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A Novel Approach for Preparation of Micrometer-sized, Monodisperse Dimple and Hemispherical Polystyrene Particles† Takuya Tanaka, Yoshifumi Komatsu, Teruhisa Fujibayashi, Hideto Minami, and Masayoshi Okubo* Graduate School of Engineering, Kobe University, Kobe 657-8501, Japan Received September 3, 2009. Revised Manuscript Received October 5, 2009 Micrometer-sized, monodisperse dimple and hemispherical polystyrene (PS) particles were successfully prepared by heating (55-70 °C) of spherical PS particles dispersed in methanol/water media (40/60 to 80/20, w/w) in the presence of decane droplets, and subsequent cooling down to room temperature. Decane was absorbed by the PS particles during the heating process. Decane-absorbed PS particles phase-separated into PS and decane phases in the inside during the cooling process, and eventually dimple and/or hemispherical particles were formed by removal of the decane phase from phase-separated PS/decane particles by evaporation. The size of the dimple, which is determined by the volume of decane phase-separated from decaneabsorbed PS particles during the cooling process, increased with increases in the heating temperature and the methanol content.
Introduction Nonspherical shape is one of the functional properties for the applications of polymer particles. Such particles were utilized to synthesize materials with unique crystal structures,1-5 light scattering properties,6 and external (e.g., shear field7 and electric field8) field-responsive materials. Thus, control of particle shape is of practical importance for industrial applications. Generally, polymer particles prepared by heterogeneous polymerizations as typified by emulsion polymerization exhibit spherical shape as a result of the minimization of the interfacial free energy. However, the formation of nonspherical polymer particles have been described in the report of the preparation of the composite Part CCCXXX of the series “Studies on Suspension and Emulsion” *To whom correspondence should be addressed. Telephone/Fax: þ81-78803-6161. E-mail:
[email protected]. †
(1) Lu, Y.; Yin, Y.; Xia, Y. Adv. Mater. 2001, 13, 415–420. (2) Lu, Y.; Yin, Y.; Li, Z.-Y.; Xia, Y. Langmuir 2002, 18, 7722–7727. (3) Mock, E. B.; Zukoski, C. F. Langmuir 2007, 23, 8760–8771. (4) Hosein, I. D.; Liddell, C. M. Langmuir 2007, 23, 8810–8814. (5) Hosein, I. D.; Liddell, C. M. Langmuir 2007, 23, 10479–10485. (6) Mishchenko, M. I.; Hovenier, J. W.; Travis, L. D., Light scattering by nonspherical particles: theory, measurements, and applications. Academic Press: San Diego, CA, 2000. (7) Jogun, S. M.; Zukoski, C. F. J. Rheol. 1999, 43, 847–871. (8) Ho, C. C.; Ottewill, R. H.; Yu, L. Langmuir 1997, 13, 1925–1930. (9) Matsumoto, T.; Okubo, M.; Shibao, S. Kobunshi Ronbunshu 1976, 33, 575– 583. (10) Okubo, M.; Ando, M.; Yamada, A.; Katsuta, Y.; Matsumoto, T. J. Polym. Sci. Polym. Lett. Ed. 1981, 19, 143–147. (11) Okubo, M.; Katsuta, Y.; Matsumoto, T. J. Polym. Sci. Polym. Lett. Ed. 1982, 20, 45–51. (12) Cho, I.; Lee, K.-W. J. Appl. Polym. Sci. 1985, 30, 1903–1926. (13) Skjeltorp, A. T.; Ugelstad, J.; Ellingsen, T. J. Colloid Interface Sci. 1986, 113, 577–582. (14) Okubo, M.; Kanaida, K.; Matsumoto, T. Colloid Polym. Sci. 1987, 265, 876–881. (15) Okubo, M. Makromol. Chem., Macromol. Symp. 1990, 35/36, 307–325. (16) Sheu, H. R.; El-Aasser, M. S.; Vanderhoff, J. W. J. Polym. Sci., Part A: Polym. Chem. 1990, 28, 629–651. (17) Sheu, H. R.; El-Aasser, M. S.; Vanderhoff, J. W. J. Polym. Sci., Part A: Polym. Chem. 1990, 28, 653–667. (18) Okubo, M.; Fujiwara, T.; Yamaguchi, A. Colloid Polym. Sci. 1998, 276, 186–189. (19) Ni, H.; Ma, G.; Nagai, M.; Omi, S. J. Appl. Polym. Sci. 2001, 80, 2002–2017. (20) Okubo, M.; Wang, Z.; Yamashita, T.; Ise, E.; Minami, H. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 3106–3111. (21) Okubo, M.; Takekoh, R.; Suzuki, A. Colloid Polym. Sci. 2002, 280, 1057– 1061.
