CHAPTER 7 BLENDS OF POLY ( εεεε-CAPROLACTONE) AND ...

Chapter 7 Blends of PCL and Intermediate Molar Mass PS

160

CHAPTER 7

BLENDS OF POLY (εεεε-CAPROLACTONE) AND INTERMEDIATE

MOLAR MASS POLYSTYRENE AS LANGMUIR FILMS AT THE

AIR/WATER INTERFACE

Most of this chapter is reproduced with permission from: Li, B.; Esker, A. R. “Blends of

Poly(ε-caprolactone) and Intermediate Molar Mass Polystyrene as Langmuir Films at the

Air/Water Interface,” Langmuir 2007, 23, 574. Copyright 2007, American Chemical

Society.

7.1. Abstract

Poly(ε-caprolactone)/polystyrene (PCL/PS) blends, where non-amphiphilic PS is

glassy in the bulk state at the experimental temperature of 22.5 °C, are immiscible as

Langmuir films at the air/water (A/W) interface. Surface pressure-area per monomer (Π-

A) isotherm analyses indicate that the surface concentration of amphiphilic PCL is the

only factor influencing the surface pressure below the collapse transition. For PS-rich

blends, Brewster angle microscopy (BAM) studies at the A/W interface and atomic force

microscopy (AFM) studies on Langmuir-Schaefer films reveal that PS nanoparticle

aggregates formed at very low surface pressures can form networks upon further

compression. The morphologies seen in PS-rich blends (networklike rings) are consistent

with a recent study of a non-amphiphilic polyhedral oligomeric silsesquioxane (POSS),

octaisobutyl-POSS, blended with amphiphilic poly(dimethylsiloxane) (PDMS),


161

suggesting that the non-amphiphilic PS aggregates at the A/W interface produce domains

with dipole densities that differ from pure PCL. In all composition regimes, the

amphiphilic PCL phase tends to spread and form a continuous surface layer at the A/W

interface, while simultaneously improving the dispersion of non-amphiphilic PS domains.

During film expansion, BAM images show a gradual change in the surface morphology

from highly continuous networklike structures (PS-rich blends) to broken ringlike

structures (intermediate composition) to small discontinuous aggregates (PCL-rich

blends). This study provides valuable information on the morphological evolution of

semicrystalline PCL-based polymer blends confined in a “two-dimensional” geometry at

the A/W interface and fundamental insight into the influence of microstructure (domain

size, phase-separated structures, crystalline morphology, etc.) on the interfacial properties

of the blends as Langmuir films.

7.2. Introduction

Polyester-based polymer blends have attracted considerable attention for improving

the properties of composite materials because of their potential applications as coatings

and drug delivery systems.177-184 High performance polyester-based polymer blends

exhibit interesting morphologies compared to homopolymers because of the

micro/nanostructures generated during phase separation.185-188 In particular, the

morphological features of binary immiscible polymer blends can be controlled through

the blend's composition and processing conditions.185-193 At extreme volume fractions,

dispersed-droplets of the minor phase within a matrix of the major phase is the normal

morphology. When the volume fraction of the dispersed phase is increased,

interconnected co-continuous biphasic microstructures form for a certain range of

compositions.189, 191 For example, an interconnected porous template of poly(L-lactic


162

acid) (PLLA) can be made by the selective extraction of the poly(ε-caprolactone) (PCL)

component in a co-continuous mixture of PLLA/PCL produced via melt-processing.185

Recent investigations of PLLA/polystyrene (PLLA/PS) blends have also shown that a

biphasic continuous morphology is observed in the composition range of ~ 40 to 80

volume% PS.191 Sarazin et al. also examined the influence of temperature on the co-

continuous morphologies in immiscible PCL/PS blends prepared by melt mixing.189

Their results indicate that the composition ranges for co-continuous microstructures are

50-65 volume% PS at 230 °C and 30 to 70 volume% PS at 155 °C. The mixing

temperature has little effect on the microstructure at lower volume fractions, while

temperature-induced coalescence becomes a dominant factor in the evolution of the

biphasic morphology when the volume fraction of PS is increased above 30 volume%.189

Furthermore, studies on spincoated thin films reveal that specific polymer-substrate

interactions can significantly affect the morphologies and physical properties of blend

materials.8-23, 194-198 Recently, Qi et al. reported the morphological evolution of PCL/PS

spincoated films prepared from tetrahydrofuran (THF).194 The morphological features of

the blend film are governed by the composition ratio at the early stages of annealing,

however; with further annealing the PCL-rich phase always becomes the continuous

phase even if it is the minor component. This observation is attributed to the fact that the

PS tends to segregate and form a dewetted layer on top of PCL because of its lower

surface energy, while PCL tends to wet the glass substrate.

Previous studies on weakly compatible bulk blends of PCL and oligomeric PS

demonstrate an upper critical solution temperature (UCST) type phase diagram with a

critical composition at a PCL composition of ~ 20 volume%.85-89 For this system, the

binodal line could not be precisely defined at temperatures below the melting curve of


163

PCL because the phase separation process is coupled with the crystallization of

semicrystalline PCL. As a result, new types of spherulitic PCL crystals were observed.

These features arose from the competition between nonequilibrium crystallization and

phase separation.

Although many interesting studies on PCL-based blends in bulk or spincoated films

have been reported because of their tailored biocompatibility and biodegradability,75, 76

little is known about the morphologies that develop within a “two dimensional” (2D)

confined dynamic field. In particular, the shear stresses and conformational constraints

placed on blends in Langmuir films during compression could potentially be used to

regulate the surface morphologies of semicrystalline PCL, leading to dramatically

different microstructures at the A/W interface. Furthermore, the morphological features

such as size, size distribution, and the microstructures of the semicrystalline and

amorphous phases are all important for controlling the physical properties of blend films.

