.51 E-4

B 3.414 E-3 ± 3.2 E-4 3.13 E-3 ± 3.21 E-4

C 1.944 E-4 ± 3.12 E-5 2.212 E-4 ± 3.17 E-5

Δn 7.5 E-5


Dektak (Å) 1,192


161

Figure 4.5.2. The refractive index curves for a PMMA-co-PVDAT 1 mol % VDAT copolymer thin film spin coated from a 3% (w.t.) toluene polymer solution exposed to 1,3-DNB for sixteen minutes.

1.46

1.47

1.48

1.49

1.5

1.51

1.52

1.53

300 400 500 600 700 800 900 1000

Ref

ract

ive I

ndex

(n)

Wavelength (nm)

Before

After

162

Table 4.5.2. The ellipsometry Cauchy model parameters, profilometer thickness, average change in refractive index (Δn), and spin coating parameters for a PMMA-co-PVDAT 1 mol % VDAT copolymer film exposed to 1,3-DNB for sixteen minutes.

Polymer: PMMA-co-PVDAT 1 mol % VDAT, 3% (w.t.) Toluene

Nitroaromatic: 1,3-DNB 16 min. exposure

Before After

MSE 2.503 2.737

Thickness (Å) 770.2 ± 0.5 769.7 ± 0.5

A 1.460 ± 1.13 E-3 1.462 ± 1.22 E-3

B 7.303 E-3 ± 3.93 E-4 6.975 E-3 ± 4.17 E-4

C -1.395 E-4 ± 3.48 E-5 -1.246 E-3 ± 3.72 E-5

Δn 4.0 E-4


Dektak (Å) 738


163

Figure 4.5.3. The refractive index curves for a PMMA-co-PVDAT 1 mol % VDAT copolymer thin film spin coated from a 3% (w.t.) toluene solution exposed to NB for twenty-five minutes.

1.46

1.465

1.47

1.475

1.48

1.485

1.49

1.495

1.5

1.505

500 550 600 650 700 750 800 850 900 950 1000

Ref

ract

ive I

ndex

(n)

Wavelength (nm)

Before

After

164

Table 4.5.3. The ellipsometry Cauchy parameters, profilometer measured thickness, average change in refractive index (Δn), and spin coating parameters for a PMMA-co-PVDAT 1 mol % VDAT copolymer film exposed to NB for twenty-five minutes.


Nitroaromatic: NB 25 min. exposure

Before After

MSE 2.631 3.399

Thickness (Å) 767.0 ± 0.4 832.8 ± 0.9

A 1.458 ± 1.46 E-3 1.454 ± 4.46 E-3

B 8.956 E-3 ± 8.82 E-4 1.854 E-2 ± 3.65 E-4

C -3.269 E-4 ± 1.23 E-4 -1.598 E-3 ± 6.93 E-4

Δn 0.009


Dektak (Å) 814


165

Figure 4.5.4. The refractive index curves for a PMMA-co-PVDAT 5 mol % VDAT copolymer film spin coated from a 1% (w.t.) toluene solution before and after exposure to NB for twenty-five minutes.

1.4

1.42

1.44

1.46

1.48

1.5

1.52

1.54

1.56

1.58

300 400 500 600 700 800 900 1000

Ref

ract

ive I

ndex

(n)

Wavelength (nm)

Before

After

166

Table 4.5.4. The ellipsometry Cauchy parameters, profilometer measured thickness, average change in refractive index (Δn), and spin coating parameters for a PMMA-co-PVDAT 5 mol % VDAT copolymer film exposed to NB for twenty-five minutes.


Nitroaromatic: NB 25 min. exposure

Before After

MSE 2.142 4.511

Thickness (Å) 333 ± 0.9 327 ± 2

A 1.432 ± 3.18 E-3 1.493 ± 7.77 E-3

B -3.672 E-3 ± 9.51 E-4 -4.648 E-3 ± 2.43 E-4

C 1.129 E-3 ± 9.80 E-5 9.555 E-4 ± 2.46 E-4

Δn 0.054


Dektak (Å) 324


167

Figure 4.5.5. The ellipsometry curves for a PMMA-co-PVDAT 5 mol % VDAT copolymer film spin coated from a 1% (w.t.) toluene solution before and after exposure to 1,3-DNB for twenty-five minutes.

1.4

1.41

1.42

1.43

1.44

1.45

1.46

1.47

1.48

400 500 600 700 800 900 1000

Ref

ract

ive I

ndex

(n)

Wavelength (nm)

Before

After

168

Table 4.5.5. The ellipsometry Cauchy parameters, profilometer measured thickness, average change in refractive index (Δn), and spin coating parameters for a PMMA-co-PVDAT 5 mol % VDAT copolymer film exposed to 1,3-DNB for twenty-five minutes.


Nitroaromatic: 1,3-DNB 25 min. exposure

Before After

MSE 2.796 2.885

Thickness (Å) 283 ± 1 283 ± 1

A 1.410 ± 4.89 E-3 1.413 ± 5.02 E-3

B 3.756 E-3 ± 1.34 E-3 4.682 E-3 ± 1.38 E-3

C 5.461 E-1 ± 1.39 E-4 3.666 E-4 ± 1.43 E-4

Δn 0.003


Dektak (Å) 204


169

4.6 P2VP Polymer Film

The P2VP-co-PVDAT copolymers did not produce homogeneous films suitable for

ellipsometry characterization. However, the P2VP homopolymer did produce a homogeneous

film that was characterized by the spectroscopic ellipsometer to determine the film's optical

constants before and after exposure to a nitroaromatic vapor (Figure 4.6.1). Table 4.6.1 lists the

Cauchy parameters, average change in refractive index, and spin coating parameters. A 3% (w.t.)

toluene P2VP polymer solution was spin coated on a silicon wafer and exposed to PNT for five

seconds. The refractive index for the P2VP film before exposure was observed to be A ≈ 1.337

with a film thickness of ≈15 nm.* After the five-second exposure to PNT, the refractive index

increased to A ≈ 1.353 with a film thickness of ≈15 nm. The MSE values for the Cauchy

parameters and optical constants indicated a reasonable fit between the experimental and

generated data for the P2VP film. Exposure to the PNT vapor produced an average change in

refractive index (Δn) of 0.019 for the ellipsometry curves. The change in refractive index for the

P2VP film occurred due to the 2-vinylpyridine moieties hydrogen bonded with PNT nitro group.

Saloni et al. performed theoretical calculations for monomers incorporated in imprinted

polymers which described their ability to imprint with TNT in different solvents.79 2-

vinylpyridine theoretically was able to hydrogen bond to the TNT nitro functional groups for an

imprinted polymer. The features observed in the ellipsometry curves indicated a polymer film

with surface roughness. The film thickness was not measured by the profilometer to confirm the

ellipsometer's measured thickness. The change in refractive index for the P2VP film exposed to

* Since the refractive index for each film varied as a function of wavelength, it was decided to use the Cauchy parameter (A). The Cauchy parameter (A) is the refractive index at very long wavelengths when the refractive index does not change with wavelength. By using the parameter (A), we could compare the values for different films without the concern of the effect of dispersion.

170

PNT was greater than the required (Δn) of 0.003 to detect a change in refractive index for the

proposed MZI sensor.

Figure 4.6.1. The before and after ellipsometry curves for a P2VP polymer film spin coated from a 3% (w.t.) toluene solution exposed to PNT for five seconds.

171

Table 4.6.1. The Cauchy parameters, average change in refractive index, and spin coating parameters for a spin coated P2VP film exposed to PNT for five seconds.

Polymer: P2VP, 3% (w.t.) Toluene


Before After

MSE 3.507 3.235

Thickness (Å) 150 ± 2 146 ± 2

A 1.337 ± 9.52 E-3 1.353 ± 9.14 E-3

B 4.786 E-3 ± 1.77 E-3 5.817 E-3 ± 1.77 E-3

C 1.038 E-3 ± 1.66 E-4 9.918 E-4 ± 1.65 E-4

Δn 0.019


Dektak (Å)


172

4.7 Commercial Polymers Films

PVI, PVI-co-PVA, and P4VP (commercial polymers) were purchased to examine their

capabilities of producing changes in refractive index after being exposed to a concentrated

nitroaromatic vapor. These polymers were chosen due to the known ability of amine bases to

form complexes with nitroaromatic species in solution. These polymers were also found to be

soluble in EtOH. Since the polymers were soluble in EtOH (b.p. ≈ 78 °C54), slower spin coating

speeds were employed for producing homogeneous films.

A 3% (w.t.) solution of P4VP was prepared in EtOH by gently heating and placing the

vial containing the polymer solution in the wrist action shaker until the polymer completely

dissolved. Before spin coating, the polymer solution was filtered twice using 0.45 μm PTFE

filters to remove any undissolved particles in the solution. The polymer solution was spin coated

by the dynamic technique. Approximately 1 mL of the polymer solution was dropped constantly

(≈ 1 drop per sec.) at 1,500 rpm for thirty seconds. After thirty seconds, the film formation was

allowed to proceed and dry at 3,000 rpm for forty-five seconds. After spin coating the polymer

film, the film was placed in an oven at ≈ 60 °C for two hours. After drying, the film appeared a

yellow-blue tint without any visible defects. The P4VP polymer film was then characterized by

ellipsometry to determine the film's thickness and optical constants. The P4VP film was exposed

to a concentrated vapor of PNT.

The refractive index of the P4VP film was A ≈ 1.580, which was similar to the reported

literature refractive index (n = 1.57280). The refractive indices as a function of wavelength

determined by spectroscopic ellipsometry are shown in Figure 4.7.1 and Table 4.7.1 provides the

Cauchy parameters, profilometer measured thickness, and spin coating parameters. The

ellipsometry curves displayed reasonable refractive indices expected for the Cauchy dispersion

173

model (refractive indices decreased toward the visible spectrum). The features observed in the

ellipsometry curves were attributed to surface roughness, similar to previously presented spectra.

When fitting the data to the Cauchy model, surface roughness was not accounted for. The small

MSE values obtained from the fitted data indicated a reasonable fit between the experimental and

generated data. The thickness measured by the profilometer differed by 4.0 nm when compared

to the thickness measured by the ellipsometer. This result was consistent with the previous

results, confirming the approximate thickness of the polymer film. Exposure to the PNT vapor

produced an average change in refractive index (Δn) of 0.014. This change in refractive index

was attributed to two types of interactions, π- π stacking or charge transfer complexes, as P4VP

does not undergo hydrogen bonding with nitroaromatics. This result was consistent with the

results Tenhaeff et al. observed with the ability of P4VP to detect a nitroaromatic vapor (TNT).80

The observed ability of P4VP to detect PNT by producing a significant change in refractive

index proposed the polymer to be an ideal material to incorporate in the MZI.

174

Figure 4.7.1. The before and after refractive index curves for a P4VP film spin coated from a 3% (w.t.) ethanol solution exposed to a concentrated PNT vapor for five seconds.

1.56

1.58

1.6

1.62

1.64

1.66

1.68

1.7

1.72

1.74

300 400 500 600 700 800 900 1000

Ref

ract

ive I

ndex

(n)

Wavelength (nm)

Before

After

175

Table 4.7.1. The ellipsometry Cauchy model MSE, film thickness, average change in refractive index (Δn), optical constants MSE, profilometer measured thickness, and spin coating parameters for the P4VP film exposed to PNT for five seconds.

Polymer: Poly(4-VP), 3% (w.t.) EtOH


Before After

MSE 1.628 1.835

Thickness (Å) 166.5 ± 0.4 161.3 ± 0.4

A 1.580 ± 5.32 E-3 1.596 ± 6.42 E-3

B 2.795 E-3 ± 1.43 E-3 2.446 E-3 ± 1.74 E-3

C 9.30 E-4 ± 1.25 E-5 8.835 E-4 ± 1.50 E-5

Δn 0.014


Dektak (Å) 121

Spin Coating 1,500 rpm for 30 sec. and 3,000 rpm for 45 sec.

176

Similar to P4VP, the PVI homopolymer and PVI-co-PVA copolymer were soluble in

EtOH. Polymer solutions were prepared by dissolving the polymers in EtOH using the wrist

action shaker until the polymer completely dissolved. After the polymers completely dissolved,

the polymer solutions were filtered removing any undissolved polymer particles. The PVI and

PVI-co-PVA solutions were spin coated by the dynamic technique, producing thin polymer

films. After spin coating, the films were then placed in an oven at 60 °C for two hours to remove

any residual EtOH. After drying, the polymer thin films were characterized by spectroscopic

ellipsometry to determine the films' thicknesses and optical constants.

