Fabrication of Hollow Polystyrene Nanospheres in Microemulsion

Jun 17, 2002 - McKelvey, C. A.; Kaler, E. W.; Zasadzinski, J. A.; Coldren, B.; Jung, H. T. .... A. Blaine; Edeki, Efe M.; Feldhaus, James C.; Feldheim...
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Langmuir 2002, 18, 5613-5618

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Fabrication of Hollow Polystyrene Nanospheres in Microemulsion Polymerization Using Triblock Copolymers Jyongsik Jang* and Hyunkyou Ha Hyperstructured Organic Materials Research Center, School of Chemical Engineering, College of Engineering, Seoul National University, Shinlimdong 56-1, Seoul 151-742, Korea Received March 13, 2002. In Final Form: May 16, 2002 A ternary microemulsion polymerization was successfully used to prepare nanosized hollow polystyrene (PS) microlatexes with triblock copolymers of poly(oxyethyene)-poly(oxypropylene)-poly(oxyethylene)[(EO)x(PO)y(EO)x]. Micelle formation using triblock copolymers is a useful nanoreactor in order to make polymer nanoparticles in oil/water (o/w) microemulsions. Poly(methyl methacrylate)/cross-linked polystyrene core/shell nanospheres have been fabricated by o/w microemulsion. PS hollow nanospheres were obtained using the polymer core etching technique. Water-soluble and oil-soluble initiators were used for the polymerization at 70 °C with moderate stirring and via modified buret feeding. After extraction of the surfactant matrix, the polymer particles contract considerably without the change of their spherical shape. The hollow sphere morphology and hollow image of the polymer particles were confirmed by field emission scanning electron microscope and transmission electron microscope. PS hollow spheres were fabricated with diameters in the range of ca. 15-30 nm, and the shell thickness was ca. 2-5 nm. The size of the hollow nanoparticle is dependent on the surfactant concentration and the weight ratio of [surfactant]/[monomer].

Introduction Organic hollow nanoparticles have a wide variety of applications1-3 such as in medical therapy, materials science, and the paint industry. In particular, they have drawn great attention in the pharmaceutical area because of the potential applications in drug delivery systems. Nanospheres and nanocapsules have already been used in the entrapment of a drug or in its adsorption. In addition, nanospheres can be used for diverse applications including encapsulation of products (for the controlled release of drugs, cosmetics, inks, and dyes), the protection of lightsensitive components, catalysis, coatings, composites, and fillers.1,2 In response to the growing need for encapsulation materials, many different approaches4-6 are used for the generation of hollow nanoparticles in order to obtain the required properties.7 In the case of making hollow ceramic and polymeric spheres, dendrimers, block copolymers, vesicles,8,9 hydrogels, and template-synthesized microtubules10 have been used and in some cases have shown to be viable encapsulants for catalytic metal clusters, small molecules, or enzymes. A recent approach to the synthesis * To whom correspondence should be addressed. Tel: 82-2-8807069. Fax: 82-2-888-1604. E-mail: [email protected]. (1) Jiang, P.; Bertone, J. F.; Colvin, V. L. Science 2001, 291, 453. (2) Wilcox, D. L.; Berg, M.; Bernat, T.; Kellerman, D.; Cochran, J. K. Hollow and Solid Spheres and Microspheres: Science and Technology Associated With Their Fabrication and Application; Materials Research Society Proceedings; Materials Research Society: Pittsburgh, PA, 1995. (3) Jin, R.; Cao, Y.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. Science 2001, 294, 1901. (4) Santa, S.; Tapec, R.; Theodoropoulou, N.; Dobson, J.; Hebard, A.; Tan, W.; Langmuir 2001, 17, 2900. (5) Huang, H.; Remsen, E. E.; Kowalewski, T.; Wooley, K. L. J. Am. Chem. Soc. 1999, 121, 3805. (6) Caruso, F. Adv. Mater. 2001, 13, 11. (7) Tiarks, F.; Landfester, K.; Antonietti, M. Langmuir 2001, 17, 908. (8) McKelvey, C. A.; Kaler, E. W.; Zasadzinski, J. A.; Coldren, B.; Jung, H. T. Langmuir 2000, 16, 8285. (9) Morgan, J. D.; Johnson, C. A.; Kaler, E. W. Langmuir 1997, 13, 6447. (10) Johnson, S. A.; Khushalani, D.; Coombs, N.; Mallouk, T. E.; Ozin, G. A. J. Mater. Chem. 1998, 8, 13.