Considering the presence of hydrophilic/hydrophobic interfaces in many biological

systems,163 fundamental studies of PCL-based polymer blends performed at the A/W

interface can provide valuable information about the interfacial phase behavior of blends

to further guide their biomedical applications. Only a few published results show that

pure PCL forms uniform Langmuir monolayers at low surface concentrations.24, 26, 75, 163

Upon compression past the “collapse” transition of PCL monolayers, the nucleation and

growth of crystals occurs, making the system particularly suitable for studying surface

pressure induced crystallization and testing models for crystallization kinetics in thin

films.24, 26 However, to the best of our knowledge, the PCL/PS blends in this chapter and

PCL/PtBA blends is Chapters 5 and 6 were the first to examine the interfacial behavior


164

and morphological evolution of semicrystalline PCL-based polymer blends at the A/W

interface.

In this chapter, the compatibility of amorphous intermediate molar mass PS and

semicrystalline PCL mixed Langmuir films at various blend ratios are quantitatively

investigated through surface pressure-area per monomer (Π-A) isotherm studies by the

Wilhelmy plate technique. Hydrophobic PS with very low surface energy does not form

stable monomer-thick films at the air/water (A/W) interface.199-202 Intermediate molar

mass PS samples used in this study refer to PS samples of weight average molar mass

from Mw = 1.56 to at least 217 kg·mol-1 with bulk glass transition temperatures (Tg)

above the experimental temperature of T = 22.5 °C. Evolving biphasic morphologies for

a series of mixed PCL/PS films are simultaneously observed by Brewster angle

microscopy (BAM) during hysteresis experiments. Langmuir-Schaefer (LS) films for

key morphologies are characterized by atomic force microscopy (AFM). At the end of

this chapter, in situ BAM studies of PCL/liquid PS (Mw = 0.74 kg·mol-1) oligomer blends

as Langmuir films are also provided to compare to the morphological features of

PCL/glassy PS blends.

7.3. Results and Discussion

7.3.1. Compression ΠΠΠΠ-A Isotherm Studies of PCL and PS Langmuir Films

Isotherms in Figure 7.1 show plots of surface pressure, Π, as a function of area per

monomer, A, for PS (Mw = 22.2 kg·mol-1) and PCL (Mw = 10 kg·mol-1). The isotherms

were obtained by compression at T = 22.5 °C and a compression rate of 20 cm2·min-1.

The shape of the isotherm for pure PS is consistent with previous reports.24, 26 After

spreading the PS solution on the water surface, PS molecules immediately aggregate and

form irregular islands upon further compression. The BAM micrograph in Figure 7.1A


165

taken at Π ~ 2 mN·m-1 during the compression of pure polystyrene films indicates that PS

films are heterogeneous for all Π at the A/W interface. This observation is reasonable

because polystyrene has no hydrophilic groups for anchoring the chains to the water

surface resulting in the aggregation of presumably glassy PS at the A/W interface.

Furthermore, the extrapolation of the steep portion of the pure PS isotherm back to Π = 0

yields an apparent limiting area of A0 ~ 2 Å2·monomer-1. The value of A0 for PS is

extraordinarily small compared to its molecular structure; consistent with the conclusion

that polystyrene molecules exist at the A/W interface as three-dimensional (3D)

aggregates rather than monomer-thick two-dimensional (2D) monolayers.

C

B

A14

12

10

8

6

4

2

0

Π

Π

Π

Π /mN•m

-1

6040200

A / Å2•monomer

-1

A

B

C

PS:22.2 k

PCL:10 k

Figure 7.1. Π-A isotherms of pure PS and pure PCL obtained by compression at 22.5 °C

and a compression rate of 20 cm2·mim-1. The letters on the Π-A isotherms correspond to

the positions where the BAM images were taken: (A) PS at Π ~ 2 mN·m-1, (B) a

homogeneous PCL monolayer at Π ~ 10 mN·m-1, and (C) PCL crystals in a Langmuir

film at Π ~ 11 mN·m-1.24, 26 All BAM images are 1.2×3.2 mm2 and solidlike domains

appear bright.


166

The isotherm for PCL in Figure 7.1 is in good agreement with those reported in the

literature.24, 26, 163 At the A/W interface, PCL forms a homogeneous liquidlike monolayer

with A0 ~ 52 Å2·monomer-1. BAM studies for the pure PCL sample show the monolayer

is homogeneous for Π < ~10 mN·m-1. On the basis of the static elasticity,

( )TS AA ∂Π∂−== −1κε as discussed in Chapter 4 and its scaling behavior,166, 171, 173 the

A/W interface is a good solvent for PCL and PCL forms a liquid-expanded monolayer.

The amphiphilic nature of PCL molecules arises from carbonyl groups capable of

adsorbing onto the water subphase interspersed with segments composed of five

hydrophobic methylene groups that prevent dissolution of the molecules.24, 26 Previous

studies have shown that nucleation sites form in a supersaturated liquid-expanded phase

at a surface pressure slightly lower than the onset of the collapse transition (ΠC ~ 11

mN·m-1 for PCL with MW = 10 kg·mol-1).24 Following nucleation, the cusp and plateau in

the compression isotherm of a pure PCL monolayer correspond to the transport of PCL

chains from the monolayer to the faces of crystallizing lamellae. Figure 7.1C is a BAM

image taken at Π ~ 11 mN·m-1 during compression past the collapse point, where the

bright domains are the growing PCL crystals. Upon further compression in the plateau

region, the crystals grow larger.24

7.3.2. Compression ΠΠΠΠ-<A> Isotherm Studies of PCL/PS Blends as Langmuir Films

Figure 7.2 provides Π-<A> isotherms of PCL/PS blends as Langmuir films with

decreasing mole fractions of polystyrene (XPS) from XPS = 1.00 (left-most curve a) to XPS

= 0.00 (right-most curve g). <A> in Figure 7.2 represents the average area per repeat unit

(expressed as monomer for short) in the mixed monolayers. One remarkable feature of

these isotherms is that they all have similar shapes to pure PCL, while <A> for each


167

isotherm shows a gradual shift to smaller values for comparable Π as the composition

shifts from pure PCL to pure PS. The qualitative behavior of these isotherms in the

monolayer regime is very different from a recent study with another blend system,

amphiphilic poly(dimethylsiloxane) (PDMS) with a non-amphiphilic polyhedral

oligomeric silsesquioxane (POSS), octaisobutyl-POSS.48(g) At first glance, the

octaisobutyl-POSS/PDMS system should be very similar to the PCL/PS system where the

bulk Tg for PS is above room temperature for several reasons: (1) PS, like octaisobutyl-

POSS, is non-amphiphilic and forms large aggregates at all A values; and (2) PCL, like

PDMS is amphiphilic at the A/W interface. The big difference between the PCL/PS and

octaisobutyl-POSS/PDMS system is the analogous plot to Figure 7.2 in the octaisobutyl-

POSS/PDMS system.48(g) Adding octaisobutyl-POSS to PDMS has no effect on the Π-

<A> isotherm.48(g) PDMS disperses octaisobutyl-POSS in the film, i.e. octaisobutyl-

POSS behaves as a 2D filler. Given the fact that Π-<A> isotherms for PCL/PS blends

shift to smaller <A> with increasing wt% PS and knowing that smaller <A> are not

physically realistic for a monomer-thick film, a different approach from the traditional

analysis in terms of area additivity used in Chapter 5 for PCL/PtBA blends is required.