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polymer particles by various seeded polymerizations.9-34 Morphology of the composite polymer particles is determined by the competition between thermodynamic and kinetic factors.35 High viscosity inside the particles during seeded polymerization prevents the particles from attaining their thermodynamic equilibrium morphology, and consequently nonspherical particles with nonequilibrium morphology dominated by kinetic factors are obtained. On the other hand, several other approaches for the preparation of nonspherical particles have also been developed; instantaneous UV-curing of nonspherical monomer droplets using microfluidic reactor with specific microchannel geometries,36-39 deformation of spherical polymer particles by external force,40-42 (22) Okubo, M.; Miya, T.; Minami, H.; Takekoh, R. J. Appl. Polym. Sci. 2002, 83, 2013–2021. (23) Wang, D.; Dimonie, V. L.; Sudol, E. D.; El-Aasser, M. S. J. Appl. Polym. Sci. 2002, 84, 2710–2720. (24) Kaneko, T.; Hamada, K.; Chen, M. Q.; Akashi, M. Macromolecules 2004, 37, 501–506. (25) Okubo, M.; Fujibayashi, T.; Terada, A. Colloid Polym. Sci. 2005, 283, 793– 798. (26) Okubo, M.; Fujibayashi, T.; Yamada, M.; Minami, H. Colloid Polym. Sci. 2005, 283, 1041–1045. (27) Stubbs, J. M.; Sundberg, D. C. Polymer 2005, 46, 1125–1138. (28) Kegel, W. K.; Breed, D.; Elsesser, M.; Pine, D. J. Langmuir 2006, 22, 7135– 7136. (29) Kim, J.-W.; Larsen, R. J.; Weitz, D. A. J. Am. Chem. Soc. 2006, 128, 14374– 14377. (30) Mock, E. B.; De Bruyn, H.; Hawkett, B. S.; Gilbert, R. G.; Zukoski, C. F. Langmuir 2006, 22, 4037–4043. (31) Zhou, L.; Shi, S.; Kuroda, S.; Kubota, H. Chem. Lett. 2006, 35, 248–249. (32) Fujibayashi, T.; Okubo, M. Langmuir 2007, 23, 7958–7962. (33) Kim, J.-W.; Larsen, R. J.; Weitz, D. A. Adv. Mater. 2007, 19, 2005–2009. (34) Yang, S.-M.; Kim, S.-H.; Yi, G.-R. J. Mater. Chem. 2008, 18, 2177–2190. (35) Sundberg, D. C.; Durant, Y. G. Polym. React. Eng. 2003, 11, 379–432. (36) Dendukuri, D.; Tsoi, K.; Hatton, T. A.; Doyle, P. S. Langmuir 2005, 21, 2113–2116. (37) Nie, Z.; Xu, S.; Seo, M.; Lewis, P. C.; Kumacheva, E. J. Am. Chem. Soc. 2005, 127, 8058–8063. (38) Xu, S.; Nie, Z.; Seo, M.; Lewis, P. C.; Kumacheva, E.; Stone, H. A.; Garstecki, P.; Weibel, D. B.; Gitlin, I.; Whitesides, G. M. Angew. Chem., Int. Ed. 2005, 44, 724–728. (39) Nisisako, T.; Torii, T.; Takahashi, T.; Takizawa, Y. Adv. Mater. 2006, 18, 1152–1156. (40) Ho, C. C.; Keller, A.; Odell, J. A.; Ottewill, R. H. Colloid Polym. Sci. 1993, 271, 469–479. (41) Alargova, R. G.; Bhatt, K. H.; Paunov, V. N.; Velev, O. D. Adv. Mater. 2004, 16, 1653–1657. (42) Sun, Z. Q.; Chen, X.; Zhang, J. H.; Chen, Z. M.; Zhang, K.; Yan, X.; Wang, Y. F.; Yu, W. Z.; Yang, B. Langmuir 2005, 21, 8987–8991.
