Coacervation Microencapsulation of Talc Particles ... - ACS Publications

We report the formation of poly(methyl methacrylate) (PMMA) microspheres and the coacervation microencapsulation of talc (Mg3Si4O10(OH)2) microparticl...
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MATERIALS AND INTERFACES Coacervation Microencapsulation of Talc Particles with Poly(methyl methacrylate) by Pressure-Induced Phase Separation of CO2-Expanded Ethanol Solutions Kiyoshi Matsuyama* and Kenji Mishima Department of Chemical Engineering, Fukuoka UniVersity, 8-19-1 Nanakuma Jonan-ku, Fukuoka 814-0180, Japan

We report the formation of poly(methyl methacrylate) (PMMA) microspheres and the coacervation microencapsulation of talc (Mg3Si4O10(OH)2) microparticles with PMMA by the pressure-induced phase separation (PIPS) of CO2-expanded ethanol solutions. PMMA is insoluble in both ethanol and CO2; however, PMMA dissolves in CO2-expanded ethanol and precipitates after the ethanol is depressurized as the solubility of PMMA in CO2-expanded ethanol solutions decreases with the pressure. Coacervation microencapsulation is achieved by mixing a suspension of talc and dissolved PMMA in CO2-expanded ethanol and then depressurizing the system to obtain PMMA microcapsules of talc in a high-pressure cell. The effects of the experimental conditionssdepressurization rate, polymer concentration, and evaporation methodson the structure and morphology of the microparticles are investigated by using a scanning electron microscope, a Fourier transform infrared spectroscope, and an electron probe microanalyzer equipped with a wavelengthdispersive X-ray spectroscope. The results reveal that the talc is thoroughly coated with PMMA. 1. Introduction Polymer-inorganic composite materials have attracted considerable attention in the field of materials science. These materials have obtained remarkable changes in properties such as thermal, electrical, and magnetic, compared with pure organic polymers.1-4 Inorganic particles are combined with polymers by either surface encapsulation or surface grafting. However, these methods often require toxic organic solvents, surfactants, stabilizers, or grafting agents. Furthermore, these chemicals contaminate the composites and must be removed as they may cause undesirable effects. To overcome these problems, many researchers have attempted the formation of polymer composites by using supercritical-fluid-based technology.5-10 In these applications, supercritical CO2 (scCO2) is the solvent of choice because it is readily available, inexpensive, nonflammable, and environmentally benign. In particular, the rapid expansion of supercritical solutions (RESS) is a well-known process, and a variety of polymer microcapsules and composite particles have been produced by several researchers.11-14 We have reported the microencapsulation of proteins, medicines, and inorganic particles with polymers by RESS with a cosolvent.15-17 However, it is difficult to inhibit the precipitation of polymer particles that do not contain the core material; such particles often cause defects in the products. In the RESS process, the core materials in the expanding jet serve as nucleating agents for precipitating the polymer, thereby aiding in a thorough encapsulation. However, polymer particles that do not contain the core material also precipitate in the expanding jet because the supersaturation leads to homogeneous nucleation and particle formation during the RESS process. To overcome these problems, we have investigated the microencapsulation of * To whom correspondence should be addressed. Tel.: +81(92)871-6631. Fax: +81(92)865-6031. E-mail: [email protected].