168

14

12

10

8

6

4

2

0

Π

Π

Π

Π /mN•m

-1

6040200

<A> / Å2•monomer

-1

a

bc d

ef

g

(A)

12

11

10

ΠΠ ΠΠc /m

N•m

-1

1.00.80.60.40.20.0XPS

(B)

Figure 7.2. (A) Π-<A> compression isotherms for various PCL/PS blends obtained at

22.5 °C and a compression rate of 20 cm2·min-1. Letters on this figure correspond to the

isotherms of PCL/PS blends with XPS = (a) 1.00 (pure PS), (b) 0.81, (c) 0.69, (d) 0.65,

(e) 0.34, (f) 0.13, and (g) 0.00 (pure PCL). (B) A ΠC-XPS graph is provided to clarify the

weak composition dependence of the collapse transition for PCL. The error bars of ± 0.2

mN·m-1 indicate the uncertainty of the surface pressure measurements by the Wilhelmy

plate technique with the plate and surface pressure sensor used in this study.


169

14

12

10

8

6

4

2

0

Π

Π

Π

Π /mN•m

-1

100806040200

APCL / Å2•monomer

-1

XPS ~ 0.81

XPS ~ 0.69

XPS ~ 0.65

XPS ~ 0.34

XPS ~ 0.13

PS

PCL

Figure 7.3. Π-APCL compression isotherms for various PCL/PS blends. This plot was

obtained by converting the <A> values from Figure 7.2A to APCL values for the isotherm

of each blend. The pure PS and PCL isotherms correspond to the labeled solid lines.

Figure 7.3 shows a plot of Π-APCL, where APCL is the area per PCL repeating unit

(expressed as monomer for short). A plot of Π-APCL is equivalent to calculating A

assuming that no PS was spread at the A/W interface. As seen in Figure 7.3, by plotting

Π vs. APCL, all of the blend isotherms match pure PCL behavior except in the vicinity of

the collapse transition. Figure 7.3 clearly shows that only PCL is responsible for the

changes in surface tension observed in the monolayer state. The fact that PS has no

quantitative effect on the Π-APCL isotherms in the monolayer state indicates that PS likely

forms a “dewetted layer” on top of PCL. This behavior is comparable to PCL/PS thin

film blends on glass substrates.194 For the PCL/PS system on glass, PCL forms a wetting


170

layer on the glass substrate, while a dewet layer of PS forms on top of PCL because of its

lower surface energy.194 Furthermore, the observation that PS has no quantitative effect

on the Π-APCL isotherms means that PS also has no effect on εs even though PS is glassy

at room temperature (Figure 7.4).

80

60

40

20

0

εε εεs /m

N•m

-1

806040200

<A> / Å2•monomer

-1

80

60

40

20

0

εε εεs/mN•m

-1

1086420Π Π Π Π /mN•m

-1

ThetaSolvent

GoodSolvent

Figure 7.4. εS vs. <A> for PCL/PS mixed Langmuir films with various PS mole

fractions. The inset is a plot of εS as a function of Π for all PCL/PS blends. The two

solid lines in the inset are theoretical curves, εS = zΠ, for good solvent conditions (z =

2.86)44 and the most extreme numerical value reported for theta solvent conditions (z =

101).46 The symbols correspond to XPS = 1.00 (�), 0.81 (�), 0.69 (�), 0.65 (▼), 0.34

(▲), 0.13(�), and 0.00 (�). All blends show behavior that is identical to the single-

component PCL film.


171

Even though PS has no effect on the monolayer state, PCL/PS blends do show small

variations in the vicinity of the film's collapse transition as seen in Figures 7.2 and 7.3.

At Π > ~10 mN·m-1, all isotherms of the mixed films show qualitatively similar collapse

behavior followed by a short plateau regime like pure PCL. Previous studies have shown

that ΠC ~11 mN·m-1 is a characteristic feature of a pure PCL (Mw = 10 kg·mol-1)

monolayer at T = 22.5 °C and corresponds to the nucleation and growth of PCL crystals

in Langmuir films.24, 26 Figure 7.2B shows a plot of the onset pressure for ΠC as a

function of XPS at a compression rate of 20 cm2·min-1. A weak composition-dependence

of ΠC for PCL/PS blend films is observed. At XPS < ~ 0.34, the dispersed PS aggregates

could provide more sites for heterogeneous nucleation of PCL in the blend films,

resulting in a slightly smaller ΠC. With further increases in XPS from 0.34 to 0.81, the

onset of the collapse transition increases to ΠC ~12 mN·m-1. However, the metastable

regime, 9 < Π < 12 mN·m-1, still falls in the crystallization window for pure PCL

Langmuir films.24, 26 This fact indicates that the collapse transition for PCL in the blend

films is not dramatically changed by adding PS, even though the nucleation rate and

mechanism (homogeneous vs. heterogeneous) may differ from that of pure PCL. The

above discussion has focused on how PS affects the thermodynamic properties of PCL at

the A/W interface. To further understand the phase behavior of PCL/PS blends, the

morphologies of mixed Langmuir films also need to be considered.


172

7.3.3. Morphological Studies of PCL/PS Blends as Langmuir Films During

Compression

In the previous section, Π-APCL isotherm studies indicated that the addition of non-

amphiphilic PS to PCL had no dramatic effects on the isotherms. In this section, BAM is

used to examine the in situ morphologies of this blend system.