Published on Web 10/16/2009
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the self-organized precipitation method,43,44 and release of a common good solvent from thermodynamically stable nonspherical polymer/solvent droplets.45,46 Recently, we have proposed a new possibility for the preparation of nonspherical polymer particles by seeded dispersion polymerization in the presence of hydrocarbon droplets. Micrometer-sized, monodisperse “golf ball-like” particles have been successfully prepared by seeded dispersion polymerization of styrene with poly(methyl methacrylate) seed particles in the presence of decalin droplets.21 Moreover, we have also succeeded in the preparation of micrometer-sized, monodisperse polymer particles having “disc-like” and “polyhedral” shapes by seeded dispersion polymerization of various methacrylic monomers with polystyrene (PS) seed particles in the presence of hydrocarbon droplets.25,32 Each hydrocarbon solvent did not dissolve the seed particles but was selectively absorbed by a separated phase in the composite particles, which was composed of a second polymer formed by seeded dispersion polymerization. Nonspherical composite particles were obtained after evaporation of the hydrocarbon solvent as a result of the volume reduction of the hydrocarbon-absorbed second polymer phase. In a series of these investigations, PS particles having a single dimple at the surface (referred to as “dimple particles”) were observed in the early stage of seeded dispersion polymerization with spherical PS seed particles in the presence of decane droplets. The formation of dimple particles cannot be explained by the above-mentioned mechanism because the size of the dimple was too large considering the amount of the second polymer (only approximately 5% monomer conversion). It would appear that the deformation of the particles occurred regardless of the polymerization process. The aim of this study is to establish a novel and simple approach for the preparation of micrometer-sized, monodisperse dimple PS particles. The heat treatment (not polymerization) of 2.54-μm-sized PS particles dispersed in a methanol/water medium in the presence of decane droplets at various heating temperatures, stirring rates and medium compositions was carried out to clarify the formation mechanism of dimple particles.
Experimental Section Materials. Styrene was distilled under reduced pressure in a
nitrogen atmosphere. Reagent-grade 2,20 -azobisisobutyronitrile (AIBN) (Wako Pure Chemical Industries, Ltd., Osaka, Japan) was purified by recrystallization with methanol. Deionized water with a specific resistance of 5 106 Ω 3 cm was used. Poly(vinyl pyrrolidone) (PVP) (weight-average molecular weights: K-90, 3.6 105), methanol and decane (Nacalai Tesque Inc., Kyoto, Japan) were used as received. Preparation of PS Particles. Monodisperse spherical PS particles were prepared under the optimum dispersion polymerization conditions determined in our previous work.26 The number-average diameter (Dn) and coefficient of variation (Cv) of the seed particles were measured with a JEOL JEM-1230 transmission electron microscope (TEM) using image analysis software (MacSCOPE, Mitani Co. Ltd., Japan) for Macintosh computer. The PS particles were used after centrifugal washing with methanol for three times. (43) Higuchi, T.; Yabu, H.; Shimomura, M. Colloids Surf., A 2006, 284-285, 250–253. (44) Tajima, A.; Higuchi, T.; Yabu, H.; Shimomura, M. Colloids Surf., A 2008, 313-314, 332–334. (45) Saito, N.; Nakatsuru, R.; Kagari, Y.; Okubo, M. Langmuir 2007, 23, 11506–11512. (46) Tanaka, T.; Nakatsuru, R.; Kagari, Y.; Saito, N.; Okubo, M. Langmuir 2008, 24, 12267–12271.
