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Transcript of Research Background
Volume-Phase Transitions in Surface-Tethered Networks and
Implications for Swelling Instabilities
Ryan Toomey
Department of Chemical & Biomedical Engineering
University of South Florida
Tampa, FL 33620
1-D Fibers 2-D Coatings 3-D Structures
5 μm 5 μm
Stimulus
Expanded Collapsed
Volume-Phase Transition
Built-In Actuation, External Cue Generates Macroscopic Response
Mechanical Micro-Switches and Actuators
Sensing Polymer Coatings Adjustable Foundations
“Buckling” Surface Coatings
PRL, 2009
PRL, 2010 10µm 10µm
Tunable Wrinkling
Surface Confined Hydrogels
Langmuir, 2010
Langmuir, 2007
Soft Matter, 2010
Soft Matter, 2013
Soft Matter, 2011
Surface-Attached Gel
Unconstrained Gel
L
L
L
L0
L0
L0
L
L0
L0
5/321 c
o
s NV
V
3/121 c
o
s NV
V
Implications of Constraints
Surface-attachment prevents attainment of
zero stress state
1E-3 0.01 0.1 11
10
100
De
gre
e o
f V
olu
me
tric
Sw
elli
ng
1/Nc
1E-4 1E-3 0.011
2
3
4
5
6
7
8
9
Lin
ear
Sw
ell R
atio
(L
/L0)
1/Nc
Unconstrained
Surface-Attached
Unconstrained
Surface-Attached
Unconstrained and Surface-Attached
Poly(dimethylacrylamide) networks
Volumetric Swelling
Linear Swelling
The surface-attached networks
experience a higher degree of
linear deformation than the
unconstrained networks
5/3
c
o
s NV
V
Vs
Vo
µ Nc
1/3
Toomey et al., Macromolecules (2004)
Can we use the phase diagram of linear polymers to predict the volume-phase transition
behavior in confined geometries?
Transition
Point
Th
ickn
ess
Stimulus
Analog
Digital
Goal: To rationally build intelligence into surface response
0.0 0.2 0.4 0.6 0.8 1.00.0
0.5
1.0
1.5
2.0
Spinodal
Coexistence Curve
2-Phase
1-Phase
T/
Polymer Volume Fraction
Critical Point
Phase Behavior of “Classical” Polymer
“Classical” Brushes
Polystyrene Brush in Cyclohexanone
Karim et al. (1994) Phys. Rev. Lett. 73:3407–10
0.0 0.2 0.4 0.6 0.8 1.0285
290
295
300
305
310
315
320
1-Phase
Polymer Volume Fraction
Coexistence Curve
Spinodal
Tem
pe
ratu
re (
K)
Critical Point
2-Phase
Phase Behavior of “Non-Classical” Polymer
Linear Poly(NIPAAm) Solution
Afroze et al. (2000) J. Mol. Struct. 554:55–68
“Non-Classical” Brushes
Poly(NIPAAm) Brush in D2O
Yim et al. (2005) Phys. Rev. E 72:051801
Phase Behavior of “Non-Classical” Polymer
0.0 0.2 0.4 0.6 0.8 1.0285
290
295
300
305
310
315
320
1-Phase
Polymer Volume Fraction
Coexistence Curve
Spinodal
Tem
pe
ratu
re (
K)
Critical Point
2-Phase
UV Cross-Linkable Copolymer Films
Benzophenone based monomers form
cross-links with free aliphatic groups
365 nm
Statistical copolymers comprising UV-sensitive benzophenone moieties are
deposited and photo-cross-linked: Strategy permits multilayer build-up of
several polymer types
• Activated at 365 nm (non-damaging)
• Provides both cross-linking and surface-attachment
• Cross-links in the presence of oxygen and water
O
R
HC
H
R
C R H
C O
R
C O
H
R
C RC O
H
R
365 nm
h
Photo-Crosslinking Benzophenone
BENZOPHENONE
Reflected
Beam
Neutron Reflection
Heated Jacket
Quartz
D2O
Poly(NIPAAm)
Incident
Beam
Heated Jacket
q q
Neutrons are incident at low angles
Reflection arises due to mismatch between averaged scattering length
densities of atomic nuclei in the direction normal to the interface
Measurement of Reflectivity versus provides “fingerprint” of
density profile with Angstrom scale resolution
Material SLD (x 106)Å2
Quartz
Poly(NIPAAm)
D2O
4.17 0.96 6.33
q sin4zq
• Plot Intensity vs.