Figure 7.5 shows representative BAM images for various PCL/PS blend compositions

obtained at constant Π values of 4 and 8 mN·m-1 during compression at 20 cm2·min-1.

Figure 7.5A shows the large solidlike aggregates of pure PS formed at the A/W interface.

In contrast, BAM images of PCL, the film-forming component in this blend system, show

uniform liquidlike films at both 4 and 8 mN·m-1 (Figure 7.5L and M). The addition of a

small amount of PCL to PS (XPS ~ 0.81) dramatically changes the surface morphologies

relative to pure PS as seen in Figure 7.5B and C. The morphologies in these images

appear to contain smaller aggregates with more regular sizes. At Π ~ 4 mN·m-1 for the

XPS ~ 0.81 blend (Figure 7.5B), the homogeneously distributed PS aggregates are

observed to coexist with some larger PS domains. Compressing this blend film to Π ~ 8

mN·m-1 leads to a higher density of the aggregates (Figure 7.5C). By systematically

decreasing the PS content from XPS ~ 0.81 to ~ 0.34, BAM images show similar

morphological features, however; the uniformity of the aggregates increases and their

density decreases as shown in Figure 7.5D through I. When the content of PS decreases

to XPS ~ 0.13 or lower, the surface morphologies become more homogeneous as seen in

Figure 7.5J and K.


173

4 mN•m-1 8 mN•m-1

B C

APure PS

~ 0.81

Pure PCL

~ 0.65

~ 0.34

~ 0.13

~ 0.69D E

F G

H I

J K

L M

Figure 7.5. BAM images obtained at Π = 4 and 8 mN·m-1 during compression at a

compression rate of 20 cm2·min-1 and a temperature of 22.5 °C for different PCL/PS

blends (XPS, Π /mN·m-1): (A) (1.00, 2), (B) (0.81, 4), (C) (0.81, 8), (D) (0.69, 4), (E)

(0.69, 8), (F) (0.65, 4), (G) (0.65, 8), (H) (0.34, 4), (I) (0.34, 8), (J) (0.13, 4), (K) (0.13,

8), (L) (0.00, 4), and (M) (0.00, 8). Solidlike aggregates appear bright in all of the

1.2×3.2 mm2 BAM images.


174

On the basis of the BAM images in Figure 7.5, the immiscible PCL/PS blends that

form at the A/W interface before compression, are comprised of strongly hydrophobic PS

domains where the PS is most likely in a solidlike state. In order to better understand the

morphology of the PS aggregates formed upon compression, AFM was used to image

LS-films transferred on silicon substrates for two representative PCL/PS blends (XPS ~

0.69 and XPS ~ 0.13). The silicon substrates were first covered with a thin layer of

spincoated PS to enhance the adhesion of transferred PS aggregates to the solid substrate.

Figure 7.6A and A′ show typical AFM images of a single layer film of PS-rich blends

(XPS ~ 0.69) transferred at Π ~ 2 mN·m-1 by the LS method. The characteristic feature of

these AFM images is that the ring-like structures of PS aggregates are present at very low

Π, where pure PCL films still exist in a homogeneous liquid-expanded monolayer state.

Figure 7.6A′, the corresponding 1×1 µm2 image, reveals that the ringlike structures are

actually composed of PS nanospheres. Upon further compression to higher Π as seen in

Figure 7.6B and C, the ring-like structures become denser. Furthermore, the AFM

images indicate that the size distribution of PS nanospheres is unaltered during

compression as seen in Figure 7.6A′, B′ and C′, suggesting that the PS nanospheres

formed at lower Π are stable under these experimental conditions and that further

coalescence is presumably inhibited by the presence of PCL. These studies clearly

indicate that adding even a small amount of PCL can dramatically alter the surface

morphology of PS by dispersing PS into nanosphere aggregates for PS-rich blends that

are resistant to further aggregation. AFM phase images corresponding to Figure 7.6 are

shown in Figure 7.7.


175

A

E

5××××5 µµµµm2 1××××1 µµµµm2

1××××1 µµµµm2

5××××5 µµµµm2

5××××5 µµµµm2

1××××1 µµµµm2

A’

B B’

C C’

Figure 7.6. AFM height images for LS-films of a representative blend, XPS ~ 0.69.

Single layer LS-films were transferred onto PS coated silicon substrates at Π ~ 2 mN·m-1:

(A) 5×5 µm2 and (A′) 1×1 µm2; Π ~ 8 mN·m-1: (B) 5×5 µm2 and (B′) 1×1 µm2; and

Π ~11 mN·m-1: (C) 5×5 µm2 and (C′) 1×1 µm2. 5×5 µm2 images are provided as they are

more easily compared to BAM images at similar Π. 1×1 µm2 images clarify how particle

sizes and size distributions are essentially independent of Π. The z-scale in all of the

AFM images are 0-60 nm.


176

A A’

B B’

C C’

Figure 7.7. AFM height (left) and phase (right) images of LS-films for a representative

PCL/PS blend, XPS ~ 0.69. Single layer LS-films were transferred onto PS coated silicon

substrates at Π ~ 2 mN·m-1 (A, A′), Π ~8 mN·m-1 (B, B′), and Π ~11 mN·m-1 (C, C′).

5×5 µm2 images are provided as they are more easily compared to BAM images at

similar Π. The z-scales in the height images are 0-60 nm and the z-scales in phase

images are 0-60 deg.


177

In contrast, the dispersed nanosphere aggregates of PS were not observed for the PCL-

rich blend, XPS ~ 0.13, as seen in Figure 7.8 and AFM images with even smaller scan

ranges (not shown). Figure 7.8A shows a BAM image taken at Π ~ 2 mN·m-1,

corresponding to the AFM images in Figure 7.8A′ and A′′ (phase images are provided in

Figure 7.9). Figure 7.8A′ and 7.8A′′ clearly indicate that ringlike PS aggregates form at

low Π, even though the corresponding BAM image is almost homogeneous in the

monolayer regime. Upon compression past ΠC for the PCL monolayer, the nucleation

and growth of PCL crystals becomes a characteristic feature for PCL-rich blends. The

BAM image in Figure 7.8B was taken at Π ~ 11 mN·m-1 upon compression past ΠC.

