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Effect of Annealing on the Surface Properties of Poly(n-butyl methacrylate) Latex Films Containing Poly(styrene/r-methylstyrene/acrylic acid) Jin-Sup Shin,† Doug-Youn Lee,† Chee-Cheong Ho,*,‡ and Jung-Hyun Kim*,† Nanospheres Process and Technology Laboratory, Department of Chemical Engineering, Yonsei University, 134 Shinchon-dong, Sudaemoon-ku, Seoul 120-749, Korea, and Department of Chemistry, University of Malaya, 50603 Kuala Lumpur, Malaysia Received May 25, 1999. In Final Form: September 22, 1999 The effect of alkali-soluble resin (ASR) postadded to emulsifier-free monodisperse poly(n-butyl methacrylate) latexes (PBMA) on the surface properties of the latex films formed was investigated using the contact angle method. The contact angles of a probe liquid (containing 50:50 wt % ethylene glycol and water) on the film surface were determined as a function of the annealing temperature of the film and the annealing duration. The ASR, poly(styrene/R-methylstyrene/acrylic acid), was found to exude to the air/polymer interface after annealing, as monitored using FTIR-ATR. Thus the surface properties of the films were dependent on the concentration of the ASR employed, the annealing temperature, and the duration. Annealing the film at a temperature approximately equal to the Tg of ASR allows them (ASR) to undergo unrestricted migration to the film surface and also to expose the hydrophobic region of their chains to the surface. The more hydrophobic character of the film annealed at 150 °C with a higher contact angle as compared to that at 50 °C is a result of the change in chain conformation of the ASR at the surface brought about by annealing. Also, the results illustrate that the hydrophobic-hydrophilic character of the PBMA-SAA film surface can be changed readily by merely adjusting the annealing temperature.
Introduction The surface properties of a polymeric material are determined essentially by the configuration of its molecules at the interface. This is the spatial arrangement of ligands and atoms in the outermost region of the material.1 However, it has often been found that the interfacial characteristics of a material in real applications, many of which involve aqueous environment, are usually quite different from those determined in air or vacuum. This situation can best be understood by considering the outermost region of a material as the “surface state”, which could be quite different from the state of the bulk material.2,3 This implies that polymeric surfaces are relatively mobile and adopt different surface configurations in different environments, both polar and nonpolar.4-6 For example, various aspects of preferential orientation of polar groups in polymer surfaces have been reported.7-10 Synthetic latexes are extensively used in many industries where the formation of a coherent film following coalescence via polymer interdiffusion across a particleparticle interface is a prerequisite in its applications. The interactions between the components in a latex dispersion † ‡
Yonsei University. University of Malaya.
(1) Langmuir. Science 1938, 87, 493. (2) Yasuda, H.; Charlson, E. J.; Charlson, E. M.; Yasuda, T.; Miyama, M.; Okuno, T. Langmuir 1991, 7, 2394. (3) Yasuda, T.; Miyama, M.; Yasuda, H. Langmuir 1992, 8, 1425. (4) Ruckenstein, E.; Lee, S. H. J. Colloid Interface Sci. 1987, 120, 153. (5) Holly. F. J.; Refojo, M. F. J. Biomed. Mater. Res. 1975, 9, 315. (6) Andrade J. D.; Gregonis. D. E.; Smith, L. M. In Physiochemical Aspects of Polymer Surfaces; Mittal, K. L., Ed.; Plenum Press: New York, 1983; Vol. 2, p 911. (7) Carre, A.; Schreiber H. P. J. Coatings Technol. 1982, 54, 31. (8) Yasuda, H.; Sharma, A. K.; Yasuda, T. J. Polym. Sci., Polym. Chem. Ed. 1981, 19, 1285. (9) Briggs D.; Rance, D. G.; Kendall, C. R.; Blythe, A. R. Polymer 1980, 21, 895. (10) Tretinnikov O. N. Langmuir 1997, 13, 2988.