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Figure 1. Optical micrograph (a) and SEM photograph (b) of PS particles after stirring at 400 rpm in the presence of decane droplets in a methanol/water (80/20, w/w) medium at 60 °C for 6 h.
Heat Treatment of Spherical PS Particles. The heat treatments of spherical PS particles were carried out in the presence of decane droplets in a methanol/water medium in the two-neck flask (closed system) with magnetic stirrer using the following recipe: PS particles, 0.5 g; methanol/water medium, 10 g; decane 1 g. Decane is partially soluble in the medium and formed droplets during the heat treatment. The heating temperature, stirring rate and methanol/water ratio were varied in the ranges 40-70 °C, 200-600 rpm and 100/0 to 0/100 (w/w), respectively. The particles sampled from the heated dispersion were observed with a MICROPHOT-FXA optical microscope and a Hitachi S-2460 scanning electron microscope (SEM) at a voltage of 20 kV, respectively. Measurements. Glass transition temperature (Tg) was measured using approximately 1 g of the PS dispersion, which was degassed in a desiccator for vacuum with stirring for 20 min, by power compensation-type high sensitivity differential scanning calorimeter (PC-DSC; nano-DSC 5100, Calorimetry Sciences Co., USA) in the temperature range of 0 to 50 or 100 °C (at 1 °C/ min. During the measurement, the dispersion was pressurized to 3 atm to prevent evaporation. The amounts of decane in the PS dispersion and in the methanol/water medium were determined by gas chromatography (GC; GC-2014, Shimadzu Corp.) with helium as the carrier gas, N,N-dimethylformamide as a solvent, and p-xylene as an internal standard. The 1.0 g decane in the methanol/water medium in the presence and absence of the 0.5 g PS particles was stirred at 400 rpm for 24 h at 60 °C to reach equilibrium. The amount of decane absorbed by PS particles was calculated by subtracting the amount of decane in the methanol/ water medium from that in the PS dispersion.
Results and Discussion Figure 1 shows an optical micrograph and a SEM photograph of PS particles after the heat treatment (not polymerization) at 60 °C for 6 h with stirring at 400 rpm in the presence of decane droplets in a methanol/water (80/20, w/w) medium. In both photographs, dimple particles were observed. Interestingly, most particles had orientation with their dimples upward at a dry state although not form long-range crystalline structure (Figure 1b). This may be explained by the opposite surface of the dimple sitting stably in the space between the particles in the layer below.4 To elucidate the optimum heat treatment conditions for the formation of dimple particles, we examined the effect of following parameters: (i) temperature, (ii) stirring rate, (iii) methanol/water composition of the medium. Figure 2 shows SEM photographs of PS particles after stirring at 400 rpm in the presence of decane droplets in the methanol/ water (80/20, w/w) medium at various temperatures for 1 h. Dimple particles were observed after the heat treatment above 55 °C although spherical particles were observed below 50 °C. The size of the dimple increased with an increase in the heat treatment DOI: 10.1021/la903309t
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Figure 2. SEM photographs of PS particles after stirring at 400 rpm in the presence of decane droplets in the methanol/water (80/20, w/w) medium for 1 h at various temperatures (°C): (a) 40; (b) 50; (c) 55; (d) 60; (e) 70.
