Facile Fabrication of Monodisperse Polymer Hollow Spheres

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Facile Fabrication of Monodisperse Polymer Hollow Spheres Hui Lv,† Quan Lin,† Kai Zhang,† Kui Yu,‡ Tongjie Yao,† Xuehai Zhang,† Junhu Zhang,† and Bai Yang*,† State Key Laboratory for Supramolecular Structure & Materials, College of Chemistry, Jilin UniVersity, Changchun 130012, People’s Republic of China, and Steacie Institute for Molecular Sciences, National Research Council of Canada, 100 Sussex DriVe, Ottawa, Ontario K1A0R6, Canada ReceiVed August 26, 2008. ReVised Manuscript ReceiVed September 19, 2008 This article reports the facile synthesis of monodisperse polymer hollow spheres by seeded emulsion polymerization without additional treatment. In this method, P(St-MMA-MAA) copolymer latex particles were first prepared by emulsifier-free emulsion polymerization and then used as seeds to carry out emulsion polymerization of methyl methacrylate (MMA), divinyl benzene (DVB), and 2-hydroxyethyl methacrylate (HEMA) with potassium persulfate (KPS) as initiator at 80 °C. The void of hollow spheres was readily adjusted by changing the monomer/seed weight ratio, and it could be enlarged while the diameters of hollow spheres changed little after etching by dimethyl formamide (DMF). The effects of synthetic parameters including the monomer composition and the properties of seeds on the morphology of hollow spheres were investigated in detail. On the basis of the experimental results, it seemed reasonable to conclude that the formation of hollow spheres was due to the “dissolution” of seeds in monomers and phase separation between the constituent polymers. As a thermodynamic factor, sodium dodecyl sulfate (SDS) would allow the preparation of solid particles depending on its level.

Introduction In recent years, polymer hollow spheres have stimulated increasing interest in the field of materials science because of their widely medicinal, biological, and industrial applications, including nano- and microreaction vessels, targeted drug delivery, controlled release, photocatalysis, macromolecule encapsulation, photonic crystals, architectural, and paper coatings.1-8 To date, significant progress has been made in the design and fabrication of polymer hollow spheres. A number of methods, such as hard template,9-15 soft template,16-23 dynamic swelling,24-26 encapsulation of nonsolvent,27-30 osmotic swelling,31,32 and polymeric micelles,33 have been developed to synthesize polymer hollow spheres. Most methods above belong to the template strategy, which is greatly popular because of perfect monodispersity of microspheres and facile control of cavity size. However, * Corresponding author. Fax: +86-431-85193423. E-mail: byangchem@ jlu.edu.cn. † Jilin University. ‡ National Research Council of Canada.

(1) Shchukin, D. G.; Sukhorukov, G. B. Langmuir 2003, 19, 4427–4431. (2) Brand, T.; Ratinac, K.; Castro, J. V.; Gilbert, R. G. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 5706–5713. (3) Cheng, D. M.; Zhou, X. D.; Xia, H. B.; Chan, H. S. O. Chem. Mater. 2005, 17, 3578–3581. (4) Dowding, P. J.; Atkin, R.; Vincent, B.; Bouillot, P. Langmuir 2004, 20, 11374–11379. (5) Shchukin, D. G.; Ustinovich, E.; Sviridov, D. V.; Lvov, Y. M.; Sukhorukov, G. B. Photochem. Photobiol. Sci. 2003, 2, 975–977. (6) Lvov, Y.; Antipov, A. A.; Mamedov, A.; Mo¨hwald, H.; Sukhorukov, G. B. Nano Lett. 2001, 1, 125–128. (7) Xu, X. L.; Asher, S. A. J. Am. Chem. Soc. 2004, 126, 7940–7945. (8) McDonald, C. J.; Devon, M. J. AdV. Colloid Interface Sci. 2002, 99, 181– 213. (9) 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, 8518–8522. (10) Wu, M. L.; O’Neill, S. A.; Brousseau, L. C.; McConnell, W. P.; Shultz, D. A.; Linderman, R. J.; Feldheim, D. L. Chem. Commun. 2000, 775–776. (11) Kamata, K.; Lu, Y.; Xia, Y. N. J. Am. Chem. Soc. 2003, 125, 2384–2385. (12) Niu, Z. W.; Yang, Z. Z.; Hu, Z. B.; Lu, Y. F.; Han, C. C. AdV. Funct. Mater. 2003, 13, 949–954. (13) Kida, T.; Mouri, M.; Akashi, M. Angew. Chem., Int. Ed. 2006, 45, 7534– 7536. (14) Jang, J.; Ha, H. Langmuir 2002, 18, 5613–5618. (15) Gao, C. Y.; Moya, S.; Donath, E.; Mo¨hwald, H. Macromol. Chem. Phys. 2002, 203, 953–960.