are important parameters influencing its film-forming properties, in addition to the latex particle size and size uniformity. This results because in contrast to the observed selective accumulation of functional groups occurring during solid surface formation of a polymer solution by casting, the latex film formation process is more complex. It includes solvent (water) evaporation, packing of particles followed by their gradual coalescence, migration, and redistribution of water-soluble components (including additives such as water-soluble polymer) among the latex particle matrix to yield a homogeneous or heterogeneous film. Whether a continuous film is formed depends on how readily the latex particles deform and coalesce with each other and the compatibility between the latex additive and the latex polymer. The polymer chains of the latex particles as well as the polymeric additives can adopt configurations or orientations that favor maximizing the film surface exposure of polar or nonpolar moieties as the film ages. For example, the surfactants, which are often found in latex dispersion, are distributed within a latex film can have a strong influence on its film properties such as adhesion and water sensitivity. Zhao et al.11-13 found that enrichment of surfactants at both the air/ polymer and polymer/substrate interfaces occurred via an exudation process of the anionic surfactant molecules to the film surface monitored using FTIR-ATR. Other studies concluded that surfactants are likely to undergo phase separation from the latex polymer during the filmforming process, resulting in surfactant domains in the bulk of the film or migration of surfactant to the interface.14-19 This is attributed to the concentration (11) Zhao, C. L.; Holl, Y.; Pith, T.; Lambla, M. Colloid Polym. Sci. 1987, 265, 823. (12) Zhoa, C. L.; Dobler, F.; Pith, T.; Holl, Y.; Lambla, M. J. Colloid Interface Sci. 1989, 128, 437. (13) Zhao, C. L.; Holl, Y.; Pith, T.; Lambla, M. Br. Polym. J. 1989, 21, 155. (14) Voyutski, S. S. J. Polym. Sci., Part A: Polym. Chem. 1958, 32, 528.
10.1021/la9906370 CCC: $19.00 © 2000 American Chemical Society Published on Web 12/15/1999
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gradient across the film providing a driving force to maintain an equilibrium concentration. More recently we have demonstrated that both the morphology and the kinetics of film formation of monodisperse surfactant-free poly(n-butyl methacrylate) (PBMA) latex prepared by emulsion polymerization are strongly influenced by the presence of postadded alkali-soluble resin (ASR).20-24 These are low molecular weight carboxylated random copolymers that have been used as polymeric surfactants, for example, in the emulsion polymerization of polystyrene and poly(methyl methacrylate) latex particles.25 Such polymers with both hydrophobic and hydrophilic groups are known to stabilize latex particles via intermolecular and/or intramolecular hydrophobic interactions.26-30 Omi et al.29 found that the random copolymers of styrene (ST) and acrylic acid (AA) possess enough heterogeneity in composition to allow PST-AA to behave as well-designed block or graft copolymers. Usually graft and block copolymers are designed as stabilizers.30 The PBMA-ASR system reveals some very interesting morphological features that could be attributed partly to the incompatibility between the two polymers and partly to the widely different glass transition temperatures of the two polymer materials. Preferential accumulation of ASR, in this case, poly(styrene/R-methylstyrene/acrylic acid), at the latex particle surface and more importantly at the outermost film surface occurred, which eventually led to a soft core-hard shell type of particle structure with a cellular type of film morphology. The effect of surface enrichment by a polymeric additive in a latex dispersion on film morphology is revealed by atomic force microscopy (AFM). Kinetics studies of this system also show that the migration of the hydrophilic ASR to the film surface is achieved within hours.24 The simple