Figure 3. Relationship between the formation percentage of nonspherical (dimple and hemispherical) particles and the temperature of the heat treatment at stirring of 400 rpm in the presence of decane droplets in the methanol/water (80/20, w/w) medium for 3 h.
temperature, resulting in the formation of hemispherical particles at 70 °C. Figure 3 shows variation of percentage of the nonspherical (dimple and hemispherical) particles as a function of the heat treatment temperature. The number of the nonspherical particles was carefully counted with observations of over 200 particles undergoing Brownian motion with an optical microscope for a certain interval so as not to overlook any particles that might have had their dimple oriented downward at any given time. The percentage of the nonspherical particles drastically increased on raising the temperature to around 55 °C. These results indicate that the heat treatment above 60 °C is required for the formation of the nonspherical particles. Heat treatment above a glass transition temperature (Tg) of polymer particles is required for the deformation of the particles without external force. Tg of the PS particles dispersed in the 3850 DOI: 10.1021/la903309t
Figure 4. SEM photographs of PS particles after stirring in the presence of decane droplets in the methanol/water (80/20, w/w) medium at 60 °C for 1 h at various stirring rates (rpm): (a) 200; (b) 300; (c) 400; (d) 600.
methanol/water (80/20, w/w) medium (TgC; subscript means “colloid”) after stirring at 400 rpm in the presence of decane droplets at 60 °C for 24 h was approximately 29 °C.32 This value was very low in comparison to Tg of PS in dry states (TgD ≈ 100 °C).47 According to a previous study,48 Tg values of submicrometer-sized polymer particles in aqueous dispersed systems, (47) Andrews, R. J.; Grulke, E. A. In Polymer handbook; 4th ed.; Brandrup, J., Immergut, E. H., Grulke, E. A., Eds.; John Wiley & Sons: New York, 1999; pp VI/ 193-277. (48) Okubo, M.; Inoue, M.; Suzuki, T.; Kouda, M. Colloid Polym. Sci. 2004, 282, 1150–1154.
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which were prepared by emulsion polymerization, were slightly lower than TgD values because of the plasticization with water, which were measured by PC-DSC. TgC of the PS particles after the heat treatment under the same conditions except for the absence of decane droplets was not less than 50 °C (PC-DSC measurement was not carried out above 50 °C so as to prevent medium composition from changing during the measurement.). These results suggest that the reduction of the TgC of the PS particles in the presence of decane is attributed to the absorption of decane into the particles (plasticization) although decane is a poor solvent for PS, consequently resulting in allowing the particle deformation. Figure 4 shows SEM photographs of PS particles after stirring at various rates in the presence of decane droplets in the methanol/water (80/20, w/w) medium at 60 °C for 1 h. The formation rate of the dimple particles increased with increasing stirring rate and the formation percentages attained 100% at Figure 7. PC-DSC 2nd heating curves of PS dispersion after stirring at 400 rpm in the presence of decane droplets at 60 °C for 24 h in the various methanol/water media (w/w): (a) 0/100; (b) 20/ 80; (c) 40/60; (d) 60/40; (e) 80/20; (f) 100/0.
Figure 5. Relationship between the formation percentages of dimple particles and the heat treatment times at 60 °C in the presence of decane droplets in the methanol/water (80/20, w/w) medium at the various stirring rates (rpm): (O) 200; (0) 300; (4) 400; (]), 600.
Figure 8. Optical micrographs of PS particles dispersed in the methanol/water (80/20, w/w) medium in the presence of decane at 60 °C for 20 min (a) and after being left to cool down to room temperature (b). Both processes were carried out under a closed system.
Figure 6. SEM photographs of PS particles after stirring at 400 rpm in the presence of decane droplets at 60 °C for 24 h in various methanol/ water media (w/w): (a) 0/100; (b) 20/80; (c) 40/60; (d) 60/40; (e) 80/20; (f) 100/0. Langmuir 2010, 26(6), 3848–3853
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Figure 9. Schematic representation of the formation mechanism of dimple and hemispherical PS particles dispersed in the methanol/water medium in the presence of decane droplets. Table 1. Differences in the Volumes of Decane (ΔVGC = V60°C V30°C) Absorbed by One PS Particle Between at 30 and 60 °C at Various Methanol/Water Ratios of the Media Measured by Gas Chromatography and the Volumes of a Single Dimple (VSEM) of the Particles Estimated from SEM Photographs
Figure 10. Amounts of decane absorbed by PS particles as functions of methanol contents (wt %) in the methanol/water medium after stirring at 400 rpm for 24 h at 30 °C (O) and 60 °C (b).
