Particle Morphology as a Control of Permeation in Polymer Films

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Particle Morphology as a Control of Permeation in Polymer Films Obtained from MMA/nBA Colloidal Dispersions David J. Lestage and Marek W. Urban* School of Polymers and High Performance Materials, Shelby F. Thames Polymer Science Research Center, The University of Southern Mississippi, Hattiesburg, Mississippi 39406 Received January 19, 2004. In Final Form: April 30, 2004 The combination of precision-controlled weight loss measurements and spectroscopic surface FT-IR analysis allowed us to identify unique behaviors of poly(methyl methacrylate) (p-MMA). When MMA and n-butyl acrylate (nBA) are polymerized into p-MMA and p-nBA homopolymer blends, MMA/nBA random copolymers, and p-MMA/p-nBA core-shell morphologies, a controlled mobility and stratification of low molecular weight components occurs in films formed from coalesced colloidal dispersions. Due to different affinities toward water, p-MMA and p-nBA are capable of releasing water at different rates, depending upon particle morphological features of initial dispersions. As coalescence progresses, water molecules are released from the high free volume p-nBA particles, whereas p-MMA retains water molecules for the longest time due to its hydrophilic nature. As a result, water losses at extended coalescence times are relatively small for p-MMA. MMA/nBA copolymer and p-MMA/p-nBA blends follow the same trends, although the magnitudes of changes are not as pronounced. The p-MMA/p-nBA core-shell behavior resembles that of p-nBA homopolymer, which is attributed to significantly lower content of the p-MMA component in particles. Annealing of coalesced colloidal films at elevated temperatures causes migration of SDOSS to the F-A interface, but for films containing primarily p-nBA, reverse diffusion back into the bulk is observed. These studies illustrate that the combination of different particle morphologies and temperatures leads to controllable permeation processes through polymeric films.

Introduction Over the past decade, numerous studies were conducted on the stratification and mobility of individual components in polymer films obtained from colloidal dispersions.1-12 During the course of these experiments, several variables were found to influence film formation processes and, more importantly, the mobility of low molecular weight species. Although overall concentration levels of such species as surfactants and other dispersion components may be considered insignificant, it is evident that their aggregation will enhance local concentration levels. One example is the formation of localized ionic clusters (LIC)12,13 near surfaces and interfaces. The main factors that contribute to this behavior are the glass transition temperature (Tg) of a polymer matrix, compatibility of individual components, and film formation environment, just to name a few. As a result, the mobility in polymeric films obtained from colloidal particles was found to be critical in * To whom correspondence should be addressed. (1) Keddie, J. L. Mater. Sci. Eng. 1997, 21, 101-170. (2) Winnik, M. A.; Feng, J. J. Coat. Technol. 1996, 68, 39. (3) Dreher, W. R.; Urban, M. W.; Zhao, C. L.; Porzio, R. S. Langmuir 2003, 19, 10254-10259. (4) Beltran, C. M.; Guillot, S.; Langevin, D. Macromolecules 2003, 36, 8506-8512. (5) Niu, B. J.; Urban, M. W. J. Appl. Polym. Sci. 1998, 70, 13211348. (6) Sethumadhavan, G. N.; Nikolov, A.; Wasan, D. Langmuir 2001, 17, 2059-2062. (7) Shin, J. S.; Lee, D. Y.; Ho, C. C.; Kim, J. H. Langmuir 2000, 16, 1882-1888. (8) Zhao, Y.; Urban, M. W. Polym. Mater. Sci. Eng. 1999, 80, 571572. (9) Zhao, Y.; Urban, M. W. Macromolecules 2000, 33, 2184. (10) Zhao, Y.; Urban, M. W. Macromolecules 2000, 33, 7573-7581. (11) Zhao, Y.; Breitenkamp, K.; Urban, M. W. Polym. Mater. Sci. Eng. 2000, 82, 380-381. (12) Dreher, W. R.; Urban, M. W. Macromolecules 2003, 36, 1228. (13) Lestage, D. J.; Urban, M. W. Proceedings from the ACS Division of Polymeric Materials Science and Engineering 2004, 90, 38.