the process of preparing core-shell particles is multistep, and the formation of the hollow structure requires the removal of the cores by dissolving or drying the particles. Itou et al. obtained polymer hollow spheres via conventional seeded emulsion polymerization without additional treatment. That was, the void of the spheres was created during polymerization, and no organic solvent was used in the system. In this article, we employ modified seeded emulsion polymerization to fabricate monodisperse polymer hollow spheres with large void. The approach has apparent differences and advantages in comparison with the conventional method. An intended swelling procedure of seeds for a long time before polymerization is ignored, which not only makes the preparation of hollow spheres time-saving, but also the proper particle viscosity is favored over complete phase separation of the constituent polymers and thus arouses a large void. At the same time, the use of surfactant is also avoided, which is beneficial to the formation of hollow (16) Han, J.; Song, G. P.; Guo, R. Chem. Mater. 2007, 19, 973–975. (17) He, X. D.; Ge, X. W.; Liu, H. R.; Wang, M. Z.; Zhang, Z. C. Chem. Mater. 2005, 17, 5891–5892. (18) Wei, Z. X.; Wan, M. X. AdV. Mater. 2002, 14, 1314–1317. (19) Zhang, L. J.; Wan, M. X. AdV. Funct. Mater. 2003, 13, 815–820. (20) Wang, Y. X.; Qiu, Z. B.; Yang, W. T. Macromol. Rapid Commun. 2006, 27, 284–288. (21) Kim, Y. B.; Yoon, K.-S. Macromol. Rapid Commun. 2004, 25, 1643– 1649. (22) McKelvey, C. A.; Kaler, E. W.; Zasadzinski, J. A.; Coldren, B.; Jung, H.-T. Langmuir 2000, 16, 8285–8290. (23) Song, L. Y.; Ge, X. W.; Wang, M. Z.; Zhang, Z. C.; Li, S. C. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 2533–2541. (24) Okubo, M.; Minami, H. Colloid Polym. Sci. 1997, 275, 992–997. (25) Minami, H.; Kobayashi, H.; Okubo, M. Langmuir 2005, 21, 5655–5658. (26) Minami, H.; Okubo, M.; Oshima, Y. Polymer 2005, 46, 1051–1056. (27) McDonald, C. J.; Bouck, K. J.; Chaput, A. B. Macromolecules 2000, 33, 1593–1605. (28) Tiarks, F.; Landfester, K.; Antonietti, M. Langmuir 2001, 17, 908–918. (29) Jang, J.; Lee, K. Chem. Commun. 2002, 1098–1099. (30) Ma, G. H.; Su, Z. G.; Omi, S.; Sundberg, D.; Stubbs, J. J. Colloid Interface Sci. 2003, 266, 282–294. (31) Pavlyuchenko, V. N.; Sorochinskaya, O. V.; Ivanchev, S. S.; Klubin, V. V.; Kreichman, G. S.; Budtov, V. P.; Skrifvars, M.; Halme, E.; Koskinen, J. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 1435–1449. (32) Yuan, C. D.; Miao, A. H.; Cao, J. W.; Xu, Y. S.; Cao, T. Y. J. Appl. Polym. Sci. 2005, 98, 1505–1510. (33) Chen, D. Y.; Jing, M. Acc. Chem. Res. 2005, 38, 494–502.