The PVI polymer film was exposed to a PNT concentrated vapor for five seconds. The

PVI refractive index curves were plotted as a function of wavelength shown in Figure 4.7.2 and

Table 4.7.2 provides the Cauchy parameters, average change in refractive index, and spin coating

parameters. The observed PVI refractive indices were consistent for the Cauchy model. Similar

to the P4VP films, the features observed in the curves were attributed to surface roughness. The

refractive index determined by the Cauchy model before exposure to PNT was approximately A

≈ 1.575. After exposure to the PNT vapor, the polymer film's refractive index increased to

approximately A ≈ 1.599. The five-second exposure to PNT produced an average change in

refractive index (Δn) of 0.014. The change in refractive index occurred due to two possible

interactions: non-covalent interactions (including hydrophilic/hydrophobic interactions,

electrostatic, hydrogen bonding, or van der Waals forces) or covalent interactions (Meisenheimer

complex). Kong et al. reported a possible Meisenheimer complex formed between a templated

cross-linked polymer containing imidazole units with TNT confirmed by 1H NMR titrations with

TNT.81 The MSE values suggested a good fit for the Cauchy model between the experimental

data and generated data. The thicknesses determined by the Cauchy model before and after

177

exposure to PNT were identical. The profilometer thickness measurement was not performed to

confirm the approximate film thickness determined by the ellipsometer.

Figure 4.7.2. The PVI thin film spectroscopic ellipsometry curves showing a change in refractive index after a five second exposure to PNT.

1.56

1.58

1.6

1.62

1.64

1.66

1.68

1.7

300 400 500 600 700 800 900 1000

Ref

ract

ive I

ndex

(n)

Wavelength (nm)

Before

After

178

Table 4.7.2. The Cauchy model parameters and spin coating parameters for a PVI polymer film spin coated from a 3% (w.t.) EtOH solution exposed to PNT for five seconds.

Polymer: PVI, 3% (w.t.) EtOH


Before After

MSE 1.951 1.377

Thickness (Å) 215.8 ± 0.5 215.4 ± 0.3

A 1.575 ± 4.85 E-3 1.599 ± 1.01 E-3

B 1.281 E-3 ± 1.38 E-3 -2.412 E-3 ± 1.01 E-3

C 8.313 E-4 ± 1.21 E-4 9.621 E-4 ± 8.78 E-5

Δn 0.014


Dektak


179

A very thin PVI-co-PVA copolymer film was characterized by spectroscopic

ellipsometry before and after exposure to PNT for five seconds (Figure 4.7.3). Table 4.7.3 lists

the Cauchy parameters, average change in refractive index, profilometer measured thickness, and

spin coating parameters. The initial refractive index measured by the ellipsometer before

exposure to the nitroaromatic vapor was observed at A ≈ 1.268. After the five-second exposure

to PNT, the film’s refractive index increased to A ≈ 1.275, which produced an average change in

refractive index (Δn) of 0.007. The features observed in the before and after ellipsometry curves

were attributed to the film's surface roughness. The MSE values suggested a good fit between the

experimental and generated data from the Cauchy model. The 0.3% (w.t) polymer solution

produced an expected very thin polymer film (≈ 16 nm). There was a 2 nm difference in film

thickness observed between the profilometer and Cauchy model due to the etching process, but

this small difference approximately confirmed the Cauchy model's predicted film thickness. The

refractive index measured for the copolymer was a low value for two monomers with higher

refractive indices. This observed low refractive index will be addressed later in this chapter.

180

Figure 4.7.3. The before and after refractive index curves for a thin PVI-co-PVA polymer film exposed to PNT for five seconds.

1.26

1.28

1.3

1.32

1.34

1.36

1.38

1.4

1.42

300 400 500 600 700 800 900 1000

Ref

ract

ive I

ndex

(n)

Wavelength (nm)

Before

After

181

Table 4.7.3. The before and after Cauchy parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a PVI-co-PVA polymer film exposed to PNT for five seconds.

Polymer: PVI-co-PVA, 0.3% (w.t.) EtOH


Before After

MSE 3.932 3.734

Thickness (Å) 167 ± 3 166 ± 3

A 1.268 ± 9.30 E-3 1.275 ± 8.91 E-3

B 5.961 E-3 ± 1.50 E-3 5.80 E-3 ± 1.45 E-3

C 4.78 E-4 ± 1.41 E-4 5.201 E-4 ± 3.16 E-4

Δn 0.007


Dektak (Å) 143


182

4.8 Polystyrene Thin Films Containing 10-Methylphenothiazine

In Chapter 5, the growth of co-crystals between 10-methylphenothiazine (10-M) and 1,3-

DNB is discussed. The co-crystals appeared red-purple in color, suggesting a strong interaction

between the electron donor and the electron deficient nitroaromatic. This result prompted the

investigations of 10-M included in polystyrene thin films to determine if a charge transfer

complex would form and cause a change in refractive index when exposed to 1,3-DNB vapors.

Polystyrene was synthesized according to the Chen procedure.49 Polystyrene was purified

by washing the polymer in EtOH in a one-neck round bottom. After purifying the polymer, the

polymer was dried at 50 °C under vacuum overnight.

3% and 1% (w.t.) stock polystyrene solutions were prepared by dissolving the polymer in

toluene. The polymer solutions were then placed in a wrist action shaker until the polymer

completely dissolved. After the polymer dissolved, the polymer solutions were filtered once to

remove any particles present in the solution.

The first experiments performed were to determine the maximum 10-M concentration

that could be included in polystyrene solutions to allow homogeneous spin coated polymer films.

3% (w.t.) polystyrene/toluene solutions containing 0.1%, 0.3%, 0.7%, 1%, 3%, 6%, and 10%

(w.t.) 10-M were prepared by dissolving 10-M in the polystyrene/toluene solutions using the

wrist action shaker. After the 10-M dissolved in the polystyrene solutions, the solutions were

filtered again to remove any particles present in the solutions. Films were spin coated by the

static technique. The films were allowed to dry for twenty-four to forty-eight hours at room

temperature in a dark area, since 10-M was sensitive to the light. After drying, the polymer films

were inspected for any defects. The films spin coated from the polystyrene solutions containing

0.1% - 1% 10-M (w.t.) produced blue films with a slight purple tint. The film spin coated from

183

the 3% (w.t.) polystyrene solution containing 3% (w.t.) 10-M produced a blue-purple tint film

with a few white, needle-like crystals protruding from the film. The spin coated film from the

polystyrene solution containing 6% (w.t.) 10-M produced a blue-purple tint film with large areas

covered with white, needle-like crystals. The 10% (w.t.) 10-M spin coated film surprisingly did

not produce white, needle-like crystals, but rather produced a blue-purple tint film with a slight

green tint with cracks throughout, creating a mosaic pattern. The same experimental procedure

was performed for 1% (w.t.) polystyrene solutions which produced similar results with 1% (w.t.)

10-M being the maximum concentration included in polystyrene films that allowed

homogeneous spin coated films. Next, the polystyrene/10-M films' optical constants were

characterized by spectroscopic ellipsometry before and after exposure to 1,3-DNB vapors from

seconds to hours. Refractive index changes were unsuccessful for short exposure times. The

exposure times were increased until a change in refractive index was observed.

A polymer film spin coated from a 3% (w.t.) polystyrene solution containing 1% (w.t.)

10-M was exposed to 1,3-DNB for two hours (Figure 4.8.1). Table 4.8.1 lists the Cauchy

parameters, average change in refractive index, profilometer measured thickness, and spin

coating parameters. The refractive index curves displayed the trend observed for a Cauchy

dispersion model. The spectrum was fitted using the Cauchy model from 400 - 1,000 nm,

excluding any absorption, which may have occurred in the UV region. The initial refractive

index of the polystyrene/10-M film was A ≈ 1.567 and increased with decreasing wavelengths.

The 10-M/polystyrene film was exposed to 1,3-DNB for two hours. After the film was exposed

to 1,3-DNB, the film's refractive index increased to A ≈ 1.582. The average change in refractive

index from 400 - 1,000 nm was (Δn) ≈ 0.005. The MSE values for the Cauchy parameters and

optical constants suggested a reasonable fit between the experimental and generated data. The

184

profilometer’s measured thickness for the film was ≈ 95.9 nm, which was 7 nm thicker than the

thickness determined by the Cauchy model (≈ 89 nm). This difference in film thickness may be

due to the etching process, where the etch penetrated the silicon wafer's surface and created a

thicker film measurement. Even though there was a small difference between the film thickness

measurements, the profilometer measurement did approximately confirm the ellipsometer's

determined film thickness. The features observed in the ellipsometry curves follow the trend

observed for a film with some surface roughness.

185

400 500 600 700 800 900 1000

1.58

1.59

1.60

1.61

1.62

1.63

1.64

1.65

1.66

Ref

ract

ive

Inde

x (n

)

Wavelength (nm)

Before After

Figure 4.8.1. The ellipsometry curves for a spin coated polystyrene/10-M film from a 3% (w.t.) polystyrene solution containing 1% (w.t.) 10-M exposed to 1,3-DNB for two hours.

186

Table 4.8.1. The before and after Cauchy model parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a polystyrene film containing 10-M exposed to 1,3-DNB for two hours.

Polymer: 3% (w.t.) PS, 1% (w.t.) 10-M

Nitroaromatic: 1,3-DNB 2 hrs. exposure

Before After

MSE 5.053 5.86

Thickness (Å) 888.3 ± 0.4 889.6 ± 0.4

A 1.567 ± 1.10 E-3 1.582 ± 2.99 E-3

B 1.651 E-2 ± 6.21 E-3 1.200 E-2 ± 1.52 E-3

C -7.346 E-4 ± 9.08 E-5 -2.279 E-4 ± 1.94 E-5

Δn 0.005


Dektak (Å) 959


187

A polystyrene/10-M film was spin coated from a 1% (w.t.) polystyrene solution

containing 0.5% (w.t.) 10-M. The polystyrene/10-M film thickness and optical constants were

characterized by ellipsometry before and after exposure to 1,3-DNB for three hours (Figure

4.8.2). Table 4.8.2 lists the Cauchy parameters, average change in refractive index, profilometer

measured thickness, and spin coating parameters for the film. The refractive index before

exposure to 1,3-DNB was A ≈ 1.470. After the film was exposed to 1,3-DNB for three hours, the

film's refractive index increased to A ≈ 1.480. The increase in refractive index was due to the 10-

M incorporated in the polymer film that hydrogen bonded with the nitro groups of 1,3-DNB. The

features observed in the refractive index curves were due to the presence of surface roughness,

which was not accounted for in the model. After exposure to the nitroaromatic, the film did not

change in color compared to the observed color change for the 10-M co-crystal with 1,3-DNB.

The before and after Cauchy MSE values described a good fit between the experimental and

generated data. The Cauchy model determined a film thickness of ≈ 27 nm, but the profilometer

measured a film thickness of ≈ 31 nm. The difference in film thickness was consistent with the

previous result where the profilometer’s measured thickness was slightly larger than the

thickness determined by the ellipsometer. Again, this difference resulted from the etching the

process. The minimal difference between both measurements did confirm the approximate film

thickness. This result led to the investigation to determine if a smaller concentration of 10-M in a

polystyrene film could produce a change in refractive index over an extended exposure period.

188

400 500 600 700 800 900 10001.46

1.48

1.50

1.52

1.54

1.56

1.58

1.60

Ref

ract

ive

Inde

x (n

)

Wavelength (nm)

Before After

Figure 4.8.2. Refractive index curves for a polystyrene/10-M film spin coated from a 1% (w.t.) polystyrene/toluene polymer solution containing 0.5% (w.t.) 10-M exposed to 1,3-DNB for three hours.

189

Table 4.8.2. The ellipsometry Cauchy parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a film spin coated from a 1% (w.t.) polystyrene solution containing 0.5% (w.t.) 10-M exposed to 1,3-DNB for three hours.