interfacial properties, particularly near the film-air (FA) and film-substrate (F-S) interfaces.14,15 Other factors that affect the mobility of individual components in colloidal films are the particle morphology and particle coalescence.9,14,16-18 In the context of these studies, mobility, and therefore diffusivity through coalesced colloidal dispersions, plays an important role in polymer matrices, especially at different stages of particle coalescence. For that reason, the effect of particle morphologies during coalescence and the effect on transient barrier properties are of interest, as these attributes will have a significant influence on film stability and the response to internal and external stimuli. If the mobility of small molecules can be controlled by morphologies of colloidal particles, it will be possible to develop separation methods based on colloidal dispersions. Due to optical transparency, resistance to discoloration, durability, and machining characteristics, p-MMA serves as a primary component in numerous biomedical applications ranging from denture bases to orthopedic bone templates and contact lenses that do not cause eye irritation or allergies.19 While these attributes make the use of p-MMA advantageous over other polymeric materials, the primary disadvantage is a lack of oxygen permeability.20 This feature in the context of stratification of low molecular weight species stimulated further studies (14) Niu, B. J.; Urban, M. W. J. Appl. Polym. Sci. 1998, 70, 1323. (15) Zhao, Y.; Urban, M. W. Langmuir 2000, 16, 9439-9447. (16) Niu, B. J.; Urban, M. W. J. Appl. Polym. Sci. 1995, 56, 377. (17) Thorstenson, T. A.; Urban, M. W. J. Appl. Polym. Sci. 1993, 47, 1381. (18) Chu, A. P.; Tebelius, L. K.; Urban, M. W. ACS Symp. Ser. 1997, 663, 212-225. (19) Ray, N. Dental Materials Science; Wilton: Ireland, 1998; pp 38-48. (20) Polymers: Biomaterials and Medical Applications; Kroschwitz, J. I., Ed.; John Wiley and Sons: New York, 1989; p 99.

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Table 1. Particle Size and Composition of p-MMA, p-nBA, Random p-(MMA-nBA), and Core/Shell p-(MMA/nBA) Colloidal Dispersions composition

p-MMA seed

p-nBA seed

copolymer p-(MMA-nBA)

core/shell p-(MMA/nBA)

DDI (%) p-MMA seed (32.2% solids) (%) methyl methacrylate (%) n-butyl acrylate (%) SDOSS (%) K2S2O8 (%) solids (%) particle size (nm)

67.6

67.6

68.9

54.5 19.8

31.2 0.78 0.42 32.4 158

14.8 14.8 0.95 0.55 31.5 157

31.2 0.78 0.42 32.4 165

24.7 1.48 0.4 32.2 295

in an effort to identify how modifications of p-MMA with a low Tg monomer such n-butyl acrylate (nBA) will alter not only particle morphologies, but also impart film properties through the formation of stratified physical barriers or altering the polymer matrix free volume, water retention, and low molecular weight permeability. Because p-MMA colloidal particles are unable to form stable films under ambient conditions, it is often necessary to alter p-MMA composition using monomers such as nBA. Because such an approach allows the formation of colloidal particles that exhibit copolymer or core-shell morphologies, these studies focus on 50/50 w/w % p-MMA/p-nBA colloidal blends, copolymers, and 20/80 w/w % core-shell dispersion films in the context of mobility of low molecular weight species and particle morphologies. Experimental Section Methyl methacrylate (MMA) and n-butyl acrylate (nBA) monomers (Aldrich Chem. Co.) were individually polymerized using a semicontinuous process outlined elsewhere21 and adapted for small-scale polymerization. After synthesis, p-MMA (particle size ) 171 nm) and p-nBA (particle size ) 158 nm) colloidal dispersions containing 32 w/w % solids were mixed in a 50/50 p-MMA/p-nBA ratio and blended for 2 h in 20 mL scintillation vials using a Barnstead/Thermolyne (Labquake) rotisserie shaker. 50/50 MMA/nBA copolymer and p-MMA/p-nBA coreshell particles were synthesized in a similar fashion, resulting in particle sizes of 157 and 295 nm, respectively. Table 1 summarizes individual dispersion compositions and corresponding particle sizes measured by a Microtrac UPA250 particle size analyzer. Such prepared dispersions were cast on a poly(vinyl chloride) (PVC) substrate using a Sheen Automatic Film Applicator 1137 with a 40 mL draw-down bar and were allowed to coalesce at 80% relative humidity (RH) for 3 days at 23 °C. An approximate film thickness of the dry films was 40 µm. Weight loss measurements were conducted using an analytical balance with an accuracy of 10-4 g. In a typical experiment, 500 µL of each specimen was deposited in a 35 × 12 mm cylindrical glass vial generating dispersion samples approximately 10 mm deep. Samples were exposed to conditions of 64% RH and 23 °C, and, in each experiment, exactly the same amount of surface was exposed to the environment (78.54 mm2). A similar procedure was employed in previous studies.2 Differential scanning calorimetry (DSC) measurements were performed on a TA Q800 Series DSC instrument, and the heating rate was 20 °C/min from -75 to 150 °C. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra were collected using a Bio-Rad FTS-6000 FTIR single-beam spectrometer equipped with a Miracle AG single reflection horizontal ATR accessory (Pike Technologies, Inc.). The films were analyzed using a 2 mm crystal with a 45° angle, which allows the analysis of the film-air (F-A) interface at approximately 200 nm below the surface. Each spectrum was recorded at 4 cm-1 resolution with 100 co-added scans ratioed to 100 scans collected on an empty ATR cell. ATR-FTIR spectra were corrected for spectral distortions using the Urban-Huang algorithm.7 (21) Davis, S. D.; Hadgraft, J.; Palin, K. J. Encylopedia of Emulsion Technology; Marcel Dekker: New York, 1985; Vol. 2.