10.1021/la802782w CCC: $40.75  2008 American Chemical Society Published on Web 10/28/2008

Fabrication of Monodisperse Polymer Hollow Spheres

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spheres kinetically. In addition, the cavity size of hollow spheres is tunable by changing the monomer/seed weight ratio, and it can be enlarged after solvent etching. The formation mechanism of hollow spheres is further discussed and confirmed on the basis of the experimental results.

Experimental Section Materials. Styrene (St), methyl methacrylate (MMA), methacrylic acid (MAA), acrylic acid (AA), and 2-hydroxyethyl methacrylate (HEMA) were distilled before use. Divinyl benzene (DVB; Fluka; 80%) and ethyleneglycol dimethacrylate (EGDMA; Aldrich) were used without further purification. Potassium persulfate (KPS) was purified by crystallization from water. Ammonium persulfate (APS), ammonium bicarbonate (NH4HCO3), sodium bicarbonate (NaHCO3), sodium chloride (NaCl), and sodium dodecyl sulfate (SDS) were chemical grade reagents and used as received. Dimethyl formamide (DMF) was analytical grade reagent. Deionized water was applied for all polymerization and treatment processes. Preparation of Seed Particles. The P(St-MMA-MAA) colloidal particles with the diameter of 230 nm were synthesized by emulsifierfree emulsion polymerization.34,35 Briefly, 25 mL of monomer mixture, consisting of St/MMA/MAA (90:5:5 v/v/v), was added to 120 mL of aqueous solution containing 0.4 g of APS and 0.8 g of NH4HCO3. After deoxygenation, the polymerization was carried out under a nitrogen atmosphere for 5 h at 70 °C. When substituting AA for MAA, 356 nm P(St-MMA-AA) particles could be obtained. When St/MMA/MAA (v/v/v) was 85:5:10, the diameter of P(StMMA-MAA) colloidal particles was 170 nm. P(St-HEMA) particles with the diameter of 300 nm were prepared using the method described in the literature.36 13.77 mL of St, 0.33 mL of HEMA, and 0.034 g of NaCl were first added to the reaction vessel containing 160 mL of deionized water. After deoxygenation, the temperature was increased to 70 °C, and a solution of 0.08 g of KPS in 5 mL of water was injected. The reaction proceeded for 24 h. The preparation of PS particles with the diameter of 457 nm was as follows: 8 mL of St was added to 50 mL of aqueous solution dissolving 0.002 g of SDS and 0.04 g of NaHCO3. After the mixture was stirred for 30 min in a nitrogen atmosphere, the polymerization was initiated by a solution of 37 mg of KPS in 5 mL of water and carried out for 12 h at 70 °C. In a typical procedure of synthesizing cross-linked PS latexes, 10 mL of St, 0.2 mL of DVB, and 1 mL of AA were added to 95 mL of deionized water in a reaction vessel equipped with a stirrer and a N2 purge. After being stirred for 30 min, the reaction was heated to 70 °C. The polymerization was initiated by the addition of 0.05 g of KPS, which was dissolved in 5 mL of deonized water. The reaction was allowed to proceed for 12 h. All of the products were washed by three cycles of centrifugation and redispersion with deionized water, and the solid content was adjusted for further use. Preparation of Polymer Hollow Spheres. In a typical experiment, the seed dispersion containing 0.523 g of particles was added to a 100 mL jacketed glass reactor equipped with a reflux condenser and diluted to 36 mL with deionized water in total volume. After deoxygenation by bubbling with nitrogen for 30 min, the temperature was heated to 80 °C. Next, 0.2 mL of HEMA and 2.8 g of monomers of MMA and DVB with the molar ratio of 2.7:1.0 were added to the above system. The polymerization was initiated with 14 mg of KPS in 4 mL of water and carried out for 5 h. Characterization. TEM images were collected using a JEOL JEM 2010 transmission electron microscope operated at an accelerator voltage of 200 kV. SEM images were obtained on a JEOL JSM6700F scanning electron microscope operated at an acceleration voltage of 3 kV. Dynamic light scattering (DLS) measurements were carried out on a Zetasizer-nanozs laser-scattering particle size distribution analyzer. Fourier transform infrared (FTIR) spectra were (34) Itou, N.; Masukawa, T.; Ozaki, I.; Hattori, M.; Kasai, K. Colloids Surf., A: Physiochem. Eng. Asp. 1999, 153, 311–316. (35) Cong, H. L.; Cao, W. X. Langmuir 2003, 19, 8177–8181. (36) Reese, C. E.; Asher, S. A. J. Colloid Interface Sci. 2002, 248, 41–46.

