Preparation of Multihollow Polymer Particles by Seeded Emulsion

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Preparation of Multihollow Polymer Particles by Seeded Emulsion Polymerization Using Seed Particles with Incorporated Nonionic Emulsifier† Hiroshi Kobayashi, Emi Miyanaga, and Masayoshi Okubo* Graduate School of Engineering, Kobe UniVersity, Kobe 657-8501, Japan ReceiVed April 13, 2007. In Final Form: June 20, 2007 Emulsion polymerizations of styrene using poly(oxyethylene) nonylphenyl ether nonionic emulsifier were carried out at different emulsifier and initiator (potassium persulfate, KPS) concentrations to prepare polystyrene (PS) seed particles with incorporated nonionic emulsifier. Seeded emulsion polymerizations of styrene using the PS seed particles with different amounts of incorporated emulsifier were carried out to develop a novel method for the preparation of multihollow particles. When seed particles with a small amount of incorporated emulsifier were used, non-hollow spherical particles were prepared. However, multihollow particles were obtained in the case of seed particles with a large amount of incorporated emulsifier. Moreover, the higher the initiator concentration in the preparation of seed particles, the more effectively were hollow particles prepared. On the basis of the above results, a mechanism for the formation of multihollow structure was suggested.

Introduction Hollow polymer particles have received much attention in many industrial fields. These particles are used as weight-saving thermal insulations, hiding and opaquifying agents1-3 through light scattering, and gloss enhancers4,5 for paper coatings. They can also potentially be used as microcapsules for controlled and sustained drug delivery systems6-9 in the pharmaceutical industry. Numerous techniques have been proposed to prepare such particles.10-20 We have succeeded in the preparation of micronsized, monodisperse, cross-linked polystyrene/polydivinylbenzene (PS/PDVB) particles having a single hollow. They were prepared by seeded polymerization of [divinylbenzene (DVB)/ hydrophobic solvent (such as toluene or p-xylene)]-swollen PS particles containing initiator prepared by the dynamic swelling method.21 Such hollow polymer particles were also prepared by †

Part CCXCV of the series “Studies on Suspension and Emulsion”. * To whom correspondence should be addressed. Tel/Fax: +81-78-8036161; e-mail: [email protected]. (1) Fasana, D. M. J. Coat. Technol. 1987, 59, 109. (2) Williams, E.; Fasana, D. M. In Handbook of Coating AdditiVes 2; Calbo, L., Ed.; Marcel Dekker: New York, 1992; p 233. (3) Anwari, F.; Arlozzo, B. J.; Kalpesh, C.; Dilorenzo, M.; Heble, M.; Knauss, C. J.; McCarthy, J.; Patterson, R.; Rozick, P.; Slifko, P. M.; Stipkovich, W.; Weaver, J. C.; Wolfe, M. J. Coat. Technol. 1993, 65, 39. (4) Shimura, Y. Japanese Patent 06280194, 1994. (5) Hultman, J. D.; Schuh, S. M. U.S. Patent 5,677,043, 1997. (6) Damge, C.; Michel, C.; Aprahamian, M.; Couvreuer, P.; Devissaguet, J. J. Controlled Release 1990, 13, 233. (7) Wilcox, D. L., Sr.; Berg, M.; Bernat, T.; Kellerman, D.; Cochran, J. K., Jr. Hollow and Solid Spheres and Microspheres: Science and Technology Associated With Their Fabrication and Application; Materials Research Society: Warrendale, PA, 1995; Vol. 372. (8) Gill, I.; Ballesteros, A. J. Am. Chem. Soc. 1998, 120, 8587. (9) Kost, J.; Langer, R. AdV. Drug DeliVery ReV. 2001, 46, 125. (10) Hislop, R. W.; McGinley, P. L. J. Coat. Technol. 1978, 50, 69. (11) Goldbrough, K.; Simpson, L. A.; Tunstall, D. F. Prog. Org. Coat. 1982, 10, 35. (12) Jain, M.; Nadkarni, S. U.S. Patent 4,782,097, 1988. (13) Kasai, K. Mater. Tech. 1993, 11, 92. (14) Taniguchi, T. U.S. Patent 5,723,077, 1998. (15) Kim, J. W.; Joe, Y. G.; Suh, K. D. Colloid Polym. Sci. 1999, 277, 252. (16) Schuler, C.; Caruso, F. Biomacromolecules 2001, 2, 921. (17) Jang, J.; Ha, H. Langmuir 2002, 18, 5613. (18) Ali, M. M.; Sto¨ver, H. D. Macromolecules 2003, 36, 1793. (19) Lee, J. E.; Kim, J. W.; Jun, J. B.; Ryu, J. H.; Kang, H. H.; Oh, S. G.; Suh, K. D. Colloid Polym. Sci. 2004, 282, 295. (20) Zhang, Y. W.; Jiang, M.; Zhao, J. X.; Wang, Z. X.; Dou, H. J.; Chen, D. Y. Langmuir 2005, 21, 1531. (21) Okubo, M.; Minami, H. Colloid Polym. Sci. 1997, 275, 992.