600 rpm stirring within a few hours as shown in Figure 5. The time to attain 100% deformation was shortened with the stirring rate. This seems to be based on the increase in the absorption of decane into the PS particles with the increase in the stirring rate. Ugelstad et al. proposed a formula indicating the absorption rate of monomer into polymer particles in the case that the diffusion of monomer through the aqueous phase is a ratedetermining step for the transport of monomer from monomer droplets to swelling particles.49 According to the formula, the absorption rate of monomer into polymer particles is accelerated by reducing size of the monomer droplets. In the current study, the “monomer” term can be replaced by “decane”, and thus the absorption rate of decane into the particles should be accelerated by reducing size of the decane droplets caused by increasing stirring rate. This is the reason that the formation of dimple particles was achieved in a short time at the high stirring rate. Figure 6 shows SEM photographs of PS particles after stirring at 400 rpm in the presence of decane droplets at 60 °C for 24 h in various methanol/water media. The spherical shape was maintained in the media below 20 wt % methanol content. On the other hand, the dimple particles were formed at the methanol contents in the range of 40 to 80 wt %, and the size of the dimple increased with an increase in the methanol content, resulting in the formation of some hemispherical particles at 80 wt %. However, at 100 wt % methanol, only spherical particles were observed. (49) Ugelstad, J.; Mørk, P. C.; Kaggerud, K. H.; Ellingsen, T.; Berge, A. Adv. Colloid Interface Sci. 1980, 13, 101–140.
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methanol/water ratios (w/w)
ΔVGC (μm3)
VSEM (μm3)
40/60 60/40 80/20
0.715 0.653 0.695
0.638 0.768 0.893
Figure 7 shows PC-DSC second heating curves of PS dispersion after stirring at 400 rpm in the presence of decane droplets at 60 °C for 24 h in the various methanol/water media. The TgC values decreased with increasing methanol content in the medium except the case of 100 wt %. This result will be discussed later. At the methanol contents in the range of 40 to 100 wt %, the TgC values of the PS particles were much lower than the heat treatment temperature of 60 °C. Popli et al.50,51 reported that small amount of ethanol was absorbed by PS particles dispersed in an ethanol/ water medium although ethanol is also a poor solvent for PS. Decane is partially miscible in methanol, and the miscibility increases with increasing temperature.52 Therefore, methanol in the particles should facilitate the absorption of decane into the particles to increase the entropy of the mixing. From above results, it is concluded that the dimple and/or hemispherical particles could be prepared by heating (55 to 70 °C) of spherical PS particles dispersed in methanol/water media (40/ 60 to 80/20, w/w) in the presence of decane droplets, and the deformation rate of the particles was accelerated by the rapid stirring. Figure 8 shows optical micrographs of PS particles dispersed in the methanol/water (80/20, w/w) medium in the presence of decane at 60 °C for 20 min and after being left to cool down to room temperature under a closed system to prevent the evaporations of methanol, water and decane. The PS particles absorbing decane exhibited a homogeneous morphology at 60 °C, subsequently phase-separated into PS and decane phases with descending temperature, resulting in PS/decane particles with hemispherical morphology at room temperature. When the cooling process was under the open system as before (the heating process was obviously under the closed system), not such phaseseparated particles but the dimple particles as shown in Figure 1b were only observed with the optical microscope. This might be explained by evaporation of the decane as a result of exposure of the dispersion to air while putting a drop of it on a slide glass for the optical microscope observation. Additionally, decane-absorbed PS dispersion of 60 °C was poured into excess methanol (50) Popli, R.; Luccas, M. H.; Tsaur, S. L. Langmuir 1991, 7, 69–72. (51) Popli, R. Langmuir 1991, 7, 73–80. (52) Matsuda, H.; Ochi, K. Fluid Phase Equilib. 2004, 224, 31–37.
