Production of Uniform-Sized Polymer Core−Shell Microcapsules by

Contrary to the usual coaxial setup, the inner nozzle was slightly bent to touch the inside wall of the outer nozzle. A polymer solution for the core ...
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Langmuir 2008, 24, 2446-2451

Production of Uniform-Sized Polymer Core-Shell Microcapsules by Coaxial Electrospraying Yoon Kyun Hwang,† Unyong Jeong,*,† and Eun Chul Cho‡ Department of Materials Science and Engineering, Yonsei UniVersity, 134 Shinchon-dong, Seoul 120-749, Korea and Amorepacific Corporation/R&D Center, 314-1, Bora-dong, Yong-in City, 449-729, Korea 1700 ReceiVed NoVember 13, 2007. In Final Form: December 10, 2007 Spherical polymeric core-shell microcapsules in uniform size were produced by electrospraying with a coaxial nozzle setup. Contrary to the usual coaxial setup, the inner nozzle was slightly bent to touch the inside wall of the outer nozzle. A polymer solution for the core was introduced through the outer nozzle, and the other solution for the shell was supplied through the inner nozzle. The setup greatly increased reproduction of the same results. As a proof of the concept, core-shell microcapsules consisting of a PS or PMMA core and a PCL shell (PS@PCL, PMMA@PCL) were produced. When the volumetric feed rate of the shell-forming PCL solution was higher than that of the coreforming PS or PMMA solution the core-shell structures in uniform size were readily obtained. In contrast, irregular morphologies were observed when the feed rate of the PCL solution was slower or equal to that of the PS or PMMA solution. The size of the colloid was dependent on the relative feed ratio between the polymer solutions as well as the magnitude of applied voltage.

Introduction Polymeric microcapsules have inspired widespread attention due to a variety of applications, including cosmetics, pharmaceuticals, food, paints, and toners in reproducing systems.1 Onestep microencapsulation by in situ polymerization, including dispersion polymerization and interfacial polycondensation, has been widely used to produce microcapsules.2 Spray-drying,3 coacervation,4 porous glass emulsification,5 and miniemulsion polymerization6 have been also employed to prepare microcapsules. As a template-mediated process, the coating of desired polymers onto pre-made colloids has been extensively studied.7 Polymerization of monomers adsorbed onto the particle surface is the most commonly used for the templating approach.8 Recent advances in the surface modification of particles originated from the sequential layer-by-layer (LBL) electrostatic deposition.9 Despite the remarkable advances on the production of microcapsules, the polymer species prepared as core-shell * To whom correspondence should be addressed. E-mail: ujeong@ yonsei.ac.kr. † Department of Materials Science and Engineering, Yonsei University. ‡ Amorepacific Corporation/R&D Center. (1) For an extensive review, see: Functional Coatings by Polymer Microencapsulation; Ghosh, S. K., Ed.; Wiley-VCH: Weinheim, 2006. (2) (a) Arshady, R. J. Microencapsulation 1989, 6, 13. (b) Shukla, P. G.; Sivaam, S. J. Microencapsulation 1999, 16, 517. (c) Ramanathan, L. S.; Sivaram, S. U.S. Patent 6022930, 2000. (3) Ninomiya, Y.; Komamura, C.; Musa, Y. German Patent DE 3417200, 1985 (CA 103:100429). (4) Bachtsi, A. R.; Kiparissides, C. J. J. Controlled Release 1996, 38, 49. (5) Ma, G. H.; Su, Z. G.; Omi, S.; Sundberg, D.; Stubbs, J. J. Colloid Interface Sci. 2003, 266, 282. (6) (a) Landfester, K. Macromol. Symp. 2000, 150, 171. (b) Landfester, K. AdV. Mater. 2001, 13, 765. (7) For a review, see: (a) Hofman-Caris, C. H. M. New J. Chem. 1994, 18, 1087. (b) Caruso, F. AdV. Mater. 2001, 13, 11, and references therein. (8) (a) Huang, C. L.; Matijevic, E. J. Mater. Res. 1995, 10, 1327. (b) Ottewill, R. H.; Schofield, A. B.; Waters, J. A.; Williams, N. St. J. Colloid Polym. Sci. 1997, 275, 274. (c) Marinakos, S. M.; Novak, J. P.; Brousseau, L. C.; House, A. B.; Edeki, E. M.; Feldhaus, J. C.; Feldheim, D. L. J. Am. Chem. Soc. 1999, 121, 8518. (d) Quaroni, L.; Chumanov, G. J. Am. Chem. Soc. 1999, 121, 10642. (9) (a) Keller, S. W.; Johnson, S. A.; Brigham, E. S.; Yonemoto, E. H.; Mallouk, T. E. J. Am. Chem. Soc. 1995, 117, 12879. (b) Caruso, F.; Donath, E.; Mohwald, H. J. Phys. Chem. B 1998, 102, 2011. (c) Dokoutchaev, A.; James, J. T.; Koene, S. C.; Pathak, S.; Prakash, G. K. S.; Thompson, M. E. Chem. Mater. 1999, 11, 2389. (d) Caruso, F.; Shuler, C.; Kurth, D. G. Chem. Mater. 1999, 11, 3394.