of hollow nanospheres is the employment of a micrometersized or nanometer-sized particle as a template.11 Organic hollow nanoparticles can be prepared using inorganic nanoparticles as a template. Silica or gold particles have been used as core materials,12-14 and polymers or ceramics encapsulated the core materials. After the core material was etched, hollow silica spheres, multifunctional hybrid silica/polymer capsules, and pyrrole-based polymer capsules were also fabricated.15 However, in doing so it is unavoidable to use very toxic HF and the procedure is very complicated. The other method is emulsion polymerization. An encapsulation of nonsolvent was used to fabricate hollow spheres using the phase separation technique. This phaseseparated polymer subsequently served as a locus for polymerization of a cross-linked network that stabilized the morphology. Typically, the encapsulated hydrocarbon was removed by vacuum or steam stripping of the latex following the polymerization process. In this case, the diameter of the hollow sphere was too large to be applied in various nanotechnology fields.16 Microemulsion polymerization17 has been quite actively studied since 1980. The enormous number (∼1015 mL-1) of either water-in-oil (w/o) or oil-in-water (o/w) microemulsion droplets is responsible for the fast polymerization of monomers and the formation of microlatex particles.18 In the beginning phases of microemulsion polymerization, the polydispersity of the droplets is still quite high, but (11) Xia, Y.; Gates, B.; Yin, Y.; Lu, Y. Adv. Mater. 2000, 12, 693. (12) Marinakos, S. M.; Shultz, D. A.; Feldheim, D. L. Adv. Mater. 1999, 11, 34. (13) Marinakos, S. M.; Novak, J. P.; Brousseau, L. C., III; House, A. B.; Edeki, E. M.; Feldhaus, J. C.; Feldheim, D. L. J. Am. Chem. Soc. 1999, 121, 8518. (14) Fleming, M. S.; Mandal, T. K.; Walt, D. R. Chem. Mater. 2001, 13, 2210. (15) Selvan, S. T.; Spatz, J. P.; Klok, H. A.; Mo¨ller, M. Adv. Mater. 1998, 10, 132. (16) McDonald, C. J.; Bouck, K. J.; Chaput, A. B. Macromolecules 2000, 33, 1593. (17) Langevin, D. Acc. Chem. Res. 1998, 21, 255. (18) Xu, J.; Chew, C. H.; Siow, K. S.; Wong, M. K.; Gan, L. M. Langmuir 1999, 15, 8067.