• Fringe spacing length
scale of sample
• Solution to the reflectivity
profile is non-unique
• Modeling profile determines
real space interpretation
zq
Neutron Reflection
Poly(NIPAAm-co-MaBP (3%))
Dry Layer Thickness = 320 Å
0.00 0.05 0.10 0.15 0.20 0.25 0.30-7
-6
-5
-4
-3
-2
-1
0
0 100 200 300 4000
1
2
3
4
SLD
(Å
-2)
x 1
06
z (Å)
Quartz Substrate
Lo
g (
R)
qz (Å
-1)
Against Dry Air
at 40 oC
0.00 0.05 0.10 0.15 0.20 0.25 0.30-9
-8
-7
-6
-5
-4
-3
-2
-1
0
0 100 200 300 4000
1
2
3
4
SLD
(Å
-2)
x 1
06
z (Å)
Quartz Substrate
Against D2O Vapor at 40
oC
Lo
g (
R)
qz (Å
-1)
Against Dry Air
at 40 oC
Neutron reflection of dry film
Neutron Reflection Profiles of Poly(NIPAAm-co-MaBP (3%))
Dry Layer Thickness = 320 Å
Scattering length density ~20% lower than expected. Suggests 2-3 water
molecules associated with each polymer segment
0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08
10-14
10-13
10-12
10-11
10-10
10-9
10-8
33oC
29oC
27oC
Re
fle
ctivity*q
4 z
Momentum Vector (Å-1)
15 oC
Best fit NR profiles for temperature range 15oC-33oC
Neutron reflection of wet films
Neutron Reflection Profiles of Poly(NIPAAm-co-MaBP (3%))
Dry Layer Thickness = 270 Å
• Approximately 2-3 D2 O molecules per polymer segment in the collapsed state
• Extended interface between swollen network and D2O
0 500 1000 1500 2000 25000.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
15 oC
22 oC
25 oC
29 oC
31 oC
33 oC
42 oC
Volu
me
Fra
ctio
n
z (Angstroms)
Segment Density Profiles
Extended profile
due to surface buckling
15 20 25 30 35 40 450
200
400
600
800
1000
1200
<Z
> (
Å)
Temperature (oC)
Thickness versus Temperature
• Gradual transition between 15-29 oC followed by strong collapse
“Dry” layer thickness
Prediction of Discontinuity?
0.0 0.2 0.4 0.6 0.8 1.0285
290
295
300
305
310
315
320
1-Phase
Volume Fraction
Binodal (prediction)
Linear poly(NIPAAm) (experimental)
3% cross-linked network
Tem
pera
ture
(K
)
Critical Point
2-Phase
Avidyasagar et al. Macromolecules (2008)
20 25 30 35
1.0
1.5
2.0
2.5
3.0
3.5
4.0
<
Z>
/Zd
ry
Temperature (oC)
Effect of cross-link density
MaBP (10%)
MaBP (3%)
Poly(NIPAAm-co-MaBP (x%))
0.0 0.2 0.4 0.6 0.8 1.0285
290
295
300
305
310
315
320
2-Phase
3% cross-link denisty
10% cross-link density
1-Phase
Volume Fraction
Te
mp
era
ture
(K
)
Critical Point
Effect of cross-linking on phase behavior
Layers prepared to the left of the critical point can enter the 2-phase region, whereas
layers prepared to the right of the critical point remain in the 1-phase region
Effect of cross-linking on phase behavior
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8285
290
295
300
305
310
315
320
10% MaBP5% MaBP3% MaBP
1% MaBP
Volume Fraction
Tem
pera
ture
(K
)
Cloud Curve2-Phase
Repeat of previous experiments: photo-crosslinked samples above demixing
temperature to drive out water
Avidysagar et al. Soft Matter (2009)
Hydrophilic Hydration
Shells
Hydrophobic Hydration
Shells (Ordered Water) Release of ordered water cannot overcome
entropy loss with collapse of network
O NH
Low Temperatures Favor Swelling
Low temperatures favor swelling
O NH
Release of ordered water from hydrophobic shells overcomes
entropy loss with collapse of network
Hydrophilic hydration
shells stay intact?