microcapsules in uniform size have not been diversified very much.10 Core-shell microcapules still need a simple technique making possible versatile combinations between polymers. A coaxial electro-jet process might be useful to diversify the polymeric combinations because the supply of two polymer solutions is an independent parameter on the structural combinations. Xia and co-workers demonstrated the fabrication of coreshell fibers by using coaxial electrospinning.11 Colloidal production employing a similar concept has been reported by Loscertales and co-workers.12 They pioneered microcapsule production from coaxial liquid jets. They obtained oil-water microcapsules and photoplymer-water microcapsules.12a The group also fabricated inorganic/organic vesicles by employing sol-gel formulation during the electrospraying.12b A general study on the process and the scaling law was performed by the same group.12c Although a few works producing microcapsules have been reported, electrospraying has attracted less attention compared with the explosive increase of publications on electrospinning.13 It is mainly due to the difficulty in controlling the uniformity in size and shape of the resulting colloids. The size of the particles can be roughly controlled by adjusting the magnitude of electric field and volumetric flow rate of polymer solutions. However, uniform-sized polymer particles by electrospraying have been rarely reported, except a limited success by applying AC voltage superimposed in DC bias voltage.14 (10) Tauer, K. Colloids and Colloid Assemblies; Caruso, F., Ed.; Wiley-VCH: Weinheim, 2004; Chapter 1. (11) (a) Li, D.; Wang, Y.; Xia, Y. Nano Lett. 2003, 3, 1167. (b) Sun, Z.; Zussman, E.; Yarin, A. L.; Wendorff, J. H.; Greiner, A. AdV. Mater. 2003, 15, 1929. (c) Li, D.; Xia, Y. Nano Lett. 2004, 4, 933. (d) Loscertales, I. G.; Barrero, A.; Marquez, M.; Spretz, R.; Velarde-Ortiz, R.; Larsen, G. J. Am. Chem. Soc. 2004, 126, 5376. (e) Hao, X.-F.; Li, Z.-Y.; Li, D.-M.; Wang, C. Chem. J. Chin. UniV. 2005, 26, 385. (12) (a) Loscertales, I. G.; Barrero, A.; Guerrero, I.; Cortijo, R.; Marquez, M.; Ganan-Calvo, A. M. Science 2002, 295, 1695. (b) Larsen, G.; Velarde-Ortiz, R.; Minchow, K.; Barrero, A.; Loscertales, I. G. J. Am. Chem. Soc. 2003, 125, 1154. (c) Lopez-Herrera, J. M.; Barrero, A.; Lopez, A.; Loscertales, I. G.; Marquez, M. J. Aerosol Sci. 2003, 34, 535. (13) For a review, see (a) Li, D.; Xia, Y. AdV. Mater. 2004, 16, 1151. (b) Ramakrishna, S.; Fujihara, K.; Teo, W.-E.; Lim, T.-C.; Ma, Z. An Introduction to Electrospinning and Nanofibers; World Scientific Publishing Co.: Singapore, 2005. (14) Balachandran, W.; Machowski, W.; Ahmad, C. N. IEEE Trans. Ind. Appl. 1994, 30, 850.

