CHAPTER 7 MOLAR MASS POLYSTYRENE AS LANGMUIR FILMS AT THE AIR/WATER INTERFACE

Transcription

1 HPTR 7 LNDS OF POLY (ε-prolton) ND INTRMDIT MOLR MSS POLYSTYRN S LNGMUIR FILMS T TH IR/WTR INTRF Most of this chapter is reproduced with permission from: Li,.; sker,. R. lends of Poly(ε-caprolactone) and Intermediate Molar Mass Polystyrene as Langmuir Films at the ir/water Interface, Langmuir 27, 23, 574. opyright 27, merican hemical Society bstract Poly(ε-caprolactone)/polystyrene (PL/PS) blends, where non-amphiphilic PS is glassy in the bulk state at the experimental temperature of 22.5, are immiscible as Langmuir films at the air/water (/W) interface. Surface pressure-area per monomer (Π- ) isotherm analyses indicate that the surface concentration of amphiphilic PL is the only factor influencing the surface pressure below the collapse transition. For PS-rich blends, rewster angle microscopy (M) studies at the /W interface and atomic force microscopy (FM) 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), 16

2 suggesting that the non-amphiphilic PS aggregates at the /W interface produce domains with dipole densities that differ from pure PL. In all composition regimes, the amphiphilic PL phase tends to spread and form a continuous surface layer at the /W interface, while simultaneously improving the dispersion of non-amphiphilic PS domains. During film expansion, M 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 (PL-rich blends). This study provides valuable information on the morphological evolution of semicrystalline PL-based polymer blends confined in a two-dimensional geometry at the /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 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 High performance polyester-based polymer blends exhibit interesting morphologies compared to homopolymers because of the micro/nanostructures generated during phase separation In particular, the morphological features of binary immiscible polymer blends can be controlled through the blend's composition and processing conditions t 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 189, 191 compositions. For example, an interconnected porous template of poly(l-lactic 161

3 acid) (PLL) can be made by the selective extraction of the poly(ε-caprolactone) (PL) component in a co-continuous mixture of PLL/PL produced via melt-processing. 185 Recent investigations of PLL/polystyrene (PLL/PS) blends have also shown that a biphasic continuous morphology is observed in the composition range of ~ 4 to 8 volume% PS. 191 Sarazin et al. also examined the influence of temperature on the cocontinuous morphologies in immiscible PL/PS blends prepared by melt mixing. 189 Their results indicate that the composition ranges for co-continuous microstructures are 5-65 volume% PS at 23 and 3 to 7 volume% PS at 155. 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 3 volume%. 189 Furthermore, studies on spincoated thin films reveal that specific polymer-substrate interactions can significantly affect the morphologies and physical properties of blend 8-23, materials. Recently, Qi et al. reported the morphological evolution of PL/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 PL-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 PL because of its lower surface energy, while PL tends to wet the glass substrate. Previous studies on weakly compatible bulk blends of PL and oligomeric PS demonstrate an upper critical solution temperature (UST) type phase diagram with a critical composition at a PL composition of ~ 2 volume% For this system, the binodal line could not be precisely defined at temperatures below the melting curve of 162

4 PL because the phase separation process is coupled with the crystallization of semicrystalline PL. s a result, new types of spherulitic PL crystals were observed. These features arose from the competition between nonequilibrium crystallization and phase separation. lthough many interesting studies on PL-based blends in bulk or spincoated films 75, 76 have been reported because of their tailored biocompatibility and biodegradability, 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 PL, leading to dramatically different microstructures at the /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. onsidering the presence of hydrophilic/hydrophobic interfaces in many biological systems, 163 fundamental studies of PL-based polymer blends performed at the /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 24, 26, 75, 163 pure PL forms uniform Langmuir monolayers at low surface concentrations. Upon compression past the collapse transition of PL 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 24, 26 films. However, to the best of our knowledge, the PL/PS blends in this chapter and PL/Pt blends is hapters 5 and 6 were the first to examine the interfacial behavior 163

