Preparation and Thermodynamic Stability of Micron-Sized

Micron-sized, monodisperse composite polymer particles having “disc-like” and “polyhedral” shapes were prepared by seeded dispersion polymeriz...
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Langmuir 2007, 23, 7958-7962

Preparation and Thermodynamic Stability of Micron-Sized, Monodisperse Composite Polymer Particles of Disc-like Shapes by Seeded Dispersion Polymerization† Teruhisa Fujibayashi‡ and Masayoshi Okubo*,‡,§ Graduate School of Science and Technology, Kobe UniVersity, Kobe 657-8501, Japan, and Department of Chemical Science and Engineering, Faculty of Engineering, Kobe UniVersity, Kobe 657-8501, Japan ReceiVed March 16, 2007. In Final Form: May 8, 2007 Micron-sized, monodisperse composite polymer particles having “disc-like” and “polyhedral” shapes were prepared by seeded dispersion polymerization of 2-ethylhexylmethacrylate (EHMA) with 2.67-µm-sized polystyrene (PS) seed particles in methanol/water media in the presence of droplets of various saturated hydrocarbons and evaporation of the hydrocarbon after the polymerization. Such nonspherical shapes were based on the volume reduction due to the evaporation. The primary factors influencing the particle shape seemed to be the absorption rate of the hydrocarbon into the resulting PS/poly(EHMA)/hydrocarbon composite particles during the polymerization, which affected the viscosities and the volumes of the PS and poly(EHMA) phases. It was found that the morphological development during the polymerization was retarded at “hamburger-like” morphology, which is a precursor of the disc-like particle, although this morphology is a thermodynamically metastable state.

Introduction Nonspherical polymer particles have been utilized to synthesize materials with unique crystal1 structures, light-scattering properties, and materials that are responsive to external fields such as shear fields2 and electric fields.3 Control of particle shape is thus of great interest both from an academic and applications perspective. Minimization of the interfacial free energy usually results in spherical particles. However, preparations of various submicron-size nonspherical composite particles by seeded emulsion polymerization (e.g., “confetti-like”,4 “raspberrylike”,5,6 “void-containing”,7 and “octopus ocellatus-like”8) have been reported since the middle of the 1970s. The morphology of composite polymer particles is determined by a combination of thermodynamic and kinetic factors.9 The high viscosity within the particles during seeded emulsion polymerization can prevent attainment of equilibrium morphology, and consequently nonspherical particles with nonequilibrium morphology dominated by kinetic factors are formed. Several other approaches to prepare submicron-sized nonspherical particles have recently been described.10-13 * To whom correspondence should be addressed. Tel/Fax: +81-78-8036161. E-mail: [email protected]. † Part CCXC of the series “Studies on Suspension and Emulsion”. ‡ Graduate School of Science and Technology, Kobe University. § Faculty of Engineering, Kobe University. (1) Yin, Y.; Xia, Y. AdV. Mater. 2001, 13, 267. (2) Jogun, S. M.; Zukoski, C. F. J. Rheol. 1999, 43, 847. (3) Ho, C. C.; Ottewill, R. H.; Yu, L. Langmuir 1997, 13, 1925. (4) Matsumoto, T.; Okubo, M.; Shibao, S. Kobunshi Ronbunshu 1976, 33, 575. (5) Okubo, M.; Katsuta, Y.; Yamada, A.; Matsumoto, T. Kobunshi Ronbunshu 1979, 36, 459. (6) Cho, I.; Lee, K.-W. J. Appl. Polym. Sci. 1985, 30, 1903. (7) Okubo, M; Ando, M.; Yamada, A.; Katsuta, Y.; Matsumoto, T. J. Polym. Sci. Polym. Lett. Ed. 1981, 19, 143. (8) Okubo, M.; Kanaida, K.; Matsumoto, T. Colloid Polym. Sci. 1987, 265, 876. (9) Sundberg, D. C.; Durant, Y. G. Polym. React. Eng. 2003, 11, 379. (10) Okubo, M.; Ichikawa, K.; Tsujihiro, M.; He, Y. Colloid Polym. Sci. 1990, 268, 791. (11) Kaneko, T.; Hamada, K.; Chen, M. Q.; Akashi, M. Macromolecules 2004, 37, 501. (12) Stubbs, J. M.; Sundberg, D. C. Polymer 2005, 46, 1125.