10.1021/la703546f CCC: $40.75 © 2008 American Chemical Society Published on Web 02/08/2008

Uniform-Sized Polymer Core-Shell Microcapsules

Micro- and nanoparticle production by electrospraying has been recently reviewed by Jaworek.15 So far, the production of the core-shell colloids by electrospraying has not been reported yet. This work aims at the development of a simple way to obtain core-shell microcapsules in uniform size by utilizing the coaxial electrospraying. The main advantage of this approach is onestep and continuous production of core-shell microcapsules by simply injecting polymer solutions. Because two solutions are separately supplied through an inner or outer nozzle, the combination of polymer species in the core and shell has a high degree of freedom. The material combination can be extended to produce multifunctional capsules composed of metal particles, ceramic or metal shells, functional polymers, and pharmaceutical drugs. As a proof of this concept, we produced spherical microcapsules consisting of polystyrene core and poly(caprolactone) shell (PS@PCL) as well as polymethylmethacrylate core and PCL shell (PMMA@PCL). The polymers used in the study have been extensively prepared in colloidal forms for the uses as magnetic beads,16a building blocks for photonic crystals,16b sacrificial material to generate hollow colloids,165c and containers in drug delivery systems.16d

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Figure 1. Schematic illustration of the coaxial electrospraying and possible structures of the colloids consisting of two polymer components. The inner nozzle was set to touch the inside wall of the outer nozzle instead of being located at the center of the outer nozzle. The morphology of the colloids could be varied by adjusting the feed ratio between the polymer solutions. Core-shell microcapsules are obtained when the feed rate (φ) of a PCL solution from the inner nozzle is higher, whereas irregular morphologies are generated when the solution through the outer nozzle is the majority.

Experimental Section Polycaprolactone (PCL, Mw ) 65 000), polystyrene (PS, Mw ) 230 000), and polymethyl methacrylate(PMMA, Mw ) 100 000) were purchased from Aldrich. Chloroform (Mallinckrodt Baker, 99.9%) and 2,2,2-trifluoroethanol (TFE, Aldrich, g99%,) were used as the solvent without any purification. All polymer solutions for electrospraying were prepared in chloroform. Small amounts (0.015 g) of a green dye (coumarin-6) and a red dye (rhodamine B) were dissolved in a PS solution (4 wt %, 52 mL) and a PCL solution (3 wt %, 52 mL), respectively. The two fluorescent dyes were purchased from Sigma Aldrich Co. Ther coaxial setup was used to produce microcapsules. Stainless needles with a size of 30G (inner diameter: 160 µm) and 22G (inner diameter: 410 µm) were employed for the inner and outer nozzle, respectively. The needle for the inner nozzle was slightly bent to touch the inside wall of the outer nozzle. The polymer solutions were loaded in plastic syringes and supplied by separate microsyringe pumps (KDS 200, Scientific). The PS solution or PMMA solution was injected through the outer nozzle and the PCL solution was supplied in the inner nozzle. The needle-to-collector distance was kept at 25 cm, and the voltage was applied in a range of 8.7-16.5 kV. Microcapsule production was carried out in stable cone-jet modes that was achieved in the range of 8-14 kV. The relative humidity was kept at 35%, and the temperature was ∼28 °C. The Field emission scanning electron microscopy(FE-SEM, S-4200, Hitachi) was used to investigate the size and morphology of the core-shell capsules. To examine the core-shell structures of the microcapsules, the PS core was crosslinked by exposing the microcapsules to UV (16W, Bogouv) at room temperature in the air and then PCL was selectively dissolved in 2,2,2-trifluoroethanol (TFE)/deionized water (1:2, v/v) for 60 min. The PCL shell was completely removed within a few min in the mixture solvent. A confocal laser microscope (LSM 510 META, Carl Zeiss, Inc.) was employed to optically check the core-shell structures of the microcapsules. The green (coumarin 6) and red dye (rhodamine B) were excited at 488 and 543 nm, respectively. The distribution and diameter of the microcapsules were measured by a size analyzer (TOMORO Scope Eye). (15) Jaworek, A. Powder Tech. 2007, 176, 18. (16) for examples, see (a) Xu, X.; Friedman, G.; Humfeld, K. D.; Majetich, S. A.; Asher, S. A. Chem. Mater. 2002, 14, 1249. (b) Park, S. H.; Qin, D.; Xia, Y. AdV. Mater. 1998, 10, 1028. (c) Graf, C.; Vossen, D. L. J.; Imhof, A.; van Blasderen, A. Langmuir 2003, 19, 6693. (d) Yang, F.; Murugan, R.; Wang, S.; Ramakrishna, R. Biomaterials 2005, 26, 2603, and references therein.