5 and morphological evolution of semicrystalline PL-based polymer blends at the /W interface. In this chapter, the compatibility of amorphous intermediate molar mass PS and semicrystalline PL mixed Langmuir films at various blend ratios are quantitatively investigated through surface pressure-area per monomer (Π-) 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 (/W) interface Intermediate molar mass PS samples used in this study refer to PS samples of weight average molar mass from M w = 1.56 to at least 217 kg mol -1 with bulk glass transition temperatures (T g ) above the experimental temperature of T = volving biphasic morphologies for a series of mixed PL/PS films are simultaneously observed by rewster angle microscopy (M) during hysteresis experiments. Langmuir-Schaefer (LS) films for key morphologies are characterized by atomic force microscopy (FM). t the end of this chapter, in situ M studies of PL/liquid PS (M w =.74 kg mol -1 ) oligomer blends as Langmuir films are also provided to compare to the morphological features of PL/glassy PS blends Results and Discussion ompression Π- Isotherm Studies of PL and PS Langmuir Films Isotherms in Figure 7.1 show plots of surface pressure, Π, as a function of area per monomer,, for PS (M w = 22.2 kg mol -1 ) and PL (M w = 1 kg mol -1 ). The isotherms were obtained by compression at T = 22.5 and a compression rate of 2 cm 2 min , 26 The shape of the isotherm for pure PS is consistent with previous reports. fter spreading the PS solution on the water surface, PS molecules immediately aggregate and form irregular islands upon further compression. The M micrograph in Figure

6 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 /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 /W interface. Furthermore, the extrapolation of the steep portion of the pure PS isotherm back to Π = yields an apparent limiting area of ~ 2 Å 2 monomer -1. The value of for PS is extraordinarily small compared to its molecular structure; consistent with the conclusion that polystyrene molecules exist at the /W interface as three-dimensional (3D) aggregates rather than monomer-thick two-dimensional (2D) monolayers. Π /mn m PL:1 k PS:22.2 k / Å 2 monomer -1 Figure 7.1. Π- isotherms of pure PS and pure PL obtained by compression at 22.5 and a compression rate of 2 cm 2 mim -1. The letters on the Π- isotherms correspond to the positions where the M images were taken: () PS at Π ~ 2 mn m -1, () a homogeneous PL monolayer at Π ~ 1 mn m -1, and () PL crystals in a Langmuir film at Π ~ 11 mn m -1 24, 26. ll M images are mm 2 and solidlike domains appear bright. 165

7 The isotherm for PL in Figure 7.1 is in good agreement with those reported in the 24, 26, 163 literature. t the /W interface, PL forms a homogeneous liquidlike monolayer with ~ 52 Å 2 monomer -1. M studies for the pure PL sample show the monolayer is homogeneous for Π < ~1 mn m -1. On the basis of the static elasticity, ε S = κ 1 = ( Π ) T as discussed in hapter 4 and its scaling behavior, 166, 171, 173 the /W interface is a good solvent for PL and PL forms a liquid-expanded monolayer. The amphiphilic nature of PL molecules arises from carbonyl groups capable of adsorbing onto the water subphase interspersed with segments composed of five 24, 26 hydrophobic methylene groups that prevent dissolution of the molecules. 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 (Π ~ 11 mn m -1 for PL with M W = 1 kg mol -1 ). 24 Following nucleation, the cusp and plateau in the compression isotherm of a pure PL monolayer correspond to the transport of PL chains from the monolayer to the faces of crystallizing lamellae. Figure 7.1 is a M image taken at Π ~ 11 mn m -1 during compression past the collapse point, where the bright domains are the growing PL crystals. Upon further compression in the plateau region, the crystals grow larger ompression Π-<> Isotherm Studies of PL/PS lends as Langmuir Films Figure 7.2 provides Π-<> isotherms of PL/PS blends as Langmuir films with decreasing mole fractions of polystyrene (X PS ) from X PS = 1. (left-most curve a) to X PS =. (right-most curve g). <> 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 PL, while <> for each 166

8 isotherm shows a gradual shift to smaller values for comparable Π as the composition shifts from pure PL 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) t first glance, the octaisobutyl-poss/pdms system should be very similar to the PL/PS system where the bulk T g for PS is above room temperature for several reasons: (1) PS, like octaisobutyl- POSS, is non-amphiphilic and forms large aggregates at all values; and (2) PL, like PDMS is amphiphilic at the /W interface. The big difference between the PL/PS and octaisobutyl-poss/pdms system is the analogous plot to Figure 7.2 in the octaisobutyl- POSS/PDMS system. 48(g) dding octaisobutyl-poss to PDMS has no effect on the Π- <> isotherm. 48(g) PDMS disperses octaisobutyl-poss in the film, i.e. octaisobutyl- POSS behaves as a 2D filler. Given the fact that Π-<> isotherms for PL/PS blends shift to smaller <> with increasing wt% PS and knowing that smaller <> are not physically realistic for a monomer-thick film, a different approach from the traditional analysis in terms of area additivity used in hapter 5 for PL/Pt blends is required. 167