Figure 1. TEM images of hollow spheres prepared with optimized molar ratio of 11.4:4.3:1.0 MMA:DVB:HEMA at different monomer/seed weight ratio: (a) 5.8:1.0, the inset shows SEM image and the scale bar is 300 nm; (b) 5.2:1.0. (c,d) TEM images of hollow spheres shown in (a) and (b) after treatment with DMF.

taken on a Nicolet Avatar 360 FTIR spectrometer. Differential scanning calorimetry (DSC) analysis was performed on a NETZSCH DSC204 thermal analyzer in a nitrogen atmosphere at a heating rate of 10 °C min-1. Elemental analysis (EA) measurements were recorded on a Perkin-Elmer 2400 series analyzer. Thermogravimetric analysis (TGA) experiments were carried out on a NETZSCHSTA 449C thermogravimetric analyzer with a heating rate of 20 °C min-1 from 35 to 800 °C in an air atmosphere.

Results and Discussion Polymer hollow spheres were prepared by seeded emulsion polymerization of methyl methacrylate (MMA), divinyl benzene (DVB), and 2-hydroxyethyl methacrylate (HEMA) in the presence of 230 nm P(St-MMA-MAA) seed particles. Figure 1a shows the TEM image of the typical hollow spheres with the diameter of 370 nm and the cavity size of 235 nm prepared under the conditions of 5.8:1.0 monomer/seed weight ratio. The SEM image (Figure 1a, inset) reveals that they had a strawberry-like surface, in which a small hole was formed as a result of osmotic pressure. By simply tailoring the monomer/seed weight ratio to 5.2:1.0, hollow spheres with a diameter of 315 nm and a void of 180 nm were obtained (Figure 1b). Dynamic light scattering (DLS) was used to determine the size distribution of the two types of spheres, and their polydispersity is 1.9% and 2.7%, respectively. After the monodisperse hollow spheres were treated with dimethyl formamide (DMF), it is seen under TEM that the cavities of the etched hollow spheres are both obviously enlarged and increase to 284 and 252 nm, respectively. At the same time, the shells turned thin and their thickness was 42 and 30 nm, respectively, while the diameters of the spheres almost were not changed (Figure 1c,d). When measured by DLS, the etched hollow spheres reduced more than 10 nm in diameter in comparison with the corresponding hollow spheres due to a decreased swelling degree. Moreover, it is also found that the polymer hollow spheres prepared out of the above range of the monomer/seed weight ratio have a relatively small void and thick shell, and there exist unexpected secondary particles in the aqueous medium. Detailed data are given in Table 1. Note that the hollow spheres were all prepared without undergoing intended swelling of seeds by monomers before polymerization, which differs from conventional seeded emulsion polymerization and is time-saving. For comparison, we also obtained hollow spheres by the swelling

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Table 1. Results of Diameter, Cavity Size, and Shell Thickness of Different Hollow Spheres and Partial Etched Hollow Spheres hollow spheres

etched hollow spheres

samples monomer/seed

diameter (nm)

cavity (nm)

shell (nm)

diameter (nm)

cavity (nm)

shell (nm)

6.3:1.0 5.8:1.0 5.2:1.0 4.9:1.0

375 370 (401) 315 (352) 330

210 235 180 160

82 68 68 85

368 (390) 312 (339)

284 252

42 30

a

a

a

secondary particles yes no no yes

The data in the brackets came from DLS.