suspension polymerization, although they were polydisperse.22 The mechanism for the creation of the polymer particles with a single hollow structure is based on the Self-assembling of Phase Separated Polymer at an interface with aqueous media,23 which was named the SaPSeP method.24,25 One of the earliest attempts to synthesize submicron-sized hollow polymer particles is based on osmotic swelling.26,27 This method usually starts with the synthesis of structured polymer particles having a carboxylated polymer-core and a thermoplastic polymer-shell by emulsion polymerization. Subsequently, the core ionized by the addition of alkali is expanded by osmotic swelling, resulting in submicron-sized hollow particles with water and ionized polymer inside. McDonald et al. have reported on the preparation of monodisperse hollow particles with diameters from 0.2 to 1 µm by the modification of an emulsion polymerization with a water-miscible alcohol and a hydrocarbon nonsolvent.28 This method consists of two stages: encapsulation of hydrocarbon and stabilization of morphology with a crosslinker. Tiarks et al. have suggested the preparation of submicronsized hollow particles by miniemulsion polymerization in the presence of larger amounts of a hydrophobe.29 We have also reported various ways to prepare submicronsized (multi-)hollow polymer particles by post-treatments (the “stepwise alkali/acid method”30 and the “alkali/cooling method”31) for styrene-methacrylic acid copolymer [P(S-MAA)] particles. In both methods, the ionized P(S-MAA) particles absorb water, resulting in numerous small water pools in the alkali treatment process. In the stepwise alkali/acid method, ionized carboxyl groups are deionized, and the small water pools coalesce to become larger during the acid treatment process, resulting in the hollow structure. Moreover, when the carboxyl group concentra(22) Okubo, M.; Konishi, Y.; Minami, H. Colloid Polym. Sci. 2000, 278, 659. (23) Okubo, M.; Konishi, Y.; Minami, H. Colloid Polym. Sci. 2001, 279, 519. (24) Okubo, M.; Minami, H.; Oshima, Y. Polymer 2005, 46, 1051. (25) Minami, H.; Kobayashi, H.; Okubo, M. Langmuir 2005, 21, 5655. (26) Kowalski, A.; Vogal, M.; Blankenship, R. M. U.S. Patent 4,427,836, 1984. (27) McDonald, C. J.; Devon, M. J. AdV. Colloid Interface Sci. 2002, 99, 181. (28) McDonald, C. J.; Bouck, K.; Chaput, A. B.; Stevens, C. J. Macromolecules 2000, 33, 1593. (29) Tiarks, F.; Landfester, K.; Antonietti, M. Langmuir 2001, 17, 908. (30) Okubo, M.; Kanaida, K.; Fujimura, M. Chem. Express 1990, 5, 797. (31) Okubo, M.; Ito, A.; Kanenobu, T. Colloid Polym. Sci. 1996, 274, 801.