9 Π /mn m a b 2 g c d e f 2 4 () 6 <> / Å 2 monomer -1 Π c /mn m ().8 1. X PS Figure 7.2. () Π-<> compression isotherms for various PL/PS blends obtained at 22.5 and a compression rate of 2 cm 2 min -1. Letters on this figure correspond to the isotherms of PL/PS blends with X PS = (a) 1. (pure PS), (b).81, (c).69, (d).65, (e).34, (f).13, and (g). (pure PL). () Π -X PS graph is provided to clarify the weak composition dependence of the collapse transition for PL. The error bars of ±.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. 168

10 Π /mn m PL PS X PS ~.81 X PS ~.69 X PS ~.65 X PS ~.34 X PS ~ PL / Å 2 monomer -1 Figure 7.3. Π- PL compression isotherms for various PL/PS blends. This plot was obtained by converting the <> values from Figure 7.2 to PL values for the isotherm of each blend. The pure PS and PL isotherms correspond to the labeled solid lines. Figure 7.3 shows a plot of Π- PL, where PL is the area per PL repeating unit (expressed as monomer for short). plot of Π- PL is equivalent to calculating assuming that no PS was spread at the /W interface. s seen in Figure 7.3, by plotting Π vs. PL, all of the blend isotherms match pure PL behavior except in the vicinity of the collapse transition. Figure 7.3 clearly shows that only PL is responsible for the changes in surface tension observed in the monolayer state. The fact that PS has no quantitative effect on the Π- PL isotherms in the monolayer state indicates that PS likely forms a dewetted layer on top of PL. This behavior is comparable to PL/PS thin film blends on glass substrates. 194 For the PL/PS system on glass, PL forms a wetting 169

11 layer on the glass substrate, while a dewet layer of PS forms on top of PL because of its lower surface energy. 194 Furthermore, the observation that PS has no quantitative effect on the Π- PL isotherms means that PS also has no effect on ε s even though PS is glassy at room temperature (Figure 7.4). ε s /mn m ε s /mn m Theta Solvent Good Solvent Π /mn m <> / Å 2 monomer -1 Figure 7.4. ε S vs. <> for PL/PS mixed Langmuir films with various PS mole fractions. The inset is a plot of ε S as a function of Π for all PL/PS blends. The two solid lines in the inset are theoretical curves, ε S = zπ, for good solvent conditions (z = 2.86) 44 and the most extreme numerical value reported for theta solvent conditions (z = 11). 46 The symbols correspond to X PS = 1. ( ),.81 ( ),.69 ( ),.65 ( ),.34 ( ),.13( ), and. ( ). ll blends show behavior that is identical to the singlecomponent PL film. 17

12 ven though PS has no effect on the monolayer state, PL/PS blends do show small variations in the vicinity of the film's collapse transition as seen in Figures 7.2 and 7.3. t Π > ~1 mn m -1, all isotherms of the mixed films show qualitatively similar collapse behavior followed by a short plateau regime like pure PL. Previous studies have shown that Π ~11 mn m -1 is a characteristic feature of a pure PL (M w = 1 kg mol -1 ) monolayer at T = 22.5 and corresponds to the nucleation and growth of PL crystals 24, 26 in Langmuir films. Figure 7.2 shows a plot of the onset pressure for Π as a function of X PS at a compression rate of 2 cm 2 min -1. weak composition-dependence of Π for PL/PS blend films is observed. t X PS < ~.34, the dispersed PS aggregates could provide more sites for heterogeneous nucleation of PL in the blend films, resulting in a slightly smaller Π. With further increases in X PS from.34 to.81, the onset of the collapse transition increases to Π ~12 mn m -1. However, the metastable regime, 9 < Π < 12 mn m -1, still falls in the crystallization window for pure PL 24, 26 Langmuir films. This fact indicates that the collapse transition for PL 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 PL. The above discussion has focused on how PS affects the thermodynamic properties of PL at the /W interface. To further understand the phase behavior of PL/PS blends, the morphologies of mixed Langmuir films also need to be considered. 171

13 Morphological Studies of PL/PS lends as Langmuir Films During ompression In the previous section, Π- PL isotherm studies indicated that the addition of nonamphiphilic PS to PL had no dramatic effects on the isotherms. In this section, M is used to examine the in situ morphologies of this blend system. Figure 7.5 shows representative M images for various PL/PS blend compositions obtained at constant Π values of 4 and 8 mn m -1 during compression at 2 cm 2 min -1. Figure 7.5 shows the large solidlike aggregates of pure PS formed at the /W interface. In contrast, M images of PL, 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 PL to PS (X PS ~.81) dramatically changes the surface morphologies relative to pure PS as seen in Figure 7.5 and. The morphologies in these images appear to contain smaller aggregates with more regular sizes. t Π ~ 4 mn m -1 for the X PS ~.81 blend (Figure 7.5), the homogeneously distributed PS aggregates are observed to coexist with some larger PS domains. ompressing this blend film to Π ~ 8 mn m -1 leads to a higher density of the aggregates (Figure 7.5). y systematically decreasing the PS content from X PS ~.81 to ~.34, M 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 X PS ~.13 or lower, the surface morphologies become more homogeneous as seen in Figure 7.5J and K. 172