inorganic micron-sized particles with a fluoropolymer by the pressure-induced phase separation (PIPS) of scCO2 solutions.18 In this case, a suspension of inorganic particles in CO2 and the dissolved fluoropolymer are mixed in scCO2. After the system is gradually depressurized to atmospheric pressure, microcapsules are obtained in a high-pressure cell. This investigation reveals that the talc is thoroughly coated with the fluoropolymer and it is possible to inhibit the precipitation of polymer particles that do not contain the core material. Recently, several researchers have reported the surface modification of materials with a low molecular weight substance using scCO2.19 Most scCO2-based techniques for polymer materials with high molecular weights have been limited by low polymer solubility in CO2 because most conventional polymers are not CO2soluble. Polysiloxanes and fluoropolymers are some of the few classes of polymers that are readily soluble in CO2 even at high molecular weights.20-22 On the other hand, CO2-expanded liquids are alternative media for polymer processes. The dielectric constants of CO2expanded liquids are considerably more tunable than the dielectric constant of scCO2;23-25 therefore, the solubility of polymers can be varied over a wider range. Recently, Ito et al. have reported a method for the formation of polymer particles by the phase separation of CO2-saturated ethanol solutions.26 We developed a method for the formation of conventional polymer poly(methyl methacrylate) (PMMA) microspheres and microcapsules of talc (Mg3Si4O10(OH)2) by the PIPS of CO2expanded ethanol solutions. In this study, ethanol was used as the solvent because it is more environmentally acceptable than many other organic solvents. Talc is used as a filler and powder foundation in cosmetics.27 Figure 1 shows the conceptual framework of the proposed process. As shown in Figure 1a, PMMA is insoluble in ethanol at atmospheric pressure and 313 K; pure ethanol is a nonsolvent for PMMA. However, PMMA

10.1021/ie070189o CCC: $37.00 © 2007 American Chemical Society Published on Web 08/09/2007

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Figure 1. Principles of the formation of (a) polymer microspheres and (b) microcapsules of talc by the pressure-induced phase separation (PIPS) of CO2expanded ethanol solutions.

readily dissolves in CO2-expanded ethanol; the solvent power of the ethanol solution increases with the concentration of CO2. During depressurization, PMMA is precipitated in ethanol and polymer microspheres are obtained. Figure 1b shows the process of the microencapsulation of talc with PMMA. During the depressurization, coacervation is induced by the decreasing solvent power of the CO2-expanded ethanol with regard to PMMA; this results in the formation of PMMA microcapsules. The objectives of this study are to form conventional PMMA microparticles and microcapsules of talc by the PIPS of CO2expanded ethanol solutions and to study the effects of the several experimental conditions on particle morphology. 2. Experimental Section 2.1. Materials. Talc (Mg3Si4O10(OH)2) was obtained from Wako Pure Chem. Ind., Ltd., and carbon dioxide (CO2) (99.9% minimum purity) was purchased from Fukuoka Sanso Co., Ltd. The PMMA samplessPMMA-15k (MW ) 15 000) and PMMA68k (MW ) 68 000)swere purchased from Aldrich Co. and Wako Pure Chem. Ind., Ltd., respectively. 2.2. Experimental Procedure. Known amounts of ethanol, PMMA powder, and talc were placed in a high-pressure cell (25 cm3) equipped with sapphire windows. The cell was placed in a water bath, and the system temperature was maintained at the desired value within (0.1 K. CO2 was pumped into the high-pressure cell through a preheater. The mixture was stirred by a magnetic agitator for 30 min, and the system was then depressurized. Subsequently, polymer microcapsules were obtained in the high-pressure cell. The structure and morphology of the products were analyzed by using a scanning electron microscope (SEM; JEOL JSM6060) and an electron probe microanalyzer (EPMA; JEOL, JXA8500F) equipped with a wavelength-dispersive X-ray spectrometer (WDX). The EPMA equipped with the WDX can be used to identify elements by using the crystal monochromator to select X-rays of a particular wavelength. For the SEM sample preparation, polymeric microparticles were mounted on a small glass plate covered with a small piece of double-sided carbon

Figure 2. Phase behavior of CO2-expanded ethanol solution containing PMMA-15k at 313 K. (a) 0.1 MPa (before agitation), (b) 0.1 MPa (with agitation), (c) 6 MPa, and (d) 0.1 MPa (after depressurization).

conductive tape. The samples were then sputter coated with silver-palladium and imaged using the SEM and EPMA. The chemical species on the surface of the products were evaluated by Fourier transform infrared spectroscopy (FT-IR; JASCO FTIR470) with the KBr method. 3. Results and Discussion 3.1. Phase Behavior. Before conducting the experiments on the formation of PMMA microparticles and microcapsules, the phase behavior of CO2-expanded ethanol containing PMMA15k at 6 MPa and 313 K was confirmed visually by using a