Polymer: 1% (w.t.) PS, 0.5% (w.t.) 10-M


Before After

MSE 2.41 2.283

Thickness (Å) 267.3 ± 0.8 270.1 ± 0.7

A 1.470 ± 5.22 E-3 1.480 ± 2.99 E-3

B 1.112 E-2 ± 2.50 E-3 1.133 E-2 ± 2.42 E-3

C 8.341 E-4 ± 3.34 E-4 8.565 E-4 ± 3.23 E-5

Δn 0.01


Dektak (Å) 307


190

A polystyrene/10-M film was spin coated on a silicon wafer from a 1% (w.t.)

polystyrene/toluene solution containing 0.1% (w.t) 10-M. The polystyrene/10-M solution

produced a blue colored film with a slight purple tint, which was characterized by ellipsometry

before and after exposure to 1,3-DNB to determine the film’s thickness and optical constants

(Figure. 4.8.3). Table 4.8.3 lists the Cauchy parameters, average change in refractive index,

profilometer measured thickness, and spin coating parameters for the polymer film. The

refractive index determined by the Cauchy model for the film before exposure to 1,3-DNB was

A ≈ 1.464. After the film was exposed to 1,3-DNB, the refractive index increased to A ≈ 1.469.

The small concentration of 10-M included in the polystyrene film showed the ability to produce

a small change in refractive index of Δn = 0.004 due to hydrogen bonding with the 1,3-DNB

nitro groups. The MSE values described a good fit between the experimental and generated data

for the Cauchy model. There was a minute difference in film thickness observed between the

Cauchy model's determined film thickness and the profilometer's measured film thickness. This

result was similar to previous film thickness measurements where the profilometer measured a

film thickness greater than the Cauchy model's determined film thickness. This minute difference

(≤ 1 nm) confirmed the approximate film thickness determined by the Cauchy model. The

features observed in the refractive index curves between 600 - 1,000 nm indicated surface

roughness for the film, since typically the refractive index curves lie flat in the near-infrared

region.

The inclusion of small concentrations of 10-M in polystyrene films exposed to 1,3-DNB

for an extended period of time showed the ability to produce changes in refractive index by non-

covalent interactions. These results provided evidence that a polymer containing 10-M moieties

in greater concentration might have a strong affinity for 1,3-DNB giving rise to a significant

191

change in refractive index with a shorter exposure time. These results suggested 10-M would be

an ideal material to be applied in the MZI sensor for detecting nitroaromatics.

400 500 600 700 800 900 1000

1.46

1.48

1.50

1.52

1.54

1.56

1.58

Ref

ract

ive

Inde

x (n

)

Wavelength (nm)

Before After

Figure 4.8.3. The refractive index curves for a polystyrene/10-M film spin coated from a 1% (w.t.) polystyrene toluene solution containing 0.1% (w.t.) 10-M exposed to 1,3-DNB for three hours.

192

Table 4.8.3. The Cauchy model parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a film spin coated from a 1% (w.t.) polystyrene solution containing 0.1% (w.t.) 10-M exposed to 1,3-DNB for three hours.

Polymer: 1% (w.t.) PS, 0.1% (w.t.) 10-M


Before After

MSE 2.215 2.119

Thickness (Å) 257.4 ± 0.8 257.7 ± 0.8

A 1.464 ± 4.86 E-3 1.469 ± 4.61 E-3

B 9.162 E-3 ± 2.26 E-3 9.333 E-3 ± 2.14 E-3

C 9.846 E-4 ± 3.03 E-4 9.830 E-4 ± 2.87 E-4

Δn 0.004


Dektak (Å) 262


193

During one experiment, a film spin coated from a 1% (w.t.) polystyrene/toluene solution

containing 0.1% (w.t.) 10-M produced a change in refractive index after a ten second exposure to

1,3-DNB (Figure 4.8.4). The refractive index before exposure to the 1,3-DNB vapors was A ≈

1.494. After the ten-second exposure, the refractive increased to A ≈ 1.499, which produced a

0.005 average refractive index change (Table 4.8.4). The MSE values for the Cauchy model

described an ideal fit between the experimental and generated data. The film thickness

determined by the Cauchy model and the profilometer measured thickness were in agreement,

which confirmed the thickness determined by the ellipsometer. Features appeared in the near-

infrared region of the refractive index curves because of the presence of surface roughness. This

result was consistent with the previous result for a film spin coated from a 1% (w.t.)

polystyrene/toluene solution containing 0.1% 10-M exposed to 1,3-DNB for three hours. This

result showed that in one instance longer exposure times were not required to produce a

detectable change in refractive index.

194

400 500 600 700 800 900 10001.48

1.50

1.52

1.54

1.56

1.58

1.60

1.62

1.64

Ref

ract

ive

Inde

x (n

)

Wavelength (nm)

Before After

Figure 4.8.4. The refractive index curves for a polystyrene/10-M film spin coated from a 1% (w.t.) polystyrene/toluene solution containing 0.1% (w.t.) 10-M exposed to 1,3-DNB for ten seconds.

195

Table 4.8.4. The Cauchy parameters before and after exposure to 1,3-DNB, average change in refractive index, profilometer measured thickness, and spin coating parameters for a film spin coated from a 1% (w.t.) polystyrene/toluene solution containing 0.1% (w.t.) 10-M exposed to 1,3-DNB for ten seconds.

Polymer: 1% (w.t.) PS, 0.1% (w.t.) 10-M

Nitroaromatic: 1,3-DNB 10 sec. exposure

Before After

MSE 2.698 3.421

Thickness (Å) 273.4 ± 0.8 271.6 ± 0.7

A 1.494 ± 5.54 E-3 1.499 ± 5.06 E-3

Δn 0.005


Dektak (Å) 272


196

Most of the refractive indices for the polymers reported in this chapter were in agreement

with the reported literature refractive indices. There were some instances where the observed

refractive indices for thin polymer films (≤ 100 nm) exhibited lower refractive indices values

than the reported literature values. The lower indices of refraction were due to a radially

symmetric segmental orientation of specific groups on the polymer chains acting as an optical

retarder, produced during the spin coating process. Schwab et al. measured the optical

birefringence of rubbed thin polystyrene films to investigate the relaxation processes of

molecules at the polymer/air interface that showed shifts in the index of refraction.82 Hu et al.

investigated anomalies of refractive index for spin coated thin polystyrene films.83 Hu found that

the index of refraction was a function of film thickness for polystyrene films less than 100 nm

and the bulk refractive index could be recovered by annealing the films above the Tg. Hu noted

that the observed change in refractive index was due to the symmetric segmental orientation

produced by the spin coating process, since the orientation induced during the spin coating

process is typically radially symmetric about the spin axis and uniformly either in or out of the

plane of the spin coated film.83

The polymer films in this chapter represented possible materials that could be

incorporated in the MZI sensor to detect nitroaromatic explosives. The PS-co-PVDAT

copolymers, P2VP, PVI, PVI-co-PVA, and polystyrene/10-M films exhibited changes in the

index of refraction after exposure to a concentrated nitroaromatic vapor, suggesting that these

materials should be considered as possible MZI sensing materials. The copolymers containing

low concentrations of the VDAT monomer produced homogeneous films with minimal surface

roughness, allowing the optical constant to be fully characterized by spectroscopic ellipsometry.

VDAT appeared to be a promising monomer to synthesize electron rich copolymers, but as the

197

concentration was increased in the copolymers, problems with solubility in an ideal spin coating

solvent and the ability to spin coat homogeneous films with minute surface roughness excluded

these polymers from possibly being used as a sensing material. These were the main

disadvantages associated with utilizing VDAT, which led to the investigations of other polymers'

abilities to sense nitroaromatics by refractive index changes. P2VP, P4VP, PVI, and PVI-co-

PVA all showed the ability to interact with the nitro groups of the nitroaromatics, which

produced changes in the refractive index curves. Most of the polymers were not soluble in an

ideal spin coating solvent, but using the static spin coating technique allowed homogeneous films

to be casted with some surface roughness that were characterized by ellipsometry.

The polystyrene films containing low concentrations of 10-M showed the unexpected

ability to interact with the 1,3-DNB vapors over long exposure periods. These initial results

provided evidence of further work needed to synthesize a polymer rich in 10-M moieties. A

polymer containing larger concentrations of 10-M may have the potential to produce significant

changes in the index of refraction after short exposure periods to 1,3-DNB.

It should be noted that some of the polymers synthesized in Chapter 3 did not show

changes in refractive index when exposed to a concentrated nitroaromatic vapor. When the

nitroaromatic (1,3-DNB) was added to the polymers, PVK and PMMA-co-PVK copolymers, in

the solution phase and dissolved, changes in color (colorless to yellow) were observed for the

polymer/nitroaromatic solutions, suggesting a strong interaction between the electron rich

polymers and 1,3-DNB. These polymers could still be used in the MZI sensor to assist in the

detection of the nitroaromatic (1,3-DNB) in the solution phase, as opposed to the vapor phase.

Lastly, a concerning problem observed for the polymers in this chapter was the variant

changes in the index of refraction after exposure to the concentrated nitroaromatic vapors.

198

Minimal, large, or no change in refractive indices after exposure to the nitroaromatic were

observed, limiting the reliability of these sensing materials. Still, there was enough data to

support the possibility of using these polymer films as nitroaromatic sensing materials for a MZI

sensor.

4.9 Polymer Thin Films Summary

Polymer thin films were investigated to determine their affinity for nitroaromatics by

measuring the change in refractive index after exposure to a nitroaromatic vapor using

ellipsometry. To demonstrate the polymer films affinities for nitroaromatics, Table 4.9.1 lists the

polymer films' average change in refractive index after a five-second exposure to a nitroaromatic

vapor. From the PVDAT copolymers, the copolymers consisting of polystyrene and PVDAT

showed the most promise as the sensing material for the MZI. The PS-co-PVDAT copolymers

allowed films to be casted from an ideal spin coating solvent with minimal surface roughness

and did exhibit an affinity for nitroaromatics. The PMMA-co-PVDAT copolymers appeared to

have a greater affinity for nitroaromatics compared to the PS-co-PVDAT copolymers, but

extreme surface roughness was observed for the PMMA-co-PVDAT copolymer films and a

change in refractive index was not observed for some of the PMMA-co-PVDAT copolymer

films after the five-second exposure to a nitroaromatic vapor. From the commercial polymers,

PVI and P4VP showed the most promise for detecting nitroaromatics due to the change in

refractive index after exposure to a nitroaromatic vapor. A change in refractive index was

observed for the PVI-co-PVA polymer film after being exposed to PNT for five seconds, but

extreme surface roughness was observed for the polymer film.

199

Table 4.9.1. Polymer films average change in refractive index after a five-second exposure to a nitroaromatic vapor.

Polymer Nitroaromatic Exposure Time (sec.) ∆n

PVI-co-PVA PNT 5 0.010

P4VP PNT 5 0.014

PVI PNT 5 0.014

PS-co-PVDAT 1 mol % PNT 5 0.002

PS-co-PVDAT 1 mol % NB 5 0.013

PS-co-PVDAT 1 mol % 1,3-DNB 5 0.010





PS-co-PVDAT 10 mol % 1,3-DNB 5 0.003



PMMA-co-PVDAT 5 mol % PNT 5 0.017

PMMA-co-PVDAT 20 mol % PNT 5 0.073

P2VP PNT 5 0.019

200

Chapter 5

Co-crystals Containing Electron Rich Aromatic Molecules and Electron Poor Nitroaromatic Molecules Nitroaromatic molecules will form molecular complexes with electron-rich aromatic

molecules, producing intense color changes. 1,3-dinitrobenzene and 2,4-dinitrotoluene both

formed jet-black molecular complexes with benzidine (4,4’-diaminobiphenyl).84 Chloroform

solutions containing aniline and either 1,3-dinitrobenzene, 1,4-dinitrobenzene or 1,3,5-

trinitrobenzene exhibited new absorptions extending from the UV region into the visible region

with extinction coefficients of 102 to 103 M-1 cm-1.85 Mixtures of picryl chloride with

hexamethylbenzene or picric acid with naphthalene formed highly colored solutions in

chloroform.86, 87

The crystal structure packing observed in complexes between an electron rich donor and

nitroaromatic exhibit a general trend with alternating stacking in charge transfer complexes. The

crystal structure of a 1:1 molecular complex between 1,4,-dinitrobenzene and phenazine showed

alternating stacks of phenazine and 1,4-dinitrobenzene molecules with a 361 pm distance

between the center of the phenazine molecule and the center of the 1,4-dinitrobenzene

molecule.88 A 1:1 molecular complex of 2-aminobenzimidazole and 1,3,5-trinitrobenzene also

showed alternating stacks.89 In each case, the electron-poor nitro aromatic rings and the electron-

rich aromatic rings were stacked face-to-face. This pattern of alternating stacking of donor and

acceptor aromatic molecules face-to-face was also observed in the crystal structure of the

molecular complex of tetrathiafulvalene and 1,3-dinitrobenzene and in the molecular complex of

201

4-iodotetrathiafulvalene and 1,4-nitrobenzene.90, 91 These intense color changes observed when

an electron donor forms a molecular complex with various nitroaromatics provide evidence of

the ability to detect nitroaromatics by using an electron rich polymer. The polymer could form a

molecular complex with the electron deficient nitroaromatics by hydrogen bonding or pi→pi*

interactions. Varieties of electron donor and acceptor combinations that were attempted are

shown in Chapter 2. This chapter will focus on the electron donors and acceptors which

produced co-crystals confirmed by 1H NMR, FTIR, UV/Vis, diffuse reflectance, and X-ray

crystallography.