Micron-sized, monodisperse, nonspherical polymer particles have also been reported: “snowman-like” particles by utilizing interpenetrating polymer networks of cross-linked polystyrene particles14-16 and “egg-like”,17 snowman-like,18,19 and confettilike19 composite particles by seeded dispersion polymerization (SDP). SDP is a powerful technique for preparation of nonspherical particles dominated by kinetic factors. The viscosity within the seed particles is high when the glass transition temperature exceeds the polymerization temperature, thus preventing equilibrium morphology to be attained.20 We recently proposed SDP in the presence of organic solvent droplets as a novel technique for preparation of nonspherical particles.21 The organic solvent was selected to be predominantly absorbed by the domain comprising the second polymer (i.e., not the seed polymer) of the composite particle. Nonspherical composite particles were obtained after evaporation of the solvent as a result of the significant volume reduction of the phase comprising the second polymer and the solvent. By utilizing this technique, various nonspherical particles of unique shapes have been prepared. Monodisperse “golf-ball-like” particles21 were prepared by SDP of styrene with poly(methyl methacrylate) (PMMA) particles in the presence of decalin droplets, and “disclike” particles22 were prepared by SDP of various methacrylate monomers with 1.57-µm-sized polystyrene (PS) seed particles (13) Mock, E. B.; Bruyn, H. D.; Hawkett, B. S.; Gilbert, R. G.; Zukoski, C. F. Langmuir 2006, 22, 4037. (14) Sheu, H. R.; El-Aasser, M. S.; Vanderhoff, J. W. J. Polym. Sci. Part A: Polym. Chem. 1990, 28, 629. (15) Kim, J.-W.; Larsen, R. J.; Weitz, D. A. J. Am. Chem. Soc. 2006, 128, 14374. (16) Kegel, W. K.; Breed, D.; Elsesser, M.; Pine, D. J. Langmuir 2006, 22, 7135. (17) Okubo, M.; Miya, T.; Minami, H.; Takekoh, R. J. Appl. Polym. Sci. 2002, 83, 2013. (18) Wang, D.; Dimonie, V. L.; Sudol, E. D.; El-Aasser, M. S. J. Appl. Polym. Sci. 2002, 84, 2710. (19) Okubo, M.; Fujibayashi, T.; Yamada, M.; Minami, H. Colloid Polym. Sci. 2005, 283, 1041. (20) Okubo, M.; Takekoh, R.; Izumi, J.; Yamashita, T. Colloid Polym. Sci. 1999, 277, 972. (21) Okubo, M.; Takekoh, R.; Suzuki, A. Colloid Polym. Sci. 2002, 280, 1057. (22) Okubo, M.; Fujibayashi, T.; Terada, A. Colloid Polym. Sci. 2005, 283, 793.