technique of contact angle measurement is often used to probe the surface energetics of polymers and could provide valuable information directly relevant to changes in surface properties as a result of a change in the surrounding medium. The interpretation of data depends entirely on the basic concept of the interfacial phenomenon. Indeed contact angle is much more sensitive to the quick changes of surface configuration than many other surface analytical means, although it cannot provide details at the molecular level of the configuration changes such as X-ray photoelectron spectroscopy (XPS) is capable (15) Isaacs, P. K. J. Macromol. Chem. 1966, 1, 163. (16) Vijayendran, B. R.; Bone, T.; Sawyer, L. C. J. Dispersion Sci. Technol. 1982, 3, 81. (17) Kast, H. Makromol. Chem. 1985, 10/11 (Suppl.), 447. (18) (a) Bindschaedler, C.; Gurny, R.; Doelker, E. J. Appl. Polym. Sci. 1987, 34, 2631; (b) 1989, 37, 173. (19) Roulstone, B. J.; Wilkinson, M. J.; Hearn, J. Polym. Int. 1992, 27, 43. (20) Park, Y. J.; Lee, D. Y.; Khew, M. C.; Ho, C. C.; Kim, J. H. Langmuir 1998, 14, 5419. (21) Park, Y. J.; Lee, D. Y.; Khew, M. C.; Ho, C. C.; Kim, J. H. Colloids Surf. A 1998, 139, 49. (22) Lee, D. Y.; Shin, J. S.; Park, Y. J.; Khew, M. C.; Ho, C. C.; Kim, J. H. Surf. Interface Anal. 1999, 28, 28. (23) Park, Y. J.; Khew, M. C.; Ho, C. C.; Kim, J. H. Colloid Polym. Sci. 1998, 276, 709. (24) Lee, D. Y.; Choi, H. Y.; Khew, M. C.; Ho, C. C.; Kim, J. H. Langmuir 1999, 15, 8252. (25) Lee, D. Y.; Kim, J. H. J. Appl. Polym. Sci. 1998, 69, 543. (26) Kuo, P. L.; Chen, C. J. J. Polym. Sci. 1993, 31, 99. (27) Astafieva, I.; Zhong, X. F.; Eisenberg, A. Macromolecules 1993, 26, 7339. (28) Wang, T. K.; Iliopoulos, I.; Audebert, R. In Water-Soluble Polymers; Shalaby, S. W., Ed.; ACS Symp. Series 467; American Chemical Society: Washington, D.C., 1991; p 218. (29) Omi, S. J. Polym. Sci., Part A: Polym. Chem. 1994, 32, 571. (30) Piirma I. Polymeric Surfactants; Surfactant Science Series 42; Marcel Dekker: New York, 1992; p 205.
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of. The contact angle method is sensitive to the upper 0.5 nm31 or so of a surface, whereas XPS provides information on the elemental ratios and functional groups of polymer surfaces within 5 nm.32 Thus, using the contact angle, information such as hydrophilicity-hydrophobicity balance, occurrence of reorientation of functional groups at the outermost surface, and microscopic roughness can be deduced. Besides the morphology of the film surface by AFM and the kinetics of film formation of these mixed latex ASR systems reported by us previously,20-24 we have now extended these studies to the surface properties of PBMA latex films containing postadded poly(styrene/R-methylstyrene/acrylic acid) (SAA) monitored using the contact angle method in order to gain more insight into the distribution of the components at the film surface. The effects of annealing on the latex films as a function of annealing temperature, its duration, and SAA concentration are investigated. The exudation of SAA molecules to the air/polymer interface is followed by FTIR-ATR. Experimental Section Materials. Distilled, deionized water was used throughout. n-Butyl methacrylate was purchased from Junsei Chemical Co., Japan. It was purified by vacuum distillation under reduced pressure and refrigeration at 4 °C until needed. The alkali-soluble resin, poly(styrene/R-methylstyrene/acrylic acid) (SAA) was purchased from Morton Inc. Ltd., U.S., and used as received. We measured molecular weight, glass transition temperature, and acid number of SAA (Mn ) 4300 g mol-1; Mw ) 8600 g mol-1, acid number ) 190, and Tg ) 115 °C). Acid number is the number of milligrams of KOH needed to neutralize 1 g of alkali-soluble resin. Therefore we can estimate the number of acrylic acid monomers incorporated in the copolymer. Potassium persulfate (KPS), from Samchun Pure Chemical Ind. Ltd., Korea, was recrystallized before use. Reagent-grade sodium bicarbonate from Samchun Pure Chemical Ind. Ltd. was used as received. Tetrahydrofuran (THF) and ethylene glycol (EG) were purchased from Aldrich Chemical Inc., U.S. Preparation and Characterization of PBMA Latexes. PBMA latexes were prepared by emulsifier-free emulsion polymerization using the following composition: 720 g of distilled, deionized water, 80.0 g of n-butyl methacrylate, 0.8 g of KPS, and 0.25 g of sodium bicarbonate. These were charged into a double-jacketed glass reactor equipped with stirrer, thermometer, condenser, and nitrogen inlet and polymerized at 70 °C for 24 h. The mean particle size (Dn) and polydispersity (Dw/Dn) of the latex particles were determined by a particle size analyzer, namely, the Capillary Hydrodynamic Fractionation (model CHDF-1100, Matec Applied Sci., U.S.) and were found to be 370.0 nm and 1.004, respectively. Molecular weight determination, using gel permeation chromatography with a Styragel series GPC column having 105-104-103-500 Å pore size from Waters and with THF as a solvent at a flow rate of 2 mL/min, shows that PBMA was obtained with Mw ) 4.4 × 105 g mol-1 and Mw/Mn ) 3.2. All latex samples were purified by ion exchange followed by serum replacement technique until the final conductivity of the serum was less than 10-5 Ω-1 cm-1.33 The latex obtained was stable without the need for any postadded stabilizer. Preparation of Latex Films. A known weight of a SAA solution at 10 wt %, obtained by dissolving the resin in distilled, deionized water adjusted to pH 9, was added slowly to 100 g of PBMA latex (solids content: 10%) at 25 °C, and then the mixed dispersion was allowed to equilibrate with mild agitation for 24 h. A series of mixed dispersions containing various weights of SAA solution was prepared in this manner. Table 1 summarizes the composition of PBMA film samples prepared in this study. (31) Volger, E. A. Interfacial Chemistry in Biomaterials Science. In Wettability; Berg, J. C., Ed.; Surfactant Science 3 Series 49; Marcel Dekker: New York, 1993. (32) Holmes-Farley, S. R.; Whitesides, G. M. Langmuir 1987, 3, 62. (33) Kim, J. H.; Chainey, M.; El-Aasser, M. S.; Vanderhoff, J. W. J. Polym. Sci., Part A: Polym. Chem. 1992, 30, 171.
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Table 1. Compositions of a Series of Mixtures of PBMA Latex and SAA Solutiona sample
PB-S03
PB-S05
PB-S10
PB-S20
PB-S40
PBMA latex SAA solution
100 3
100 5
100 10
100 20
100 40
a Unit: gram. Solid content of PBMA latex: 10 wt %. Solid content of SAA solution: 10 wt %.
The films studied by FTIR-ATR and contact angle methods were deposited as thin films on a glass surface. All films were first dried at 50 °C for 24 h. The other films, after they were dried, were annealed at the respective temperature in an oven for fixed duration. FTIR-ATR Spectroscopy. A FTIR spectrophotometer (model 2000, Perkin-Elmer, U.K.) with a total internal reflectance or attenuated total reflectance (ATR) accessory was used to obtain the spectra. The penetration depth of the measurements was 5-10 µm from the film surface. A total of 128 scans was accumulated at 4 cm-1 resolution. Contact Angle Measurement. A contact angle goniometer (Kruss, model G23, Germany) was used to measure the contact angle of a drop of a probe liquid (containing 50/50 wt % mixture of ethylene glycol and water) delivered from a syringe onto the film surface at 25 °C. The average of the angles obtained at more than ten different locations on each sample surface was reported. Atomic Force Microscopy (AFM). The latex films were prepared by depositing a few drops of the mixed dispersion of PBMA and SAA onto a freshly cleaved mica surface (ca. 10 mm × 10 mm) and allowed to dry at 25 °C in a desiccator. All films were imaged in air at 25 °C using a Nanoscope III AFM (Digital Instruments, Inc., Santa Barbara, CA) operated in the Tapping Mode. In the Tapping Mode, the cantilever on which the tip is mounted is oscillated at a frequency of ca. 250 kHz. The oscillation is driven by a constant driving force, and the amplitude of its oscillation is monitored. The scans were done under ambient conditions without any sample surface treatment.