Results and Discussion In an electro-jet process, the high electric field builds up an electric charge on liquid flowing out from the capillary nozzles and leads to elongation along the electric field. Polymer solutions with high viscosity produces continuous fibers, but the jet of a dilute polymer solution breaks into droplets. Several groups reported that the electric field strength has a significant effect on the mode of spray.17 The mode normally changes from dripping to a cone-jet as the field strength increased. Overly high voltage makes the liquids unstable and generates a multi-jet spray that degrades the size uniformity of the particles.17 The cone-jet mode is desirable for the production of stable droplets with uniform size. The voltage window of the cone-jet mode becomes wider as the viscosity of the solution increases. Therefore, the concentration of the solutions in this study was set as high as possible, but not to produce any fiber. Figure 1 shows the schematic drawing of the coaxial electrospraying used in this study. Instead of being located at the center of the outer nozzle as Xia group proposed,11 the inner nozzle touched the inside wall of the outer nozzle. We found it difficult to repetitively locate the inner nozzle at the center of the outer nozzle, which obstructs the reproduction of the microcapsules with the same size and shape. Slight bending of the inner needle to the wall of outer nozzle made possible the alignment of the needles at the same position and led to reproduction of the same colloids. A shell-forming liquid was supplied to the inner nozzle and a core-forming liquid was fed through the outer nozzle. In order to facilitate the solution from the inner nozzle (Sin) to surround the solution from the outer nozzle (Sout) during electrospraying, the inner needle was located somewhat inwardly, as depicted in Figure 1. Viscoelastic liquids coming out from a small nozzle experience a large volume expansion due to the first normal stress, which is called “die swell”. The polymer solution coming out from the indented inner nozzle expands at the nozzle tip and fills up the vacancy between Sout. When the volumetric flow rate of Sin is higher than that of (17) (a) Jaworek, A.; Krupa, A. J. Aerosol Sci. 1999, 30, 873. (b) Lenggoro, W.; Okuyama, K.; Fernandez de la Mora, J.; Tohge, N. J. Aerosol Sci. 2000, 31, 121. (c) Grigoriew, A.; Edirisinghe, M. J. J. Appl. Phys. 2002, 91, 437. (d) Smith, K. L.; Alexander, M. S.; Stark, J. P. W. J. Appl. Phys. 2006, 99, 064909.

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Figure 2. SEM images (A-C) and confocal laser scanning microscope image (D) of the microcapsules. The PCL solution including rhodamin B was fed at 6 µL/min through the inner nozzle and PS solution possessing coumarin 6 was supplied at 3 µL/min through the outer nozzle. (A) The as-produced colloids with high uniformity in size. (B) The higher magnification showing the embossing golf-ball structure of the colloidal surface. (C) PS core obtained by selectively dissolving PCL shell. (D) localized distribution of the dyes, verifying the core-shell structure of the colloids.

Sout, the stream of Sin builds up pressure to the wall of the outer needle. The flow of Sin experiences another volume expansion at the exit of the outer nozzle. The larger volumetric expansion of Sin than Sout at the outer nozzle tip makes possible Sin to surround Sout in the cone-jet mode. When the relative flow rate of Sin is not higher than that of Sout by a certain value, the volume expansion of Sin is not enough to surround Sout. If the inner nozzle is not located inward, then the surrounding Sout by Sin is not guaranteed because the two solutions are released separately to the air, thus Sin is more probable to be neighboring with Sout rather than encapsulating. The possibility of the wrapping becomes lower or impossible in a setup in which the inner nozzle is protruded outward because the electro-jets from PS and PCL solutions can be generated separately. Although nozzles with a small diameter are more desirable for the stable production of microparticles, viscoelastic solutions made of polymers with a high glass transition temperature (Tg) readily clogged fine nozzles during the electrospraying. PS or PMMA solution coming out from a single nozzle of the 30G stainless needle (inner diameter: 160 µm) blocked the nozzle tip within a few minutes after applying electric field. The considerable volume expansion of viscoelastic polymer solutions exiting from such a fine nozzle is considered to promote the clogging because the abrupt volume increase at the nozzle tip can enhance wetting of the solution to the outside surface of the outer needle. Under high electric field, the fast evaporation of the solvent accelerates the solidification of the polymers around the nozzle tip, forming a big precipitation. Meanwhile, the same polymer solutions supplied through a single nozzle of the 22G stainless needle (inner diameter: 410 µm) did not clog the nozzle. Therefore, viscous solutions from polymers with a high Tg need to be supplied through needles with relatively large diameters. In contrast, PCL solutions showed stable electrospraying without the clogging problem regardless of the needle diameter. Such experimental conditions required the supply of shell-forming PCL solutions