Bright domains are growing PCL crystals, while the darker areas around bright domains

are composed of PS aggregates as seen in Figure 7.8B′ and B′′. In comparison with the

A′ and A′′ images in Figure 7.8, the B′ and B′′ images show that PS aggregates become

denser upon compression to higher Π, i.e. a higher PS surface concentration. Figure

7.8B′ also shows the coexistence of PS aggregates with PCL lamellae (right-hand side of

Figure 7.8B′). AFM phase images corresponding to Figure 7.8 are shown in Figure 7.9.

PCL lamellae grown in PCL-rich blends are shown in Figure 7.10. Cross-section

analyses of PCL lamellae yield a lamellar thickness of ~ 8 nm. This value is comparable

to the literature value for PCL crystallized at the A/W interface.24, 26


178

B’’B

A A’ A’’

B’

Figure 7.8. In situ BAM images and AFM height images of single layer LS-films for a

representative blend of XPS ~ 0.13 highlighting PS-rich domains. Individual images

correspond to Π ~ 2 mN·m-1: (A) 1.2×3.2 mm2 BAM image, (A′) 10×10 µm2 AFM height

image and (A′′) 5×5 µm2 AFM height image; and Π ~ 11 mN·m-1: (B) 1.2×3.2 mm2

BAM image, (B′) 10×10 µm2 AFM height image and (B′′) 5×5 µm2 height AFM image.

Bright features in (B) and the features on the right-hand side of (B') represent PCL

crystals. The z-scales for all of the AFM images are 0-60 nm.


179

B B’

A A’

Figure 7.9. AFM height (left) and phase (right) images of LS-films for a representative

PCL/PS blend, XPS ~ 0.13. Single layer LS-films were transferred onto PS coated silicon

substrates at Π ~ 2 mN·m-1 (A, A′) and Π ~11 mN·m-1 (B, B′). 5×5 µm2 images are

provided as they are more easily compared to BAM images at similar Π. Z-scales: (A, B)

0-60 nm, (A′) 0-40 deg, and (B′) 0-60 deg.


180

25 nm

-25

0

10 µµµµm

A

B C

10××××10 µµµµm2 10××××10 µµµµm2

Vertical Distance ~ 8.322 nm

Figure 7.10. AFM images and a cross-section analysis of a PCL crystal for a LS-film

prepared from a XPS ~ 0.13 PCL/PS blend. Single layer LS-films were transferred onto

PS coated silicon substrates at Π ~ 11 mN·m-1 (above ΠC). (A) Cross-section analysis

yielding a lamellar thickness of ~ 8 nm. (B) 10×10 µm2 height image (Z-scale: 0-60 nm)

and (C) 10×10 µm2 phase image (Z-scale: 0-40 deg).


181

A

B

C

D

E

F

G

H

14

12

10

8

6

4

2

0

Π

Π

Π

Π /mN•m

-1

2520151050

<A> / Å2•monomer

-1

AB

C

D

E

FG H

Figure 7.11. BAM images for a XPS ~ 0.81 PCL/PS blend film obtained at 22.5 °C and

an expansion rate of 20 cm2·min-1. The letters on the isotherm indicate where the BAM

images were taken during the hysteresis experiments and correspond to (<A>

/Å2·monomer-1, Π /mN·m-1) for compression: A (22.6, 0), B (12.9, 0.7), C (8.3, 2.9), and

D (5.8, 6.9); and expansion: E (5.1, 5.3), F (10.1,1.2), G (17,0.3), and H (25.5, 0).

Solidlike domains appear bright in all of the 1.2×3.2 mm2 BAM images.

7.3.4. BAM Studies for Hysteresis Experiments

In the preceding paragraphs, representative BAM and AFM images obtained during

compression experiments show that PCL disperses PS into nanoscale aggregates.

Hysteresis experiments provide further insight into the aggregation process for PCL/PS

blends at the A/W interface. Figures 7.11 through 7.14 show four representative

hysteresis loops (1st cycle) for different PCL/PS blends as Langmuir films. Figure 7.11

shows a hysteresis loop for a XPS ~ 0.81 PCL/PS blend, where PS is the major

component. The morphologies of this blend during compression (Figure 7.11A through


182

D) are clearly different from pure PS (Figure 7.1A). Upon expansion of the compressed

films, the surface density of the PS aggregates decreases with increasing <A>.

Meanwhile, the aggregates self-assemble into networklike structures as seen in Figure

7.11F through H. The “mesh sizes” of the networks increase with increasing <A> during

expansion. Furthermore, the networklike structures remain even at Π ~ 0 mN·m-1 (Figure

7.11H).

The surface morphologies observed for XPS ~ 0.65 during compression (Figure 7.12A

through D) are very similar to those for XPS ~ 0.81. However, the BAM images taken

during expansion show that some of the networklike morphologies break-up (Figure

7.12E through H) during expansion, indicating that increasing the PCL content may

weaken dipolar interactions responsible for networklike structures of PS aggregates. The

ringlike morphological features are very similar to those reported for blends of PDMS

and octaisobutyl-POSS at the A/W interface.48(g) In that study, the authors claim that the

formation of ringlike structures in the PDMS/octaisobutyl-POSS system at compositions

between ≅ 40 ~ 70 wt % POSS was attributed to a nonequilibrium coexistence of a

liquidlike PDMS-rich film with solidlike POSS aggregates. The similarity in

morphology between the PDMS/octaisobutyl-POSS and PCL/PS systems most likely

arises from the presence of a hydrophobic non-amphiphilic component in both blends

(octaisobutyl-POSS and PS).48(g) Another feature to note for the hysteresis of the XPS ~

0.65 blend, is that the area inside the hysteresis loop is much larger than for the XPS ~

0.81 PCL/PS blend.


183

B

C

D

E

F

G

H

A

14

12

10

8

6

4

2

0

Π

Π

Π

Π /mN•m

-1

403020100

<A> / Å2•monomer

-1

AB

C

D

E

F

G H




/Å2·monomer-1, Π /mN·m-1) for compression: A (23.7, 1.0), B (17.6, 2.5), C (10.6, 8.3)

and D (7.0, 10.8); and expansion: E (7.5, 5.0), F (15.8, 2.6), G (28.7, 0.6), and H (35.5,

0.3). Solidlike domains appear bright in all of the 1.2×3.2 mm2 BAM images.