14 Pure PS ~.81 ~.69 ~.65 ~.34 ~.13 Pure PL D F H J L G I K M 4 mn m -1 8 mn m -1 Figure 7.5. M images obtained at Π = 4 and 8 mn m -1 during compression at a compression rate of 2 cm 2 min -1 and a temperature of 22.5 for different PL/PS blends (X PS, Π /mn m -1 ): () (1., 2), () (.81, 4), () (.81, 8), (D) (.69, 4), () (.69, 8), (F) (.65, 4), (G) (.65, 8), (H) (.34, 4), (I) (.34, 8), (J) (.13, 4), (K) (.13, 8), (L) (., 4), and (M) (., 8). Solidlike aggregates appear bright in all of the mm 2 M images. 173

15 On the basis of the M images in Figure 7.5, the immiscible PL/PS blends that form at the /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, FM was used to image LS-films transferred on silicon substrates for two representative PL/PS blends (X PS ~.69 and X PS ~.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 7.6 and show typical FM images of a single layer film of PS-rich blends (X PS ~.69) transferred at Π ~ 2 mn m -1 by the LS method. The characteristic feature of these FM images is that the ring-like structures of PS aggregates are present at very low Π, where pure PL films still exist in a homogeneous liquid-expanded monolayer state. Figure 7.6, the corresponding 1 1 µm 2 image, reveals that the ringlike structures are actually composed of PS nanospheres. Upon further compression to higher Π as seen in Figure 7.6 and, the ring-like structures become denser. Furthermore, the FM images indicate that the size distribution of PS nanospheres is unaltered during compression as seen in Figure 7.6, and, 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 PL. These studies clearly indicate that adding even a small amount of PL can dramatically alter the surface morphology of PS by dispersing PS into nanosphere aggregates for PS-rich blends that are resistant to further aggregation. FM phase images corresponding to Figure 7.6 are shown in Figure

16 5 5 µm µm µm µm µm µm 2 Figure 7.6. FM height images for LS-films of a representative blend, X PS ~.69. Single layer LS-films were transferred onto PS coated silicon substrates at Π ~ 2 mn m -1 : () 5 5 µm 2 and ( ) 1 1 µm 2 ; Π ~ 8 mn m -1 : () 5 5 µm 2 and ( ) 1 1 µm 2 ; and Π ~11 mn m -1 : () 5 5 µm 2 and ( ) 1 1 µm µm 2 images are provided as they are more easily compared to M images at similar Π. 1 1 µm 2 images clarify how particle sizes and size distributions are essentially independent of Π. The z-scale in all of the FM images are -6 nm. 175

17 Figure 7.7. FM height (left) and phase (right) images of LS-films for a representative PL/PS blend, X PS ~.69. Single layer LS-films were transferred onto PS coated silicon substrates at Π ~ 2 mn m -1 (, ), Π ~8 mn m -1 (, ), and Π ~11 mn m -1 (, ). 5 5 µm 2 images are provided as they are more easily compared to M images at similar Π. The z-scales in the height images are -6 nm and the z-scales in phase images are -6 deg. 176

18 In contrast, the dispersed nanosphere aggregates of PS were not observed for the PLrich blend, X PS ~.13, as seen in Figure 7.8 and FM images with even smaller scan ranges (not shown). Figure 7.8 shows a M image taken at Π ~ 2 mn m -1, corresponding to the FM images in Figure 7.8 and (phase images are provided in Figure 7.9). Figure 7.8 and 7.8 clearly indicate that ringlike PS aggregates form at low Π, even though the corresponding M image is almost homogeneous in the monolayer regime. Upon compression past Π for the PL monolayer, the nucleation and growth of PL crystals becomes a characteristic feature for PL-rich blends. The M image in Figure 7.8 was taken at Π ~ 11 mn m -1 upon compression past Π. right domains are growing PL crystals, while the darker areas around bright domains are composed of PS aggregates as seen in Figure 7.8 and. In comparison with the and images in Figure 7.8, the and images show that PS aggregates become denser upon compression to higher Π, i.e. a higher PS surface concentration. Figure 7.8 also shows the coexistence of PS aggregates with PL lamellae (right-hand side of Figure 7.8 ). FM phase images corresponding to Figure 7.8 are shown in Figure 7.9. PL lamellae grown in PL-rich blends are shown in Figure 7.1. ross-section analyses of PL lamellae yield a lamellar thickness of ~ 8 nm. This value is comparable 24, 26 to the literature value for PL crystallized at the /W interface. 177