through the inner nozzle and the core-forming solutions through the outer nozzle. The formation of a core-shell structure using a coaxial electrospraying assumes that the two polymer solutions are immiscible during electrospraying. When the PS solution including a dye, of the same concentration used in this study, was slowly poured into the PCL solution of the study, we found the interface clearly sustained more than 5 min unless they were agitated. The increased viscosity hinders the inter-diffusion of the molecules. We suppose the charge difference between the solutions under high electric field may increase the phase separation of the solutions during the electrospraying. The generation of uniform-sized regular drops in the electrospraying is possible only in a limited range of voltage and flow rate. The solutions in this study experienced a dripping mode at low voltages, and the microdripping was observed until the voltage was raised up to 6.7 kV. Large particles (10∼100 µm) generated in the microdripping mode were not dried during the elecrospraying and spread out on the collector substrate. When the electric field was over 8 kV, the meniscus of the solution at the nozzle tip changed to the cone-jet mode. The length of the cone-jet gradually decreased as the electric field was raised. The cone-jet mode changed to a multiple jet at 14.5 kV. The generation of stable and regular droplets was only possible in the range of the cone-jet mode (8∼14.5 kV in this study). The morphology of the colloids was dependent on the relative feed rate of the two polymer solutions. When the feed rate of a shell-forming solution (PCL solution in Figure 1) is higher than that of the core-forming solution (PS solution in Figure 1), the cone-jet mode will be composed of an inner stripe of the PS solution surrounded by the PCL solution. The droplets from the cone-jet may produce PS@PCL core-shell microcapsules. The thickness of the shell should increase as the feed rate of PCL solution is raised. In contrast, when the feed rate of PCL is equal to or slower than that of PS solution, the PCL solution may be insufficient to effectively surround the PS solution, which possibly

Uniform-Sized Polymer Core-Shell Microcapsules

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Figure 3. SEM images describing the shape of the colloids depending on the feed ratio between the solutions (PS:PCL, vol/vol): (A) 1:0, (B) 2:1, (C) 1:1, (D) 1:3, (E) 1:4, and (F) 0:1. The total volume through the nozzles was fixed 9 or 10 µL/min. When the feed rate of PCL solution was less than or comparable with that of PCL solution (A-C), the colloids were irregular in size and shape. In contrast, the spherical microcapsules were generated when the PCL solution was majority in the cone-jet. The insets in (A) and (F) are the as-produced colloids and the two insets in (B-E) represent the as-prepared colloids (left) and PS core (right) obtained by selectively removing PCL.

results in stacks or interconnection between the polymers as normally observed in immiscible polymer blends. Figure 2 shows SEM images (A-C) and a confocal laser scanning microscope image (D) of the PS@PCL core-shell microcapsules. The PCL solution was fed by 6 µL/min through the inner nozzle and the PS solution was supplied by 3 µL/min through the outer nozzle. The voltage was fixed at 10.5 kV to form a cone-jet mode. In order to optically monitor the location of the polymer molecules, a green dye (coumarin 6) was dissolved in PS solution and a red dye (rhodamine B) was mixed with PCL solution. The concentration of rhodamine B and coumarin 6 in both solutions was kept 0.018 wt % in chloroform. Figure 2A shows the as-produced spherical colloids with good size uniformity. The diameter of the colloids was about 3 µm. The higher magnification of the colloids, as displayed in Figure 2B, revealed the embossing pattern like a golf ball on the colloidal surface. The surface morphology is attributed to the decreased solubility of PCL in chloroform as the concentration increased due to the fast evaporation of the solvent.18 In the late stage of the solvent evaporation, the polymer molecules crystallize, and thus the solvent is localized at the surface of the microparticle.