Figure 7.13 (XPS ~ 0.34) and Figure 7.14 (XPS ~ 0.13) provide representative

morphologies for blends where PCL is the major phase. The nucleation and growth of

PCL crystals in the “homogeneously heterogeneous monolayer” were clearly observed

upon compression above ΠC as shown in Figure 7.13D for XPS ~ 0.34 and Figure 7.14D

for XPS ~ 0.13 (bright objects). It is clear from Figures 7.11 through 7.14 that the stability

of the network structures decreases with increasing PCL content in the blend. As seen in

Figure 7.14E, XPS ~ 0.13, the networklike structures break apart immediately upon

expansion. In comparison to the blends with higher PS contents, only discontinuous

smaller aggregates with weak BAM contrast are observed when the barriers start to open


184

during the expansion process. It is also worth noting that the area inside the hysteresis

loop systematically increases with increasing PCL content from the smallest area for the

XPS ~ 0.81 (Figure 7.11) to the largest area for the XPS ~ 0.13 blend (Figure 7.14). The

increasing area and changing shape of the hysteresis loops with increasing PCL content is

attributed to the crystallization of PCL in blends where PCL is the major component. For

PCL-rich blends, once PCL crystallizes, it is necessary for PCL to “melt” before it can re-

spread. The plateau in the expansion isotherms has been identified as the “melting”

transition for PCL crystals that form during compression.24, 26 For PS rich blends, the

near absence of a plateau in the compression isotherms means that PS inhibits the

nucleation and growth of PCL crystals, even though other aspects of the Π-<A> isotherm

are completely controlled by PCL for all of the blends.


185

B

C

A

D

E

F

G

H

14

12

10

8

6

4

2

0

Π

Π

Π

Π /mN•m

-1

50403020100

<A> / Å2•monomer

-1

A

B

C

D

EF

GH




/Å2·monomer-1, Π /mN·m-1) for compression: A (47.8, 0.7), B (27.3, 4), C (19,8.4), and D

(10, 10.8); and expansion: E (10.2, 4.9), F (22.9, 3.6), G (31, 2.0), and H (46, 0.6).



186

A

B

C

D

E

F

G

H

12

10

8

6

4

2

0

Π

Π

Π

Π /mN•m

-1

6040200

<A> / Å2•monomer

-1

A

BC

D

EF

G

H




/Å2·monomer-1, Π /mN·m-1) for compression: A (43.7, 2.3), B (20, 10.5), C (13, 10.7),

and D (10.8, 10.7); and expansion: E (10, 5.5), F (18.1, 4.8), G (29, 3.9) and H (49, 1.3).



187

Additional features of these hysteresis loops are further illustrated in Figure 7.15, where

the compression isotherms are represented by dotted lines and solid lines highlight the

expansion isotherms. In Figure 7.15A, the arrow inside the figure indicates the starting

and ending points of each individual hysteresis loop as well as the composition (XPS).

All blend films were compressed to sufficiently small <A> values to ensure that all films

have gone through the collapse transition within the limitation of the compression ratio

for the Langmuir trough. The expansion isotherm for pure PCL is analogous to a

"melting" process. The surface pressure corresponding to the plateau in the expansion

isotherm is characteristic of the “melting pressure” of PCL crystals. As seen in Figure

7.15A, there is a definite change in the expansion isotherm with a change in blend

composition. On the basis of the observation that the surface activity of the blends is

controlled by the PCL component Π-APCL plots were constructed. Figure 7.15B clearly

shows that the expansion isotherms for blends of XPS ~ 0.13 and ~ 0.34 are consistent

with pure PCL where the plateau corresponds to the melting of PCL crystals formed

during compression. In contrast, the hysteresis loops for binary blends where PS is the

major component have smaller areas without clearly observable plateaus. As expected,

the surface tension and surface area of the blend films during expansion are only

controlled by the surface-active PCL component, even though there are dramatic

morphological changes when non-amphiphilic PS is blended with PCL.


188

A

B

14

12

10

8

6

4

2

0

Π

Π

Π

Π /mN•m

-1

6040200

<A> / Å2• monomer

-1

PS

0.81

0.65

0.34

0.13

PCL

14

12

10

8

6

4

2

0

Π

Π

Π

Π /mN•m

-1

100806040200

APCL / Å2• monomer

-1

0.81

0.65

PCL

0.130.34

Figure 7.15. (A) Π-<A> hysteresis loops (1st cycle) for PCL/PS blends and pure PCL

obtained at 22.5 °C and a compression rate of 20 cm2·min-1. The numbers with arrows

inside the figure indicate the XPS value of each blend as well as the starting and ending

points of each hysteresis loop. (B) Π-APCL hysteresis loops for different PCL/PS blends

and pure PCL. The numbers with arrows indicate the XPS value of each blend and the

corresponding expansion isotherm. The compression isotherms are represented by dotted

lines and solid lines highlight the expansion isotherms in both (A) and (B).


189

14

12

10

8

6

4

2

0

Π

Π

Π

Π /mN•m

-1

403020100

<A> / Å2•monomer

-1

XPS ~ 0.65

1st cycle 2nd cycle 3rd cycle

Figure 7.16. Multiple Π-<A> hysteresis loops for a XPS ~ 0.65 PCL/PS blend obtained

at 22.5 °C and a compression rate of 20 cm2·min-1. The 1st hysteresis loop is represented

by solid line, a dotted line indicates the 2nd cycle, and the dashed line shows the 3rd cycle.