5.1 1,3-Dinitrobenzene Crystals (1,3-DNB)

1,3-DNB crystals were produced by dissolving 1,3-DNB in EtOH and allowing the EtOH

to evaporate at room temperature for two days. Needle-like crystals formed with a faint white-

yellow color shown in Figure 5.1.1.

Figure 5.1.1. Image of 1,3-DNB crystals.

202

The 1H NMR (360 MHz, CDCl3) spectrum of the 1,3-DNB crystals was recorded shown

in Figure 5.1.2. The 1,3-DNB proton signals were characterized by 1H NMR. The reported 1,3-

DNB proton signals were in agreement with reported literature values. The positions of the

proton signals were used as a reference to determine the presence of 1,3-DNB in the co-crystals.

Figure 5.1.2. 1H NMR spectrum of 1,3-DNB crystals (360 MHz, CDCl3).

1,3-DNB: 1H NMR (360 MHz, CDCl3, δ): 9.06 (t, 1.00H), 8.57 (dd, 2.04H), 7.80 (t, 1.05H).

The integration values and peak positions were used as a reference when comparing 1H NMR

spectra in order to determine the ratio between 1,3-DNB and the electron donors.

0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.59.0ppm

H1

H2

H3

H2O

CDCl3

203

The FTIR spectrum for the 1,3-DNB crystals is shown Figure 5.1.3. The peaks located at

1540 and 1347 cm-1 were assigned to the NO2 asymmetric and symmetric stretching vibrations.

The C-H stretching vibrations appeared at 2873, 3049, and 3108 cm-1. The benzene ring

stretching vibrations were observed at 1614 and 1602 cm-1 with the benzene ring overtones

appearing at 2873, 3049, and 3108 cm-1. The NO2 asymmetric and symmetric stretching

vibrations were used as a reference to determine if a complex formed between the electron

donor and electron acceptor, producing a shift in the vibrational bands.

4000 3500 3000 2500 2000 1500 1000 500

20

30

40

50

60

70

80

90

100

% T

rans

mitt

ance

Wavenumbers (cm-1)

1540NO2 vas

1347NO2 vsym

CH stretchingvibrations

Benzene RingOvertones

Figure 5.1.3. The FTIR spectrum of the 1,3-dinitrobenzene crystals.

204

The electronic absorption spectrum for the 1,3-DNB crystals (Figure 5.1.4) was recorded

in acetonitrile at a concentration of 2.0 x 10-5 M. Acetonitrile was chosen as the applicable

solvent due to its UV/Vis solvent cut-off (≈ 190 nm for a 1 cm cuvette). The 1,3-DNB crystals'

electronic absorption spectrum displayed a λmax at 237 nm (ε = 1.80 x 104 M-1cm-1).

200 300 400 500 600 700 8000

5000

10000

15000

20000

ε (M

-1 c

m-1)

Wavelength (nm)

2.0 x 10-5 M 1,3-DNB crystals

Figure 5.1.4. Electronic absorption spectrum of 1,3-DNB crystals in acetonitrile.

205

The diffuse reflectance spectrum for the 1,3-DNB crystals (Figure 5.1.5) was recorded at

room temperature. The reflectance was more than 50% in the visible region from 450 - 800 nm.

The reflectance decreased to approximately 5% from 450 - 400 nm, which gave rise to the

yellow tint for the 1,3-DNB crystals.

200 300 400 500 600 700 8000

10

20

30

40

50

60

% R

efle

ctan

ce

Wavelength (nm)

Figure 5.1.5. Diffuse reflectance spectrum of the 1,3-DNB crystals.

The melting point for the 1,3-DNB crystals was measured for comparison with the

melting point for co-crystals containing 1,3-DNB and electron-rich aromatic molecules. The

melting range was defined at the temperature that liquid formation was visible until the crystals

completely melted and formed a meniscus. The observed melting range was 90.9 - 91.3 °C (lit.

206

89 °C54). The narrow melting range provided evidence of crystals with few inhomogeneities or

individual components.

5.2 9-Ethylcarbazole (9-EC) Co-Crystals with Nitroaromatics

Attempts to prepare 1:1 co-crystals with 9-EC and either 2-NT, 3-NT, and PNT were not

successful. The solutions evaporated, producing a mixture of crystals of the pure compounds.

However, when the EtOH solutions of 9-EC and 1,3-DNB were mixed, a yellow-orange color

rapidly appeared. The EtOH was allowed to evaporate for two days, producing yellow-orange

needle-like crystals shown in Figure 5.2.1. 9-EC crystals were prepared by the same procedure

producing, white-brown, needle-like crystals.

Figure 5.2.1. 9-EC + 1,3-DNB crystals after drying for two days, producing yellow-orange tint crystals.

The 1H NMR spectra for the 9-EC co-crystals, 9-EC crystals, and 1,3-DNB crystals

(Figure 5.2.2) were recorded in CDCl3 to determine the ratio between the electron donor and

acceptor. Table 5.2.1 lists the peak positions, splitting patterns, and integration values for the 1,3-

DNB crystals, 9-EC crystals, and 9-EC co-crystals. The peaks observed in the 9-EC co-crystals

207

1H NMR spectrum at 9.06, 8.56, and 7.79 ppm were assigned to the 1,3-DNB incorporated in the

co-crystals. The 9-EC proton signals were observed at 8.09, 7.45-7.39, 7.23-7.19, 4.36, and 1.42

ppm. Neither the 9-EC nor 1,3-DNB peaks exhibited a chemical shift in the spectrum. The

integration value for the 9-EC proton signal at 7.22-7.20 ppm in the 9-EC and 9-EC co-crystals

spectra was not an accurate integration due to the CDCl3 solvent peak overlapping. The

integration of the spectrum revealed an approximate 1:1 ratio between 9-EC and 1,3-DNB.

Figure 5.2.2. 1H NMR (360 MHz, CDCl3) spectra for the 9-ethylcarbazole crystals (9-EC), the co-crystals (9-EC co-crystals with 1,3-DNB), and 1,3-dinitrobenzene crystals (1,3-DNB).

0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.59.0ppm

1,3-DNB

9-EC

9-EC co-crystals

208

Table 5.2.1. 1H NMR peak positions, splitting patterns, and integration values of the 1,3-DNB crystals, 9-EC co-crystals, and 9-EC crystals.

1,3-DNB 9-EC co-crystals 9-EC δ S.P. Int. δ S.P. Int. δ S.P. Int.

9.06 t 1.00 9.06 t 1.00 8.57 dd 2.04 8.56 dd 2.02

8.09 dt 1.86 8.10 dt 1.90 7.80 t 1.05 7.79 t 1.12

7.45-7.38 m 3.77 7.48-7.39 m 3.89 7.23-7.19 m 3.42 7.22-7.20 m 2.48 4.36 q 1.84 4.37 q 2.00 1.42 t 2.67 1.43 t 2.93

S.P. - Splitting Pattern Int. - Integration values

The FTIR spectra for the 9-EC crystals and co-crystals of 9-EC with 1,3-DNB are shown

in Figure 5.2.3. The symmetric and asymmetric stretching modes for the NO2 groups in the co-

crystal were shifted to lower energy, compared to those for the 1,3-DNB crystals. This shift

indicated intermolecular interactions between 9-EC and 1,3-DNB in the co-crystals. Table 5.2.2

shows the comparison between the NO2 asymmetric and symmetric stretching modes before and

after incorporation into the co-crystal.

209

4000 3500 3000 2500 2000 1500 1000 500

% T

rans

mitt

ance

(A.U

.)

Wavenumbers (cm-1)

3420

31183104

2981 1601

1536

1452

1345

30472978

2931

1594

14541377

1326

Free OH

CH stretching vibrations

9-EC

9-EC + 1,3-DNB

Figure 5.2.3. FTIR spectra of KBr pellets containing either 9-EC crystals (black curve) or the co-crystals containing 9-EC and 1,3-DNB (red curve).

Table 5.2.2. Comparison of NO2 asymmetric and symmetric stretching vibrations between 1, 3-DNB crystals and 9-EC + 1, 3-DNB co-crystals.

Crystals Color NO2 vas ( cm-1 ) NO2 vs ( cm-1 )

1,3-DNB white-yellow 1540 1347

9-EC white-brown N/A N/A

9-EC + 1,3-DNB yellow-orange 1536 1345

210

The electronic absorption spectrum of the 9-EC co-crystals with 1,3-DNB was recorded

in acetonitrile to determine if a charge complex formed in the dilute solutions. Figure 5.2.4

displays the electronic absorption spectra for the 1,3-DNB crystals, 9-EC crystals, and 9-EC co-

crystals with 1,3-DNB. There was no new absorption band that would be expected if a charge

transfer complex formed. There were no significant chemical shifts observed in the spectra

between 9-EC crystals and 9-EC co-crystals.

200 300 400 500 600 700 8000

10000

20000

30000

40000

50000

60000

ε (M

-1 c

m-1)

Wavelength (nm)

2.0 x 10-5 M 1,3-DNB crystals 9-EC crystals 9-EC co-crystals

Figure 5.2.4. Electronic absorption spectra of 1,3-DNB crystals (black), 9-EC crystals (red), and 9-EC co-crystals with 1,3-DNB (blue) in acetonitrile.

211

To determine if the spectrum for the 9-EC co-crystals was the result of a charge transfer

complex or just the combination of free 9-EC and 1,3-DNB molecules in solution, the spectra of

the 1,3-DNB crystals and 9-EC crystals were combined and compared against the 9-EC co-

crystals spectrum (Figure 5.2.5). The similarities between the 9-EC co-crystals electronic

absorption spectrum and the combined 1,3-DNB and 9-EC crystals electronic absorption

spectrum was the result of the independent 1,3-DNB and 9-EC molecules in solution rather than

the formation of a charge transfer complex. Clearly, 1,3-DNB and 9-EC did not form a charge

transfer complex in dilute ~10-5 M acetonitrile solutions.

200 300 400 500 600 700 8000

10000

20000

30000

40000

50000

60000

ε (M

-1 c

m-1)

Wavelength (nm)

2.0 x 10-5 M 1,3-DNB + 9-EC 9-EC co-crystals

Figure 5.2.5. Electronic absorption spectra in acetonitrile for the 9-EC co-crystals (red) and the sum of the spectra for 9-EC and 1,3-DNB crystals (black).

212

The diffuse reflectance spectra were measured for the 9-EC crystals and 9-EC co-crystals

with 1,3-DNB (Figure 5.2.6). The reflectance for the 9-EC crystals was greater than 70%

throughout the visible region. Below 400 nm, the reflectance decreased to less than 40% as the

UV light was absorbed by the crystals. The reflectance spectrum for the co-crystals had a

reflectance of less than 40% in the near infrared and red region of the spectrum. The reflectance

decreased to 10% at wavelengths below 500 nm. The difference in absorption was expected,

since the 9-EC crystals were white-brown, compared to the co-crystals which were yellow-

orange in color.

200 300 400 500 600 700 8000

20

40

60

80

100

% R

efle

ctan

ce

Wavelength (nm)

9-EC crystals 9-EC co-crystals

Figure 5.2.6. Diffuse reflectance spectra for 9-EC crystals (black) and the co-crystals of 9-EC and 1,3-DNB (red).

213

The melting points of the 9-EC crystals and 9-EC co-crystals were measured for

comparison. Table 5.2.3 lists the melting points for the 1,3-DNB crystals, 9-EC crystals, and 9-

EC co-crystals. The co-crystals with 1,3-DNB had a much lower melting range (48.4 - 50.1 °C)

compared to the 9-EC crystals (68 - 70 °C) and 1,3-DNB crystals (89 °C) melting ranges. This

lower, narrow melting point range indicated that there were few inhomogeneities or individual

components present. During the melt, the co-crystals produced a color change from a yellow-

orange to a red-orange color.