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the size and polydispersity decrease until the microemulsion reaches a steady state. Therefore, micelle formation in o/w microemulsons has been a useful nanoreactor for fabricating polymer nanoparticles.19 The polymerization created enormous numbers of nanocompartments which are separated from each other by a continuous phase.20 Another fabrication method, in which toxic materials were used, was complicated. Unlike the cationic and anionic surfactants, the block copolymers can be nearly continuously tuned during the micelle formation by adjusting composition, molecular weight, or molecular architecture. The characteristic of our article is that the hollow sphere size is smaller than that found in other works. Thus, we propose a new hollow sphere fabrication method with organic core etching using solvent after organic core/organic shell fabrication. Moreover, the block copolymers can be used for large-scale polymer hollow nanosphere production compared to the ionic surfactants because of the lower critical micelle concentration (cmc).21-24 We have sought to use the triblock copolymers for several reasons, including production cost and environmental and biomimetic applications. The purpose of this study is to control the size of hollow nanospheres with low-cost, nontoxic, and biodegradable nonionic organics under relatively dilute aqueous conditions. In particular, we investigate the use of block copolymers in order to extend the range and control of the hollow organic nanoparticle size from the ca. 15-30 nm scale. In addition, we present the facile methodology for the synthesis of organic hollow nanospheres in microemulsion polymerization using triblock copolymers. Several parameters for polymer hollow nanosphere synthesis are envisaged from the viewpoint of the nanoparticle size. The relationship between the size variation of hollow nanospheres and the surfactant types, surfactant concentration, and [surfactant]/[monomer] weight ratio can also be considered. Experimental Section Materials. Methyl methacrylate (MMA) from Junsei Chemical, styrene from Kanto Chemical, and divinylbenzene (DVB) from Aldrich were used. Inhibitors were removed by using the Aldrich inhibitor (hydroquinone) remover column. The watersoluble initiator potassium persulfate (KPS) from Aldrich was used as received. The oil-soluble initiator azobis(isobutyronitrile) (AIBN) from Aldrich was purified by precipitation from methanol.25 Block copolymers having a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (EO-PO-EO) sequence centered on a hydrophobic poly(propylene glycol) nucleus terminated by two primary hydroxyl groups.22 Surfactants, which were Pluronic P65 (EO20PO30EO20, Mav ) 3400), P123 (EO20PO70EO20, Mav ) 5800), and L121 (EO5PO70EO5, Mav ) 4400), were purchased from BASF AG. The solvent for poly(methyl methacrylate) (PMMA) core etching was methylene chloride (MC) from Aldrich. Distilled water and ethanol for the nonsolvent of the polymer were used in all experiments. Instrumentation. A Bomem MB 100 FT-IR spectrometer was used to characterize the polymer materials fabricated at each step. The IR spectra were obtained using the transmission (19) Meier, W. Colloid Interface Sci. 1999, 4, 6. (20) Landfester, K. Adv. Mater. 2001, 13, 765. (21) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (22) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024. (23) Chu, B.; Zhou, Z. In Nonionic Surfactants: Polyoxyalkylene Block Copolymers; Nace, V. M., Ed.; Surfactant Science Series Vol. 60; Marcel Dekker: New York, 1996. (24) Sjo¨blom, J.; Stenius, P.; Danielsson, I. In Nonionic Surfactants: Physical Chemistry; Schick, M. J., Ed.; Surfactant Science Series Vol. 23; Marcel Dekker: New York, 1987. (25) Hotz, J.; Meier, W. Langmuir 1998, 14, 1031.

Jang and Ha Scheme 1 . Schematic Diagram of Polystyrene Hollow Nanosphere Fabrication

technique. Thirty-two scans were collected with a spectral resolution of 4 cm-1. The morphology of polymer nanospheres was observed with a field emission scanning electron microscope (FE-SEM, JEOL JSM-6330F). The polymer nanospheres were diluted in ethanol, and the diluted solution was cast onto carbon tape. After drying, the specimens were coated with a thin layer of gold to eliminate charging effects and the morphology was examined at a voltage of 5.0 kV. The images of hollow nanospheres were observed with a transmission electron microscope (TEM, JEOL JEM-200CX and JEOL JEM-2000EXII). The hollow nanospheres were diluted in ethanol, and the diluted solution was cast onto carbon-coated copper grids. After drying, the images were examined at a voltage of 200 kV. Microemulsion Polymerization and Fabrication of Polystyrene (PS) Hollow Nanospheres.26-29 The synthesis of hollow nanoparticles consists of several stages, A variable amount of surfactant was magnetically stirred in 40 mL of distilled water at room temperature. MMA monomer (1.0 g) was added dropwise to the surfactant solution. The solution was heated to 70 °C. Then, 0.01 g of AIBN or KPS was added into the surfactant/ MMA solution. Microemulsion polymerization proceeded with magnetic stirring at 70 °C for 2 h. The solution containing PMMA latexes was quenched using water to room temperature and continuously stirred. Styrene and DVB were incorporated into the solution dropwise. The solution was heated to 70 °C again. Then, the initiator was added into the solution. Styrene and DVB monomers were polymerized at 70 °C for 2 h. To fabricate PS hollow nanospheres, MC was used to dissolve the PMMA core. The solution was moved to a separation funnel, and then ethanol was added to remove the surfactants. PS hollow nanospheres were precipitated after 1 day, and the upper solution containing surfactants and PMMA was discarded. The nanoparticle products were dried at room temperature. In our experiment, PMMA was completely dissolved in MC. IR data showed that PMMA was removed by solvent etching.