Furthermore, during subsequent hysteresis loops (2nd and 3rd), there does not appear to

be any significant alteration of PS aggregation by BAM. Nonetheless, PCL molecules

must form some long-lived three-dimensional structures at the end of the 1st compression

step as subsequent hysteresis loops progressively shift towards smaller <A> with each

cycle (Figure 7.16), much like pure PCL.24, 26

In short, the surface morphologies of PCL/PS blends show composition dependent

changes even if all of the isotherms can be collapsed down to a single Π-APCL curve for

the initial compression step in the monolayer regime as done in Figure 7.3. To better

compare the effects of morphological changes with increasing PS content, representative


190

BAM images obtained during expansion at constant Π values for all blend films are

shown in Figure 7.17. For pure PS (not shown here), the surface pressure immediately

drops to zero upon expansion. The large aggregates (Figure 7.5A) do not re-spread

during expansion, indicating irreversible aggregation behavior that is consistent with the

absence of hydrophilic groups for PS. In contrast to PS, the surface pressure in

expansion isotherms of PCL decreases slowly and shows larger hysteresis.24, 26 The

BAM images of pure PCL show that the PCL crystals grown during compression become

smaller and smaller with decreasing surface concentration during expansion and finally

disappear at lower Π. This behavior indicates that the compressed single-component

PCL films are able to melt and re-spread upon expansion as seen in Figure 7.17M

through O. In Figure 7.17, the BAM images of the PCL/PS blends clearly show that

there is a gradual change in surface morphology from highly continuous networklike

structures (PS-major component) to broken ringlike structures (intermediate composition)

to small discontinuous aggregates (PCL-major component).


191

~0.34

Pure PCL

~0.81

~0.65

~0.13

A B C

D E F

G H I

J K L

M N O

Figure 7.17. BAM images obtained during expansion experiments at an expansion rate

of 20 cm2·min-1 and a temperature of 22.5 °C for different PCL/PS blends and pure PCL

(XPS, Π /mN·m-1): (A) (0.81, 5), (B) (0.81, 4), (C) (0.81, 0), (D) (0.65, 5), (E) (0.65, 4),

(F) (0.65, 0.4), (G) (0.34, 5), (H) (0.34, 4), (I) (0.34, 0.6), (J) (0.13, 5), (K) (0.13, 4), (L)

(0.13, 0.4), (M) (0, 5), (N) (0, 4), and (O) (0, 1.1). Solidlike domains appear bright in all



192

Furthermore, similar morphological features were also observed for PS (Mw ~ 1.56

kg·mol-1, Mw ~ 64.4 kg·mol-1, and Mw ~ 217 kg·mol-1) blends with PCL as seen in Figure

7.18 through 7.23. This similarity leads to a more general conclusion that PS of

intermediate molar mass from Mw ~ 1.56 to at least 217 kg·mol-1 (glassy in the bulk state

at 22.5 °C) will exhibit surface behavior at the A/W interface that is similar to the results

presented here for PCL/PS blends (PS Mw = 22.2 kg·mol-1). The similarity in

networklike morphology seen here for PCL/PS binary polymer blends also strongly

supports the interpretation provided for the PDMS/octaisobutyl-POSS system.48(g) This

observation also reinforces the conclusion drawn on the basis of isotherm studies that

both PS and PCL show independent surface behavior, even though the weak hydrophobic

cohesive forces at PCL/PS domain boundaries help disperse the PS aggregates and inhibit

further coalescence.


193

A

B

C

D

E

F

G

H

14

12

10

8

6

4

2

0

Π

Π

Π

Π /mN•m

-1

3020100

<A> / Å2•monomer

-1

A

B

C

D

E

F

G H

Compression

Expansion

Figure 7.18. BAM images for a XPS ~ 0.81 PCL (Mw = 10 kg·mol-1)/PS (Mw = 1.56

kg·mol-1, Mw/Mn = 1.06) blend film obtained at 22.5 °C and an expansion rate of 20

cm2·min-1. The letters on the isotherm indicate where the BAM images were taken during

the hysteresis experiments and correspond to the average surface area (<A>

/Å2·monomer-1) for compression: A (28.8), B (8), C (5.8), and D (4.7); and expansion: E

(4.8), F (7.9), G (17.7), and H (25). Solidlike domains appear bright in all of the 2.4×3.2

mm2 BAM images.


194

A

B

C

D

E

F

G

H

14

12

10

8

6

4

2

0

Π

Π

Π

Π /mN•m

-1

806040200

<A> / Å2•monomer

-1

ABCD

E

FG

H

Compression

Expansion





/Å2·monomer-1) for compression: A (~18.7), B (~12), C (~10.4), and D (~9.8); and

expansion: E (~9.6), F (~16.3), G (~35), and H (~64). Solidlike domains appear bright in

all of the 2.4×3.2 mm2 BAM images.


195

14

12

10

8

6

4

2

0

Π

Π

Π

Π /mN•m

-1

3020100

<A> / Å2•monomer

-1

Compression

Expansion AB

C

D

E

F G H

A

B

C

D

E

F

G

H





/Å2·monomer-1) for compression: A (34.4), B (10.8), C (7), and D (5.1); and expansion: E

(6.2), F (14.6), G (19.4), and H (25). Solidlike domains appear bright in all of the 2.4×3.2

mm2 BAM images.


196

14

12

10

8

6

4

2

0

Π

Π

Π

Π /mN•m

-1

6040200

<A> / Å2•monomer

-1

Compression

Expansion

AB

CD

E

FG

H

A

B

C

D

E

F

G

H





/Å2·monomer-1) for compression: A (~22.2), B (~15.1), C (~11), and D (~9); and

expansion: E (~12.2), F (~22.3), G (~30.9), and H (~57.6). Solidlike domains appear

bright in all of the 2.4×3.2 mm2 BAM images.


197

14

12

10

8

6

4

2

0

Π

Π

Π

Π /mN•m

-1

3020100

<A> / Å2•monomer

-1

Compression

Expansion

A

B

C

D

E

FG H

A

B

C

D

E

F

G

H

Figure 7.22. BAM images for a XPS ~ 0.81 PCL (Mw = 10 kg·mol-1)/PS (Mw = 217




/Å2·monomer-1) for compression: A (21.3), B (9.7), C (7.2), and D (6.2); and expansion:

E (6.4), F (10.7), G (18.6), and H (25.5). Solidlike domains appear bright in all of the



198

14

12

10

8

6

4

2

0

Π

Π

Π

Π /mN•m

-1

806040200

<A> / Å2•monomer

-1

Compression

Expansion

ABCD

EF

GH

A

B

C

D

E

F

G

H

Figure 7.23. BAM images for a XPS ~ 0.13 PCL (Mw = 10 kg·mol-1)/PS (Mw = 217




/Å2·monomer-1) for compression: A (~16.3), B (~12.5), C (~8.6), and D (~7.9); and

expansion: E (~10.4), F (~16.5), G (~38), and H (~49). Solidlike domains appear bright

in all of the 2.4×3.2 mm2 BAM images.