Scheme 1. Schematic Illustration of the Formation of Hollow Spheres by Seeded Emulsion Polymerization

Figure 2. TEM images of the samples extracted from different polymerization time: (a) 35 min; (b) 40 min; (c) 45 min, the inset is SEM image; (d) 1 h. The polymerization was carried out under the conditions of 5.8:1.0 monomer/seed weight ratio and 11.4:4.3:1.0 MMA:DVB: HEMA molar ratio. The scale bar is 200 nm for all of the images.

method, but the void of the spheres diminished obviously (Supporting Information). The further experimental results reveal that the cavity size of the hollow spheres strongly depends on the viscosity within the particle during polymerization. Neither high nor low intraparticle viscosity is helpful to create a large void. Detailed work will be reported in the future. Formation Mechanism of Polymer Hollow Spheres. To monitor the void formation, time-dependent polymerization was carried out, and the taken samples were characterized via TEM and SEM. When polymerization is carried out for 35 min, the monomer conversion is 5.6% and the composite particles still remain spherical in shape, but their diameter has an increase of ca. 20 nm as compared to seeds (Figure 2a). As polymerization proceeds, the seriously deformed particles are observed under TEM, indicating that the seed polymer begins migrating toward the particle surface (Figure 2b). At 45 min (13% of monomer conversion), the seed polymer completely moves out of the particle center to the shell, and the void is produced accordingly (Figure 2c). However, the formation of the cavity in the spheres leads to cracking of the shell, which is fairly thin during vacuum drying (Figure 2c, inset). Subsequently, the void of the microspheres is developed and their morphology is improved (Figure 2d). The formation process of hollow spheres is described in Scheme 1. In this system, polymerization preferentially takes place on the seed surface because the aqueous radicals are adsorbed and anchored on it, leading to the formation of the shell. As the reaction proceeds, seeds are highly swollen by monomers, and the formed cross-linked shell serves as the following polymerization locus for the infused monomers from the interior of the particles. Gradually, the seed polymer moves from the center of seeds to the shell together with monomers, and the void is produced. As a result of osmotic pressure, a hole is created in

the shell. The process is accompanied by phase separation, which is induced due to the incompatibility between the seed polymer and the cross-linked polymer. After complete phase separation, the seed polymer is distributed on the inner wall of the shell in the end.24,27 Characterization of Polymer Hollow Spheres. FTIR spectra were provided to confirm the composition of seeds, hollow spheres, and the etched hollow spheres. In Figure 3a, the characteristic peaks at 1731 and 698 cm-1 correspond to the CdO stretching of PMMA and the out-of-plane bending of the benzene ring, respectively. The asymmetric peak between 1700 and 1731 cm-1 is attributed to the overlapping of the two different carbonyl groups of PMMA and PMAA. In the spectrum of hollow spheres (Figure 3b), the CdO and C-O stretching peaks at 1277, 1241, 1195, and 1150 cm-1 were obviously strengthened in intensity, indicating the effective polymerization of MMA monomer. Comparing (c) with (b), the band at 698 cm-1 becomes weak after dissolution of the linear polymer by DMF, and the result displays this band originated from the benzene ring bending of the polymerized DVB monomer. Thus, it can be suggested that PMMA, PS, and PDVB exist in hollow spheres. To evaluate the thermal behavior of the samples, DSC thermograms were measured under a flowing high-purity nitrogen atmosphere. Figure 4 shows that seeds and hollow spheres give a characteristic temperaturesglass transition temperature (Tg) at 106.6 and 108.3 °C, respectively. Tg of hollow spheres is slightly higher than that of seeds as a result of interface effect between constituent polymers. For comparison, Tg of the etched hollow spheres was not measured because of high cross-linking of the

Figure 3. FTIR spectra of (a) seed particles; (b) hollow spheres; and (c) the etched hollow spheres.