19 Figure 7.8. In situ M images and FM height images of single layer LS-films for a representative blend of X PS ~.13 highlighting PS-rich domains. Individual images correspond to Π ~ 2 mn m -1 : () mm 2 M image, ( ) 1 1 µm 2 FM height image and ( ) 5 5 µm 2 FM height image; and Π ~ 11 mn m -1 : () mm 2 M image, ( ) 1 1 µm 2 FM height image and ( ) 5 5 µm 2 height FM image. right features in () and the features on the right-hand side of (') represent PL crystals. The z-scales for all of the FM images are -6 nm. 178

20 Figure 7.9. FM height (left) and phase (right) images of LS-films for a representative PL/PS blend, X PS ~.13. Single layer LS-films were transferred onto PS coated silicon substrates at Π ~ 2 mn m -1 (, ) and Π ~11 mn m -1 (, ). 5 5 µm 2 images are provided as they are more easily compared to M images at similar Π. Z-scales: (, ) -6 nm, ( ) -4 deg, and ( ) -6 deg. 179

21 25nm Vertical Distance ~ nm µm 1 1 µm µm 2 Figure 7.1. FM images and a cross-section analysis of a PL crystal for a LS-film prepared from a X PS ~.13 PL/PS blend. Single layer LS-films were transferred onto PS coated silicon substrates at Π ~ 11 mn m -1 (above Π ). () ross-section analysis yielding a lamellar thickness of ~ 8 nm. () 1 1 µm 2 height image (Z-scale: -6 nm) and () 1 1 µm 2 phase image (Z-scale: -4 deg). 18

22 14 12 Π /mn m D F G H <> / Å 2 monomer -1 D F G H Figure M images for a X PS ~.81 PL/PS blend film obtained at 22.5 and an expansion rate of 2 cm 2 min -1. The letters on the isotherm indicate where the M images were taken during the hysteresis experiments and correspond to (<> /Å 2 monomer -1, Π /mn m -1 ) for compression: (22.6, ), (12.9,.7), (8.3, 2.9), and D (5.8, 6.9); and expansion: (5.1, 5.3), F (1.1,1.2), G (17,.3), and H (25.5, ). Solidlike domains appear bright in all of the mm 2 M images M Studies for Hysteresis xperiments In the preceding paragraphs, representative M and FM images obtained during compression experiments show that PL disperses PS into nanoscale aggregates. Hysteresis experiments provide further insight into the aggregation process for PL/PS blends at the /W interface. Figures 7.11 through 7.14 show four representative hysteresis loops (1 st cycle) for different PL/PS blends as Langmuir films. Figure 7.11 shows a hysteresis loop for a X PS ~.81 PL/PS blend, where PS is the major component. The morphologies of this blend during compression (Figure 7.11 through 181

23 D) are clearly different from pure PS (Figure 7.1). Upon expansion of the compressed films, the surface density of the PS aggregates decreases with increasing <>. 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 <> during expansion. Furthermore, the networklike structures remain even at Π ~ mn m -1 (Figure 7.11H). The surface morphologies observed for X PS ~.65 during compression (Figure 7.12 through D) are very similar to those for X PS ~.81. However, the M images taken during expansion show that some of the networklike morphologies break-up (Figure 7.12 through H) during expansion, indicating that increasing the PL 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 /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 4 ~ 7 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 PL/PS systems most likely arises from the presence of a hydrophobic non-amphiphilic component in both blends (octaisobutyl-poss and PS). 48(g) nother feature to note for the hysteresis of the X PS ~.65 blend, is that the area inside the hysteresis loop is much larger than for the X PS ~.81 PL/PS blend. 182

24 14 Π /mn m D F 1 2 G 3 H <> / Å 2 monomer -1 4 D F G H Figure M images for a X PS ~.65 PL/PS blend film obtained at 22.5 and an expansion rate of 2 cm 2 min -1. The letters on the isotherm indicate where the M images were taken during the hysteresis experiments and correspond to (<> /Å 2 monomer -1, Π /mn m -1 ) for compression: (23.7, 1.), (17.6, 2.5), (1.6, 8.3) and D (7., 1.8); and expansion: (7.5, 5.), F (15.8, 2.6), G (28.7,.6), and H (35.5,.3). Solidlike domains appear bright in all of the mm 2 M images. Figure 7.13 (X PS ~.34) and Figure 7.14 (X PS ~.13) provide representative morphologies for blends where PL is the major phase. The nucleation and growth of PL crystals in the homogeneously heterogeneous monolayer were clearly observed upon compression above Π as shown in Figure 7.13D for X PS ~.34 and Figure 7.14D for X PS ~.13 (bright objects). It is clear from Figures 7.11 through 7.14 that the stability of the network structures decreases with increasing PL content in the blend. s seen in Figure 7.14, X PS ~.13, the networklike structures break apart immediately upon expansion. In comparison to the blends with higher PS contents, only discontinuous smaller aggregates with weak M contrast are observed when the barriers start to open 183