At higher temperatures hydrophobic forces dominate and drive collapse
High Temperatures Favor Collapse
4000 3000 2000 10000
2
4
6N-D
Amide II
N-H
Amide II
D2O
102OC (Saturated Vapor)
50OC (Liquid)
A
bsorb
ance
Wavenumber(CM-1)
H2O
Is water left in the collapse?
Isotopic substitution of N-H with D2O readily observed in collapsed state
0 10 20 30 40 50 60 70 80
1.0
1.5
2.0
2.5
3.0
3.5
4.0
S
we
llin
g R
atio
(H
w/H
dry)
Temperature(oC)
nPAAm
DEAAm
PNIPAAm
cPAAm
NVIBAAm
Effect of monomer structure
NH O
NH
O
NH O
NH O
10 20 30 40 50 60 70
1.0
1.5
2.0
2.5
3.0
3.5
4.0
poly(NVIBm)
poly(NVIBm)
poly(CPAAm)
poly(CPAAm)
H/H
dry
Temperature (oC)
Cross-linked Films of Poly(cPAAm) and poly(NVIBm)
Poly(NVIBAm) has a discontinuous transition with hysteresis. Poly(CPAAm) has a
continuous transition with no hysteresis. Both cross-linked with 3 % mole MnBP.
Soft Matter, 2013
Cloud Point Curves
0 5 10 15 20 25 30 35 40 45 5010
15
20
25
30
35
40
45
50
Poly(CPAAm)
Poly(NVIBm)
Tem
pera
ture
(oC
)
Polymer Weight %
Poly(NVIBm) has an off-zero critical point, allowing a discontinuous transition.
Poly(cPAAm) has a critical point near zero concentration, preventing a discontinuous
transition.
1700 1650 1600 1550 1500
0.0
0.5
1.0
1.5
2.0
Inte
nsity
Wavenumber (cm-1)
25 oC
40 oC
70 oC
2nd Order Derivarive of 70 oC
1700 1650 1600 1550 1500
0
1
2
Inte
nsity
Wavenumber (cm-1)
FTIR of poly(PVIBm) Coating
NH
O O2H
H2O
(Amide II)
(Amide I)
(Amide I)
(Amide II)
Subbands
1650 1625 1540
Two distinct hydrogen bonding populations are observed at Amide I. The populations
change through the phase transition
20 30 40 50 60 700.30
0.32
0.34
0.36
0.38
0.40
0.42
0.44
0.46
0.48
A1
65
0 / A
tota
l
Temperature oC
Change in area of
1650 sub-band
to total Amide I
1700 1650 1600 1550 1500
0.0
0.5
1.0
1.5
2.0
Inte
nsity
Wavenumber (cm-1)
25 oC
40 oC
70 oC
2nd Order Derivative 70 oC
FTIR of poly(CPAAm) coating
NH O O2H
O2H (Amide II)
(Amide I)
(Amide I) (Amide II)
1630
1540
In contrast to poly(NVIBm), only a single distinct sub-band found for the Amide I.