Table 5.2.3. Melting points of 1,3-DNB crystals, 9-EC crystals, and 9-EC co-crystals with 1,3-DNB.

Crystals Melting Point (°C) Lit. Value (°C)

1,3-DNB 90.9 - 91.3 89 54

9-EC 71.0 - 71.8 68 - 70 92

9-EC co-crystal 48.4 - 50.1

The 9-EC co-crystal structure was analyzed by X-ray diffraction. A survey scan of a co-

crystal revealed that the crystal structure was 1,3-DNB. Ito et. al. reported similar results with

carbazole derivative co-crystals with 1,3-DNB.92 Ito made reference that the crystal adducts

might be too small for X-ray diffraction characterization.

5.3 9-Vinylcarbazole (9-VC) Co-Crystals with 1,3-DNB

9-VC did not form co-crystals with 2-NT or NB. It did form a co-crystal with 1,3-DNB,

which was confirmed by an intense color change. When the two solutions of 9-VC and 1,3-DNB

were combined in a crystallization dish, a bright yellow color rapidly appeared. The EtOH

evaporated at room temperature for two weeks, producing yellow crystals with spots as shown in

214

Figure 5.3.1. 9-VC crystals were prepared by the same procedure producing white needle-like

crystals.

Figure 5.3.1. Co-crystals of 9-VC and 1,3-DNB.

The 1H NMR spectra of the 9-VC co-crystals, 9-VC crystals, and 1,3-DNB crystals

(Figure 5.3.2) were recorded in CDCl3 to determine the ratio between the electron donor and

acceptor. The peak positions, multiplicities, and integrations are listed in Table 5.3.1. The 1H

NMR spectrum for the 9-VC co-crystals with 1,3-DNB showed peaks located at 9.07, 8.57, and

7.79 ppm assigned to 1,3-DNB incorporated in the co-crystal. The 9-VC proton signals were

observed at 8.06, 7.65, 7.46, 7.32-7.25, 5.54, and 5.15 ppm. Neither the 9-VC nor 1,3-DNB

peaks exhibited a chemical shift in the spectrum. The integration values for the 9-VC proton

signals located between 7.32-7.25 ppm in the 9-VC and 9-VC co-crystal spectra were not an

accurate integration due to the overlapping CDCl3 solvent peak. The integration of the spectrum

revealed an approximate 1:2 ratio between 9-VC and 1,3-DNB.

215

Figure 5.3.2. 1H NMR spectra of the 1,3-DNB crystals, 9-VC crystals, and 9-VC co-crystals with 1,3-DNB (360 MHz, CDCl3).

Table 5.3.1. 1H NMR peak positions, splitting patterns, and integration values of 1,3-DNB crystals, 9-VC co-crystals, and 9-VC crystals.

1,3-DNB 9-VC co-crystals 9-VC δ S.P. Int. δ S.P. Int. δ S.P. Int.

9.06 t 1.00 9.07 t 1.71 8.57 dd 1.98 8.57 dd 3.28

8.06 d 1.81 8.07 d 1.93 7.80 t 1.04 7.79 t 1.90

7.65 d 1.94 7.65 d 1.95 7.46 td 2.04 7.46 td 1.98 7.32-7.25 m 3.71 7.33-7.27 m 2.88 5.54 dd 1.02 5.55 dd 1.01 5.15 dd 1.00 5.16 dd 1.00

S.P. - Splitting Pattern Int. - Integration value

4.44.85.25.66.06.46.87.27.68.08.48.89.2ppm

1,3-DNB

9-VC co-crystals

9-VC

216

In Figure 5.3.3, the FTIR spectra for the 9-VC crystals (red) and the 9-VC co-crystals

with 1,3-DNB (black) are shown. The NO2 peaks at 1536 (asymmetric stretching mode) and

1345 cm-1 (symmetric stretching mode) were red shifted (4 and 2 nm) to lower energy compared

to the corresponding peaks in the FTIR spectrum for the 1,3-DNB crystals. This shift indicated

an intermolecular interaction between 9-VC and 1,3-DNB in the co-crystals. Table 5.3.2 lists the

NO2 asymmetric and symmetric stretching vibrations for the 1,3-DNB crystals and 9-VC co-

crystals with 1,3-DNB.

4000 3500 3000 2500 2000 1500 1000 500

% T

rans

mitt

ance

(A.U

.)

Wavenumbers (cm-1)

CH Bonding

1536 1345

9-VC co-crystals

9-VC

Figure 5.3.3. FTIR spectra for KBr pellets containing either 9-VC crystals (red) or the co-crystals of 9-VC and 1,3-DNB (black).

217

Table 5.3.2. NO2 asymmetric and symmetric stretching vibrations for 1,3-DNB crystals and 9-VC co-crystals.

The co-crystals were not soluble in ethanol or acetonitrile; therefore, the electronic

absorption spectrum in solution was not obtained. As a result, we could not obtain co-crystals

suitable for single crystal X-ray diffraction. Ito et. al. reported similar results with co-crystals of

1,3-DNB and carbazole derivatives.92

The melting ranges were measured for the 9-VC crystals and 9-VC co-crystals with 1,3-

DNB (Table 5.3.3). The liquid formation temperature observed in the co-crystals (79.7 °C) was

higher than the liquid formation temperature for the 9-VC crystals (61.8 °C), but lower than the

liquid formation temperature for the 1,3-DNB crystals (90.9 °C). The co-crystals' observed

melting temperature (formation of meniscus) was higher compared to both the 1,3-DNB crystals

and 9-VC crystals. This broad melting range indicated the presence of inhomogeneities or

individual components. During the melt, the co-crystals produced color changes from yellow to

orange at approximately 80 °C before liquid formation.

Table 5.3.3. 1,3-DNB crystals, 9-VC crystals, and 9-VC co-crystals melting points.

Crystals Melting Point (°C) Lit. Value (°C)

1,3-DNB 90.9 - 91.3 89 54

9-VC 61.8 - 62.9

9-VC co-crystal 79.7 - 116.9


1,3-DNB White-yellow 1540 1347

9-VC White N/A N/A

9-VC + 1,3-DNB Yellow 1536 1345

218

5.4 Carbazole (CBZ) Co-Crystals with 1,3-DNB

Attempts to prepare co-crystals between CBZ and either 2-NT or NB were not successful.

When the solutions of CBZ and 1,3-DNB were combined in a crystallization dish, the solution

did not produce a color change. The color of the solution was similar to the CBZ solution (light

brown). The EtOH was allowed to evaporate for two days, producing small, needle-like crystals

with large flakes as shown in Figure 5.4.1. CBZ crystals were prepared by the same procedure,

producing small, light brown, needle-like crystals with brown flakes. Figure 5.4.1 shows images

of CBZ crystals (A) and CBZ co-crystals (B).

Figure 5.4.1. Crystals of carbazole (A) and co-crystals of carbazole and 1,3-DNB (B).

The 1H NMR spectra of the co-crystals, CBZ crystals, and 1,3-DNB crystals (Figure

5.4.2) were recorded in CDCl3 to determine the ratio between the electron donor and acceptor.

The peak positions, peak multiplicities, and peak integrations are listed in Table 5.4.1. The peaks

observed in the 1H NMR spectrum for the CBZ co-crystals with 1,3-DNB located at 9.06, 8.56,

and 7.79 ppm were assigned to 1,3-DNB incorporated in the co-crystals. The CBZ proton signals

were observed at 8.07-8.04, 7.43-7.38, and 7.24-7.20 ppm. Neither the CBZ nor 1,3-DNB peaks

(A) (B)

219

exhibited a chemical shift in the spectrum. The integration value for the CBZ proton signals

located between 7.24-7.20 ppm in the CBZ and CBZ co-crystal spectra were not an accurate

integration due to the overlapping CDCl3 solvent peak. The integration of the spectrum revealed

an approximate 1.2:1.0 ratio between CBZ and 1,3-DNB.

Figure 5.4.2. 1H NMR spectra of CBZ crystals, CBZ co-crystals with 1,3-DNB, and 1,3-DNB crystals (360 MHz, CDCl3).

6.36.77.17.57.98.38.79.1ppm

1,3-DNB

CBZ co-crystals

CBZ

220

Table 5.4.1. 1H NMR peak positions, splitting patterns, and integration values of 1,3-DNB crystals, CBZ co-crystals, and CBZ crystals.

1,3-DNB CBZ co-crystals CBZ δ S.P. Int. δ S.P. Int. δ S.P. Int.

9.06 t 1.00 9.06 t 1.00 8.57 dd 2.04 8.56 dd 1.94

8.07-8.04 m 3.49 8.07-8.03 m 2.94 7.80 t 1.05 7.79 t 1.07

7.43-7.38 m 4.80 7.43-7.38 m 4.00 7.24-7.20 m 4.50 7.23-7.21 m 3.85


The infrared spectra of the CBZ crystals and the co-crystals are shown in Figure 5.4.3

and Table 5.4.2 lists the NO2 asymmetric and symmetric stretching vibrations for the 1,3-DNB

crystals and CBZ co-crystals. The spectrum for the CBZ co-crystals showed that the NO2

asymmetric stretching mode (1537 cm-1) was red-shifted (3 nm) to lower energy, which indicated

that a weak complex occurred during the formation of the co-crystals. The NO2 symmetric

stretching mode (1347 cm-1) was at the same position as the symmetric stretch in the 1,3-DNB

crystals. Only one of the NO2 stretching vibrations exhibited a shift, suggesting a weak

intermolecular interaction between CBZ and 1,3-DNB.

221

4000 3500 3000 2500 2000 1500 1000 500

% T

rans

mitt

ance

(A.U

.)

Wavenumbers (cm-1)

Free OH

3419NH

1537

1347

CBZ co-crystals

CBZ crystals

Figure 5.4.3. FTIR spectra for KBr pellets containing CBZ crystals (black) and the CBZ co-crystals with 1,3-DNB (red).

Table 5.4.2. NO2 asymmetric and symmetric stretching vibrations for 1,3-DNB crystals and CBZ co-crystals.



CBZ Light brown N/A N/A

CBZ + 1,3-DNB Light brown 1537 1347

222

The electronic absorption spectrum of the CBZ co-crystals with 1,3-DNB was recorded

in acetonitrile to determine if a charge transfer complex formed within the co-crystals. Figure

5.4.4 shows the electronic absorption spectra for the 1,3-DNB crystals, CBZ crystals, and CBZ

co-crystals with 1,3-DNB. The expected weak broad absorption band for a charge transfer

complex was not observed in the spectrum, indicating that a charge transfer complex did not

form. None of the CBZ peaks in the co-crystal spectrum exhibited any significant chemical

shifts. To determine if the CBZ co-crystal spectrum was the result of a charge transfer complex

or just the interaction between CBZ and 1,3-DNB molecules in dilute solutions, the sum of the

1,3-DNB and CBZ electronic absorption spectra were compared with the CBZ co-crystals

spectrum (Figure 5.4.5). The combined spectra closely matched the spectrum for the CBZ co-

crystals. From this result, it was assumed that the spectrum for the co-crystals with a

concentration of 10-5 M was simply the sum of the spectra for free CBZ and free 1,3-DNB

molecules. There was no charge transfer complex formed at that concentration.

223

200 300 400 500 600 700 8000

5000

10000

15000

20000

25000

30000

35000

40000

45000

50000

55000

60000

65000

ε (M

-1 c

m-1)

Wavelength (nm)

2.0 x 10-5 M 1,3-DNB crystals CBZ crystals CBZ co-crystals

Figure 5.4.4. Electronic absorption spectra in acetonitrile for 1,3-DNB crystals (black), CBZ crystals (red), and CBZ co-crystals containing 1,3-DNB and CBZ (blue).

224

200 300 400 500 600 700 8000

10000

20000

30000

40000

50000

60000

ε (M

-1 c

m-1)

Wavelength (nm)

2.0 x 10-5 M 1,3-DNB + CBZ CBZ co-crystals

Figure 5.4.5. Comparison of the electronic absorption spectrum in acetonitrile for the co-crystals containing 1,3-DNB and CBZ (red) and the sum of the spectrum for the 1,3-DNB crystals and the spectrum for the CBZ crystals (black).