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Figure 3. SEM photographs of PMMA-15k microspheres produced by PIPS of CO2-expanded solutions. Drying method after depressurization: vacuum drying. Depressurization rate: (a) 6 and (b) 0.22 MPa‚min-1. Preexpansion conditions: temperature, 313 K; pressure, 6 MPa; PMMA, 0.5 wt % (CO2-free).

high-pressure vessel equipped with sapphire windows. The concentration of PMMA-15k in ethanol was 0.5 wt %. As shown in Figure 2a,b, the PMMA powder is insoluble in ethanol at 313 K and 0.1 MPa (atmospheric pressure). By agitation with a magnetic stirrer, the PMMA powders were dispersed in ethanol (Figure 2b) because ethanol is a nonsolvent for PMMA at 313 K and atmospheric pressure. The volume of ethanol increased with CO2 pressure because CO2 dissolves in ethanol. The details of the expansion ratio of ethanol solution with CO2 have been reported by Kitamura et al.28 PMMA-15k completely dissolved in the CO2-expanded ethanol solution at 6 MPa and 313 K (Figure 2c). CO2 in expanded liquids interacts with PMMA and reduces the glass transition temperature of the polymer, thereby leading to the plasticization of PMMA.29 The affinity between CO2-expanded ethanol and plasticized PMMA increases by the addition of CO2. Therefore, the solvent power of the CO2expanded ethanol solution with regard to PMMA increased with CO2 concentration. After depressurization, PMMA precipitated in ethanol, as shown in Figure 2d. 3.2. Formation of PMMA Microparticles. The observed PMMA particle morphology (primary particle diameter (PPD), particle size distribution (PSD), the coefficient of variation (CV)) by the PIPS of CO2-expanded solutions is shown in Table 1. Figure 3a shows SEM photographs of the PMMA-15k microparticles that were produced by the PIPS of the CO2expanded ethanol solutions. The system was rapidly depressurized from 6 MPa to atmospheric pressure in approximately 1 min (6 MPa‚min-1). The feed concentration of PMMA-15k in pure ethanol was 0.5 wt % (CO2-free). After depressurization, the ethanol solution containing the PMMA particles was dried

Figure 4. SEM photographs of PMMA-15k microspheres produced by the PIPS of CO2-expanded solutions. Drying method after depressurization: drying at atmospheric pressure for 12 h. Depressurization rate: (a) 6 and (b) 0.22 MPa‚min-1. Preexpansion conditions: temperature, 313 K; pressure, 6 MPa; PMMA, 0.5 wt % (CO2-free).

under vacuum to separate the ethanol from the PMMA particles. The products were microspheres with sizes on the order of less than 1 µm and not adhesive to each other because ethanol is a nonsolvent for PMMA. The effects of the experimental conditions on particle morphology, such as the method for the removal of ethanol, were investigated. The particle morphology was strongly affected by the removal method. After depressurization, the ethanol solution containing PMMA was dried at room temperature and atmospheric pressure for 12 h. The products were microspheres with sizes on the order of 10-30 µm and they adhered to each other, as shown in Figure 4a. These polymer particles were evidently larger than those obtained by vacuum drying, as shown in Figure 3a. The CV values of polymer particles are larger than those obtained by vacuum drying, as shown in Table 1. This may be due to the slow increase of supersaturation in the drying process, which results in a low nucleation rate (similar to slow cooling in cooling crystallization30). Although PMMA was partially precipitated in ethanol during the depressurization, it was also partially dissolved in ethanol because the ethanol solution became unstable after depressurization. Therefore, relatively large polymer particles were precipitated. On the other hand, very small particles were obtained after vacuum drying, as shown in Figure 3a. On vacuum drying, the rapid evaporation of ethanol caused a rapid cooling of the solution. Consequently, the supersaturation and nucleation rate increased rapidly, resulting in the precipitation

Table 1. Observed PMMA Particle Morphology Produced by the PIPS of CO2-Expanded Solutionsa no.

polymer

polymer concn [%] CO2-free

depressurizing rate [MPa‚min-1]

drying method after depressurization

PPDb [µm]