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at room temperature to remove the decane quickly, eventually the dimple particles formed as expected. On the basis of these findings, it seems that the formation of the dimple was caused by the phase separation between the PS and the decane phases, and the subsequent removal of the decane by evaporation from the decane phase, as illustrated in Figure 9. The phase-separated morphology of the PS/decane particles was determined by the minimization of the interfacial free energy. In other words, varying each interfacial tension can control the phaseseparated morphology, resulting in tuning the final particle shape. Figure 10 shows amounts of decane absorbed by the PS particles at 30 and 60 °C at various methanol/water ratios of the media measured by GC. The amounts of decane in the PS particles increased with increasing methanol content of the media except the case of 100 wt %. In the case of 100 wt %, decane was completely dissolved in the methanol medium (no decane droplets), and thus preferentially partitioned into the medium rather than the particles compared with the case of 80 wt % methanol content. Consequently, the TgC of the PS particles at the methanol content of 100 wt % was slightly higher than that of 80 wt %. Furthermore, the differences in the volumes of decane (ΔVGC) in the PS particles between at 30 and 60 °C at the various methanol contents in the range of 40 to 80 wt % were significantly larger than those at the other methanol contents. Compared to the SEM photographs (Figure 6), the deformation of the particles occurred in the cases of large ΔVGC. Table 1 shows ΔVGC and volumes of a single dimple (VSEM) of the particles formed at various methanol/water ratios estimated using the SEM photographs by subtraction of the particle volume before deformation (spherical particle) from that after deformation (dimple particle) on the assumption that dimple particles were spherical shape including dimple part. ΔVGC values were in good agreement with VSEM values at all methanol/water ratios. Thus, dimple and hemispherical particles would be formed by removal of the decane phase by evaporation from phase-separated PS/decane particles during the cooling process. (53) Lu, Y.; Yin, Y.; Xia, Y. Adv. Mater. 2001, 13, 34–37. (54) Sakurai, Y.; Okuda, S.; Nagayama, N.; Yokoyama, M. J. Mater. Chem. 2001, 11, 1077–1080.
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We have proposed a novel and simple approach for the preparation of dimple and hemispherical polymer particles: Heating of the spherical polymer particles dispersed in the methanol/water medium stirred with poor solvent for the particles, and subsequent cooling down to room temperature (not polymerization process). Removal of the solvent by evaporation from phase-separated polymer/solvent particles during cooling process led to the formation of the dimple and/ or hemispherical particles. These nonspherical particles, in particular, hemispherical particles will be oriented on planar surfaces with curved surfaces pointing upward and act as microlens.53,54 This simple preparation method might be applicable to other polymer and poor solvent system, and can thus be a very useful tool for industrial applications. On the basis of the formation mechanism, we are currently going to extend the range of possible nonspherical particles by controlling the phase-separated morphology from the viewpoint of thermodynamic considerations.
Conclusions Micrometer-sized, monodisperse dimple and hemispherical PS particles were successfully prepared by heating spherical PS particles at higher temperature than a glass transition temperature of the PS particles dispersed in a methanol/water medium in the presence of decane droplets, and subsequent cooling down to room temperature. The optimum conditions for the formation of the dimple and hemispherical particles including temperature, stirring rate and composition of the medium were demonstrated. Formation of the dimple and hemispherical particles was caused by absorption of decane into the PS particles during the heating process, phase separation of PS/decane particles during the cooling process and removal of the decane phase by evaporation. The size of the dimple depended on the amount of decane in the PS particles. This method would be an innovative technique for the preparation of nonspherical polymer particles. Acknowledgment. This work was supported by Grant-in-Aid for Scientific Research (Grant 21245050) from the Japan Society for the Promotion of Science (JSPS).
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