Results and Discussion Mechanism for Organic Hollow Nanoparticle Formation. A schematic diagram illustrating the formation of PS hollow nanospheres is shown in Scheme 1. To tailor the hollow nanoparticles, synthetic parameters such as surfactant/water ratio, stirring speed, nanoparticle recovery, and drying technique were carefully controlled. A block copolymer molecule must contain at least 10 PO groups in order to form micelles in aqueous solutions.30 (26) Gan, L. M.; Lian, N.; Chew, C. H.; Li, G. Z. Langmuir 1994, 10, 2197. (27) Full, A. P.; Kaler, E. W.; Arellano, J.; Puig, J. E. Macromolecules 1996, 29, 2764. (28) Okubo, M.; Konishi, Y.; Minami, H. Colloid Polym. Sci. 1998, 276, 638. (29) Okubo, M.; Minami, H.; Yamamoto, Y. Colloids Surf., A 1999, 153, 405. (30) Wanka, G.; Hoffmann, H.; Ulbricht, W. Macromolecules 1994, 27, 4145.

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Figure 1. FT-IR spectra of the PMMA/PS core-shell structure and the PS hollow nanosphere: (a) spectrum after polymerization of MMA as the core, (b) spectrum of the PMMA/PS core/ shell structure, and (c) spectrum of the PS hollow nanosphere.

In this study, all surfactants had more than 10 PO groups. P65 (cmc ) 1.5 × 10-1 wt % at 40 °C), P123 (cmc ) 3 × 10-4 wt % at 40 °C), and L121 (cmc ) 3 × 10-4 wt % at 40 °C) were used for the spherical micelle formation in our experimental conditions.30 The MMA monomer penetrated inside micelles by diffusion, and the core part was polymerized in the micelle. Consecutively, the polymerization proceeded at the shell layer using polystyrene and the PMMA core was removed after the fabrication of the PMMA/PS core-shell nanosphere. In Figure 1, Fourier transform infrared (FT-IR) spectra demonstrate the formation of PMMA as a core, PMMA/PS core/shell structure, and PS hollow nanospheres. The band at 1640 cm-1, which was attributed to the CdC stretching mode, disappeared completely after polymerization of MMA and styrene. This means that PMMA and PS nanospheres were effectively fabricated. In Figure 1a, the C-O stretching peaks of PMMA appeared at 1277, 1241, 1195, and 1150 cm-1. In addition, the band at 1731 cm-1 was assigned to the CdO stretching peak of PMMA. In the IR spectrum of (b), a peak at 698 cm-1 (the benzene ring out-of-plane bending peak) appeared clearly after polymerization of styrene. This implies that there exist both PMMA and PS in polymer materials. Comparing (b) with (c), the band at 1731 cm-1 (the CdO stretching peak) and the peaks between 1150 and 1300 cm-1 (the C-O stretching peaks) became weak after PMMA dissolution using methylene chloride. Judging from those spectra, PMMA was dissolved by MC and PS hollow spheres were formed. The crosslinked hollow polymer nanoparticles were robust and would withstand complete core etching, drying, and resuspension with no apparent change in the nanosphere structure. Influence of the Surfactant Concentration and Type on the Size of Hollow Organic Nanoparticles.30-33 Figure 2 presents FE-SEM images of polystyrene hollow nanospheres fabricated with the L121 template. When the surfactant concentration changed from 2.5 to 12.5 wt %, the size of PS nanoparticles decreased from ca. 30 to ca. 20 nm. To measure the average size of hollow nanoparticles, 50 PS nanoparticles were counted by TEM images. (31) Alexandridis, P.; Zhou, D.; Khan, A. Langmuir 1996, 12, 2690. (32) Debuigne, F.; Jeunieau, L.; Wiame, M.; Nagy, J. B. Langmuir 2000, 16, 7605. (33) Xu, X. J.; Siow, K. S.; Wong, M. K.; Gan, L. M. Langmuir 2001, 17, 4519.

Figure 2. SEM images of hollow polystyrene nanoparticles obtained from different weight percents of L121 surfactant: (a) 2.5 wt %; (b) 5.0 wt %; (c) 7.5 wt %; (d) 12.5 wt %. Nanoparticles were fabricated with 1 g (9.9 × 10-3 mol) of MMA monomer, 1 g (9.6 × 10-3 mol) of styrene monomer, 0.1 g (7.6 × 10-4 mol) of DVB, and 0.01 g of AIBN (3.6 × 10-5 mol) in 40 mL (2.2 mol) of H2O at 70 °C.