PSDc [µm]

CV valued [%]

1 2 3 4 5 6 7

PMMA-15k PMMA-15k PMMA-15k PMMA-15k PMMA-15k PMMA-15k PMMA-68k

0.5 0.5 0.5 0.5 1.0 2.0 0.5

6.0 0.22 6.0 0.22 6.0 6.0 6.0

vacuum drying vacuum drying drying at atmospheric pressure drying at atmospheric pressure vacuum drying vacuum drying vacuum drying

1.2 1.4 19.0 19.7 1.2 1.5 1.1

0.22 0.30 8.2 6.2 0.25 0.48 0.27

18 21 43 31 21 32 25

a

Figure 3a 3b 4a 4b 5a 5b 6

n Preexpansion conditions: temperature, 313 K; pressure, 6 MPa. b Primary particle diameter (PPD) D h; D h ) (1/n)∑i)1 Di, where n is the number of

k particles and Di is the particle diameter. c Particle size distribution (PSD) σ; σ ) x(1/n)∑i)1 (Di-D h )2. d Coefficient of variation (CV); CV (%) ) 100(σ/D h ).

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Figure 5. Effect of polymer concentration on morphology of PMMA-15k microspheres produced by the PIPS of CO2-expanded solutions. Drying method after depressurization: vacuum drying. Depressurization rate: 6 MPa‚min-1. Preexpansion conditions: temperature, 313 K; pressure, 6 MPa; PMMA, (a) 1 and (b) 2 wt % (CO2-free). Polymer concentration, 0.5 wt % (see Figure 3a).

Figure 6. SEM photographs of PMMA-68k microspheres produced by the PIPS of CO2-expanded solution. Drying method after depressurization: vacuum drying. Depressurization rate: 6 MPa‚min-1. Preexpansion conditions: temperature, 313 K; pressure, 6 MPa; PMMA, 0.5 wt % (CO2-free).

of very small polymer microspheres (similar to the rapid cooling in cooling crystallization30). We investigated the effect of the depressurization rate on the particle morphology. The system was slowly depressurized from 6 MPa to atmospheric pressure in approximately 30 min (0.21 MPa‚min-1). After the PIPS of the scCO2 solution containing fluoropolymers,18 the particle morphology was found to be strongly affected by the depressurization rate. However, in comparison with the effects of the drying method, the particle morphology is independent of the depressurization rate for the PIPS of the CO2-expanded solutions, as shown in Figures 3b and 4b. Further, it is indicated that the polymer particles mainly precipitate through the drying process because the ethanol solution is unstable after depressurization. Next, we discuss the effect of the polymer concentration on the particle morphology. When the polymer concentration is in

Figure 8. FT-IR spectra of (a) talc and (b) PMMA-15k microcapsules (drying at atmospheric pressure of talc) produced by the PIPS of CO2expanded solution. Depressurization rate: 6 MPa‚min-1. Preexpansion conditions: temperature, 313 K; pressure, 6 MPa; talc, 0.05 wt %; PMMA, 0.5 wt % (CO2-free).

the range of 0.5-2.0 wt % (CO2-free), the particle morphology barely changes, as shown in Figure 5 (polymer concentration 0.5 wt % (see Figure 3a). However, the CV value increases with the increase in the polymer concentration. In this concentration region, PMMA-15k completely dissolves in the CO2expanded ethanol solution at 313 K and 6 MPa. When the polymer concentration exceeds 3 wt %, it is difficult to completely dissolve PMMA-15k in the CO2-expanded ethanol at 313 K. In addition, we attempted to produce PMMA particles with higher molecular weights. Figure 6 shows SEM photographs of the PMMA-68k microparticles produced by the PIPS of the CO2-expanded ethanol solution. The concentration of PMMA68k in ethanol was 0.5 wt %. The products were small particles with sizes on the order of 1 µm. The values of PPD and CV for PMMA-68k are similar to those values for PMMA-15k. The results show that it is possible to produce polymer particles with high molecular weights by the PIPS of CO2-expanded solutions. 3.3. Formation of PMMA Microcapsules of Talc. Figure 7 shows SEM photographs of the talc and PMMA microcapsules containing talc that were produced by the PIPS of the CO2expanded ethanol solution. The system was rapidly depressurized from 6 MPa to atmospheric pressure in approximately 1 min (6 MPa‚min-1) at 313 K. The feed concentrations of the talc and the PMMA in ethanol were 0.5 wt % and 0.05 wt % (CO2-free), respectively. Ethanol was dried at atmospheric pressure and room temperature for 12 h. The talc had a platelike configuration and a smooth surface, as shown in Figure 7a. A comparison of the SEM photographs reveals that the configurations of the talc and the PMMA microcapsules containing talc are similar, as shown in Figure 7b. The surface morphology of the microcapsules reflects the configuration of talc present in them because the coating thickness of the talc is very small. Evidence for the formation of the PMMA microcapsules of talc can be obtained using FT-IR and EPMA. Figure 8 shows