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tion is above some critical value, the small water pools can coalesce during the alkali treatment (the alkali/cooling method). Throughout these studies, it was observed that poly(oxyethylene) nonylphenyl ether nonionic emulsifier [Emulgen 911, hydrophilic-lipophilic balance (HLB) 13.7], used in the emulsion copolymerization for the preparation of P(S-MAA) (MAA, 10 mol %) particles, was incorporated inside the polymer particles during the polymerization.32 Surprisingly, 75% of the emulsifier was incorporated inside the particles. In further studies, incorporation was also clearly observed in the emulsion homopolymerization of styrene using two kinds of poly(oxyethylene) lauryl ether nonionic emulsifiers having different HLB values: Emulgen 109P (HLB 13.6) and Emulgen 150 (HLB 18.3).33 Additionally, 30 and 15% of the total mass were respectively incorporated inside the PS particles, although the levels of incorporation were lower than for Emulgen 911, as mentioned above. Moreover, the incorporation of two emulsifiers (Emulgen 911 and Emulgen 109P) having different hydrophobic portions in the emulsion polymerizations of methacrylic monomers (i.e., methyl methacrylate, ethyl methacrylate, or i-butyl methacrylate) was investigated. In the case of Emulgen 911, incorporation occurred in all cases. On the other hand, incorporation of Emulgen 109P was not observed in the cases of ethyl methacrylate and i-butyl methacrylate.34 Incorporation of emulsifier induces a decrease in the stability of polymer colloids (because of a decrease in the amount of emulsifiers used for stabilizing the particles) when using normal emulsifier concentrations. Moreover, incorporation causes several problems such as making control of particle size distribution and polymerization kinetics difficult, causing a lack of information about the required amount of emulsifier to stabilize the particles, and wasting emulsifier even if the particle surface is fully covered with emulsifier. In addition, in film applications, the existence of emulsifier in the final polymer particles (i.e., incorporation of emulsifier) causes a reduction in water resistance. However, we also found that the nonionic emulsifier incorporated inside the P(S-MAA) particles promotes the formation of (multi-)hollow structures by the alkali/cooling method.35 Furthermore, it was found that PS particles with incorporated poly(oxyethylene) lauryl ether nonionic emulsifier had the domains with less contrast (less electron density) with a transmission electron microscope (TEM) in our previous study.33 This result suggests that the incorporation of nonionic emulsifier can be useful for the preparation of hollow particles. In this article, we carried out seeded emulsion polymerization of styrene using PS seed particles with incorporated nonionic emulsifier in order to develop a novel method to prepare submicron-sized, hollow polymer particles without alkali (acid) swelling. Experimental Section Materials. Styrene was purified by distillation under reduced pressure. Potassium persulfate (KPS) of analytical grade (Nacalai Tesque, Inc., Kyoto, Japan) was purified by recrystallization. Commercial-grade poly(oxyethylene) nonylphenyl ether nonionic emulsifiers (Emulgen 911, HLB 13.7) with averages of 10.9 ethylene oxides per molecule were used as received (Kao Co., Tokyo, Japan). Sodium thiosulfate pentahydrate (Na2S2O8 5H2O), copper (II) chloride dihydrate (CuCl2 2H2O), 2-propanol, and tetrahydrofuran (THF) of guaranteed reagent grade were used as received (Nacalai Tesque). (32) Okubo, M.; Furukawa, Y.; Shiba, K.; Matoba, T. Colloid Polym. Sci. 2003, 281, 182. (33) Okubo, M.; Kobayashi, H.; Matoba, T.; Oshima, Y. Langmuir 2006, 22, 8727. (34) Chaiyasat, A.; Kobayashi, H.; Okubo, M. Colloid Polym. Sci. 2007, 285, 557. (35) Okada, M.; Matoba, T.; Okubo, M. Colloid Polym. Sci. 2003, 282, 193.