Figure 1. Differential scanning calorimetry (DSC) of colloidal dispersions: (A) 50/50 MMA/nBA copolymer; (B) MMA/nBA core-shell; and (C) 50/50 MMA/nBA blend.

Results Although the primary objectives of these studies are to understand the mobility of low molecular weight components in p-MMA matrices, as a starting point let us consider thermal characteristics of MMA/nBA films obtained from copolymer, core-shell, and blended colloidal dispersions utilizing DSC. As shown in Figure 1, trace A, 50/50 w/w % copolymer films exhibit a Tg at 10 °C, while core-shell films have two separate Tg’s at 126 and -46 °C (trace B). MMA/nBA blends at a 50/50 w/w % ratio exhibit the same two Tg’s as shown in trace C. As indicated in the Introduction, p-MMA has inherent barrier properties making it impermeable to oxygen and possibly other species, and the colloidal dispersions utilized in these studies seem to exemplify this behavior, especially at various stages of particle coalescence. Figure 2 illustrates the weight loss plotted as a function of time for MMA and nBA homopolymer, copolymer, core-shell, and blended colloidal dispersions cast in 12 × 35 mm glass vials and measured with an analytical balance.2 Indeed, at the early stages, from 0 to 100 h, there is no difference among the weight loss for all colloidal particle morphologies. This is labeled as zone I in Figure 2 and indicates that this stage is vapor pressure controlled and independent of the particle content. However, surprisingly, during the next 100-350 h, the fastest water loss is exhibited by the MMA homopolymer, whereas the slowest rate is observed for p-MMA/p-nBA core-shell morphologies. At extended times over 400 h, the p-MMA weight loss levels off (stage III), but an apparent crossover occurs reversing the order observed during stage II. These simple experiments clearly demonstrate that this behavior is

Particle Morphology as a Control of Permeation

Figure 2. Gravimetric analyses of weight loss versus time of (A) MMA homopolymer; (B) 50/50 MMA/nBA copolymer; (C) MMA/nBA core-shell; (D) 50/50 MMA/nBA blend; and (E) nBA homopolymer.

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Figure 4. (A) Micro ATR-FTIR spectra of the F-A interface of MMA homopolymer annealed at: (A) 25; (B) 60; (C) 90; (D) 120; and (E) 150 °C. (B) Micro ATR-FTIR spectra of the F-A interface of nBA homopolymer annealed at: (A) 25; (B) 60; (C) 90; (D) 120; and (E) 150 °C.

Figure 3. Micro ATR-FTIR spectra recorded from the F-A interface of 50/50 w/w % MMA/nBA copolymer annealed at: (A) 25; (B) 60; (C) 90; (D) 120; and (E) 150 °C.