10.1021/la7007842 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/16/2007

Disc-like, Monodisperse Composite Polymer Particles

in the presence of hydrocarbon droplets. Disc-like particles23,24 (approximately 50 µm) have also been prepared recently with a microfluidic reactor by using a combination of soft lithography and complex pattern formation in two-phase flows. There are also reports describing the preparation of nonspherical particles by similar techniques utilizing phase separation of polymers and solvent and evaporation of solvent; Kumacheva and co-workers prepared hemispherical polymer particles by utilizing phase separation between silicon oil and monomer in a microfluidic reactor,25 and we investigated the influence of solvent distribution in separated PS and PMMA phases on the shape of PS/PMMA composite particles prepared by evaporation of solvent from solvent droplets containing dissolved PS and PMMA.26 In the preparation of disc-like particles by SDP in the presence of organic solvent droplets, the particle shape is primarily affected by (i) the alkyl chain length of ester group of the methacrylate, (ii) the type of hydrocarbon, and (iii) the methanol/water ratio of medium. When gradually changing one of these factors (for example, by decreasing the length of the hydrocarbon from hexadecane to hexane), the particle shape changes from golfball-like to disc-like via “polyhedron-like”.22 It is at present believed that the particle shape changes in such a manner as a result of the effect of the above parameters on the morphology prior to solvent evaporation and the distribution of solvent between seed polymer and the second polymer. The key factors controlling the particle shape and the formation mechanism of nonspherical particles during SDP in the presence of organic solvent droplets are at present poorly understood. In order for this technique to realize its potential for the design of the functional nonspherical particles, a mechanistic understanding of the process is important. To this end, the formation mechanism of disc-like particles during SDP of 2-ethylhexyl methacrylate (EHMA) with 2.67-µm-sized PS seed particles in the presence of various hydrocarbons droplets in methanol/water has been examined. The seed particles employed were larger than those in previous studies, in order to be able to study the morphology development by optical microscopy. The results are discussed in terms of the thermodynamic stabilities of the various morphologies of the PS/PEHMA/hydrocarbon composite particles. Experimental Section Materials. Styrene was purified by distillation under reduced pressure in a nitrogen atmosphere. Reagent grade 2,2′-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·cm was distilled. EHMA, poly(N-vinylpyrrolidone) (PVP) (weight-average molecular weights: K-90, 3.6 × 105; K-30, 4 × 104), methanol, butanol, hexane, octane, decane, dodecane, tetradecane, hexadecane (Nacalai Tesque Inc., Kyoto, Japan), and 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) (Aldrich) were used as received. Seed Particles. Monodisperse PS seed particles were prepared under the optimum dispersion polymerization conditions determined in a previous article.19 The number-average diameter (Dn) and coefficient of variation (Cv) of the seed particles were measured with a transmission electron microscope (TEM) (H-7500, Hitachi Science Systems Ltd., Ibaraki, Japan) using image analysis software (MacSCOPE, Mitani Co. Ltd., Fukui, Japan) for a Macintosh (23) Dendukuri, D.; Tsoi, K.; Hatton, T. A.; Doyle, P. S. Langmuir 2005, 21, 2113. (24) 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. (25) Nie, Z.; Xu, S.; Seo, M.; Lewis, P. C.; Kumacheva, E. J. Am. Chem. Soc. 2005, 127, 8058. (26) Okubo, M.; Saito, N.; Fujibayashi, T. Colloid Polym. Sci. 2005, 283, 691.

Langmuir, Vol. 23, No. 15, 2007 7959 computer. PS seed particles were used after centrifugal washing three times with methanol. Seeded Dispersion Polymerization. SDPs of EHMA were carried out in the presence of various hydrocarbons in sealed glass tubes under nitrogen atmosphere using the following recipe: PS seed particles, 0.25 g; EHMA, 0.125 g; AIBN, 3 mg; PVP K-30, 0.05 g; methanol, 8 g; water, 2 g; hydrocarbon, 1.25 g. The sealed glass tubes were shaken horizontally (3-cm strokes) at 60 °C at 60 cycles/ min in a water bath. The hydrocarbons used were hexadecane, tetradecane, dodecane, decane, octane, and hexane. The methanol/ water ratio and shaking rate were varied between 90/10 and 70/30 (w/w) (total amount of methanol/water, 10 g) and 20 and 100 cycles/ min, respectively. The obtained composite particles were observed with a scanning electron microscope (SEM) (S-2500, Hitachi Science Systems Ltd., Ibaraki, Japan) after extraction of poly(EHMA) (PEHMA) and hydrocarbon with 1-butanol. The conversion of EHMA was measured by gas chromatography. Measurement of Amount of Hydrocarbon Absorbed into PS Particle by Gas Chromatography. A mixture of 8 g of methanol, 2 g of water, and 1.25 g of hydrocarbon was allowed to reach equilibrium at 60 °C by shaking at 60 cycles/min for 24 h. The solubility of the hydrocarbon in the methanol/water medium was measured by gas chromatography. A dispersion of 0.25 g of PS seed particle, 8 g of methanol, 2 g of water, and 1.25 g of hydrocarbon was kept under the same conditions. The amount of hydrocarbon in the PS dispersion was measured by gas chromatography, and the amount of absorbed hydrocarbon into PS particles was calculated by assuming that the medium was saturated with the hydrocarbon. Measurement of Glass Transition Temperature by Power Compensation-Type High Sensitivity Differential Scanning Calorimeter (PC-DSC). The above PS dispersion was degassed in a desiccator for vacuum with stirring for 20 min. The glass transition temperature (Tg) was measured using approximately 1 g of the dispersion by PC-DSC (nano-DSC 5100, Calorimetry Sciences Co.) in the temperature range from 0 to 60 °C at 1 °C/min. During the measurement, the dispersion was pressurized to 3 atm to prevent evaporation. Interfacial Tension Measured by the Pendant Drop Method. Interfacial tensions between hydrocarbons and a methanol/water (80/20, w/w) medium were measured by the pendant drop method using a Drop Master 500 (Kyowa Interface Science Co., Ltd.). Pendant drops of the hydrocarbons were formed at the tip of the stainless steel needle in a glass cell filled with methanol/water (80/20, w/w) medium. All the measurements were performed at room temperature (ca. 20 °C). The accuracy of the measured interfacial tensions was on the order of (0.2 mN/m.