Letters Deionized water with a specific resistance of 5 × 106 Ω cm was distilled before use. Preparation of PS Seed Particles. PS seed particles were prepared by emulsion polymerization at 70 °C for 24 h under a nitrogen atmosphere in a four-necked 1-L round-bottom flask equipped with an inlet of nitrogen gas and a reflux condenser. Water (120 g) and Emulgen 911 (1, 5, and 10 wt % based on the total amount of styrene) were added to the reactor, and the solution was stirred with a half-moon-type stirrer at 240 rpm under a nitrogen atmosphere. After the solution was heated to 70 °C, styrene (4 g) was poured into the reactor. The mixture was deoxygenated with a stream of nitrogen gas for 30 min. Subsequently, a KPS aqueous solution (0.5, 2, and 8 wt % aliquots of KPS, based on the total amount of styrene, were dissolved in 20 g of water, respectively) was added to the reactor to initiate polymerizations. The conversions were over 90% according to gravimetric measurements. Seeded Emulsion Polymerization. The seed emulsion was centrifugally washed with distilled water to remove undecomposed initiator. PS seed particles were swollen with styrene (PS seed/ styrene, 1/0.25, 0.5, 0.75, w/w) in a two-necked 30-mL reactor at 30 °C for 24 h before starting polymerization. After swelling with styrene, a KPS/CuCl2 aqueous solution (10 mg of KPS and 2 mg of CuCl2 were dissolved in 0.75 g of water) and a Na2S2O8 aqueous solution (4 mg of Na2S2O8 dissolved in 1.25 g of water) as a redox initiator were added to the seed emulsions (7.2 g; solid content, 2.8%). Seeded emulsion polymerization was carried out under a nitrogen atmosphere at 40 °C for 24 h, with the same amounts of initiator solutions described above being added to the reactor every 6 h. Particles were observed with a Hitachi H-7500 TEM. Each emulsion was diluted to about 50 ppm, and a drop was placed onto a carbon-coated copper grid and allowed to dry at room temperature in a desiccator. The weight-average particle diameter and particle size distributions were determined by dynamic light scattering (DLS, DLS-7000, Otsuka Electronics Co., Ltd., Osaka, Japan). Quantitative Analysis of Nonionic Emulsifier Inside PS Seed Particles. The PS seed emulsions were centrifugally washed with 2-propanol three times to remove emulsifier from the particle surfaces, and subsequently dried at room temperature under reduced pressure for a few days. It was confirmed that the emulsifiers were completely removed using the above technique in the previous work.33,34 The validity of direct quantitative analysis by gel permeation chromatography (GPC) or 1H NMR was also confirmed by the fact that the results obtained by direct analysis were essentially identical with those obtained by indirect analysis based on mass balance. (The amount of emulsifier inside the particles was obtained by subtracting the amounts of emulsifiers in the medium and on the particle surfaces from the total amount.) The dried particles (25 mg) were dissolved in THF (2.0 g). The sample solutions were filtered using a 0.45 µm poly(tetrafluoroethylene) (PTFE) membrane. Subsequently, a 0.5 wt % THF solution (0.5 g) of super-high molecular weight (Mw ) 5.48 × 106) PS was added as an internal standard to the filtered solutions. The samples were analyzed by GPC using two styrene/ DVB gel columns (TOSOH Corporation, TSKgel GMHHR-H, 7.8 mm i.d. × 30 cm; separation range per column: approximately 50 to 4 × 108 g/mol (exclusion limit)) using THF as eluent at 40 °C at a flow rate of 1.0 mL/min employing refractive index (TOSOH RI-8020/21). The absolute amounts of nonionic emulsifier incorporated inside particles were obtained by use of super-high molecular weight PS as an internal standard (added to the filtered solutions). The calibration curve for the quantitative analysis of the incorporated emulsifier was constructed as follows: THF solutions (containing 0.1 wt % of super-high molecular weight PS as an internal standard) of emulsifier with concentrations ranging from 0.08 to 0.65 mg of emulsifier/g-THF were prepared. These samples were injected into the GPC column, and the areas of emulsifier and super-high molecular weight PS under the detector response were determined. The area ratios of emulsifier to super-high molecular weight PS were proportional to the mass concentration of emulsifier in the injected sample.

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Figure 1. TEM photographs of PS particles (a′,b′,c′) prepared by seeded emulsion polymerization (PS seed/styrene ) 1/0.5, w/w) at 40 °C with PS seed particles (a,b,c). The PS seed particles were prepared by emulsion polymerization with KPS (8.45 mmol/L-water) at 70 °C at different Emulgen 911 contents [mmol/L-water (wt% based on styrene)]: (a) 0.4 (1); (b) 2.0 (5); (c) 4.1 (10).

Results and Discussion Figure 1a-c shows TEM photographs of PS seed particles prepared by emulsion polymerization with KPS initiator (8.45 mmol/L-water) at different Emulgen 911 concentrations. In the case of an Emulgen 911 content of 1 wt % (based on styrene), monodisperse spherical particles with homogeneous contrast were obtained (Figure 1a). Using 5 wt %, slightly nonspherical particles having a small number of domains with less contrast were obtained. It is difficult to confirm the domains in Figure 1b, but they were clearly observed during the actual observation using the TEM instrument. Using 10 wt %, the particles were more irregular, and a large number of domains with less contrast were clearly observed (Figure 1c). Using 5 and 10 wt %, smaller particles were obtained as a byproduct. Figure 2 shows particle size distributions (weightaverage) (a,b) obtained by DLS and TEM photographs (c,d) of the seed particles (a,c) prepared by emulsion polymerization using 10 wt % and the hollow particles (b,d) prepared by seeded emulsion polymerization. Using 5 wt %, the data were basically similar to that observed at 10 wt %; however, the amount of secondary particles decreased compared to that at 10 wt %. Such secondary nucleation arising from a continuous release of emulsifier from the monomer droplets to the aqueous phase is often seen in emulsion polymerizations with nonionic emulsifier.33,36,37 This nonspherical shape may be related to heterocoagulation between large pre-existing particle and small byproduct particles due to secondary nucleation, but a sufficient understanding requires further investigations. Under certain (36) Piirma, I.; Chang, M. J. Polym. Sci., Polym. Chem. Ed. 1982, 20, 489. (37) O ¨ zdegˇer, E.; Sudol, E. D.; El-Aasser, M. S.; Klein, A. J. Polym. Sci., Part A: Polym. Chem. 1997, 35, 3813.