Figure 5. Micro ATR-FTIR spectra of the F-A interface of MMA/nBA core-shell films annealed at: (A) 25; (B) 60; (C) 90; (D) 120; and (E) 150 °C.

influenced by the presence of high Tg p-MMA. The literature22 data indicate that p-MMA may form unreacted carbon double bonds, thus suggesting the possibility of branching/cross-linking reactions resulting in slower molecular diffusion rates, which inhibit the mobility and permeability of water. In addition, previous work has revealed that homopolymer dispersions of hard (high modulus) components such as p-MMA dry faster as compared to soft component dispersions (p-nBA).2 However, the question is a crossover of the evaporation rates, as demonstrated in Figure 2. Let us induce the mobility of other components in MMA/ nBA colloidal films during and after film formation. For that reason, 50/50 w/w % MMA/nBA copolymer films were annealed at 25, 60, 90, 120, and 150 °C for 2 h. Figure 3, traces A-E, illustrates a series of ATR-FTIR spectra in the 1350-900 cm-1 region recorded from the F-A interface of coalesced copolymer films. Two bands at 1232 and 1046 cm-1 labeled in Figure 3 attributed to asymmetric stretching modes of H-bonded C-O-C entities of SDOSS and the S-O symmetric stretching mode of SO3-Na+‚‚‚H2O associations,2 however, are not detected. MMA and nBA homopolymers exhibit different behaviors. Figure 4A and B illustrates ATR FT-IR spectra recorded approximately

200 nm from the F-A interface of MMA and nBA homopolymer films annealed between 25 and 150 °C for 2 h. As seen in Figure 4A, for p-MMA, vibrational modes pertinent to SDOSS are not detected even after annealing at 150 °C, thus indicating no SDOSS migration to the F-A interface. Although p-MMA does not form a uniform film at 25 °C, these experiments utilized micro-ATR with a 2000 µm Ge crystal (Experimental Section) which allows us to compare noncoalesced and partially coalesced films. In contrast, as shown in Figure 4B, for p-nBA, SDOSS migration to the F-A interface can be induced by temperature, which is confirmed by the increased intensity of the bands at 1046 and 1232 cm-1. However, interestingly enough, the strongest increase of the 1046 cm-1 band is observed at 90 °C, which becomes weaker at 120 °C, followed by its disappearance at 150 °C. In an attempt to establish the importance of MMA and SDOSS interactions, the SDOSS mobility in the p-MMA/ p-nBA core-shell films was examined. Figure 5, traces A-D, illustrates ATR FT-IR spectra recorded from the F-A interface of core-shell films annealed at 25-150 °C for 2 h. As shown in trace A, at 25 °C, SDOSS is detected at the F-A interface, thus resembling the nBA homopolymer behavior, where SO3-Na+‚‚‚H2O associations are detected. However, at 60 °C, the 1046 cm-1 band increases, followed by a decrease at 90 and 120 °C, and, at 150 °C, this band is not detected.

(22) Nising, P.; Zeilmann, T.; Meyer, T. Chem. Eng. Technol. 2003, 26-5, 599-604.

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Figure 6. Micro ATR-FTIR spectra of the F-A interface of 50/50 wt % MMA/nBA blend annealed at: (A) 25; (B) 60; (C) 90; (D) 120; and (E) 150 °C.

Figure 6, traces A-D, illustrates ATR FT-IR spectra recorded from the F-A interface of a 50/50 w/w % MMA/ nBA latex blend. While the film should consist of hard p-MMA particles suspended in a p-nBA matrix in which SDOSS freely migrates to the F-A interface (Figure 4B), traces A-C reveal that annealing at 25-90 °C does not mobilize SDOSS, as demonstrated by the lack of the 1046

Lestage and Urban

cm-1 band. However, at 120 °C (trace D), the 1046 cm-1 band is detected, followed by an increase at 150 °C (trace E). Although similar trends are observed for p-MMA/p-nBA core-shell films, their magnitudes are different. Figure 7A illustrates a series of spectra recorded from the F-A interfaces of core-shell films, and, for comparison, traces A, B, and C are reference spectra for p-MMA, 50/50 p-MMA/p-nBA blends, and 50/50 MMA/nBA copolymers. As seen, the 1148 and 1162 band intensities progressively change when going from p-MMA to MMA/nBA copolymer, respectively. For the p-MMA/p-nBA core-shell films, the spectra recorded from 230 to 700 nm below the F-A interface are illustrated in traces E-H, and trace D represents the spectrum of p-nBA. The analysis and deconvolution of the bands in the core-shell film at 1162 and 1148 cm-1 is depicted in Figure 7B and shows that, going further into the F-A interface, the dominating component is p-nBA. In contrast, under the same conditions, 50/50 p-MMA/p-nBA blends display a homogeneous distribution of both components at the F-A interface, which is shown by the presence of broad bands at 1162 and 1148 cm-1 (Figure 6, trace A). Figure 8 illustrates the SDOSS concentration at the F-A interface for p-MMA (A), MMA/nBA copolymer (B), p-MMA/p-nBA core-shell (C), p-MMA/p-nBA blended (D), and p-nBA (E) colloidal films. As seen, when MMA/nBA copolymer (B) and p-MMA/p-nBA blends (D) are analyzed, only traces of SDOSS are released to the F-A interface. In contrast, for p-MMA/p-nBA core-shell (C) and p-nBA (E) particles, significant amounts of SDOSS are present at the F-A interface, and above 100 °C SDOSS migrates