Results and Discussion Although 1.57-µm-sized PS particles were used in a previous article,22 PS particles having larger diameter (Dn, 2.67 µm; Cv, 3.61%), which were prepared by dispersion polymerization, were used in order to facilitate in situ observation of the morphology of composite particle consisting of PS and PEHMA phases by optical microscope. As shown in the previous article, the disclike particle had hamburger-like morphology before evaporation of the hydrocarbon. Thus, observation of the composite particles dispersed in the medium is important to understand the relationship between the particle shape and the morphology. Figure 1 shows SEM photographs of PS particles after the extraction of PEHMA and the hydrocarbon with 1-butanol from PS/PEHMA/hydrocarbon composite particles prepared by SDPs in the presence of various hydrocarbon droplets in methanol/ water (80/20, w/w). The extraction of PEHMA and the hydrocarbon was conducted to observe the particle shape obviously, because particle adhesion occurred during drying due to the low Tg of PEHMA.22 However, this process is not a requirement for preparation of the dispersion of nonspherical particles, which are prepared by simple evaporation of the

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Figure 1. SEM photographs of PS particles after the extraction of PEHMA and the hydrocarbon with 1-butanol from PS/PEHMA/ hydrocarbon composite particles prepared by seeded dispersion polymerizations in methanol/water (80/20, w/w) medium in the presence of various kinds of saturated hydrocarbon: (a) hexadecane, (b) tetradecane, (c) dodecane, (d) decane, (e) octane, (f) hexane.

Figure 2. SEM photographs of PS particles after the extraction of PEHMA and decane with 1-butanol from PS/PEHMA composite particles prepared by seeded dispersion polymerizations in the presence of decane droplets at various ratios of methanol/water (w/ w) media: (a) 70/30, (b) 75/25, (c) 80/20, (d) 85/15, (e) 90/10.

hydrocarbon. Golf-ball-like particles were observed in the case of hexadecane (see Figure 1a). With decreasing length of the alkyl chain of the hydrocarbon, the particle shapes changed from golf-ball-like to disc-like via polyhedron-like. Disc-like particles were prepared in the cases of decane and octane. The conversions in parts a and d of Figure 1 were 81 and 69%, respectively, and the conversions in the other polymerizations are anticipated to be similar. The effect of the conversion on the particle shape will be discussed later. Figure 2 shows SEM photographs of PS particles after extraction of PEHMA and decane from PS/PEHMA/decane composite particles prepared by SDPs in methanol/water media in weight ratios of 70/30 to 90/10 in the presence of decane droplets. The particle shapes changed from polyhedron-like to disc-like with increasing methanol content. The results shown in Figures 1 and 2, which were similar to the results reported in the previous article,24 and the volume of disc-like particle were calculated from SEM photograph, and the result of the calculation gave good agreement with the volume of polystyrene seed particle. Thus, the spherical PS seed particle changed their shape during SDPs, and PEHMA existed at the PS particle surfaces. Although it was observed that the size of PS seed particle affected on the shape of PS/PnBMA composite particles prepared by SDP,15 in