Figure 1. FTIR-ATR spectra showing the 710-690 cm-1 region (typical absorption peak for benzene ring) of the air/ polymer interface of (A) pure PBMA film and (B) pure SAA film.
Results and Discussion FTIR-ATR on Films from Mixed Dispersions. Before the analysis of the surface infrared spectra of SAA and PBMA latex films, the relevant features in the infrared spectra of all individual components of the latex films will be defined. Figure 1 illustrates the FTIR-ATR spectra of the PBMA latex film (trace A) and the SAA film (trace B), respectively. The SAA spectrum shows the characteristic strong absorbance band for the aromatic ring (due to styrene groups) at 710-690 cm-1, which is absent in the spectrum of the PBMA latex film. Thus the absorbance bands in this region are unique for SAA and will be taken to reflect the presence of SAA due to their migration and exudation to the film surface. The FTIR-ATR spectra of films of PBMA latex containing postadded SAA at different concentrations after drying at 50 °C for 24 h are presented in Figure 2. It is clear that as the concentration of SAA in the latex increases, the SAA concentration in the surface of the film also increases. In the mixed dispersions of PBMA-SAA, the adsorption of SAA on the latex particle is quite different from those of conventional stabilizers. SAA copolymers are not a salt form of carboxylic acid stabilizer; they are only soluble in alkali conditions and then have a peculiar adsorption behavior. The partitioning of the SAA copolymer between the continuous phase and the latex particles is strongly dependent on the pH of the dispersion. The lower the pH, the more hydrophobic the SAA, and then the more the adsorption of SAA is occurred. When SAA is added to surfactant-free PBMA latex, part of the SAA is adsorbed on the PBMA particle surface and the rest remains in the aqueous phase at pH 9. During drying, the pH of the mixed dispersion decreases, resulting in a higher amount of SAA being adsorbed on the particle surface. Eventually all the SAA would fill the space
Figure 2. FTIR-ATR spectra in the 1300-650 cm-1 region of the air/polymer interface of PBMA latex films containing (A) 3 wt % SAA, (B) 10 wt % SAA, (C) 30 wt % SAA, and (D) 40 wt % SAA.
between the PBMA particles when all the water has evaporated. Thus, the higher the concentration of SAA used, the more the SAA would be present between the PBMA particles. The presence of this SAA on and between the PBMA particles changes the interaction between them. As the particles deform and coalesce, more SAA, being hydrophilic, will migrate to and accumulate at the air/ polymer interface. This behavior is confirmed by performing the annealing of the films at elevated temperature as shown in Figure 3. When annealing the film for 1 h at 90 °C which is below the Tg of SAA (Tg ) 130 °C), some exudation of the SAA was noted compared to those given in Figure 2. The AFM images shown in Figure 4 clearly depict the morphology of the film surface before and after annealing at 90 °C for
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Figure 3. FTIR-ATR spectra in the 1300-650 cm-1 region of the air/polymer interface of PBMA latex films containing 10 wt % SAA: (A) dried film before annealing; (B) after annealing at 90 °C for 1 h.