Figure 7. (A) ATR-FTIR spectra recorded from the F-A interface of (A) MMA homopolymer; (B) 50/50 MMA/nBA blend; (C) 50/50 MMA/nBA copolymer; (D) nBA homopolymer; (E) MMA/nBA core-shell polymer 0.23 µm from the F-A interface; (F) MMA/nBA core-shell polymer 0.45 µm from the F-A interface; (G) MMA/nBA core-shell polymer 0.54 µm from the F-A interface; and (H) MMA/nBA core-shell polymer 0.69 µm from the F-A interface. (B) Spectral deconvolution in the 1200-1100 cm-1 region of core-shell MMA/nBA spectra from parts E, F, G, and H, respectively.

Particle Morphology as a Control of Permeation

Figure 8. SDOSS surface concentration plotted as a function of annealing temperature.

back into the polymer matrix. Initially, as shown in Figure 8, the core-shell (C) films exhibit the largest volume concentration of SDOSS at the F-A interface, which is due to increased particle size and therefore SDOSS concentration (Table 1) required to stabilize particle interfaces. Discussion The presence of two Tg’s (Figure 1) as well as a single particle size (Table 1) demonstrates that distinct phases of p-MMA and p-nBA coexist, and the magnitude of relative heat flow for core-shell particles is higher for the p-nBA component (shell) Tg, and significantly smaller for p-MMA, confirming that the core p-MMA component is significantly smaller. Similarly, 50/50 w/w % latex blends exhibit Tg’s at 126 and -46 °C, but the magnitude of heat flow for each component is the same, indicating that p-MMA and p-nBA phases coexist. Although experimental approaches13,23,24 and theories24-27 concerning film formation of colloidal dispersions have been investigated, variables responsible for the effect of particle size and morphology have not been explored for p-MMA-containing particles. Following the wt % mass lost as a function of coalescence time (Figure 2) for MMA and nBA homopolymer, copolymer, core-shell, and blended colloidal dispersions, one can relate the evaporation rate of water to the amount of p-MMA in the dispersion and its affinity to solubility in an aqueous phase. Considering Hansen solubility parameters,28 p-MMA in comparison to p-nBA has enhanced polar (δp) and hydrogen-bonding (δh) components, which suggests greater solubility and interactions with an aqueous phase. Furthermore, the smaller dispersive (δd) value of 13.5 indicates stronger self-interactions of p-MMA, thus suggesting a more efficient packing mechanism. Although the above considerations suggest that p-MMA exhibits higher compatibility toward water, and therefore higher water retention of the coalesced films, the crossover of weight loss requires further analysis. It should be noted that the sample thickness with respect to the overall volume plays an important role, as formation of the “skin”29-31 at the early stages may decrease the total weight (23) van Tent, A.; te Nijenhuis, K. J. Colloid Interface Sci. 2000, 232, 350-363. (24) Doubler, F.; Holl, Y. ACS Symp. Ser. 1996, 648, 22-43. (25) Vanderhoff, J. W.; Tarkowski, H. L.; Jenkins, M. C.; Bradford, E. B. J. Macromol. Chem. 1966, 1, 131. (26) Brown, G. L. J. Polym. Sci. 1956, 22, 423. (27) Dillon, R. E.; Matheson, L. A.; Bradford, E. B. J. Colloid Sci. 1951, 6, 108. (28) Barton, A. F. Handbook of Solubility Parameters and other Cohesion Parameters; CRC Press: Boca Raton, RL, 1983; p 272. (29) Croll, S. G. J. Coat. Technol. 1986, 58, 41.