25 during the expansion process. It is also worth noting that the area inside the hysteresis loop systematically increases with increasing PL content from the smallest area for the X PS ~.81 (Figure 7.11) to the largest area for the X PS ~.13 blend (Figure 7.14). The increasing area and changing shape of the hysteresis loops with increasing PL content is attributed to the crystallization of PL in blends where PL is the major component. For PL-rich blends, once PL crystallizes, it is necessary for PL to melt before it can respread. The plateau in the expansion isotherms has been identified as the melting 24, 26 transition for PL crystals that form during compression. For PS rich blends, the near absence of a plateau in the compression isotherms means that PS inhibits the nucleation and growth of PL crystals, even though other aspects of the Π-<> isotherm are completely controlled by PL for all of the blends. 184

26 14 Π /mn m D F G 2 3 H 4 5 <> / Å 2 monomer -1 D F G H Figure M images for a X PS ~.34 PL/PS blend film obtained at 22.5 and an expansion rate of 2 cm 2 min -1. The letters on the isotherm indicate where the M images were taken during the hysteresis experiments and correspond to (<> /Å 2 monomer -1, Π /mn m -1 ) for compression: (47.8,.7), (27.3, 4), (19,8.4), and D (1, 1.8); and expansion: (1.2, 4.9), F (22.9, 3.6), G (31, 2.), and H (46,.6). Solidlike domains appear bright in all of the mm 2 M images. 185

27 Π /mn m D F G 2 4 H 6 <> / Å 2 monomer -1 D F G H Figure M images for a X PS ~.13 PL/PS blend film obtained at 22.5 and an expansion rate of 2 cm 2 min -1. The letters on the isotherm indicate where the M images were taken during the hysteresis experiments and correspond to (<> /Å 2 monomer -1, Π /mn m -1 ) for compression: (43.7, 2.3), (2, 1.5), (13, 1.7), and D (1.8, 1.7); and expansion: (1, 5.5), F (18.1, 4.8), G (29, 3.9) and H (49, 1.3). Solidlike domains appear bright in all of the mm 2 M images. 186

28 dditional 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 7.15, the arrow inside the figure indicates the starting and ending points of each individual hysteresis loop as well as the composition (X PS ). ll blend films were compressed to sufficiently small <> 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 PL is analogous to a "melting" process. The surface pressure corresponding to the plateau in the expansion isotherm is characteristic of the melting pressure of PL crystals. s seen in Figure 7.15, 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 PL component Π- PL plots were constructed. Figure 7.15 clearly shows that the expansion isotherms for blends of X PS ~.13 and ~.34 are consistent with pure PL where the plateau corresponds to the melting of PL 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. s expected, the surface tension and surface area of the blend films during expansion are only controlled by the surface-active PL component, even though there are dramatic morphological changes when non-amphiphilic PS is blended with PL. 187

29 Π /mn m -1 Π /mn m PS PL <> / Å 2 monomer PL PL / Å 2 monomer -1 Figure () Π-<> hysteresis loops (1 st cycle) for PL/PS blends and pure PL obtained at 22.5 and a compression rate of 2 cm 2 min -1. The numbers with arrows inside the figure indicate the X PS value of each blend as well as the starting and ending points of each hysteresis loop. () Π- PL hysteresis loops for different PL/PS blends and pure PL. The numbers with arrows indicate the X PS 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 () and (). 188

30 Π /mn m X PS ~.65 1st cycle 2nd cycle 3rd cycle 3 4 <> / Å 2 monomer -1 Figure Multiple Π-<> hysteresis loops for a X PS ~.65 PL/PS blend obtained at 22.5 and a compression rate of 2 cm 2 min -1. The 1 st hysteresis loop is represented by solid line, a dotted line indicates the 2 nd cycle, and the dashed line shows the 3 rd cycle. Furthermore, during subsequent hysteresis loops (2 nd and 3 rd ), there does not appear to be any significant alteration of PS aggregation by M. Nonetheless, PL molecules must form some long-lived three-dimensional structures at the end of the 1 st compression step as subsequent hysteresis loops progressively shift towards smaller <> with each 24, 26 cycle (Figure 7.16), much like pure PL. In short, the surface morphologies of PL/PS blends show composition dependent changes even if all of the isotherms can be collapsed down to a single Π- PL 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 189