Phase diagram of (some) linear polymers may
serve to predict (or approximate) volume-phase
transition behavior in confined systems
In the single-phase region, swelling is determined
by a balance between chain elasticity and mixing
If the system enters the two-phase region, the
swelling jumps discontinuously to the polymer rich
binodal of the phase diagram
Summary … Part I
Adjustable Foundations
• Model of biological membranes – Barrier between bulk phases
• Current status – Bilayer on solid substrate easy to deposit but poor biological
mimic
– Bilayer on thick hydrated support difficult to deposit
• Solution: Adjustable cushion that provides a viable
surface for both deposition and natural mimicry
-1
0
1
2
3
4
5
6
7
0 50 100 150 200 250
Neutron Reflectometry Measurements of Polymer / Membrane (37 °C)
10-10
10-9
10-8
0 0.03 0.06 0.09 0.12 0.15
NIPAAm D2O
NIPAAm
Qz (Å-1)
Reflectivity (
R×
Q4)
SLD
(×
10
-6 Å
-2)
Thickness (Å)
Reflectivity Profiles
Scattering Length Density vs
Thickness
Poly(NIPAAm) (37° C)
Poly(NIPAAm) + DPPC (37° C)
Quartz
10-10
10-9
10-8
0 0.025 0.05 0.075
1
2
3
4
5
6
0 200 400 600 800
Neutron Reflectometry Measurements of Polymer / Membrane (25 °C)
NIPAAm D2O
NIPAAm
Qz (Å-1)
Reflectivity (
R×
Q4)
SLD
(×
10
-6 Å
-2)
Thickness (Å)
Reflectivity Profiles
Scattering Length Density vs
Thickness
Poly(NIPAAm) (25° C)
Poly(NIPAAm) + DPPC (25° C)
Quartz
10-9
10-8
0 0.02 0.04 0.06 0.08 0.1
1
2
3
4
5
6
7
0 200 400 600 800
10-10
10-9
10-8
0 0.03 0.06 0.09 0.12 0.15
NIPAAm + DPPC
(before temp. cycle)
NIPAAm + DPPC (after)
Qz (Å-1)
Reflectivity (
R×
Q4)
SL
D (
×1
0-6
Å-2
)
Thickness (Å)
Qz (Å-1)
Re
fle
ctivity (
R×
Q4)
25 °C
32 °C
37 °C
25 °C
32 °C
37 °C
Polymer / Membrane Temperature Dependence
Smith et al. PRL (2009)
0.05 0.10 0.15 0.20 0.25 0.30
0.05
0.10
0.15
0.20
0.25
0.30
1.000E-61.687E-62.847E-64.804E-68.106E-61.368E-52.308E-53.894E-56.571E-51.109E-41.871E-43.157E-45.326E-48.988E-40.0015170.0025590.0043180.0072850.012290.020740.03500
Off-Specular Neutron Scattering
• Probes in-plane structure
• Can indentify 2 length scales
0.005 0.010 0.015 0.020 0.025 0.030
1E-4
1E-3
0.01
0.1
1
0.005 0.010 0.015 0.020 0.025 0.030
1E-4
1E-3
0.01
0.1
1
8 and 18 μm
pf (Å
-1)
pi (Å-1)
pi (Å-1)
Inte
nsity 18 μm
8 μm
Mikhail Zhernenkov (Lujan, Los Alamos National Labs)
Boris Toperverg (Ruhr-University Bochum and Nuclear Physics Institute)
3-D Structure at 25 °C
1000~ScaleLengthplaneofOut
ScaleLengthplaneIn
What is happening?
1. How tightly bound is the lipid bilayer to the poly(NIPAAm) layer?
2. Are concentration fluctuations in the bilayer independent of the
underlying poly(NIPAAm) layer?
3. Or are concentration fluctuations in the poly(NIPAAm) layer
transmitted to the bilayer?
Poly(NIPAAM) coatings were imaged with AFM, but no
discernable out-of-plane structure could be measured,
… however poly(NIPAAm) coatings of 100x thickness
present significant creasing effects
10 m We will deuterate the outer edge of the
poly(NIPAAm) coating to better understand fluctuations
In ultrathin filmsu
1 m 1 m
30 nm thick 180 nm thick
Biaxial stress results in bicusps
Effects of Surface Confinement
3 General Forms of Non-Uniform Swelling
Bulk Buckling
Differential Lateral Swelling
Edge Buckling
High Aspect Ratio Low Aspect Ratio
Bulk Buckling in pNIPAAm Structures
• Buckling occurs when: – compressive stresses overcome the energy required to
bend the structure
DuPont et al. Soft Matter (2010)
Linear Swelling Model
• Developed by Mora et al. (2006) • Describes onset and characteristics of bulk buckling • Linearization of Föppl–von Kármán equations
T. Mora et al. Eur. Phys. J. E (2006)
Buckling Onset: Pc = 0.867 w2/h2
Wavelength: = 3.256h
Model Assumptions
• σ =f(ε) is linear (stress is a linear function of strain) • kc is small (low magnitude strain)
Bulk Buckling in pNIPAAm Structures
Wavelength is only a function of height?