The evaporation of the solvent leaves the humps on the colloidal surface. The same morphology has been reported in PCL particles obtained by electrospraying chloroform solutions.18b However, the golf ball shape was not observed when PCL was highly miscible with solvents.18a The structure of the core and shell of the colloids was examined by dissolving PCL with a mixture solvent of 2,2,2-trifluoroethanol (TFE)/deionized water (1:2, v/v) for 60 min. PCL was readily dissolved within a few minutes in the mixture solvent. PS was crosslinked by UV exposure before soaking the colloids into the mixture solvent to prevent any possible dissolution of PS. Figure 2C shows the crosslinked PS particles after dissolving PCL. They still maintained the spherical shape, and the size was reduced about 18% from the as-produced colloids, indicating the core-shell structure of electrosprayed colloids. The golf ball structure on the surface was not observed in the PS core, which informs that the two solutions were well (18) (a) Zhou, X. D.; Zhang, S. C.; Huebner, W.; Ownby, P. D. J. Mater. Sci. 2001, 36, 3759. (b) Wu, Y.; Clark, R. L. J. Colloid Interface Sci. 2007, 310, 529. (19) (a) Koombhongse, S.; Liu, W.; Reneker, D. H. J. Polym. Sci. Polym. Phys. 2001, 39, 2598. (b) Fong, H.; Liu, W. D.; Wang, C. S.; Vaia, R. A. Polymer 2002, 43, 775. (c) Koski, A.; Yim, K.; Shivkumar, S. Mater. Lett. 2004, 58, 493.

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Figure 4. Dependence of the size and shape of the PS@PCL microcapsules on the feed ratio between PS and PCL solutions. The diameter of the microcapsules approached ∼3 µm as the feed rate of PCL solution was increased.

separated and the interface was sharply maintained during the electrospraying. The core-shell structure of the colloids was optically confirmed as shown in Figure 2D. The simultaneous excitation of the dyes at 488 and 543 nm revealed that the green dye was confined in the core and the red dye was localized in the shell. The position of the dyes reflects the location of the polymer molecules, which clarifies the core-shell structure of the colloids. Even when the dyes appear mixed with each other, the polymer molecules will still probably be separated because the small dye molecules can diffuse faster through the interface between the polymer solutions. In a coaxial system using distinct polymer solutions, the relative volumetric feed ratio between the solutions considerably influences the shape and overall structure. The SEM images in Figure 3 display the shape of the colloids depending on the relative feed ratio between the PS solution and the PCL solution. In order to produce spherical core-shell microcapsules with uniform size, the electric field was fixed at 10.5 kV to achieve the cone-jet mode. The total feed rate through the two nozzles was kept at 9 µL/min or 10 µL/min to remove the effect of solution volume on the size and shape of the colloids. Figure 3A shows the electrosprayed colloids of the pure PS solution from a single nozzle of the 22G stainless needle that was used as the outer nozzle in the coaxial setup. Giant irregular-shaped particles with broad size distribution were obtained. The wrinkled structure is attributed to the early formation of the PS skin layer that is often observed in electrospinning of high molecular weight polymer solutions. The skin layer collapses in a wrinkled fashion as the solvent evaporates from the core. Contrary to PS particles, PCL colloids by electrospraying through a single nozzle of the 30G needle were generated in a solid spherical form, as shown in Figure 3F. Spherical colloids of PCL could be obtained from other single nozzles regardless of the nozzle diameter. For the PS solution, as stated above, nozzles with a smaller diameter could not be used because the nozzle tip was clogged by the polymer precipitation. Figure 3B-E have been obtained by varying the volumetric feed ratio of the PS solution to the PCL solution (2:1, 1:1, 1:3, and 1:4 in the order). The insets in Figure 3B-E are high magnification of the as-produced particles (left) and the UVcrosslinked PS core after dissolving the PCL shell (right). The size of the particles decreased as the feed ratio of PCL solution was raised. When the feed rate of the PCL solution was equal to or less than that of PS solution, the shape of the particles were irregular and the wrinkled surface was not improved. Alternatively, spherical core-shell microcapsules with greatly improved size uniformity were obtained when the feed rate of the PCL

Figure 5. Dependence of the size and shape of PS@PCL microcapsules on the concentration of PCL solution at a fixed conentration of PS solution (4 wt %). The concentration of PCL solution was varied from 1 to 4 wt %. The volumetric flow rate of PS solution and PCL solution were fixed at 3 µL/min and 6 µL/min, respectively.