Figure 3. TEM image of PMMA/PS core/shell nanoparticles before extraction with methylene chloride.

Figure 3 illustrates a TEM photograph of PMMA/PS core/shell nanoparticles before the extraction with methylene chloride. Circa 10 vol % nanoparticles were not a core/shell structure. These particles consisted of only PS by generation of by-produced particles. From the consideration of thermodynamics as described in a published paper,35 PMMA, which is more hydrophilic than PS, is more likely to locate at the interface between the polymer and water than PS. So, if the morphology is thermodynamically controlled, it should consist of a PS core and a PMMA shell, whereas if it is kinetically (34) Zhou, Z.; Chu, B. J. Colloid Interface Sci. 1988, 126, 171. Yu, G.; Deng, Y.; Dalton, S.; Wang, Q.; Attwood, D.; Price, C.; Booth, C. J. Chem. Soc, Faraday Trans. 1992, 88, 2537. Mortensen, K.; Pedersen, J. S. Macromolecules 1993, 26, 805. Brown, W.; Schillen, K.; Almgren, M.; Hvidt, S.; Bahadur, P. J. Phys. Chem. 1991, 95, 1850. Brown, W.; Schillen, K. J. Phys. Chem. 1992, 96, 6038. Linse, P.; Malmsten, M. Macromolecules 1992, 25, 5434. Malmsten, M.; Lindman, B. Macromolecules 1992, 25, 5446. (35) Winzor, C.; Sundberg, D. J. Appl. Polym. Sci. 1992, 33, 3797.

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Figure 4. TEM images of hollow polystyrene nanoparticles obtained from different weight percents of L121 surfactant: (a) 2.5 wt %; (b) 5.0 wt %; (c) 7.5 wt %; (d) 12.5 wt %. Nanoparticles were fabricated with 1 g of MMA, 1 g of styrene, 0.1 g of DVB, and 0.01 g of AIBN in 40 mL of H2O at 70 °C.

Figure 5. TEM images of hollow polystyrene nanoparticles obtained from different weight percents of P123 surfactant: (a) 2.5 wt %; (b) 5.0 wt %; (c) 7.5 wt %; (d) 12.5 wt %. Nanoparticles were fabricated with 1 g of MMA, 1 g of styrene, 0.1 g of DVB, and 0.01 g of AIBN in 40 mL of H2O at 70 °C.

controlled, it should consist of a PMMA core and a PS shell.36 In our experiment, a large amount of surfactant was used in microemulsion polymerization to form a compact micelle. From the viewpoint of micelle formation, the inside of the micelle consists of a lot of hydrophobic tails of surfactants and polymerization time was relatively short. Therefore, the interface of the micelle’s inner part is more hydrophobic compared to the outer part of the micelle. In this case, PMMA is located in the core part and PS is polymerized in the shell part. Okubo et al. reported that an electron beam degrades PMMA.36 Figure 4 shows TEM images of PS nanoparticles fabricated with L121 templates after the solvent etching process. All of the operations were conducted at an acceleration voltage of 200 kV. IR data showed that PMMA was removed by solvent etching. Judging from Figures 3 and 4, these facts indicate that the PS hollow structure was not formed from degradation by the electron beam. The diameters of hollow nanospheres were ca. 30-20 nm, and the shell thickness was ca. 2-5 nm. As the concentration of block copolymer increased, smaller micelles were generated30 due to the decrease in the chemical potential of water and the dehydration of the PO groups was enhanced. It is obvious that smaller nanoparticles generate smaller micelles. TEM images of polystyrene hollow nanospheres fabricated with P123 templates are shown in Figure 5. The EO/PO ratio has a major effect on the size variation of the hollow organic nanospheres. P123 generated smaller nanoparticles than L121. The small size of the hollow nanospheres was mainly attributed to the relatively long hydrophilic EO blocks of P123. Alexandridis et al.31 reported that self-organization of A-B block copolymers stemmed from the segregation between the different blocks in the copolymer. The segregation of A-B block copolymers depended on block-block interactions, described in the