31 M images obtained during expansion at constant Π values for all blend films are shown in Figure For pure PS (not shown here), the surface pressure immediately drops to zero upon expansion. The large aggregates (Figure 7.5) 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 24, 26 expansion isotherms of PL decreases slowly and shows larger hysteresis. The M images of pure PL show that the PL 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 PL films are able to melt and re-spread upon expansion as seen in Figure 7.17M through O. In Figure 7.17, the M images of the PL/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 (PL-major component). 19

32 ~.81 ~.65 ~.34 ~.13 Pure PL D F G H I J K L M N O Figure M images obtained during expansion experiments at an expansion rate of 2 cm 2 min -1 and a temperature of 22.5 for different PL/PS blends and pure PL (X PS, Π /mn m -1 ): () (.81, 5), () (.81, 4), () (.81, ), (D) (.65, 5), () (.65, 4), (F) (.65,.4), (G) (.34, 5), (H) (.34, 4), (I) (.34,.6), (J) (.13, 5), (K) (.13, 4), (L) (.13,.4), (M) (, 5), (N) (, 4), and (O) (, 1.1). Solidlike domains appear bright in all mm 2 M images. 191

33 Furthermore, similar morphological features were also observed for PS (M w ~ 1.56 kg mol -1, M w ~ 64.4 kg mol -1, and M w ~ 217 kg mol -1 ) blends with PL as seen in Figure 7.18 through This similarity leads to a more general conclusion that PS of intermediate molar mass from M w ~ 1.56 to at least 217 kg mol -1 (glassy in the bulk state at 22.5 ) will exhibit surface behavior at the /W interface that is similar to the results presented here for PL/PS blends (PS M w = 22.2 kg mol -1 ). The similarity in networklike morphology seen here for PL/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 PL show independent surface behavior, even though the weak hydrophobic cohesive forces at PL/PS domain boundaries help disperse the PS aggregates and inhibit further coalescence. 192

34 14 12 F Π /mn m D F xpansion ompression G H G <> / Å 2 monomer -1 D H Figure M images for a X PS ~.81 PL (M w = 1 kg mol -1 )/PS (M w = 1.56 kg mol -1, M w /M n = 1.6) blend film obtained at 22.5 and an expansion rate of 2 cm 2 min -1. The letters on the isotherm indicate where the M images were taken during the hysteresis experiments and correspond to the average surface area (<> /Å 2 monomer -1 ) for compression: (28.8), (8), (5.8), and D (4.7); and expansion: (4.8), F (7.9), G (17.7), and H (25). Solidlike domains appear bright in all of the mm 2 M images. 193

35 14 Π /mn m D F 2 G xpansion ompression <> / Å 2 monomer -1 H D F G H Figure M images for a X PS ~.13 PL (M w = 1 kg mol -1 )/PS (M w = 1.56 kg mol -1, M w /M n = 1.6) blend film obtained at 22.5 and an expansion rate of 2 cm 2 min -1. The letters on the isotherm indicate where the M images were taken during the hysteresis experiments and correspond to the average surface area (<> /Å 2 monomer -1 ) for compression: (~18.7), (~12), (~1.4), and D (~9.8); and expansion: (~9.6), F (~16.3), G (~35), and H (~64). Solidlike domains appear bright in all of the mm 2 M images. 194

36 14 12 F Π /mn m D ompression xpansion F G H G <> / Å 2 monomer -1 D H Figure 7.2. M images for a X PS ~.81 PL (M w = 1 kg mol -1 )/PS (M w = 64.4 kg mol -1, M w /M n = 1.3) blend film obtained at 22.5 and an expansion rate of 2 cm 2 min -1. The letters on the isotherm indicate where the M images were taken during the hysteresis experiments and correspond to the average surface area (<> /Å 2 monomer -1 ) for compression: (34.4), (1.8), (7), and D (5.1); and expansion: (6.2), F (14.6), G (19.4), and H (25). Solidlike domains appear bright in all of the mm 2 M images. 195

37 Π /mn m D 4 F G 2 xpansion 2 ompression 4 6 <> / Å 2 monomer -1 H D F G H Figure M images for a X PS ~.13 PL (M w = 1 kg mol -1 )/PS (M w = 64.4 kg mol -1, M w /M n = 1.3) blend film obtained at 22.5 and an expansion rate of 2 cm 2 min -1. The letters on the isotherm indicate where the M images were taken during the hysteresis experiments and correspond to the average surface area (<> /Å 2 monomer -1 ) for compression: (~22.2), (~15.1), (~11), and D (~9); and expansion: (~12.2), F (~22.3), G (~3.9), and H (~57.6). Solidlike domains appear bright in all of the mm 2 M images. 196