Figure 6. Dependence of the size and shape of PS@PCL microcapsules on the concentration of PS solution at a fixed conentration of PCL solution (3 wt %). The concentration of PCL solution was varied from 1 to 5 wt %. The volumetric flow rate of PS solution and PCL solution were fixed at 3 µL/min and 6 µL/min, respectively.

solution was higher than that of the PS solution. Figure 2 shows the colloids produced at 1:2 feed ratio, and Figure 3D,E shows the results at 1:3 and 1:4. The insets of Figure 3D,E prove the core-shell structure of the colloids. The surface structure of the microcapsules was varied from an embossing pattern at 1:2 to a wrinkled surface at 1:4 feed ratio. Figure 4 depicts the size change of the colloids according to the feed ratio between PS and PCL solutions. The size and distribution were measured by a size analyzer (TOMORO Scope Eye). The diameter of the particles gradually decreased to that of the pure PCL particles (∼2.76 µm). When the PCL solution was less supplied than PS solution the droplets had various blend morphologies of immiscible polymer pairs such as the isolated, stacked, or interconnected structure. Meanwhile, feeding PCL solution more than PS counterpart stabilized the cone-jet mode, hence spherical core-shell microcapsules were reproducibly generated. The morphology and size of the colloids should also depend on the viscosity of the solutions. We examined the effect of the

Uniform-Sized Polymer Core-Shell Microcapsules

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Figure 7. SEM images of PMMA@PCL microcapsules obtained by coaxial electrospraying. Similar to the results in PS@PCL, the morphology was dependent on the feed ratio between the solutions: (A) 2:1, (B) 1:1, (C) 1:2, and (D) 1:4.

polymer concentration on the morphology of the microcapsules. First, the concentration of the PS solution was fixed at 4 wt %, at which the cone-jet mode was achieved. The concentration of the PCL solution was varied from 1 wt % to 4 wt %. The other spraying conditions were the same as those obtained in Figure 2. The volumetric flow rate of PS solution and PCL solution were fixed at 3 µL/min and 6 µL/min, respectively. The results are shown in Figure 5. When the PCL solution was 1 wt % in chloroform, the shapes of the colloids were irregular, and the sizes were broadly distributed. The use of 2 wt % PCL solution improved the size uniformity, but the spherical microcapsules were not produced. An increase in the concentration to 3 wt % produced spherical microcapsules with uniform size, as shown in Figure 2. Higher concentrations (4 wt %) started to generate fibers. Second, the concentration of PCL was fixed at 3 wt %, and PS solution was varied from 1 wt % to 5 wt %. The volumetric flow rate of PS solution and PCL solution were fixed at 3 µL/min and 6 µL/min, respectively. The results are displayed in Figure 6. The diameter of the resulting colloids increased as the concentration of PS solution was raised. The microcapsules were readily produced even when the PS concentration was 1 wt % that is not high enough for the cone-jet mode. The spherical microcapsules were produced until the concentration of the PS solution was raised up to 4 wt %. The PS solution with concentration more than 5 wt % generated irregular colloids mixed with fibers. It is notable that the morphology of microcapsules was also affected by the evaporation rate of the solvent in the chamber. The solvent had to be evacuated by continuous suction to prevent the build-up of the spatial charge. Otherwise, the production of the microcapsules was not repetitively achieved. The coaxial electrospraying can be used to produce other pairs of polymer microcapsules. We obtained spherical PMMA@PCL

core-shell microcapsules by using a PMMA solution (3 wt %) as shown in Figure 6. Similar to the results in the PS@PCL case, microcapsules were obtained in a limited range of voltages, concentration of polymer solutions, relative feed ratio, and volumetric feed rate of each solution. The process can be further extended to produce various microcapsules composed of other polymer pairs or polymer/inorganic hybrid coatings.

Conclusions In summary, polymeric spherical microcapsules were produced by the coaxial electrospraying of concentrated polymer solutions. In order to improve the reproducibility, we had the inner nozzle contact the inside surface of the outer nozzle. The concept was verified by generating core-shell microcapsules of PS@PCL and PMMA@PCL. The experimental results revealed that the relative feed ratio is an important factor for the formation of core-shell structure. Spherical microcapsules could be obtained when the feed rate of the shell-forming solution was higher than that of the core-forming solution. Otherwise, blend morphologies shown in immiscible polymer pairs were generated. The size of the particles could be controlled in the range of 3-6 µm in diameter by adjusting the applied voltage and volumetric feed rate. Acknowledgment. This work was supported in part by the IT R&D program of Korean Ministry of Information and Communication (MIC/IITA) [2007-S-078-0] and by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MOST) (R11-2007-050-02004-0). Y.K.H. and E.C.C. thank Amorepacific Corporation/R&D Center for the financial support. LA703546F