Fabrication of Monodisperse Polymer Hollow Spheres

Figure 4. DSC curves of seed particles, hollow spheres, and the etched hollow spheres.

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Figure 6. TEM images of the polymer spheres with different MMA: DVB molar ratio: (a) 100:0; (b) 11.7:1.0; (c) 2.7:1.0; (d) 0:100. All of the polymerizations proceeded in the presence of 0.50% (v/v) of HEMA.

Figure 7. TEM images of hollow spheres prepared with 2.7:1.0 MMA: DVB molar ratio at different HEMA concentration: (a) 0.25% (v/v); (b) 0.75% (v/v). The inset in (a) shows the TEM image of the etched hollow spheres with DMF.

Figure 5. Carbon (C1s; a) and oxygen (O1s; b) spectra from the XPS analysis of hollow spheres (upper) and the etched hollow spheres (lower).

polymer by a large amount of DVB (about 31 wt %). On the basis of the results, it is proved that the seed polymer is present within hollow spheres. TGA spectra further hold out the conclusion (Supporting Information). XPS was used to determine the surface composition of the hollow spheres and the etched hollow spheres by comparing the ratios of the peak areas obtained for carbon and oxygen of the two samples. The carbon peaks and the oxygen peaks from the XPS analysis are shown in Figure 5. The C1s/O1s of the surface of the hollow spheres is 4.94. In the case of the etched hollow spheres, the peak area ratio is 4.80. The two values are very close. So we speculate that the surface composition for the two types of hollow spheres is identical and should be cross-linked polymer, and the seed polymer is distributed on the inner shell of hollow spheres, inducing their cavity enlarged after solvent etching. Effect of Monomer Composition. It is found that the DVB content in monomers has an important effect on creating hollow spheres at a constant HEMA concentration of 0.50% (v/v), and their ultimate morphologies can be controlled by changing the molar ratio of MMA to DVB. When no DVB was added, the resulting products were solid particles (Figure 6a). At the same time, a large number of secondary particles were produced due to the water solubility of MMA. When a small quantity of DVB

Figure 8. TEM images of hollow spheres prepared using different seeds with varied diameters: (a) 170 nm P(St-MMA-MAA); (b) 356 nm P(StMMA-AA); (c) 300 nm P(St-HEMA); and (d) 457 nm PS under the conditions of 5.8:1.0 monomer/seed weight ratio and optimized monomer composition. The insets in (c) and (d) show the corresponding etched hollow spheres, and the bar is 100 nm.

was added, polymer hollow spheres were formed (Figure 6b). When the molar ratio of MMA to DVB was 2.7:1.0, the products were optimized; that is, the hollow spheres had relatively large cavity and the secondary particles were restricted to generate (Figure 6c). If monomers were composed of DVB and HEMA, the polymer spheres were still hollow (Figure 6d). The reason

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Table 2. Dependence of the Cavity Size of Hollow Spheres (HSs) on the Diameter and the Zeta Potential of Seeds diameter of seeds seeds

zeta potential

DLSa

SEM

cavity size of HSs (TEM)a

P(St-MMA-MAA)

-56.4 mV -57.4 mV -53.4 mV -42.2 mV -28.5 mV

200 nm (2.2%) 257 nm (1.7%) 393 nm (1.4%) 338 nm (6.4%) 480 nm (5.1%)

170 nm 230 nm 356 nm 300 nm 457 nm

132nm(5.6%) 235nm(1.9%) 275nm (28%) 305nm(7.5%) 320nm(6.8%)

P(St-MMA-AA) P(St-HEMA) PS a

The results in the brackets are polydispersity of seeds and hollow spheres, respectively.