225

The melting point ranges were measured for the CBZ crystals and the CBZ co-crystals

(Table 5.4.3). The co-crystals have a broad melting point range, compared with that of the 1,3-

DNB crystals and the CBZ crystals. The observed co-crystals' liquid formation temperature (82.4

°C) was lower than the liquid formation temperatures for the 1,3-DNB crystals (90.9 °C) and the

CBZ crystals (245.5 °C). The observed temperature when the co-crystals completely melted

forming a meniscus (209.3 °C) was higher compared to the 1,3-DNB crystals (91.3 °C), but

lower than the CBZ crystals (249.1°C). This broad melting range indicated the presence of

inhomogeneities or individual components within the co-crystals. During the melt, the CBZ co-

crystals changed from light brown to yellow in color before the first signs of the melt. Above 150

°C, the co-crystals changed color again from yellow to orange.

Table 5.4.3. 1,3-DNB crystals, CBZ crystals, and CBZ co-crystals melting points.

Crystals Melting Point (°C) Lit. Values (°C)

1,3-DNB 90.9 - 91.3 89 54

CBZ 245.5 - 249.1 245 54

CBZ co-crystal 82.4 - 209.3

226

5.5 Phenothiazine (PHZ) Co-Crystals with 1,3-DNB

Attempts to prepare (1:1) co-crystals of PHZ and either 2-NT, 3-NT, or PNT were

unsuccessful. When the two solutions of PHZ and 1,3-DNB were combined in a crystallization

dish, the solution did not produce a color change. The color of the solution was similar to the

PHZ solution (light brown). The EtOH was allowed to evaporate for two days, producing needle-

like crystals with large flakes as shown in Figure 5.5.1 (A). PHZ crystals (Figure 5.5.1 (B)) were

prepared by the same procedure producing small, white-brown, needle-like crystals with flakes.

Figure 5.5.1. Images of PHZ co-crystals (A) and PHZ crystals (B).

1H NMR spectra were recorded of the PHZ crystals and PHZ co-crystals containing 1,3-

DNB in order to determine the ratio between the electron donor and acceptor (Figure 5.5.2).

Table 5.5.1 lists the peak positions, peak multiplicities, and peak integrations. The 1,3-DNB

proton signals in the co-crystals' spectrum were observed at 9.07, 8.57, and 7.79 ppm. The PHZ

aromatic and NH proton signals were located at 7.00-6.95, 6.81, 6.54, and 5.78 ppm. The co-

crystals spectrum integration revealed an approximate 1.3:1.0 ratio between 1,3-DNB and PHZ.

(A) (B)

227

Figure 5.5.2. 1H NMR spectra (360 MHz, CDCl3) of the 1,3-DNB crystals, PHZ crystals, and the co-crystals made from PHZ and 1,3-DNB.

Table 5.5.1. 1H NMR peak positions, splitting patterns, and integration values of 1,3-DNB crystals, PHZ co-crystals, and PHZ crystals.

1,3-DNB PHZ co-crystals PHZ δ S.P. Int. δ S.P. Int. δ S.P. Int.

9.06 t 1.00 9.07 t 1.33 8.57 dd 2.04 8.57 dd 2.67 7.80 t 1.05 7.79 t 1.42

7.00-6.95 m 3.87 6.99-6.96 m 3.96 6.81 td 2.05 6.81 s 1.96 6.54 dd 2.00 6.54 dd 2.00 5.78 s 0.95 5.77 s 1.14


5.56.06.57.07.58.08.59.0ppm

1,3-DNB

PHZ co-crystals

PHZ

228

Figure 5.5.3 shows the infrared spectra for the PHZ crystals and co-crystals made from

PHZ and 1,3-DNB. Table 5.5.2 lists the NO2 asymmetric and symmetric stretching modes for the

1,3-DNB crystals and the PHZ co-crystals. The NO2 asymmetric stretching mode (1539 cm-1)

was red shifted (1 nm) to lower energy in the co-crystal spectrum, which suggests a weak

intermolecular interaction between the electron donor and acceptor. The NO2 symmetric

stretching mode (1347 cm-1) was located at the same position for the 1,3-DNB crystals

symmetric stretching mode.

4000 3500 3000 2500 2000 1500 1000 500

% T

rans

mitt

ance

(A.U

.)

Wavenumbers (cm-1)

PHZ PHZ co-crystal

1539

13473340 NH

Figure 5.5.3. FTIR spectra of KBr pellets containing either PHZ (black) or the co-crystal of PHZ and 1,3-DNB (blue).

229

Table 5.5.2. NO2 asymmetric and symmetric stretching modes for the PHZ co-crystals and 1,3-DNB crystals.

To determine if a charge complex formed within the co-crystals, the electronic absorption

spectrum was recorded in acetonitrile. Figure 5.5.4 shows the electronic absorption spectra for

the 1,3-DNB crystals, PHZ crystals, and the co-crystals. There were no shifts in the peak

positions for the co-crystal spectrum, compared to the spectrum for the PHZ crystals. There was

no observable charge transfer band present in the PHZ co-crystals spectrum. To determine if a

charge complex formed within the co-crystals, the sum of the PHZ crystals and 1,3-DNB crystals

electronic absorption spectra were compared to the co-crystals electronic absorption spectrum

shown in Figure 5.5.5. The two spectra were identical, with no observable differences. The

spectra had similar results as the CBZ co-crystals, which suggested that the PHZ co-crystals

electronic absorption spectrum was primarily the result of free PHZ molecules and 1,3-DNB

molecules in dilute solutions.



PHZ White brown N/A N/A

PHZ co-crystal Light brown 1539 1347

230

200 300 400 500 600 700 8000

10000

20000

30000

40000

50000

60000

ε (M

-1 c

m-1)

Wavelength (nm)

2.0 x 10-5 M 1,3-DNB crystals PHZ crystals PHZ co-crystals

Figure 5.5.4. Electronic absorption spectra recorded in acetonitrile for 1,3-DNB crystals (black), PHZ crystals (red), and the co-crystals containing PHZ and 1,3-DNB (blue).

231

200 300 400 500 600 700 8000

10000

20000

30000

40000

50000

60000

ε(M

-1 c

m-1)

Wavelength (nm)

Sum of 1,3-DNB and PHZ PHZ co-crystals

Figure 5.5.5. Electronic absorption spectra in acetonitrile for the PHZ co-crystals (red) and the sum of the spectra for 1,3-DNB crystals and PHZ crystals (black).

232

The diffuse reflectance spectra for the PHZ crystals and the PHZ co-crystals containing

1,3-DNB are shown in Figure 5.5.6. The spectrum for the PHZ crystals showed a diffuse

reflectance of approximately 80% from the NIR (800 nm) to 500 nm. Below 500 nm, the

reflectance decreased to less than 10% below 400 nm. This was consistent with the light brown

color of the PHZ crystals. The diffuse reflectance for the co-crystals showed a decrease in the

reflectance from the NIR to 400 nm. This difference cannot be explained as simply due to the

innocent presence of 1,3-DNB. The diffuse reflectance spectrum for 1,3-DNB (Figure 5.1.5)

showed a high reflectance greater than 50% through 800 - 500 nm region. Here a new feature

was observed suggesting a strong intermolecular interaction between PHZ and 1,3-DNB.

200 300 400 500 600 700 8000

20

40

60

80

100

% R

efle

ctan

ce

Wavelength (nm)

Phenothiazine crystals Phenothiazine co-crystals

Figure 5.5.6. Diffuse reflectance spectra for PHZ crystals (black) and co-crystals containing PHZ and 1,3-DNB (red).

233

Table 5.5.3 shows the melting ranges for the 1,3-DNB crystals, PHZ crystals, and PHZ

co-crystals. The co-crystals had a broad melting point range, compared to those for the 1,3-DNB

and PHZ crystals. The co-crystals' observed liquid formation temperature (71.4 °C) was lower

than those for the 1,3-DNB crystals (90.9 °C) and the PHZ crystals (186.9 °C). The temperature

at which the co-crystals completely melted and formed a meniscus (145.7 °C) was higher than

the 1,3-DNB crystals' melting temperature (91.3 °C), but lower than the PHZ crystals' melting

temperature (189.4 °C). This broad melting range indicated the presence of inhomogeneities or

individual components within the co-crystals. During the melt, the PHZ co-crystals began

shrinking and changing color from light brown to a dark-red before the first signs of liquid

formation.

Table 5.5.3. Melting points of the 1,3-DNB crystals, PHZ crystals, and PHZ co-crystals.


1,3-DNB 90.9 - 91.3 89 54

PHZ 186.9 - 189.4 184.9 54

PHZ co-crystal 71.4 - 145.7

234

5.6 10-Methylphenothiazine (10-M) Co-Crystals with 1,3-DNB

As with other electron rich aromatic molecules, attempts to prepare co-crystals with 10-

M and 2-NT, 3-NT, and PNT were not successful. However, when the two solutions of 10-M and

1,3-DNB were combined in the crystallization dish, a dark red color rapidly appeared. The EtOH

evaporated for two days, producing reddish-purple crystals as shown in Figure 5.6.1 (B). 10-M

crystals were prepared by the same experimental procedure, which produced white, needle-like

crystals as shown in Figure 5.6.1(A).

Figure 5.6.1. Images of 10-M crystals (A) and 10-M co-crystals with 1,3-DNB (B).

The 1H NMR spectra of the 10-M crystals and co-crystals containing 10-M and 1,3-DNB

were recorded in CDCl3 in order to determine the ratio between the electron donor and acceptor

(Figure 5.6.2). The peak positions, peak multiplicities, and peak integrations are listed in Table

5.6.1. The 1,3-DNB peaks were observed at 9.06, 8.56, and 7.79 ppm. The 10-M peaks were

located at 7.17-7.11, 6.91, 6.80, and 3.36 ppm. The co-crystal spectrum integration revealed an

approximate 1.0:1.1 ratio between 1,3-DNB and 10-M. Neither the 1,3-DNB nor the 10-M

proton signals in the co-crystals spectrum displayed a shift in peak positions.

(A) (B)

235

Figure 5.6.2. 1H NMR spectra of 1,3-DNB crystals, 10-M crystals, and co-crystals containing 10-M and 1,3-DNB (360 MHz, CDCl3).

Table 5.6.1. 1H NMR peak positions, splitting patterns, and integration values of 1,3-DNB crystals, 10-M co-crystals, and 10-M crystals.

1,3-DNB 10-M co-crystals 10-M δ S.P. Int. δ S.P. Int. δ S.P. Int.

9.06 t 1.00 9.06 t 1.00 8.57 dd 2.04 8.56 dd 2.01 7.80 t 1.05 7.79 t 1.07

7.17-7.11 m 4.34 7.18-7.12 m 4.06 6.91 td 2.16 6.93 td 2.09 6.80 d 2.16 6.80 d 2.06 3.36 s 3.43 3.36 s 3.00


3.54.04.55.05.56.06.57.07.58.08.59.0ppm

1,3-DNB

10-M co-crystals

10-M

236

Figure 5.6.3 shows the FTIR spectra for 10-M and the co-crystals. Table 5.6.2 lists the

NO2 asymmetric and symmetric stretching vibrations for the 1,3-DNB crystals and the PHZ co-

crystals. The NO2 asymmetric and symmetric stretching modes were shifted 4 nm and 5 nm to

lower energy in the co-crystals, indicating a strong intermolecular interaction between the

electron donor and acceptor.

4000 3500 3000 2500 2000 1500 1000 500

% T

rans

mitt

ance

(A.U

.)

Wavenumbers (cm-1)

1536 1342

10-M co-crystal

10-M

Figure 5.6.3. Infrared spectra of KBr pellets containing either 10-M crystals (black) or the co-crystals containing 10-M and 1,3-DNB (red).

237

Table 5.6.2. NO2 asymmetric and symmetric stretching vibrations for the 1,3-DNB crystals and 10-M co-crystals.


1,3-DNB white-yellow 1540 1347

10-M White N/A N/A

10-M co-crystals red-purple 1536 1342

The electronic absorption spectra for the 1,3-DNB crystals, 10-M crystals, and the co-

crystals were recorded in acetonitrile (Figure 5.6.4). 10-M showed two peaks, an intense

absorption observed at 254 nm (ε=3.77 x 104 M-1 cm-1) and a weak absorption at 308 nm (ε=5.00

x 104 M-1 cm-1). 1,3-DNB had a single broad absorption at 237 nm (ε=1.80 x 104 M-1 cm-1). The

spectrum for the co-crystals made from 1,3-DNB and 10-M was simply the sum of the spectra

for 1,3-DNB and 10-M (Figure 5.6.5). This indicated that there was no intermolecular interaction

between 1,3-DNB and 10-M in the acetonitrile solution at 2.0 x 10-5 M concentration.