conditions, emulsion polymerization of styrene with ethoxylated nonionic emulsifier potentially proceeds with a homogeneous nucleation mechanism (like emulsifier-free emulsion polymerization) due to predominant partitioning of nonionic emulsifier to the monomer phase,33,37 even if the concentration of emulsifier initially added to the aqueous phase is much higher than the critical micelle concentration (cmc). This would cause the similarity in the size of each large particle. It has been reported that particles with a less-contrast domain (hollow) were obtained in emulsifier-free emulsion polymerization of styrene with KPS.38-44 Emulsion polymerization of styrene with ethoxylated nonionic emulsifier potentially proceeds with a nucleation mechanism similar to that of emulsifier-free emulsion polymerization as described above when the predominant partitioning of emulsifier to the monomer phase occurs. Thus, the less-contrast domains inside particles in the present work (Figure 1b,c) may have some relation to the hollow in the previous studies of emulsifier-free emulsion polymerization. However, it was observed at only low conversion in almost all the previous reports,38,39,41-44 with a single exception of the report40 where emulsifier-free emulsion polymerization of styrene with KPS using microwave heating was carried out (there is no (38) Goodall, A. R.; Wilkinson, M. C.; Hearn, J. J. Polym. Sci., Polym. Chem. Ed. 1977, 15, 2193. (39) Cox, R. A.; Wilkinson, M. C.; Creasey, J. M.; Goodall, A. R.; Hearn, J. J. Polym. Sci., Polym. Chem. Ed. 1977, 15, 2311. (40) Zhang, W.; Gao, J.; Wu, C. Macromolecules 1997, 30, 6388. (41) Tauer, K.; Mu¨ller, H.; Rosengarten, L.; Riedelsberger, K. Colloid Surf. 1999, 153, 75. (42) Tauer, K.; Deckwer, R.; Ku¨hn, I.; Schellenberg, C. Colloid Polym. Sci. 1999, 277, 607. (43) Tauer, K.; Riedelsberger, K.; Deckwer, R.; Zimmermann, A. Macromol. Symp. 2000, 155, 95. (44) Tauer, K. Macromolecules 2006, 39, 2007.

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Figure 2. Particle size distributions (a,b) obtained by DLS and TEM photographs (c,d) of the seed particles (a,c) prepared by emulsion polymerization with KPS (8.45 mmol/L-water) and Emulgen 911 (4.1 mmol/L-water) at 70 °C and the hollow particles (b,d) prepared by seeded emulsion polymerization at 40 °C after swelling for 24 h.

discussion about a less-contrast domain in ref 40). Cox et al. concluded that the domain (hollow) was created by the volatilization of monomer from inside particles at low conversion.39 However, in the present work, only less than 5% of total monomer existed in the emulsion (almost all monomer would dissolve in the aqueous phase) after polymerization according to gas chromatography. Therefore, it appears unlikely that the domains with less contrast can be ascribed to volatilization of monomer from inside the particles in the present work. Another explanation for the formation of domains by Tauer44 is that hydrophilic chain ends buried inside particles may gather in a particular spot and water accumulates there (note that the particles were swollen with monomer or organic solvent in all his cases, but were not swollen in the present work). A more detailed discussion of the domains will be given in the following section. According to the measurement by GPC, 2.3 (46) and 3.2 (32) wt% of Emulgen 911 [relative to the weight of PS (relative to the total added Emulgen 911)] were incorporated inside the PS seed particles at Emulgen 911 contents of 5 and 10 wt %, respectively. In the case of 1 wt %, the amount of incorporated emulsifier was too small to be detected by GPC. We also measured the amount of incorporated emulsifier by 1H NMR (method described in detail in ref 33), which gives only the amount of noncovalently incorporated emulsifier. The results of both measurements were very similar, and thus the amount of grafted emulsifier is negligible. It was clarified that the incorporated emulsifiers were homogeneously mixed with P(S-MAA) by turbidity measurement of the film containing emulsifiers in ref 35. They were also homogeneously mixed with PS to some extent (approximately 5%). Moreover, the glass-transition temperature (Tg) in the dry state of the particles with 3.2 wt % of incorporated Emulgen 911 (relative to the weight of PS) was approximately 10 °C lower than that of the particles without incorporated emulsifier, but differential scanning calorimetry resulted in only one peak. Therefore, incorporated emulsifiers would be compatible with the polymer in dry state. In the emulsion state, the