context of the Flory-Huggins theory by N(A+B)χAB, as well as polymer-solvent interactions, described by NAχAS + NBχBS (where N(A+B) is the number of segments in the polymer, NA and NB are the numbers of segments in the A and B blocks, respectively, χij is the Flory-Huggins interaction parameter, and S denotes the solvent). At high temperature, interaction parameters of PEO-water and PPO-water increased and the PEO-PPO interaction parameter decreased. Consequently, the contribution to segregation of the term NAχAS + NBχBS increased and that of the term N(A+B)χAB decreased.31 From the viewpoint at the molecular level, P123 and L121 had the same 70 PO block length. P123 had 20 EO groups, whereas L121 had 5 EO groups. P123 had more EO groups compared to L121. In the previous explanation, the number increment of EO blocks (NB is increased) increased the term NBχBS. This means that increase in the EO blocks results in the more hydrophilic characteristic of micelles and the water molecule was able to easily penetrate the micelle. Water molecules pushed the spherical micelle from the outside to the inside. Therefore, the core part packed to form the micelle tightly due to outside pressure of the water molecules and the smaller micelles were generated. Smaller sphere particles were favored due to the smaller micelle formation. The smallest nanoparticles were obtained from P65 templates. In Figure 6, hollow PS nanoparticle average size decreased from ca. 25 to 15 nm. The hydrophobic PO groups are located in the micellar core which is surrounded by the hydrophilic shell formed by the EO groups. This model is generally accepted by various research groups.34 The micellization process has some connection with the size of the PO block. The inner part of the micelle consists of PO groups, and the aggregation number of the spherical micelle was determined by the length of the PO block.30 The size difference of blocks in the copolymer and block position played an important role in determining phase behavior, by affecting the curvature and packing symmetry

(36) Okubo, M.; Izumi, J.; Hosotani, T.; Yamashita, T. Colloid Polym. Sci. 1997, 275, 797.

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Figure 6. TEM images of hollow polystyrene nanoparticles obtained from different weight percents of P65 surfactant: (a) 2.5 wt %; (b) 5.0 wt %; (c) 7.5 wt %; (d) 12.5 wt %. Nanoparticles were fabricated with 1 g of MMA, 1 g of styrene, 0.1 g of DVB, and 0.01 g of AIBN in 40 mL of H2O at 70 °C.

Figure 7. Variation of the average size of the hollow nanospheres as a function of different surfactant concentrations.

of the ordered microstructures. When the apparent volume of the soluble blocks was much larger than that of the insoluble blocks (PPO), spherical or cylindrical structures were more favored. The lamellar arrangement was preferred when the relative length of the insoluble blocks was sufficiently large.31 In the case of the large PO blocks in the inner part, it was difficult to pack much more densely into the interface due to steric hindrance and the micelle size increased. P123 and L121 had 70 PO groups, whereas P65 had 30 PO groups. P65 generated smaller nanoparticles than P123 and L121. To our knowledge, this is the first evidence that polymeric hollow nanoparticles with dimensions of less than 20 nm have been fabricated. Figure 7 summarizes the size variation of hollow organic nanoparticles fabricated using triblock copolymers as a function of the different EO/PO ratios and surfactant concentration. As we mentioned previously, smaller micelles were generated by increasing the composition of the EO blocks. Judging from this explanation, P123 generated smaller nanoparticles than L121. Lowering the

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Figure 8. TEM images of hollow polystyrene nanoparticles obtained from different concentrations of [L121 surfactant]/ [monomer]: (a) 3 wt ratio; (b) 2 wt ratio; (c) 1 wt ratio. L121 surfactant concentration (3 g, 7.75 wt %) was fixed in 40 mL of H2O at 70 °C. Monomer concentration is changed from 1 to 3 g.