Figure 7. SEM photographs of (a) talc, (b) PMMA-15k microcapsules (drying at atmospheric pressure), and (c) PMMA-15k microcapsules (drying under vacuum) produced by the PIPS of CO2-expanded solution. Depressurization rate: 6 MPa‚min-1. Preexpansion conditions: temperature, 313 K; pressure, 6 MPa; talc, 0.05 wt %; PMMA, 0.5 wt % (CO2-free).

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Figure 9. EPMA images of PMMA-15k microcapsules of talc produced by the PIPS of CO2-expanded ethanol solutions. See Figure 8 for the experimental conditions. (b) and (c) show the distributions of C and Si, respectively.

Figure 10. Stability of microcapsules in pure water. (a) Talc and (b) PMMA-15k microcapsules produced by the PIPS of CO2-expanded ethanol solutions. See Figure 8 for the experimental conditions.

Figure 11. Effect of polymer concentration on the morphology of the particles formed by the PIPS of CO2-expanded ethanol solutions (drying at atmospheric pressure). Depressurization rate: 6 MPa‚min-1. Preexpansion conditions: temperature, 313 K; pressure, 6 MPa; talc, 0.5 wt %; PMMA, (a) 0.02 wt %, (b) 0.05 wt %, (c) 0.2 wt %, and (d) 0.5 wt % (CO2-free).

the FT-IR spectra of the talc and microcapsules. Although the peak (ca. 1730 cm-1) corresponding to the CO group caused by PMMA can be observed in the case of the microcapsules, it cannot be detected for the talc because it does not contain a CO group. The surface distributions of the C and Si were mapped in an EPMA image, as shown in Figure 9. Although the distribution of C in the microcapsules, which indicates the presence of PMMA, was fairly sharp (Figure 9b), it was not detected on the talc surface. The distribution of Si, indicating talc, was also detected in the microcapsules, as shown in Figure 9c. It can be considered that the talc was completely encapsulated by a thin PMMA film. This revealed that the talc was uniformly encapsulated by the PMMA. It was difficult to determine the coating performance for all the collected microcapsules by using FT-IR and EPMA because, in the proposed process, an extremely large number of microcapsules were produced. To evaluate the performance of the polymer coating, we examined the stability of the microcapsules in pure water. The talc particles or microcapsules were added to pure water (particle concentration 0.5 wt %), and the suspended solution was shaken by a mechanical shaker. Figure 10 shows the stable conditions of the talc and microcapsules in water. Pure talc was dispersed in pure water for more than 3 min, as shown in Figure 10a, because talc possesses hydrophilic surfaces. On the other hand, the microcapsules were not