location of incorporated emulsifier in particles containing water pools inside is under investigation. The PS particles prepared by seeded emulsion polymerization with the PS seed particles (Figure 1a-c) are shown in Figure 1a′-c′, respectively. Nonhollow and spherical particles were observed using the seed particles prepared at an Emulgen 911 content of 1 wt % (Figure 1a′). However, larger domains than those of the seed particles were observed using the seed particles containing 2.3 (Figure 1b′) and 3.2 wt % (Figure 1c′). In particular, multihollow structure was clearly formed in the case of 3.2 wt %. The volume percentage of domains in the particles as shown in Figure 1c′ (approximately 32%) was much higher than that in ref 40 (approximately 4%), in which the domains were observed even after polymerization completely finished. The percentages were calculated from each TEM photograph. In our experiment, the calculation was carried out for the single hollow particles obtained by heat treatment of the multihollow particles, assuming that the percentage remains unchanged before and after heating. This result suggests the necessity of incorporating emulsifier inside the seed particles to a certain extent when preparing hollow polymer particles by seeded emulsion polymerization. The hollows were filled with water (i.e., water pools) before drying because they contained no monomer and the volume percentage of incorporated emulsifier was only approximately 3% based on the hollow particle. Tauer investigated hydrophilic regions inside polymer particles dispersed in water and stated that the anomalous morphologies (less-contrast domain) are determined on the molecular scale by the fraction of hydrophobic and hydrophilic groups and on the particle scale by the geometric size.44 He suggested the ratio nP/nSM as an indicator of whether water domains are formed inside particles, where nP is the number of polymer chains per particle and nSM is the maximum number of polymer chains if the particle surface is completely covered with sulfate groups due to KPS. When nP/nSM is greater than 1, water penetrates into particles to minimize free energy because hydrophilic chain end

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Figure 3. TEM photographs of PS particles (a′,b′,c′) prepared by seeded emulsion polymerization (PS seed/styrene ) 1/0.5, w/w) at 40 °C with PS seed particles (a,b,c). The PS seed particles were prepared by emulsion polymerization with Emulgen 911 (4.1 mmol/L-water, 10 wt % based on styrene) at 70 °C at different KPS concentrations (mmol/L-water): (a) 0.53; (b) 2.11; (c) 8.45.

groups are surrounded with hydrophobic polymer chains inside the particles, and consequently water domains are formed inside the particles. In order to investigate the influence of sulfate end groups buried inside the particles on the formation of domains and multihollow structure, PS seed particles were prepared by emulsion polymerization at different KPS concentrations: 0.53, 2.11, and 8.45 mmol/L-water (Figure 3a-c). In all cases, nonspherical particles with less electron dense domains were obtained. After seeded emulsion polymerizations with the PS particles, multihollow polymer particles having spherical shape were obtained (Figure 3a′-c′). The number of domains inside the seed particles decreased with decreasing KPS concentration. Moreover, the size and volume percentages of hollows inside the particles after seeded emulsion polymerization appeared to decrease upon decreasing the KPS concentration. However, no multihollow particles were obtained when the amount of incorporated emulsifier was low, even if the KPS concentration was high (Figure 1a′). When Tauer’s treatment is applied to our case (an approximate treatment because the seeded particles are nonspherical), the values of nP/nSM for the seed particles shown in Figure 3a-c are 0.33, 0.62, and 1.31, respectively. Even though nP/nSM is less than 1 (i.e., only few or no sulfate groups are buried inside the particles), water domains appeared in the cases of seed particles prepared at 0.53 and 2.11 mM KPS concentrations. Moreover, the values of nP/nSM for the particles after seeded emulsion polymerization are 0.61, 0.92, and 1.24, respectively. Also after seeded emulsion polymerization, hollow structures were observed for nP/nSM < 1. These results suggests that hydrophilic chain ends buried inside particles promote the formation of domains inside the seed particles and the multihollow structure after seeded emulsion polymerization to some extent,