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loss of water, thus slowing down coalescence. As a matter of fact, samples remained opaque after 800 h of coalescence time. While vibrational modes pertinent to SDOSS are not detected for p-MMA homopolymer, even when annealed at 120 and 150 °C forming a partially coalesced film, Figure 4B, p-nBA homopolymer exhibits multiple IR absorbance changes at the F-A interface. The initial increase of the 1046 cm-1 band at 90 °C is attributed to the fact that water is being driven upward to the F-A interface as a result of increased evaporation. As the film dehydrates, SO3-Na+‚‚‚H2O interactions are destroyed, leaving free SDOSS which, in turn, is responsible for the slight band broadening as unrestrained SDOSS exhibits a symmetric S-O stretching mode at 1050 cm-1. At 150 °C, the disappearance of the 1046 cm-1 band results from reverse migration of SDOSS segments due to thermal transitions of the p-nBA phase,32 which will be described in the discussion of core-shell films which behave very similar to the p-nBA homopolymer. Previous studies1-4 showed that surfactant molecules may be largely, if not completely, displaced from colloidal particle surfaces as a result of interdiffusion of polymeric particle segments during latex coalescence. However, an accurate assessment of the location of these molecules after coalescence has yet to be determined. The data shown in Figure 4B illustrate that SDOSS can be effectively driven to the F-A interface in p-nBA, but again this behavior is not detected when p-MMA or MMA/nBA copolymer serve as a matrix. The absence of SO3-Na+‚‚‚H2O associations (1046 cm-1) in MMA/nBA copolymer films shown in Figure 3 is intriguing as this behavior contrasts the previous studies on Sty/nBA dispersions which revealed significant amounts of SDOSS migrating to the F-A interface upon annealing to elevated temperatures.2 Although the mobility of SDOSS for Sty/nBA was attributed to the increase of free volume above the Tg of p-Sty, this is not the case for p-MMA. Because there are no acid groups in this system, which at or above 120 °C would result in the release of SDOSS to the surface as in the Sty/nBA system, the question concerning the SDOSS response to temperature in MMA/nBA matrices remains open. In an effort to understand the molecular interactions between MMA and SDOSS species, let us consider SDOSS mobility in the p-MMA/p-nBA core-shell films. As shown in Figure 5, these results indicate that the primary phase responsible for SDOSS release is the p-nBA shell, which behaves similarly to the nBA homopolymer. As the temperature approaches 100 °C, solubilized SDOSS is driven to the surface with water, and, when approaching 150 °C, the depolymerization temperature and the boiling point of nBA are reached.32 This process can be envisioned as a displacement of nBA monomer that results in the formation of micropores and voids which are then filled with SDOSS in an attempt to lower the increased, voidinduced surface energy, which is confirmed by the disappearance of the 1046 cm-1 band at 0.2 µm from the F-A interface. As indicated in Figure 6, significantly different behavior is observed for the blended dispersions. Here, one can envision hard p-MMA spheres in a coalesced p-nBA matrix which is similar to the coalesced core-shell films, unless, as a result of particle rearrangement, the p-MMA phase (30) Eckersley, S. T.; Rudin, A. Prog. Org. Coat. 1994, 23, 387. (31) Okubo, M.; Takeya, T.; Tsutsumi, Y.; Kadooka, T.; Matsumoto, T. J. Polym. Sci.: Polym. Chem. Ed. 1981, 19, 1. (32) Brandrup, J.; Immergut, E. H. Polymer Handbook, 3rd ed.; John Wiley and Sons: New York, 1989; p 491.

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Figure 9. Schematic representation of the mobility of SDOSS and H2O at different stages of film formation.