dispersed in water, as shown in Figure 10b, because the PMMA microcapsules of talc have hydrophobic surfaces. The stability analysis revealed that most of the talc particles were coated with PMMA and were present inside the resulting microcapsules. 3.4. Effect of Various Factors on the Morphology of Microcapsules. The effects of experimental conditions such as the drying method, polymer concentration, and depressurizing rate on the morphology of the microcapsules were investigated. First, we discuss the influence of the drying method on the particle morphology. As shown in Figure 7c, the polymer particles that were precipitated on the talc surface through vacuum drying were smaller than those obtained by drying under atmospheric pressure (Figure 7b). During rapid depressurization, both the talc and the polymer precipitated from the solutions, and the talc particles were formed in the ethanol solution. During vacuum drying, some polymer was coated on the talc particles, and some fine polymer particles were formed. To prevent the formation of polymer particles that do not contain talc, we must inhibit the supersaturation of the solute and the homogeneous nucleation caused by the vacuum drying and the rapid evaporation of ethanol. The control of the degree of supersaturation and the nucleation rate caused by the drying method is the key novel feature of this study regarding the inhibition of the formation of polymer particles that do not contain talc.

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Figure 12. Effect of polymer concentration on the morphology of particles formed by the PIPS of CO2-expanded ethanol solutions (drying at vacuum condition). Depressurization rate: 6 MPa‚min-1. Preexpansion conditions: temperature, 313 K; pressure, 6 MPa; talc, 0.5 wt %; PMMA, (a) 0.02 wt %, (b) 0.05 wt %, (c) 0.2 wt %, and (d) 0.5 wt % (CO2-free).

Figure 11 shows the effect of the polymer concentration on the particle morphology of the microcapsules produced by the PIPS of the CO2-expanded ethanol solutions. Ethanol was dried under atmospheric pressure and room temperature for 12 h. At low polymer concentrations, the microcapsules reflect the configuration of the talc surface because the coating thickness is very small. However, the microcapsules adhere to each other at high polymer concentrations. The film shown in Figure 11d is obtained when the feed composition ratio of the talc and the polymer is 0.5:0.5. The talc is completely buried in the PMMA solid, and we cannot obtain any microcapsules or microparticles. Finally, we discuss the effects of the polymer concentration and the drying method on the particle morphology, as shown in Figure 12. Ethanol was removed by vacuum drying. In the low polymer concentration region, we cannot observe small polymer particles on the talc surface, as shown in Figure 12a. However, as the polymer concentration increases, small polymer particles are precipitated on the talc surface. Furthermore, the microcapsules adhere to each other at high polymer concentrations. If the feed composition ratio of the talc and the polymer is 0.5:0.5, the polymer microcapsules will aggregate with small polymer particles. 4. Conclusions To obtain the fundamental information for polymer microspheres and microcapsules formation by the PIPS of CO2expanded ethanol solution, the phase behavior of CO2-expanded ethanol solution containing PMMA was discussed. Although both ethanol and CO2 are nonsolvent for PMMA at 313 K, PMMA completely dissolved in the CO2-expanded ethanol solution at 6 MPa and 313 K. CO2 in expanded liquids interacts with PMMA and reduces the glass transition temperature of the polymer, thereby leading to the plasticization of PMMA. The affinity between CO2-expanded ethanol and plasticized PMMA increases by the addition of CO2. Therefore, the solvent power of the CO2-expanded ethanol solution with regard to PMMA increased with CO2 concentration. Furthermore, PMMA microspheres and microcapsules of talc were produced by the PIPS of CO2-expanded ethanol solutions. In the experiment for PMMA microsphere formation, the particle morphology is strongly affected by the removal method of