199

In picking the title for this chapter, care was taken to restrict the lower bound for

defining “intermediate” molar mass to ensure that PS was in the glassy state at T = 22.5

°C. The reason for this is that PCL blends with a PS oligomer (Mw = 0.74 kg·mol-1) that

is in the liquid state at T = 22.5 °C exhibit dramatically different phase behavior. BAM

images obtained from two representative blends as Langmuir films are provided in

Figures 7.24 and 7.25 to highlight some of these differences. In contrast to the

PCL/glassy PS blends, the PCL/liquid PS oligomer blends may exhibit interesting

features consistent with phase separation by “nucleation and growth” and “spinodal”

decomposition mechanisms.

For strongly PS-rich blends, where PCL crystallization was inhibited, BAM images

captured during dynamic compression reveal a homogeneous surface morphology at very

low Π (Figure 7.24A). Further compression of the PS-rich blend film causes the films to

turn “cloudy” at intermediate Π values as seen in Figure 7.24B and C, heterogeneously

homogeneous surface morphologies with fine structures are observed. If the film is

compressed a bit more, the features coarsen and small droplets, presumably

corresponding to phase separation by a nucleation and growth mechanism, are observed

as seen in Figure 7.24D, E, and F. At even higher Π, the droplet morphology gives way

to elongated, curved domains that are surprisingly similar to what one may expect for

phase separation by spinodal decomposition (Figure 7.24G through I).


200

A B C

D E F

G H I

12

8

4

0

Π

Π

Π

Π /mN•m

-1

2520151050

<A> /Å2•monomer

-1

XPS ~ 0.89

AB

CD E

FG

HI

Figure 7.24. BAM images obtained at 22.5 °C and a compression rate of 20 cm2·min-1

during the 1st compression step for a PCL/PS blend with XPS ~ 0.89. The letters on the

Π–<A> compression isotherm indicate where the individual BAM images were taken and

correspond to (Image, <A>/Å2·monomer-1): (A, 25), (B, 6.4), (C, 4.9), (D, 4.6), (E, 4.4),

(F, 4.2), (G, 4.1), (H, 3.8), and (I, 3.6). All BAM images are 3.2 × 2.4 mm2 in size.


201

A

E F

B C

D

G H

12

8

4

0

Π

Π

Π

Π /mN•m

-1

6040200

<A> /Å2•monomer

-1

XPS ~ 0.22

A

BCD

EF

G

H

Figure 7.25. BAM images obtained at 22.5 °C and a compression rate of 20 cm2·min-1

during the 1st compression step for a PCL/PS blend with XPS ~ 0.22. The letters on the

Π–<A> compression isotherm indicate where the individual BAM images were taken and

correspond to (Image, <A>/Å2·monomer-1): (A, 41), (B, 16), (C, 11.8), (D, 10.5), (E, 9.8),

(F, 9.2), (G, 8.9), and (H, 8.6). All BAM images are 3.2 × 2.4 mm2 in size. Bright

domains in Figure D→H are PCL crystallites.


202

While Figure 7.24 provided morphological data for a PS-rich blend of PCL/liquid PS

oligomer, Figure 7.25 provides representative data for a PCL-rich blend (XPS ~ 0.22) of

the same blend system. In the XPS ~ 0.22 blend, the films do not turn cloudy until Π is

much larger (Figure 7.25A through D). Unlike the PS-rich blend, droplets formed in the

PCL-rich blend may have much lower optical contrast (Figure 7.25E and F). At even

higher Π, it is clear that domains are no longer droplets (Figure 7.25G and H). One other

interesting feature of Figure 7.25 is the very bright spots that signify the nucleation and

growth of some PCL crystals.

At this stage, Figures 7.24 and 7.25 are only provided as proof that liquid PS

oligomers have very different properties in blends with PCL than higher molar mass PS.

While it is tempting to claim that the PCL/liquid PS oligomer system exhibits Π-induced

liquid-liquid phase separation, a complete understanding of the observed behavior is still

lacking. As such further speculation is saved for Chapter 9, where some suggestions for

future work are made.

7.4. Conclusions

This study demonstrates surface morphologies of immiscible intermediate molar mass

(glassy) PS blends with PCL as Langmuir films at the A/W interface. Π-<A> isotherm

studies indicate that the surface activity of the blend films is only controlled by the

amphiphilic PCL component. The addition of glassy PS to PCL has no effect on the Π-A

isotherm of PCL in the monolayer state if one ignores the PS component. Instead,

solidlike nanoscale PS aggregates tend to exist on top of a liquidlike PCL film. The

better dispersed PS aggregates for all the blends apparently arise from more favorable PS

wetting of the PCL layer than the water surface. BAM studies show that PCL-rich blends

exhibit nucleation and growth of PCL crystals upon collapse of the monolayer, whereas


203

PS inhibits PCL crystallization when PS is the major component. In PS-rich blends, the

composition-dependent morphological features show PS aggregates can self-assemble

over longer length scales. The networklike morphologies observed for PS-rich blends

show similarity to morphological features observed in blends of amphiphilic PDMS and

non-amphiphilic octaisobutyl-POSS,48(g) and blends of amphiphilic trisilanolisobutyl-

POSS with amphiphilic PDMS, where non-amphiphilic trisilanolisobutyl-POSS may

form non-amphiphilic hydrophobic dimers upon collapse.48(c) Hence, it appears that

networklike morphologies of hydrophobic nanoscale aggregates are a common feature

when non-amphiphilic materials (POSS and PS) are blended with amphiphilic materials

that form liquidlike monolayers (PDMS and PCL) at the A/W interface.

CHAPTER 7 BLENDS OF POLY ( εεεε-CAPROLACTONE) AND ...

Documents

Transcript of CHAPTER 7 BLENDS OF POLY ( εεεε-CAPROLACTONE) AND ...