30 min.20,21 The accumulation of SAA at the interstitial space between the arrays of PBMA particles on the surface is clearly discernible in Figure 4b. The AFM results are consistent with the formation of a hard shell of SAA over the soft PBMA particles and the migration of SAA to the film surface during film formation as described previously.20-22 The deformation and eventual coalescence of the PBMA particles occur below the surface layer of SAA during annealing. As the annealing temperature is raised, more SAA molecules would migrate to the air/polymer interface. The exudation of the SAA is the direct consequence of the incompatibility between PBMA and SAA.22 Our findings are in general agreement with recent work by Urban et al.34,35 on the migration of surfactants to the air/polymer interface in latex films monitored using FTIRATR. It was also concluded that incompatibility between the polymer and surfactant was one of the driving forces for preferential segregation of surfactant at the film surface. By annealing the film at 150 °C, which is well above the Tg of SAA, the whole process of migration of SAA to the surface is accelerated as shown by the very much higher SAA concentration found at the film surface after annealing for 2 h (see Figures 5 and 6). Both PBMA and SAA soften and flow at this temperature, making it easier for SAA to migrate to the surface. It was also found that the SAA concentration at the film surface did not change much after annealing at 150 °C for 2 h. This means that an equilibrium was reached after which no further migration of SAA to the film surface occurred with prolonged annealing. Contact Angles on Films from Mixed Latex Dispersions. Figure 7 reveals that the contact angles of the probe liquid (EG to water at 50:50 w/w) on pure SAA films prepared from aqueous solution were very much lower than those on SAA films prepared from THF as a solvent. The contact angles on both types of films decrease with time, and a constant equilibrium value was reached after (34) Evanson, K. W.; Urban, M. W. J. Appl. Polym. Sci. 1991, 42, 2287. (35) Urban, M. W. Polym. Mater. Sci., Eng. 1995, 73, 137.
Figure 4. Three-dimensional AFM surface images of PBMA latex film with 10 wt % SAA (a) before and (b) after annealing at 90 °C for 30 min.
ca. 20 min. However, the SAA film prepared from THF had a much bigger equilibrium contact angle compared to that for the aqueous system. This difference in behavior of the contact angle is directly related to the different conformations of the SAA molecules in water and in THF as solvent, respectively. SAA, being an amphiphilic polymer, forms aggregate-like micelles in water.25 Thus the hydrophilic carboxylic acid groups of the SAA molecule are exposed to the aqueous phase, and the hydrophobic groups remain in the interior of the aggregates. In contrast, SAA is soluble in a random structure in THF, an organic solvent. The chain conformations of SAA in both solvents would be maintained during drying, respectively. Thus the hydrophobic aromatic rings of SAA could induce bigger contact angles on SAA film prepared from THF, whereas the SAA film prepared from water is more hydrophilic and the contact angles of the probe liquid on them are lower. The contact angles of the probe liquid on PBMA latex film are found to be intermediate between those on SAA film prepared from water and from THF, respectively.
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Figure 5. FTIR-ATR spectra in the 1300-650 cm-1 region of the air/polymer interface of PBMA latex films containing 20 wt % SAA: (A) dried film before annealing; after annealing at 150 °C for (B) 1 h and (C) 2 h.
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Figure 7. Decay characteristics of the contact angle of the probe liquid (EG: water (50:50 wt %)) on pure SAA films and pure PBMA latex film after drying at 50 °C. Table 2. Contact Angles of a Probe Liquid (Containing 50:50 wt % Ethylene Glycol and Water) on Film Samples after Annealing at 90 and 150 °C, Respectively, for 1 h 50 °Ca
90 °C
150 °C
film samples
θI
θF
θI
θF
θI
θF
SAA prepared from water SAA prepared from THF pure PBMA PB-S05 PB-S10 PB-S20 PB-S40
62.3 81.7 76.3 67.5 65.2 62.5 62.7
56.8 71.7 68.4 60.9 57.1 57.1 56.4
72.0 76.0 76.5 76.5
63.6 67.5 68.1 68.6
78.2 78.6 78.1 78.9
70.5 70.3 70.5 70.4
a Contact angle measure after drying at 50 °C. θ : Initial contact I angle value measured after dropping. θF: Final contact angle value measured after 40 min.