Fujibayashi and Okubo

Figure 3. SEM photographs of PS particles after the extraction of PEHMA and decane with 1-butanol from PS/PEHMA composite particles prepared by seeded dispersion polymerization in the presence of decane droplets in methanol/water (80/20, w/w) at various conversions (%): (a) 5, (b) 13, (c) 34, (d) 49, (e) 57, (f) 69.

this case the effect of seed particle size was not significant in the range of 1.57 and 2.67 µm. Figure 3 shows SEM photographs of PS particles after the extraction of PEHMA and decane with 1-butanol from PS/ PEHMA/decane composite particles at various conversions of SDPs in the methanol/water (80/20, w/w) in the presence of decane droplets. The golf-ball-like particles were observed at 13% conversion and changed their shape to polyhedron-like and finally to disc-like with increasing conversion. Disc-like particles were observed at 69% conversion (24 h). This indicates that PEHMA/decane domains grew larger by polymerization therein, absorption of decane, and consolidation of domains. It has previously speculated that the phase-separated domains were consolidated by the coalescence27-29 and Ostwald ripening.9 Interestingly, dimpled particles were observed at 5% conversion. Because their formation mechanism seemed to be different from those of golf-ball-like, polyhedron-like, and disc-like particles and unrelated to the polymerization, it will be discussed in the near future. From the result shown in the Figure 3, there is a possibility of further change of the morphology in the case of using hexadecane, which gives golf-ball-like particles at 24 h. Figure 4 shows optical micrographs (a-c) of PS/PEHMA/hexadecane composite particles prepared by SDP at various times in the methanol/water (80/20, w/w) medium in the presence of hexadecane droplets, and SEM photographs (d-f) of PS particles after extraction of PEHMA and hexadecane with 1-butanol from them. Although the conversion of EHMA and the particle size after 24 h did not change significantly, morphologies of PS/ PEHMA/hexadecane composite particles and the shape of the PS particles obviously changed (from golf ball to disc via polyhedron), which were confirmed by optical microscopy and SEM, respectively. This indicates that the morphology of the PS/PEHMA/hexadecane composite particles changed to a thermodynamically stable state because the interfacial free energy decreased with decreasing the number of PEHMA/hydrocarbon domains due to the decrease in the interfacial area between PS and PEHMA/hydrocarbon phases. It is noteworthy to point out that the morphology change was not necessarily accompanied by polymerization, and thus, the particle shape was affected by not only the conversion but also the rate of consolidation of (27) Gonzalez-Ortiz, L. J.; Asua, J. M. Macromolecules 1995, 28, 3135. (28) Gonzalez-Ortiz, L. J.; Asua, J. M. Macromolecules 1996, 29, 383. (29) Gonzalez-Ortiz, L. J.; Asua, J. M. Macromolecules 1996, 29, 4520.

Disc-like, Monodisperse Composite Polymer Particles

Figure 4. Optical micrographs (a-c) of PS/PEHMA/hexadecane composite particles and SEM photographs (d-f) of PS particles after the extraction of PEHMA and hexadecane with 1-butanol from PS/PEHMA composite particles prepared at various times of seeded dispersion polymerizations in methanol/water (80/20, w/w) medium in the presence of hexadecane droplets: (a, d) 40 h, (b, e) 56 h, (c, f) 72 h.