38 Π /mn m xpansion ompression D F G H F G <> / Å 2 monomer -1 D H Figure M images for a X PS ~.81 PL (M w = 1 kg mol -1 )/PS (M w = 217 kg mol -1, M w /M n = 1.5) blend film obtained at 22.5 and an expansion rate of 2 cm 2 min -1. The letters on the isotherm indicate where the M images were taken during the hysteresis experiments and correspond to the average surface area (<> /Å 2 monomer -1 ) for compression: (21.3), (9.7), (7.2), and D (6.2); and expansion: (6.4), F (1.7), G (18.6), and H (25.5). Solidlike domains appear bright in all of the mm 2 M images. 197

39 Π /mn m D F 2 G xpansion 2 4 ompression 6 8 <> / Å 2 monomer -1 H D F G H Figure M images for a X PS ~.13 PL (M w = 1 kg mol -1 )/PS (M w = 217 kg mol -1, M w /M n = 1.5) blend film obtained at 22.5 and an expansion rate of 2 cm 2 min -1. The letters on the isotherm indicate where the M images were taken during the hysteresis experiments and correspond to the average surface area (<> /Å 2 monomer -1 ) for compression: (~16.3), (~12.5), (~8.6), and D (~7.9); and expansion: (~1.4), F (~16.5), G (~38), and H (~49). Solidlike domains appear bright in all of the mm 2 M images. 198

40 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 = The reason for this is that PL blends with a PS oligomer (M w =.74 kg mol -1 ) that is in the liquid state at T = 22.5 exhibit dramatically different phase behavior. M 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 PL/glassy PS blends, the PL/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 PL crystallization was inhibited, M images captured during dynamic compression reveal a homogeneous surface morphology at very low Π (Figure 7.24). Further compression of the PS-rich blend film causes the films to turn cloudy at intermediate Π values as seen in Figure 7.24 and, 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,, and F. t 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). 199

41 12 X PS ~.89 Π /mn m D I H G F <> /Å 2 monomer D F G H I -1 Figure M images obtained at 22.5 and a compression rate of 2 cm 2 min during the 1 st compression step for a PL/PS blend with X PS ~.89. The letters on the Π <> compression isotherm indicate where the individual M images were taken and correspond to (Image, <>/Å 2 monomer -1 ): (, 25), (, 6.4), (, 4.9), (D, 4.6), (, 4.4), (F, 4.2), (G, 4.1), (H, 3.8), and (I, 3.6). ll M images are mm 2 in size. 2

42 Π /mn m H F G D X PS ~ <> /Å 2 monomer -1 D F G H -1 Figure M images obtained at 22.5 and a compression rate of 2 cm 2 min during the 1 st compression step for a PL/PS blend with X PS ~.22. The letters on the Π <> compression isotherm indicate where the individual M images were taken and correspond to (Image, <>/Å 2 monomer -1 ): (, 41), (, 16), (, 11.8), (D, 1.5), (, 9.8), (F, 9.2), (G, 8.9), and (H, 8.6). ll M images are mm 2 in size. right domains in Figure D H are PL crystallites. 21

43 While Figure 7.24 provided morphological data for a PS-rich blend of PL/liquid PS oligomer, Figure 7.25 provides representative data for a PL-rich blend (X PS ~.22) of the same blend system. In the X PS ~.22 blend, the films do not turn cloudy until Π is much larger (Figure 7.25 through D). Unlike the PS-rich blend, droplets formed in the PL-rich blend may have much lower optical contrast (Figure 7.25 and F). t 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 PL crystals. t this stage, Figures 7.24 and 7.25 are only provided as proof that liquid PS oligomers have very different properties in blends with PL than higher molar mass PS. While it is tempting to claim that the PL/liquid PS oligomer system exhibits Π-induced liquid-liquid phase separation, a complete understanding of the observed behavior is still lacking. s such further speculation is saved for hapter 9, where some suggestions for future work are made onclusions This study demonstrates surface morphologies of immiscible intermediate molar mass (glassy) PS blends with PL as Langmuir films at the /W interface. Π-<> isotherm studies indicate that the surface activity of the blend films is only controlled by the amphiphilic PL component. The addition of glassy PS to PL has no effect on the Π- isotherm of PL in the monolayer state if one ignores the PS component. Instead, solidlike nanoscale PS aggregates tend to exist on top of a liquidlike PL film. The better dispersed PS aggregates for all the blends apparently arise from more favorable PS wetting of the PL layer than the water surface. M studies show that PL-rich blends exhibit nucleation and growth of PL crystals upon collapse of the monolayer, whereas 22

44 PS inhibits PL 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 PL) at the /W interface. 23