Figure 6. FTIR-ATR spectra in the 1300-650 cm-1 region of the air/polymer interface of PBMA latex films containing 40 wt % SAA: (A) dried film before annealing; after annealing at 150 °C for (B) 1 h and (C) 2 h.
The contact angles of the probe liquid on PBMA latex films containing different amount of SAA after drying at 50 °C for 24 h and annealing at 90 °C, 150 °C for 1 h are listed in Table 2. It can be seen that, after drying at 50 °C for 24 h, the equilibrium contact angle decreases with increasing SAA concentration, i.e., the surface of the PBMA latex film becomes increasingly more hydrophilic, and eventually the equilibrium contact angle of PB-S40 approaches that of the pure SAA film (prepared from water) itself (see Figure 8a). This strongly suggests that the chain configuration of SAA adsorbed on the surface of the PBMA particles in the PBMA latex films is similar to that of the pure SAA film prepared from water as a solvent. Upon blending of the SAA solution with the PBMA latex, the SAA molecules adsorb on the surface of the PBMA particles and stabilize them via intermolecular
and/or intramolecular hydrophobic interactions. Annealing the mixed latex film at 50 °C did not change the chain conformation of SAA in the aqueous phase, and the concentration of hydrophilic groups on the film surface remain high giving rise to low equilibrium contact angles. Also, the above results suggest that with increasing SAA concentration greater coverage of the PBMA latex particles is achieved, and eventually the PBMA particles are completely covered by SAA molecules at 40 wt % and the film surface essentially reflects that of SAA. At 5.0 wt % SAA, both SAA and PBMA characteristics are expected at the film surface, as illustrated by the intermediate values of the contact angles. On the other hand, the behavior of the PBMA latex films containing SAA annealed at 90 °C for 1 h is completely different. The equilibrium contact angles of the probe liquid on the PBMA latex film increase with increasing SAA concentration in the mixture. The contact angle values eventually reached a value similar to that of the pure SAA film prepared from THF (Figure 8b). The difference in contact angle values before annealing and after annealing at 90 °C was related to the change of the chain conformation of SAA molecules. It is clear that the higher kinetic energy associated with the higher annealing temperature has enabled unfolding of the SAA molecules to occur, resulting in an increase in the hydrophobicity of the film surface. At 40 wt % SAA, the PBMA latex film
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Figure 9. Equilibrium contact angle of the probe liquid (EG: water (50:50 wt %)) on PBMA latex films containing different amounts of SAA after annealing at 90 °C and 150 °C for 1 h.
behaves like that of the pure SAA film prepared from THF. Increasing the annealing temperature of the film to 150 °C speeds up remarkably the rate of change in the conformation of the SAA molecules. The equilibrium contact angles of the probe liquid on all latex films, irrespective of the SAA concentration, converge to values similar to that of the pure SAA film prepared from THF (Figure 8c). This clearly suggests that the much greater mobility and unrestricted migration of the SAA molecules at 150 °C to the film surface has resulted in a film surface bearing the properties of the exuded SAA, even at the low SAA concentration of 5 wt %. Figure 9 shows briefly the equilibrium contact angle of PBMA latex films containing SAA with different annealing temperatures. The equilibrium contact angle increases during the annealing process which results in the change of the chain conformation of SAA molecules. In the case of PB-S05, before annealing it showed intermediate values between that of PBMA film and that of SAA film prepared from water owing to the incomplete coverage of SAA on the PBMA film surface. After the annealing process, however, the exudation of SAA to the air/polymer interface and the chain conformation change of SAA caused a contact angle value similar to those of other samples to be reached. Figure 10 depicts schematically the exudation of SAA to the air/polymer interface and the change of chain conformation of SAA molecules in latex film during annealing. Some information on the contact angle of latex films or their surface energetics seems to be available in the literature.36,37 Information on the effect of polymeric additives on the contact angles of monodisperse latex films is even more scarce. Dobler et al.38 described work on the contact angles of air bubbles or alkane on styrene/butyl acrylate coploymer latex films under water and reported that the hydrophilicity of the film increases with the methacrylic acid (MAA) content of the shell material of the core-shell latex particles used. The trend of the change in hydrophilicity of the latex film surface with MAA content of the shell bears some resemblance to the influence of the SAA content on the hydrophilicity of the Figure 8. Decay characteristics of the contact angle of the probe liquid (EG: water (50:50 wt %)) on PBMA latex films containing SAA (a) after drying at 50 °C, after annealing (b) at 90 °C for 1 h, and (c) at 150 °C for 1 h.