238

200 300 400 500 600 700 8000

5000

10000

15000

20000

25000

30000

35000

40000

45000

50000

55000

ε (M

-1 c

m-1)

Wavelength (nm)

2.0 x 10-5 M 1,3-DNB crystals 10-M crystals 10-M co-crystals

Figure 5.6.4. Electronic absorption spectra for 1,3-DNB crystals (black), 10-M crystals (red), and the co-crystals (blue).

239

200 300 400 500 600 700 8000

10000

20000

30000

40000

50000

ε (M

-1 c

m-1)

Wavelength (nm)

Sum of 1,3-DNB and 10-M 10-M co-crystals

Figure 5.6.5. Electronic absorption spectra for the co-crystals (red) and the sum of the spectra for 1,3-DNB crystals and 10-M crystals (black).

240

The diffuse reflectance spectrum (Figure 5.6.6) for the co-crystals containing 10-M and

1,3-DNB provided strong evidence for intermolecular interactions in the solid state. The

reflectance for the 10-M crystals was greater than 70% from 800 nm to 400 nm. Below 400 nm,

the reflectance dropped to less than 10%. This was consistent with the white color of the 10-M

crystals. The diffuse reflectance spectrum for the co-crystals was dramatically different. The

reflectance was slightly above 40% in the region from 800 nm to 650 nm. Below 650 nm, the

reflectance dropped below 10%. This was consistent with the dark red color of the co-crystals.

200 300 400 500 600 700 8000

20

40

60

80

100

% R

efle

ctan

ce

Wavelength (nm)

10-M crystals

10-M co-crystals

Figure 5.6.6. Diffuse reflectance spectra for 10-M crystals (black) and the co-crystals containing 10-M and 1,3-DNB (red).

241

This also indicated a strong intermolecular interaction between the 10-M and 1,3-DNB in the

solid state.

Table 5.6.3 lists the melting ranges measured for the 10-M crystals and 10-M co-crystals

with 1,3-DNB. The melting range for the co-crystals was significantly lower than the melting

range for the 10-M crystals and the 1,3-DNB crystals. The narrow melting range indicated the

existence of a co-crystal with few inhomogeneities or individual components. During the melt,

the co-crystals did not exhibit any color change.

Table 5.6.3. Melting points of the 1,3-DNB crystals, 10-M crystals, and 10-M co-crystals.


1,3-DNB 90.9 - 91.3 89 54

10-M 101.7 - 104.1 99-100 93

10-M co-crystal 61.7 - 63.4

The co-crystals were suitable for single crystal X-ray diffraction in order to determine the

structure. Steven Kelley obtained and interpreted the X-ray diffraction data. The following is his

interpretation of the structure. The 1:1 co-crystal of 1,3-DNB and 10-M crystallized in the chiral,

orthorhombic space group P212121 with two symmetry-independent formula units (Z = 8). None

of the atoms or molecules reside on special positions. The 1,3-DNB molecules were planar,

except for the nitro groups, which are twisted slightly out-of-plane. There were no statistically

significant differences in bond lengths for the two 1,3-DNB molecules, and the nitro groups on

both molecules had approximately the same orientation relative to the ring. The 10-M molecules

had the typical geometry of phenothiazine and its derivatives, with both of the phenyl rings

joining at an acute angle. The corresponding bond distances and N- and S-centered bond angles

of both 10-M molecules were statistically equivalent to each other and very similar to those in

the reported crystal structure of 10-M.94

242

Figure 5.6.7. 50% probability ellipsoid plot of the asymmetric unit of the co-crystal. The dashed lines indicate distances that were less than the sum of the van der Waals radii.

The short contact environments around the symmetry-independent molecules were

different (Figure 5.6.7). Both 1,3-DNB molecules made short contacts to five 10-M molecules,

but no 1,3-DNB molecules. Both accepted hydrogen bonds through either nitrate group. This

interaction explained the decrease in the peak positions for the asymmetric and symmetric

vibrational modes for the nitro groups in the co-crystals. The major difference was that 1,3-DNB

molecule A formed π-π contacts with the end of a 10-M molecule, while molecule B formed

those contacts with the center.

243

Figure 5.6.8. Short contact environment around 1,3-DNB A (left) and B (right). The green lines indicate distances that were less than the sum of the van der Waals distance.

The 10-M molecules also had different short contact environments (Figures 5.6.8 and

5.6.9). 10-M (A) only interacted with 1,3-DNB molecules through hydrogen bonding to the nitro

groups or inter-ring π-π stacking. The nitrogen and sulfur atoms of 10-M (A) were not involved

in short contacts. Molecule (B) made short contacts to two 10-M molecules as well as four 1,3-

DNB molecules. The 10-M (B) molecules interacted with each other through herringbone-type

C-H---π interactions between the phenyl rings. 10-M (B) did not π-stack with any 1,3-DNB

molecules; instead, it donated hydrogen bonds to nitro groups on two 1,3-DNB molecules and

accepted hydrogen bonds from 1,3-DNB molecules at the N and S atoms.

244

Figure 5.6.9. Short contact environment around 10-M A (left) and B (right). The green lines indicate distances that were less than the sum of the van der Waals contacts.

Infinite hydrogen bonded chains along b, formed by one of the 10-M phenyl rings

donating hydrogen bonds to 1,3-DNB molecules on either side of it, was a major structural

feature (Figure 5.6.10). Each of these chains only involved hydrogen bonds between 1,3-DNB

(A) and 10-M (A) or 1,3-DNB (B) and 10-M (B). The A and B chains were interdigitated with

each other, which allowed for extra hydrogen bonding between the chains as shown in Figure

5.6.10. The molecular recognition, which allowed co-crystallization, may stem from this

hydrogen bonding, as neither 1,3-DNB nor 10-M can form these chains without the other.

Figure 5.6.10. The 1,3-DNB-10-M H-bonded chain along b. The green lines indicate the distances that were less than the sum of the van der Waals contacts.

245

Other supramolecular structures can be described in terms of how the “A” and “B”

chains, the hydrogen bonded chains formed between 1,3-DNB and 10-M (A) respectively,

interacted with each other. Two adjacent A chains formed a dimer through π-π stacking. These

chain dimers interacted with each other through π- π stacking as well forming a 3-D network.

The interactions between these dimers are shown in Figure 5.6.11. The B chains were

interwoven into the A chain network through π- π stacking between 1,3-DNB (B) molecules and

10-M (A) molecules as well as hydrogen bonding of 1,3-DNB (A) molecules to the sulfur atom

on 10-M (B) molecules. The B chains also formed a network with each other through the 10-M-

10-M C-H π contact and hydrogen bonding between 1,3-DNB (B) molecules and the nitrogen

atom on 10-M. Figure 5.6.12 shows the network of A chain dimers network and the packing

down the b axis.

Figure. 5.6.11. View along b axis of π- π stacking interactions between A chains. The green lines indicate the distances that were less than the sum of the van der Waals contacts.

246

Figure 5.6.12. Packing down b axis showing only A chains (left) and all atoms (right). A chains are colored blue in both pictures. B chains are colored red. Crystallographic axes are color coded as a = red, b = green, c = blue.

247

Chapter 6

Conclusions and Future Works

Random copolymers of styrene or methyl methacrylate and the VDAT monomer showed

the potential to sense nitroaromatics by changes produced in the index of refraction after

exposure to a concentrated nitroaromatic vapor. The electron rich structure of VDAT presented a

problem for the solubility of copolymers in a suitable spin coating solvent. This solubility

dilemma limited the synthesis of copolymers with larger concentrations of VDAT due to the

insolubility of PVDAT. Copolymers rich in PVDAT moieties may be capable of producing

larger changes in the index of refraction after exposure to a nitroaromatic vapor, but a polar

solvent with an ideal boiling point with the ability to dissolve these electron rich copolymers

needs to be identified in order to allow the spin coating of homogeneous films. A disadvantage

of VDAT observed when synthesizing copolymers with other electron rich monomers was a

cross-linking effect, making the copolymers insoluble in solvents at or near room temperature.

VDAT appeared to have the ideal structure for sensing nitroaromatics due to its electron rich ring

containing amino functional groups, but these characteristics might be the monomer's downfall

as it limits the synthesis and solubility of polymers that would allow the production of thin films

for the MZI sensor.

The problems associated with the use of the PVDAT copolymers led to the investigations

of other polymers to determine their potential to detect nitroaromatics by changes in the

248

refractive index. Pyridine and imidazole based polymers (P4VP, PVI, and PVI-co-PVA) showed

the ability to sense nitroaromatics by changes in the refractive index after exposure to a

nitroaromatic vapor. These polymers were not soluble in an ideal spin coating solvent, but

homogeneous films were casted by adjusting the spin coating parameters for the dynamic

technique. There was surface roughness observed for these polymer films, which was expected

due to the low boiling point of EtOH. The change in the refractive index after exposure to a

nitroaromatic results attracted interest to synthesize copolymers with pyridine or imidazole

monomers with VDAT. These investigations resulted in brittle hard polymers, which had limited

solubility in spin coating solvents. Due to limited solubility, full characterization of the optical

constants was not possible; however, if suitable solvents were found for casting films of these

copolymers, these films would potentially have an affinity for nitroaromatics based on previous

results.

The interactions between the electron rich polymers and electron deficient nitroaromatics

led to research and the production of co-crystals between electron rich reagents and a

nitroaromatic. Attempts to produce co-crystals between VDAT and nitroaromatics were

unsuccessful due to VDAT solubility in polar organic solvents with high boiling points or H2O at

elevated temperatures. These unsuccessful attempts led to studies using other electron rich

reagents. Co-crystals were produced between 1,3-DNB with 9-VC, 9-EC, and 10-M. The color

changes associated with the formation of the complexes with 1,3-DNB suggested a strong

interaction between the reagents, which was confirmed by FTIR. The nitro groups' asymmetric

and symmetric stretching modes were red shifted to lower energy. The electronic absorption

spectra did not confirm a charge transfer complex in dilute solutions, but rather showed the

interaction between the electron donor molecules and the nitroaromatic molecules in the solution

249

phase. Only the 10-M co-crystal with 1,3-DNB was able to characterized by X-ray diffraction

allowing a crystal structure to be determined. The 10-M molecules interacted with the 1,3-DNB

molecules through hydrogen bonding and π-π contacts. At this time, no crystal structures have

been determined between the carbazole derivatives with 1,3-DNB. This obstacle may be due to

co-crystals' size produced during the slow evaporation process. The strong affinity the electron

donors had for 1,3-DNB suggested they should be considered as sensing materials.

The results for the 10-M co-crystals with 1,3-DNB led to the development of polystyrene

films containing small concentrations of 10-M to determine if the 10-M would interact with the

1,3-DNB vapors to produce a change in the index of refraction for the films. After the maximum

concentration of 10-M that could be included in a polystyrene film was determined, the optical

constants of the polystyrene/10-M film were characterized after long exposure times to 1,3-DNB.

Unexpectedly, the low concentrations of 10-M were capable of producing a change in refractive

index. This result confirmed that 10-M would be an ideal sensing material.

Future investigations should still focus on synthesizing electron rich copolymers with an

affinity for nitroaromatics. Polymers containing carbazole or phenothiazine derivatives should be

considered, since previous results showed the strong affinity these reagents had for 1,3-DNB.

One would expect these polymers to show a significant change in the index of refraction after

exposure to 1,3-DNB if the electron donating properties of reagent are not altered significantly.

These types of polymers should also be investigated as a solution phase nitroaromatic sensor due

to the color changes observed during the growing of the co-crystals.

Another important part of this project that must be considered is the development of

imprinted polymers from the homopolymers and copolymers that exhibited an affinity for

nitroaromatics. The polymer would be imprinted with a specific nitroaromatic, creating a specific

250

cavity for the targeted analyte in the polymer. After the targeted analyte is removed from the

polymer, a specific imprint site will be left where only the targeted analyte can enter the site and

interact at the recognition site. The development of imprinted polymer films would allow the

detection of a specific nitroaromatic and eliminate any false positives. A foreseeable problem

that must be considered is the solubility of VDAT in polar organic solvents. Removing the

majority of the solvent could be problematic for producing a thin imprinted polymer film.