is stratified closer to the F-A interface of the blended film and inhibitsz SDOSS mobility. P-MMA/p-nBA coreshell films analyzed at various depths from the F-A interface (Figure 7, traces E-H) indeed indicated there is a stratification of p-MMA and p-nBA with p-MMA absorbance (1148 cm-1) increasing with increasing distance from the F-A interface. Thus, coalesced p-MMA/ p-nBA core-shell and blended dispersions exhibit contrastingly different chemical makeups near the F-A interface, which also indicates film formation is affected by p-MMA. Because mobility is affected by free volume as a function of temperature, we conducted a series of annealing experiments. As shown in Figure 8, the relationship between the volume concentration of SDOSS plotted as a function of temperature at 200 nm from the F-A interface for p-MMA (A), MMA/nBA copolymer (B), p-MMA/p-nBA core-shell (C), p-MMA/p-nBA blended (D), and p-nBA (E) colloidal films illustrates that SDOSS is not released to the F-A interface when p-MMA is the matrix. Due to impermeable barrier properties,20 rapid migration of low molecular weight species around colloidal particles leads to diffusion to the F-A interface, but, annealed above its Tg, p-MMA exhibits enhanced surfactant compatibility and low permeability. In view of the data shown in the Results section and the above discussion, the following scenario for diffusion of SDOSS carried by H2O at various stages of coalescence of p-MMA homopolymer, MMA/nBA copolymer, p-MMA/ p-nBA core-shell, p-MMA/p-nBA blend, and p-nBA homopolymer is proposed. As indicated earlier, at the early stages of film formation, the morphologies of particles play no role in small molecule mobility, and the rate of weight loss changes depends on vapor pressure. However, during this wet stage, both homo- and copolymers are permeable to water molecules, but, due to significant free volume and hydrophobicity differences, p-MMA will permeate significantly less than p-nBA. Thus, there is partitioning of water molecules between the surrounding aqueous phase and water molecules retained by polymer particles. While at the earlier stages water molecules surrounding colloidal particles evaporate first due to vapor pressure, at the later stages, where polymers are considered “dry”, the situation is quite different. Limited absorption of water

and particle deformation of hard p-MMA allows water molecules to diffuse faster through the matrix around particles, yet water that was absorbed will remain longer in the particles. In contrast, at this stage, p-nBA particles are capable of absorbing water molecules and deforming. As a result, the observed water loss is slower as diffusion rates are decreased due to changes in free volume and capillary pressures between soft particles.2 As coalescence progresses, water molecules are released from the high free volume p-nBA particles, whereas p-MMA will retain water molecules for the longest time due to their hydrophilic nature. As a result, its water loss is the smallest. This is illustrated schematically in Figure 9, where the crossover of the weight lost is presented in the context of particle morphologies. This figure also illustrates the behavior of MMA/nBA copolymer and p-MMA/p-nBA blends with the same trends, although the magnitude of the changes is not as pronounced. The p-MMA/p-nBA core-shell behavior falls very close to the copolymer, which is attributed to the significantly lower content of the p-MMA component in its particle morphology. Finally, the mobility of SDOSS needs to be addressed in the context of weight loss of water. SDOSS does not contribute to the weight loss measurements, and, as ATR FT-IR experiments showed, SDOSS is capable of migrating to the F-A interface only when particles contain p-nBA and will vary with the particle morphology. These observations indicate that the excess of free volume and weaker nBA-SDOSS interactions allow SDOSS to be released to the surface at relatively early stages of film formation. In contrast, the presence of p-MMA inhibits SDOSS mobility entirely, even at elevated temperatures. In summary, these studies show that colloidal dispersions containing different particle morphologies are capable of controlling nanoscale permeation, which is accomplished by the chemical design of particle morphologies. Such particles are capable of selective permeation, following principles of gel permeation chromatography at nano-levels. It should be also pointed out that these experiments were performed under strictly controlled environmental conditions, which are necessary for sufficient accuracy to observe this phenomenon.

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Conclusions During film formation at ambient conditions, the large free volume content of the p-nBA homopolymer matrix, due to its low Tg, allows migration of low molecular weight species to the F-A interface. However, due to p-nBA particle permeability and deformation, H2O and therefore SDOSS diffusion are slowed, and extended coalescence times are required for p-nBA to undergo the greatest weight loss. Upon annealing, free volume increases and compatibility with surfactant decreases, thereby releasing more species to the F-A interface facilitated by the removal of water. In contrast, p-MMA allows rapid migration of species around colloidal particles to the F-A interface, but above its Tg, p-MMA does not exhibit an increased migration of low molecular weight species due to enhanced surfactant compatibility and low permeability. Although blends primarily indicate an entrapment

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of species until annealed above the Tg of p-MMA, minute quantities are exhibited at the F-A interface prior to p-MMA coalescence due to diffusion of components through an intricate path around the high Tg component. Coreshell latex films exhibit a quicker release of surfactant to the F-A interface due to the larger surface-stabilized shell component. However, pertinent band intensities decrease due to the coalescence of the more compatible component, p-MMA. In essence, complex macromolecular rearrangements occur in multicomponent colloidal films, and future studies will be conducted to establish improved understanding of these processes. Acknowledgment. This work was supported primarily by the MRSEC Program of the National Science Foundation under Award Number DMR 0213883. LA049823I