PEHMA/solvent domains. This result has good agreement with the fact that the conversion of disc-like particles was less than that of the golf-ball-like particles in Figure 1. In addition, it is confirmed that the nonspherical particles were induced by the phase-separated structure consisting of PS and PEHMA/ hydrocarbon phases and volume reduction of the PEHMA/ hydrocarbon phase due to the evaporation of hydrocarbon. From above results, it seems that the variations of shape of the particle prepared by SDP at 24 h result from the rate of changing morphology of the PS/PEHMA/solvent by the growth of the PEHMA/solvent domains. Both the PS and the PEHMA phases absorb solvent, resulting in an increase in volume and a decrease in viscosity of each phase. However, the amount of absorbed solvent per original unit volume was greater for the PEHMA phase, and thus, the PEHMA/solvent domain grew larger. The decrease of viscosity in the PS phase accelerates coalescence of PEHMA domains by collision and Ostwald ripening between the domains. As a result, disc-like particles are formed when the amount of absorbed solvent is sufficiently large. The mechanism of the absorption of hydrocarbon by the PS particles is considered to be similar to that of the absorption of monomer by the particles in emulsion polymerization. The absorption rate of monomer depends on solubility, mass transfer coefficient, total interfacial area, volume fraction, and the difference in chemical potential between monomer droplet and swollen particle of the monomer.30 If the absorption rate of monomer is enough fast to reach equilibrium state, the monomer amount in the particle is expressed in the competition between the reduction of free energy caused by mixing polymer with monomer and the increase in the interfacial free energy due to an increase in the interfacial area of the swollen particles results from the increase of the volume.31 The interfacial tensions between hexadecane, tetradecane, dodecane, decane, octane, hexane, and methanol/water (80/20, w/w) medium are, respectively, 8.9, 8.5, 8.2, 7.5, 6.5, and 5.2 mN/m. The decrease in the alkyl chain length of the hydrocarbon gives lower interfacial energy between the swollen particles and the medium, which seemed to promote swelling of the particles. However, the Morton equation31 reveals that an increase in interfacial free energy is enough small to be (30) Ugelstad, J.; Mørk, P. C.; Herder Kaggerud, K.; Ellingsen, T.; Berge, A. AdV. Colloid Interface Sci. 1980, 13, 101. (31) Morton, M.; Kaizerman, S.; Altier, M. W. J. Colloid Sci. 1954, 9, 300.

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Figure 5. PC-DSC second heating curves of PS particles swollen by (a) hexadecane, (b) tetradecane, (c) dodecane, and (d) decane.

ignored in the case of micron-sized particle. Thus, the free energy of mixing determines the amount of hydrocarbon in the particle. The experimental results indicate that the amounts of the hexadecane, tetradecane, dodecane, decane, octane, and hexane absorbed by the PS particles were, respectively, 0.12, 0.14, 0.16, 0.27, 0.39, and 0.42 g per 1 g of PS particles. This indicates that the affinity between hydrocarbon and PS increases with a decrease in the alkyl chain length of the hydrocarbons. The absorbed hydrocarbon behaves as plasticizer and the degree of plasticization for PS influences the morphological development. Figure 5 shows PC-DSC second heating curves of PS particles swollen by various hydrocarbons. The inflection points of molar heat capacity due to the glass transition of the PS shifted to lower temperature with decreasing alkyl chain length of the absorbed hydrocarbon. Tg values of PS particles swollen by octane and hexane were too low to be detected. These results indicate that the viscosity of the PS phase, which affected the rates of domain coalescence and Ostwald ripening between the domains, lowered with the decrease in the alkyl chain length of the hydrocarbons. This is the reason why the particle shape changed from golfball-like to disc-like via polyhedron-like (Figure 1). The viscosity of the PS particle swollen by hexadecane, tetradecane, and dodecane was too high to form a disc-like particle in 24 h. These considerations are consistent with the results shown in Figure 2. The solubilities of the hydrocarbons increased with the increasing methanol content of the medium, resulting in increases of the absorption rate and the amount of the hydrocarbons in the particle. Thus, variation of the particle shapes shown in Figures 1 and 2 can be explained similarly. Figure 6 shows PS particles after extraction of PEHMA and decane from PS/PEHMA/decane composite particles prepared by SDPs at various shaking rates in the presence of decane droplets in methanol/water (80/20, w/w). The interfacial area between decane and the medium increased with increasing shaking rate. The particle shapes were similar to those shown in Figures 1 and 2. The particle shapes changed from golf-ball-like to disc-like upon increasing the shaking rate. The swollen particles could not reach equilibrium state at the low shaking rate because of the low absorption rate of decane. The absorbed amount of the decane, which influenced the viscosity of the PS phase and volume of the PHMA/decane phase, was an important factor of determination of the particle shape. Finally, the thermodynamic stability of the hamburger-like morphology, which was precursor of the disc-like particle, was investigated. The hemispherical morphology seemed to be more stable because the interfacial area between PS and PEHMA