For the last part of this research project, the copolymer films should be applied to a MZI

to determine the sensor's sensitivity. From experimental results in the literature, this will be an

exciting part of the project to see how sensitive the MZI is to changes in the index of refraction

after exposure to a concentrated nitroaromatic vapor. To determine the MZI sensitivity and limit

of detection, a vapor generator will need to be constructed to control the amount of a

nitroaromatic vapor required to determine a certain change in refractive index.

An additional side note for this project might be to evaluate the thermal properties of the

copolymers for heat resistant materials. The TGA characterization of the PS-co-PVDAT

copolymers and PMMA-co-PVDAT copolymers showed a significant increase in the

decomposition temperatures for the copolymers. Even though some of the copolymers were not

characterized by TGA to determine their decomposition temperatures, they may possess high

decomposition temperatures similar to liquid crystal polymers.

251

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259

APPENDIX

Appendix Figure 1. VDAT 1H NMR spectrum (360 MHz, DMSO-d6).

3.23.64.04.44.85.25.66.06.46.87.27.68.0ppm

5.60

5.61

5.63

5.63

6.26

6.29

6.31

6.33

6.37

6.38

6.42

6.42

6.67

D

C, B

A

260

Appendix Figure 2. PS-co-PVDAT 10 mol % VDAT copolymer 1H NMR spectrum (360 MHz, CDCl3).

0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.0ppm

CDCl3

DMSO

ETOH

H2O

AcetoneETOHSi grease

CH2 CH CH2 CH

N

N

N

H2N NH2

m n

261


0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.0ppm

CDCl3

DMSO

Acetone

H2O

Si greaseETOH

ETOH

CH2 CH CH2 CH

N

N

N

H2N NH2

m n

262


0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.0ppm

CDCl3

Acetone

DMSO

H2O

Si grease

CH2 CH CH2 CH

N

N

N

H2N NH2

m n

263

Appendix Figure 5. Polystyrene 1H NMR spectrum (360 MHz, CDCl3).

0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.0ppm

CDCl3

DMSO

Acetone

H2O

Si grease

CH2CH

n

264

Appendix Figure 6. Polystyrene 13C NMR spectrum (500 MHz, CDCl3).

30405060708090100110120130140150160170180ppm

C2

C1

C3

C4, C5

C6

265

Appendix Figure 7. PS-co-PVDAT 1 mol % VDAT copolymer 13C NMR spectrum (500 MHz, CDCl3).

30405060708090100110120130140150160170180ppm

C3

C4, C5

C6

C2

C1

266

Appendix Figure 8. PS-co-PVDAT 5 mol % VDAT copolymer 13C NMR spectrum (500 MHz, CDCl3).

30405060708090100110120130140150160170180ppm

C3

C4, C5

C6

C2

C1

267

Appendix Figure 9. PS-co-PVDAT 10 mol % VDAT 13C NMR spectrum (500 MHz, CDCl3).

30405060708090100110120130140150160170180190ppm

C1

C2

C6

C4, C5

C3C7

268

Appendix Figure 10. VDAT 13C NMR spectrum (500 MHz, DMSO-d6).

405060708090100110120130140150160170ppm

C1C2C3

C4

269

100 200 300 400 500 6000

20

40

60

80

100

Wei

ght L

oss

(% )

Temperature (oC)

Appendix Figure 11. Polystyrene TGA curve.

270

100 200 300 400 500 6000

20

40

60

80

100

Wei

ght L

oss

(%)

Temperature (oC)

Appendix Figure 12. PS-co-PVDAT 1 mol % VDAT copolymer TGA curve.

271

100 200 300 400 500 600

20

40

60

80

100

Wei

ght L

oss

(%)

Temperature (οC)

Appendix Figure 13. PS-co-PVDAT 5 mol % VDAT copolymer TGA curve.

272

100 200 300 400 500 600

20

40

60

80

100

Wei

ght L

oss

(%)

Temperature (οC)

Appendix Figure 14. The PS-co-PVDAT 10 mol % VDAT copolymer TGA curve.

273

Appendix Figure 15. The PS-co-PVDAT 1 mol % VDAT copolymer GPC data.


274



275

4000 3500 3000 2500 2000 1500 1000 5000

20

40

60

80

100

% T

rans

mitt

ance

Wavenumbers (cm-1)

3425 - 3228 NH2

2998 - 2843 CH

1637, 1570 in-plane

1733C=O

1544

C=N

Appendix Figure 19. FTIR spectrum of the PMMA-co-PVDAT 20 mol % VDAT copolymer.

276

4000 3500 3000 2500 2000 1500 1000 5000

10

20

30

40

50

60

70

80

90

100

% T

rans

mitt

ance

Wavenumbers (cm-1)

3425 - 3228 NH2

2997 - 2842 CH

1728C=O

1636, 1570

in-plane 1545

C=N


277

4000 3500 3000 2500 2000 1500 1000 5000

10

20

30

40

50

60

70

80

90

100

% T

rans

mitt

ance

Wavenumbers (cm-1)

3420 NH2

2999 - 2842 CH

1727C=O

1638, 1570in - plane 1543

C=N


278

4000 3500 3000 2500 2000 1500 1000 5000

20

40

60

80

100

% T

rans

mitt

ance

Wavenumbers (cm-1)

NH

3378, 3439

2995 - 2842

CH

1730

C=O

1609, 1569

in - plane

1548

C=N


279

Appendix Figure 23. The 1H NMR spectrum of PMMA in DMSO-d6 using the 500 MHz spectrometer.

-0.50.00.51.01.52.02.53.03.54.04.55.05.56.06.5ppm

TMS

H3

H1

H2

280

Appendix Figure 24. The 1H NMR spectrum for the PMMA-co-PVDAT 1 mol % VDAT copolymer in CDCl3 using the 500 MHz spectrometer.

-0.50.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.5ppm

H4

H3

H1M

DMSO

281

Appendix Figure 25. The 1H NMR spectrum for the PMMA-co-PVDAT 1 mol % VDAT copolymer with the spectrum intensity increased showing the vinyl protons for either MMA or VDAT (6.18, 5.48, and 5.45 ppm) suggesting unreacted monomer present within the polymer matrix.

4.44.85.25.66.06.46.8ppm

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

282

Appendix Figure 26. The 1H NMR spectrum of the PMMA-co-PVDAT 5 mol % VDAT copolymer in CDCl3 using the 500 MHz spectrometer.

-0.50.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.5ppm

DMSO

H4

H3H1M

283

Appendix Figure 27. The 1H NMR spectrum of the PMMA-co-PVDAT 10 mol % VDAT copolymer in CDCl3 using the 500 MHz spectrometer.

0.01.02.03.04.05.06.07.08.09.0ppm

H5

H3, H1V

DMS0

H4

H1M, H2V

284

Appendix Figure 28. The 13C NMR spectrum of PMMA in DMSO-d6 using the 500 MHz spectrometer.

102030405060708090100110120130140150160170180190ppm

C4 C1

C5

C3C2

DMSO-d6

285

Appendix Figure 29. The 13C NMR spectrum of the PMMA-co-PVDAT 10 mol % VDAT copolymer in CDCl3 using the 500 MHz spectrometer.

102030405060708090100110120130140150160170180ppm

C4C9

C1

C5

C3C2

286

Appendix Figure 30. The 13C NMR spectrum of the PMMA-co-PVDAT 5 mol % VDAT copolymer in CDCl3 using the 500 MHz spectrometer.

102030405060708090100110120130140150160170180190ppm

C4C9

C2C3

C1

C5

287

Appendix Figure 31. The 13C NMR spectrum of the PMMA-co-PVDAT 1 mol % VDAT copolymer in DMSO-d6 using the 500 MHz spectrometer.

102030405060708090100110120130140150160170180ppm

C4

C2

C5

C1

C3

288

100 200 300 400 500 6000

20

40

60

80

100

Wei

ght L

oss

(%)

Temperature (oC)

Appendix Figure 32. TGA curve for the PMMA-co-PVDAT 1 mol % VDAT copolymer.

289

100 200 300 400 500 6000

20

40

60

80

100

Wei

ght L

oss

(%)

Temperature (oC)

Appendix Figure 33. The TGA curve for the PMMA-co-PVDAT 5 mol % VDAT copolymer.

290

100 200 300 400 500 6000

20

40

60

80

100

Wei

ght L

oss

(%)

Temperature (oC)

Appendix Figure 34. The TGA curve for the PMMA-co-PVDAT 20 mol % VDAT copolymer.

291

Appendix Figure 35. GPC curve and data for the PMMA-co-PVDAT 20 mol % VDAT copolymer.

292

Appendix Figure 36. The GPC curve and data for the PMMA-co-PVDAT 10 mol % VDAT copolymer.

293

Appendix Figure 37. The GPC curve and data for the PMMA-co-PVDAT 5 mol % VDAT copolymer.

294

Appendix Figure 38. The PMA 1H NMR spectrum (360 MHz, CDCl3).

0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.0ppm

H3

DMSO

H2

H1

295

Appendix Figure 39. The PMA 13C NMR spectrum (500 MHz, DMSO-d6).

2030405060708090100110120130140150160170180ppm

34.2

7

40.7

6

51.5

2

174.

36

C3

C4C2

C1

296

Appendix Figure 40. The P2VP 1H NMR spectrum (360 MHz, CDCl3).

0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.59.0ppm

H1H3

H2H4

DMS0

CH2CH

N

n

H4

H3

H2

H1

297

Appendix Figure 41. The P2VP-co-PVDAT 5 mol % VDAT copolymer 1H NMR spectrum (360 MHz, DMSO-d6).

0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.59.09.5ppm

H1 H3H2

H4, H5

298

Appendix Figure 42. The P2VP-co-PVDAT 1 mol % VDAT copolymer 1H NMR spectrum (360 MHz, DMSO-d6).

0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.5ppm

H1 H3H2

H4, H5

299

Appendix Figure 43. The P2VP 13C NMR spectrum (500 MHz, DMSO-d6).

2030405060708090100110120130140150160170180190ppm

C2C6

C4

C3

C5

CH2CH

N

n

C3

C4

C5

C6

C2

300

Appendix Figure 44. The P2VP-co-PVDAT 5 mol % VDAT copolymer 13C NMR spectrum (500 MHz, DMSO-d6) from 180 - 110 ppm.

115120125130135140145150155160165170175ppm

C2C6

C4 C3 C5

301

Appendix Figure 45. The P2VP-co-PVDAT 20 mol % VDAT copolymer 13C NMR spectrum (500 MHz, DMSO-d6) from 190 - 110 ppm.

110115120125130135140145150155160165170175180185ppm

C8

C2

C6C4

C3

C5

302

Appendix Figure 46. The PAM polymer 1H NMR spectrum (360 MHz, D2O).

0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.0ppm

H2

H1

DMSO

303

Appendix Figure 47. The PAM-co-PVDAT 1 mol % VDAT copolymer 1H NMR spectrum recorded in D2O (360 MHz).

0.00.51.01.52.02.53.03.54.04.55.05.56.06.5ppm

304


0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.0ppm

305


0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.0ppm

306

Appendix Figure 50. The Poly(acrylamide) (PAM) 13C NMR spectrum (500 MHz, D2O).

2030405060708090100110120130140150160170180ppm

C3 C2C1

DMSO

307

Appendix Figure 51. The PAM-co-PVDAT 1 mol % VDAT copolymer 13C NMR spectrum (500 MHz, D2O).

2030405060708090100110120130140150160170180ppm

308

Appendix Figure 52. The PAM-co-PVDAT 5 mol % VDAT copolymer 13C NMR spectrum (500 MHz, D2O).

2030405060708090100110120130140150160170180ppm

309

Appendix Figure 53. The PAM-co-PVDAT 10 mol % VDAT copolymer 13C NMR spectrum (500 MHZ, D2O) from 190 - 150 ppm showing the PMA carbonyl carbon signal (C3) and the PVDAT carbon signal (C7) confirming the presence of PVDAT in the copolymer.

160166172178184190ppm

C3

C7

310

Appendix Figure 54. The 13C NMR spectrum for the PMMA-co-PVK 20 mol % vinylcarbazole recorded in CDCl3 (500 MHz).

102030405060708090100110120130140150160170180ppm

20 mol % PVK

C=O1a 8a

7,25

4,6,3

5a 4a

8 1

(V+M)

10

9

CH (V)

C(M)

311

100 120 140 160 180-2.8

-2.6

-2.4

-2.2

-2.0

-1.8

-1.6

-1.4

Hea

t Flo

w (W

/g)

E

ndot

herm

Temperature (οC)

Appendix Figure 55. The DSC curve for the PVI homopolymer.

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