but this is not the main factor in this system. The nonionic emulsifier incorporated inside the particles is considered the main factor for the formation of the small domains and the hollow. Further investigations are required to clarify the mechanism in more detail. Figure 4 shows TEM photographs of PS particles prepared by seeded emulsion polymerization at different weight ratios of styrene to the PS seed particles. With increasing styrene content, the size of hollows inside the particles increased while their number decreased, and they finally disappeared at 1/0.75 (Figure 4d). This is explained as follows: the small water pools inside the seed particles coalesce to minimize the free energy during the swelling process (the period from the monomer addition until initiator addition) and polymerization, where the Tg of the monomer-swollen particles must be below the temperatures at swelling and polymerization. If coalescence of the water pools proceeds excessively, the water eventually leaves the particles. The viscosity inside the particles decreases with increasing styrene content, hence the coalescence rate of the small water pools becomes faster. As a result, the difference of morphology inside the particles appears. Reducing the viscosity inside the particles (to increase the added styrene content) at the same swelling time and lengthening the swelling time at the same viscosity basically have identical effects on the coalescence of the small water pools. Therefore, if the particles are swollen for an adequate time, the water domains will get out of the particles even if the amount of added styrene is small. We carried out seeded emulsion polymerizations (PS seed/styrene ) 1/0.5, w/w) after swelling for different times (1, 12, 24, and 48 h) with the seed particle prepared by emulsion polymerization with KPS (8.45 mmol/ L-water) and Emulgen 911 (4.1 mmol/L-water) at 70 °C. As a

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Letters Scheme 1. Formation of Multihollow Structure during Seeded Emulsion Polymerization

Figure 4. TEM photographs of PS particles (b,c,d) prepared by seeded emulsion polymerization at 40 °C with the PS seed particles (a). The PS seed particles were prepared by emulsion polymerization with Emulgen 911 (4.1 mmol/L-water, 10 wt % based on styrene) with KPS (8.45 mmol/L-water) at 70 °C. PS seed/styrene (w/w): (b) 1/0.25; (c) 1/0.5; (d) 1/0.75.

result, there is little change between 12, 24, and 48 h, and longer times would be needed for the water to exit the particles. These results indicate that hollow structure is thermodynamically unstable (i.e., not reached at thermodynamic equilibrium), which is consistent with our previous report for the “stepwise alkali/ acid method”.45 The coalescence level of water pools (i.e., the number and size of hollows) is determined by the rate of coalescence and the time coalescence can proceed. Therefore, the hollow structure is greatly affected by polymerization kinetics and dynamics. The rate of coalescence mainly depends on the viscosity inside the particles, and the time for coalescence to (45) Okubo, M.; Sakauchi, A.; Okada, M. Colloid Polym. Sci. 2002, 280, 303.

stop mainly depends on the polymerization rate and swelling time. Thus, it is important for the preparation of hollow polymer particles to control the coalescence of the water pools. From the above results, the mechanism proposed for the formation of multihollow structures during the seeded emulsion polymerization of styrene with PS seed particles with incorporated nonionic emulsifier is illustrated in Scheme 1. The PS seed particles contained numerous small water pools due to absorption of water into the particles during the polymerization. They coalesce to minimize interfacial free energy during the seeded emulsion polymerization until the particles solidify, resulting in a clear hollow structure. The number and size of hollows are mainly determined by a rate of coalescence and the time coalescence can proceed. It will be discussed in the near future whether further absorption of water by particles during seeded emulsion polymerization occurs. It is concluded that multihollow PS particles can be prepared by seeded emulsion polymerization of styrene using PS seed particles with incorporated nonionic emulsifier. This method potentially enables the preparation of hollow particles with less dependency on the kind of base-polymer used and without the use of ingredients having an adverse effect on environment. Moreover, it is favorable for this method that emulsion polymerization is environmentally friendly and widely used in industry. Acknowledgment. This work was supported by the Creation and Support program for Start-ups from Universities (No. 1509) from the Japan Science and Technology Agency (JST) and by Research Fellowships of the Japan Society for the Promotion of Science (JSPS) for Young Scientists (given to H.K.). LA7010748