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Figure 6. SEM photographs of PS particles after the extraction of PEHMA and decane with 1-butanol from PS/PEHMA composite particles prepared by seeded dispersion polymerizations in the presence of decane droplets in methanol/water (80/20, w/w) medium at various shaking rates (cycles/min): (a) 20, (b) 40, (c) 60, (d) 80, (e) 100.

Fujibayashi and Okubo

droplets. Unexpectedly, no morphological change was observed during polymerization for 72 h, but the particles were enlarged by the further absorption of decane. For further examination, the dispersion of hamburger-like particles was heated after removal of excess decane and addition of TEMPO as inhibitor in order to prevent unnecessary enlargement of the particles and secondary nucleation. TEMPO is considered not to influence the morphological development caused by coalescence and/or Ostwald ripening of PEHMA/solvent because it was not necessarily accompanied by polymerization from the results shown in Figure 3. As shown in Figure 7c, the hamburger-like morphology changed to a hemispherical one after heat treatment at 150 °C for 24 h. This morphological change proves above prediction that the hemispherical morphology is more stable than the hamburger-like one. However, the hamburger-like morphology did not change when the particles were treated at 60 °C for 7days (See Figure 7d). These results perhaps indicate that the hamburgerlike morphology was stable at 60 °C. However, it is too difficult to consider that the hamburger-like morphology is more stable than the hemispherical one. Thus, the possible explanation is that the morphological development is too slow to change the morphology in the later stage of polymerization at 60 °C due to the high viscosity of the PS phase, which is caused by exhaustion of monomer. Perhaps there is an energy barrier between the hamburger-like and the hemispherical morphologies, which prevents the morphological development via coalescence by collision and/or the Ostwald ripening of PEHMA/decane domains at 60 °C. Future research will aim to further elucidate the mechanism of morphological development during polymerization and the reason for preservation of the hamburger-like morphology.

Conclusions

Figure 7. Optical micrographs of PS/PEHMA/decane composite particles prepared by seeded dispersion polymerizations in methanol/ water (80/20, w/w) medium in the presence of decane droplets for 24 h (a) and 72 h (b) and those after heat treatment of former particles (a) at 150 °C for 24 h (c) and at 60 °C for 7 days (d).

phases, which absorbed hydrocarbon, is smaller. Thus, one would anticipate that the hamburger-like morphology would eventually change to hemispherical one with or without polymerization. Figure 7 shows optical micrographs of PS/PEHMA/decane composite particles prepared by SDPs for 24 h (a) and 72 h (b) at 60 °C in methanol/water (80/20, w/w) in the presence of decane

Micron-sized, monodisperse polymer particles having disclike and polyhedral shapes were prepared by SDP of EHMA with 2.67-µm-sized polystyrene seed particles in the presence of saturated hydrocarbon droplets in methanol/water. Particle shapes were influenced by the type of hydrocarbon, shaking rate, and methanol content of the medium. These results indicate that the particle shape is governed by the viscosity of the PS phase and the hydrocarbon absorption rate. The hamburger-like morphology, which is a precursor of the disc-like particles, is less thermodynamically stable than the hemispherical morphology. However, hamburger-like morphology was maintained for several days, although the Tg of the swollen PS phase was below the polymerization temperature. The morphological development from hamburger-like morphology to hemispherical one is enough slow to be convenient for selective preparation of the disc-like particles. Acknowledgment. This work was partially supported by Creation and Support Program for Start-ups from Universities (No. 1509) from the Japan Science and Technology Agency (JST). LA7007842