(36) Noda, I. Nature 1991, 350, 143. (37) Noda, I. J. Adhes. Sci. Technol. 1992, 6, 467. (38) Dobler. F.; Affrossman, S.; Holl, Y. Colloids Surf. 1994, 89, 23.
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In yet another related situation, Keddie41 examined the film formation of acrylic latexes in the presence of nonfilm-forming latex of methyl methacrylate-2-ethylhexyl acrylate. Again the surface properties were derived from ellipsoimetry and environmental SEM. On the other hand, the exudation of surfactants in the presence of a coalescing aid in latex film was studied by AFM.42 It was found that the presence of the coalescing aid promoted the exudation of the surfactant to the film surface. Conclusions
Figure 10. Schematic representation of (a) the exudation of SAA to the air/polymer interface during the annealing process and (b) the change of chain conformation of SAA molecules in latex film during annealing.
PBMA film annealed at 50 °C in the present system. The findings of Joanicot and co-workers39,40 on the cellular type of latex film using core-shell particles that the thermal fragmentation of the cellular film depends on the mobility of the shell and core materials, in addition to whether the shell material is anchored or merely adsorbed onto the core, is in general agreement with our conclusions. However, these results were derived from small angle neutron scattering, and no contact angle data were given. (39) Wong, K.; Joanicot, M.; Richard, J.; Maquet, J.; Cabane, B. Macromolecules 1993, 26, 3168. (40) Joanicot, M.; Wong, K.; Cabane, B. Macromolecules 1996, 29, 4976. (41) Keddie, J. L.; Meredith, P.; Jones, R. A. L.; Donald, A. M. Langmuir 1996, 12, 3793. (42) Juhue, D.; Wang, Y. C.; Lang, J.; Leung, O. M.; Goh, M. C.; Winnik, M. A. J. Polym. Sci., Part B: Polym. Phys. 1995, 33, 1123.
The FTIR-ATR results are clearly indicative of the exudation of the SAA to the surface of the PBMA latex film under thermal influence. At a temperature above the Tg of the SAA, (i) its mobility is high enough to effect the unrestricted migration of SAA to the film surface, and (ii) the energy is sufficient to cause a change in the chain configuration of SAA, thereby exposing the hydrophobic region of the molecules to the film surface. The contrasting behavior of the SAA in water and in THF, and the striking similarity of the surface properties of the annealed PBMA latex film containing SAA with those of the SAA in water and THF, is a manifestation of reorientation at the molecular level on the film surface. This also means that the surface properties of the same latex film can be manipulated at will to produce film surfaces of different characters by just choosing an annealing temperature commensurable with the Tg of the postadded ASR. The preferential enrichment of the film surface by SAA via thermal influence results in a rather unique film structure cross-sectionally. The technique could possibly be adopted to produce latex films that can meet variable application environments resulting from thermal changes. Further exploration in this direction is worth pursuing. Acknowledgment. The authors thank Young-Jun Park and Mei-Ching Khew for their valuable discussions. The authors acknowledge the financial support of (i) the Korea Institute of S&T Evaluation and Planning (KISTEP) made in the program year of 1999 and (ii) the Ministry of Science, Technology and the Environments, Malaysia, under the IRPA program, project number 03-02-03-0226. LA9906370