◊ - Dry Height ● - Swollen Height
DuPont et al. Soft Matter (2010)
Differential Lateral Swelling
Large Expansion of Structure’s Surface (ε: 0.1 – 3.5)
Low aspect ratio structures likely best for rapid cell release
50μm
Engineering Goal
• Culture organized μ-tissues of various shapes/sizes • Rapid • Minimize low temperature exposure
Develop Platform for: Organization, Rapid Release, and Direct Stamping of μ-Tissues
Cell Release from pNIPAAm Surfaces
• Thin film pNIPAAm surfaces (<30nm) • Very little geometric deformation
• Mostly normal to the surface
• Mostly hydration/dehydration of pNIPAAm chains
• Release is slow (>10 min)
μm
T<32°C
T>32°C
DuPont et al. Soft Matter, 2010.
• 3D Micro-scale Surface Structures • High geometric deformation
• In all spatial dimensions
• Induces stress in overlying cell sheet
• Lateral swelling of the gel
Types of Organized μ-Tissues μ-Tissue Building Blocks
100μm 1000μm
Closely spaced arrays (4 - 8μm) Wide, isolated, structures (20 - 100μm)
100μm
Aligned Sheets
100μm
50μm
“fiber-like” μ-tissues
“ribbon-like” μ-tissues
Release is Fast
Testing Detachment Mechanism
• Thin film release: requires metabolic activity • Release inhibited by NaN3 (Sodium Azide)
– Inhibits cytochrome c in mitochondria (ATP production)
• Hypothesis: Cell release is mechanical in nature – Surface strain (pNIPAAm structure)
– Strain can be controlled by crosslink density
Sodium Azide • Inhibits ATP production • Diminishes metabolic activity
Y-27632 • Inhibits Rho-associated protein kinases
(ROCKs) • Allows depolymerization of stress fibers • Reduces cell stiffness
Strain Induced Cell Detachment
Control
Sodium Azide (NaN3)
Y-27632 (ROCK inhibitor)
pre-swell
post-swell
pre-swell
post-swell
Cell Release from pNIPAAm Structures
pre-swell post-swell (3 min)
Control
NaN3
(+)
Y-27632 (+)
Surface Strain > 0.3
μ-contact Printing of Aligned μ-tissues
• Strain induced cell detachment allows for rapid direct stamping of μ-tissues
NIH 3T3 fibroblasts transferred to fn treated PDMS (~5 minutes)
NIH 3T3 fibroblasts cultured atop pNIPAAm structures
50μm
μ-contact Printing of Aligned μ-tissues Stamping of various shapes by controlling pNIPAAm structure geometry
100 μm
Cultured Stamp Post-printing Printed μ-microtissue
μ-tissue “Ribbons”
75μm width
Printed μ-microtissue
50 μm
Cultured Stamp
μ-tissue “Strands”
20μm width
μ-contact Printing of Aligned μ-tissues
• Global pattern transfer of cells
500 μm
Post Transfer Alignment
Alignment is Gradually Lost after 4 Hours
58
0 h
ou
rs
4 h
ou
rs
Cell Culture (Summary)
• Alignment of NIH 3T3 fibroblasts on micron-scale pNIPAAm structures
• Ranging from 1 - 30μm thick, 4 - 100μm wide
• Rapid release of aligned μ-tissues • Release occurs in the time scale of swelling (~ 3min) • Temperature reduction minimized (28°C)
• Release of μ -tissues is primarily mechanical in nature • Does not require metabolic activity • Cell contractility effects detachment
• Micron-scale pNIPAAm structures as a platform for direct stamping of μ-tissues
• μ-tissues maintain organization upon transfer
Final Remarks…
• Soft “messy” systems with externally cued responses can show rich range of behavior
• Can we control, understand, and predict “specificity” in response and behavior?
• How do we control differential swelling in confined geometries to be able to produce 3D patterns using standard 2D processing techniques?
Ajay Vidyasagar (Ph.D.) – phase behavior
Leena Patra (Ph.D.) – phase behavior
Samuel DuPont (Ph.D.) – confined structures and cells
Alejandro Castellanos (Ph.D.)
Martiza Muniz (Ph.D.)
Vinicio Carias (Ph.D.)
Ophir Ortiz (Ph.D.)
Gulnur Efe (Ph.D.)
Ryan Cates (M.S.)
Carlos Bello (M.S.)
Prof. Nathan Gallant
USF, Mechanical Engineering
Camille and Henry Dreyfus Foundation
Draper Laboratory
NSF CMMI 107671
NSF CAREER DMR 0645574
NSF EEC 0530444
USF-BITT Center of Excellence
SmartMaterialsResearchGroup