EO/PO ratio of the triblock copolymer spacer promotes the formation of smaller hollow nanoparticles, while a higher ratio of EO/PO favors bigger hollow nanospheres.22 In triblock copolymers, the core of the micelles consists of PO groups. In the case of P65, a small number of PO groups easily form the smaller micelle compared to P123 and L121. Influence of the [Surfactant]/[Monomer] Weight Ratio on the Organic Hollow Nanoparticle Size. Figure 8 shows TEM images of hollow nanoparticles obtained from different [surfactant]/[monomer] concentrations of L121 templates. The surfactant concentration was fixed at 7.5 wt %, and the monomer concentration changed with the [surfactant]/[monomer] weight ratio of up to 3:1. As the surfactant-to-monomer weight ratio increased from 1:1 to 3:1, the hollow nanosphere size decreased from ca. 30 to ca. 20 nm. During the microemulsion polymerization, the droplets became polydisperse due to two droplet growth mechanisms: One is Ostwald ripening (τ1 mechanism) and the other is collisions (coalescence) (τ2 mechanism). Suppression of these two processes was a prerequisite for the formation of a stable microemulsion. High surfactant-to-monomer weight ratio means that surfactant effectively interfered with the monomer diffusion from one droplet to the other droplet. Therefore, the smaller nanoparticles were fabricated with the high [surfactant]/[monomer] weight ratio. Figures 9 and 10 demonstrate TEM images of hollow nanospheres obtained from P123 and P65 templates, respectively. As the [surfactant]/[monomer] weight ratio increased up to 3:1, the hollow nanoparticle size decreased from ca. 30 to 15 nm. It is confirmed that hollow nanoparticle size decreased with increasing the [surfactant]/[monomer] weight ratio. Compared to the P123 template, P65 had the higher EO/PO ratio. This means that it favors the formation of smaller hollow nanospheres. The average hollow nanoparticle sizes are shown as a function of weight ratio of [surfactant]/[monomer] in

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Figure 9. TEM images of hollow polystyrene nanoparticles obtained from different concentrations of [P123 surfactant]/ [monomer]: (a) 3 wt ratio; (b) 2 wt ratio; (c) 1 wt ratio. P123 surfactant concentration (3 g, 7.75 wt %) was fixed in 40 mL of H2O at 70 °C. Monomer concentration is changed from 1 to 3 g.

Jang and Ha

Figure 10. TEM images of hollow polystyrene nanoparticles obtained from different concentrations of [P65 surfactant]/ [monomer]: (a) 3 wt ratio (1 g of monomer); (b) 2 wt ratio (2 g of monomer); (c) 1 wt ratio (3 g of monomer). P65 surfactant concentration (3 g, 7.75 wt %) was fixed in 40 mL of H2O at 70 °C. Monomer concentration is changed from 1 to 3 g.

Figure 11. The particle sizes decreased with increasing [surfactant]/[monomer] ratio. This implies that a higher [surfactant]/[monomer] weight ratio generates smaller nanoparticles. The hollow nanoparticle size decreased from ca. 30 to 15 nm with increasing [surfactant]/[monomer] weight ratio. Considering the composition ratio of EO/ PO, it can also be concluded that the average hollow nanoparticle size decreases with increasing EO spacer groups in the triblock copolymer: P65 < P123 < L121. Conclusions Polymer hollow spheres with dimensions of less than 20 nm were successfully prepared by microemulsion polymerization using commercially available nonionic poly(alkylene oxide) block copolymer surfactants. The size of hollow organic nanoparticles was adjustable by changing the surfactant concentration, surfactant type, and weight ratio of [surfactant]/[monomer]. The high surfactant concentration generated smaller nanoparticles. A smaller number of PO groups in the triblock copolymer was seen to favor the formation of the smaller hollow nanospheres, and high EO/PO ratios, such as EO20PO70EO20, compared to small ratios, such as EO5PO70EO5, fabricated the smaller nanoparticles. A high weight ratio of [surfactant]/ [monomer] can obtain a small diameter of hollow organic nanospheres. The diameters of hollow nanospheres were ca. 15-30 nm, and the shell thickness was ca. 2-5 nm. The cross-linked hollow polymer nanoparticles were robust

Figure 11. Variation of the average size of the hollow nanospheres as a function of different weight ratios of [surfactant]/[monomer]. Nanoparticles were fabricated with 3 g (7.75 wt %) of surfactant in 40 mL (2.2 mol) of H2O at 70 °C.

and withstand complete core etching, drying, and resuspension with no apparent change in nanosphere structure. Acknowledgment. This work was supported in part by the Brain-Korea 21 Program of the Korea Ministry of Education and by the Hyperstructured Organic Materials Research Center at Seoul National University. LA0257283