ethanol. To obtain microspheres of sizes on the order of 1 µm and without adhesion, ethanol solutions containing PMMA must dry under vacuum. This may be due to the degree of supersaturation of polymer solution in the drying process, which results in a low nucleation rate. In the experiment for microcapsule formation, the polymer and talc were mixed in the CO2-expanded ethanol solutions. PMMA coacervation was achieved during the depressurization. After the coacervation, we obtained PMMA microcapsules of talc. However, some polymer was coated on the talc particles, and some fine polymer particles were formed after vacuum drying. To prevent the formation of polymer particles that do not contain talc, ethanol was dried at atmospheric pressure and room temperature. We must inhibit the supersaturation of the solute and the homogeneous nucleation caused by the vacuum drying and the rapid evaporation of ethanol. The control of the degree of supersaturation and the nucleation rate caused by the drying method is the key novel feature of this study regarding the inhibition of the formation of polymer particles that do not contain talc. Acknowledgment This study was partly supported by a Grant-in-Aid for Young Scientists (Grant 19760537) and the JST. Literature Cited (1) Lu, X.; Winnik, M. A. Luminescence Quenching in Polymer/Filler Nanocomposite Films Used in Oxygen Sensors. Chem. Mater. 2001, 13, 3449. (2) Walcarius, A. Electrochemical Applications of Silica-Based OrganicInorganic Hybrid Materials. Chem. Mater. 2001, 13, 3351. (3) Schmidt, G.; Malwitz, M. M. Properties of Polymer-Nanoparticle Composites. Curr. Opin. Colloid Interface Sci. 2003, 8, 103. (4) Reculusa, S.; Mingotaud, C.; Bourgeat-Lami, E.; Duguet, E.; Ravaine, S. Synthesis of Daisy-Shaped and Multipod-Like Silica/Polystyrene Nanocomposites. Nano Lett. 2004, 4, 1677. (5) Jung, J.; Perrut, M. Particle Design Using Supercritical Fluids: Literature and Patent Survey. J. Supercrit. Fluids 2001, 20, 179. (6) Sun, Y.-P.; Rollins, H. W.; Bandara, J.; Meziani, M. J.; Bunker, C. E. In Supercritical Fluid Technology in Materials Science and Engineering: Synthesis Properties and Applications; Sun, Y.-P., Ed.; Marcel Dekker: New York, 2002; p 491. (7) Hakuta, Y.; Hayashi, H.; Arai, K. Fine Particle Formation Using Supercritical Fluids. Curr. Opin. Solid State Mater. Sci. 2003, 7, 341. (8) Beckman, E. J. Supercritical and Near-critical CO2 in Green Chemical Synthesis and Processing. J. Supercrit. Fluids 2004, 28, 121. (9) Yeo, S. D.; Kiran, E. Formation of Polymer Particles with Supercritical Fluids: A Review. J. Supercrit. Fluids 2005, 34, 287. (10) Reverchon, E.; Adami, R. Nanomaterials and Supercritical Fluids. J. Supercrit. Fluids 2006, 37, 1. (11) Tom, J. W.; Debenedetti, P. G.; Jerome, R. Precipitation of Poly(L-lactic acid) and Composite Poly(L-lactic acid)-Pyrene Particles by Rapid Expansion of Supercritical Solutions. J. Supercrit. Fluids 1994, 7, 9. (12) Kim, J.-H.; Paxton, T. E.; Tomasko, D. L. Microencapsulation of Naproxen Using Rapid Expansion of Supercritical Solutions. Biotechnol. Prog. 1996, 12, 650. (13) Meziani, M. J.; Sun, Y. P. Protein-conjugated Nanoparticles from Rapid Expansion of Supercritical Fluid Solution into Aqueous Solution. J. Am. Chem. Soc. 2003, 125, 8015. (14) Levit, N.; Guney-Altay, O.; Pestov, D.; Tepper, G. Development of Layered Polymer Nanocomposites Using Supercritical Fluid Technology. Macromolecules 2005, 38, 6528. (15) Mishima, K.; Matsuyama, K.; Tanabe, D.; Yamauchi, S.; Young, T. J.; Johnston, K. P. Microencapsulation of Proteins by Rapid Expansion of Supercritical Solution with a Nonsolvent. AIChE J. 2000, 46, 857. (16) Matsuyama, K.; Mishima, K.; Hayashi, K.; Matsuyama, H. Microencapsulation of TiO2 Nanoparticles with Polymer by Rapid Expansion of Supercritical Solution. J. Nanopart. Res. 2003, 5, 87. (17) Matsuyama, K.; Mishima, K.; Hayashi, K. I.; Ishikawa, H.; Matsuyama, H.; Harada, T. Formation of Microcapsules of Medicines by the Rapid Expansion of a Supercritical Solution with a Nonsolvent. J. Appl. Polym. Sci. 2003, 89, 742.

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ReceiVed for reView February 2, 2007 ReVised manuscript receiVed June 29, 2007 Accepted July 9, 2007 IE070189O