Figure 9. TEM images of the polymer microspheres prepared using 230 nm P(St-MMA-MAA) as seeds at various SDS concentration: (a) 2.2 mM; (b) 4.3 mM; and (c) 6.5 mM.

is that DVB can highly swell linear seed latexes based on compatibility and similitude principle. When a similar reaction was performed on the cross-linked PS seeds, or DVB was substituted for ethyleneglycol dimethacrylate (EGDMA), hollow spheres could not be obtained (Supporting Information). It is obvious that the “dissolution” of seeds in monomers is crucial to the formation of hollow spheres. It is also noticed that the cavity size of the hollow spheres can be tuned through the HEMA amount at the optimized molar ratio of 2.7:1.0 MMA:DVB. In the case of the HEMA concentration of 0.25% (v/v), the hollow spheres only gave a small void of 146 nm (Figure 7a). As the HEMA concentration increases to 0.50% (v/v), the cavity size of the hollow spheres was maximized and about 235 nm (Figure 6c). It is because an adequate amount of HEMA produces more -COOH groups on P(MMA-DVBHEMA) chains36,37 so that the complete phase separation happens between the seed polymer and the cross-linked polymer as a result of their enhanced incompatibility, whereas at a small amount of HEMA, the constituent polymers interpenetrated due to incomplete phase separation, leading to the emergence of multivoid in the hollow spheres after the solvent etching process (Figure 7a, inset). In addition, excessive HEMA could not enlarge the void but induced undesired nucleation in the medium (Figure 7b). Effect of the Properties of Seeds. The cavity size of hollow spheres not only depends on the monomer composition, but also on the properties of seeds. When the zeta potential of the seed dispersion was high, we could obtain hollow spheres with the void of 132 nm using 170 nm P(St-MMA-MAA) seeds (Figure 8a), and the cavity size of hollow spheres increases with the diameter of seeds (Table 2). However, when seeds got to 356 nm in diameter, hollow spheres were deformed badly due to decreased mechanical strength of the cross-linked shell (Figure 8b). Also, this is seen more clearly from the etched hollow spheres (Figure 8b, inset). At the same time, the deformation made hollow spheres have increased polydispersity (28%) and a small cavity of 275 nm corresponding to the seed diameter. In comparison, as the zeta potential of the seed dispersion decreased, the resulting hollow spheres still held a wonderful spherical shape even if the

diameter of seeds was more than 300 nm (Figure 8c,d). Different from the above, the etched hollow spheres gave semidouble or double shell structure, and the cavity was not developed (Figure 8c,d, insets). In fact, the variation of the zeta potential of the seed dispersion suggests that the composition of the seeds is changed. Therefore, the differences in intrinsic morphology of the hollow spheres show that the seed composition also affects the phaseseparation extent of the polymers. When the zeta potential is high, phase separation is complete due to poor compatibility, which is helpful in preparing hollow spheres with the void of around 200 nm. When the zeta potential is low, phase separation is incomplete, but otherwise it enhances the mechanical strength of the shell and avoids the deformation of hollow spheres. Especially, when using P(St-HEMA) particles with the appropriate zeta potential as seeds, the cavity size of hollow spheres is close to the diameter of seeds (Figure 8c), and it is beneficial to the preparation of hollow spheres with a large void. Effect of Surfactant. In this study, sodium dodecyl sulfate (SDS) play a key role in affecting the sphere morphology. Although the spheres were hollow at 2.2 mM of SDS, their cavity size had a visible reduction at about 194 nm (Figure 9a). Multiple voids occurred in the spheres when more SDS was added (Figure 9b). When the SDS concentration reached 6.5 mM, solid spheres were generated (Figure 9c). As we know, cross-linking agent is the kinetic factor and surfactant is the thermodynamics factor. When SDS is used in our system, it will counteract the formation of hollow spheres, which possess inner and outer surfaces and have high interface tension with aqueous medium due to decreased interface tension. So, the gathering of a small amount of SDS molecules on the seed surface prevents from the construction of the cross-linked shell and the proceeding of phase separation, leading to the production of hollow spheres with reduced cavity. As more SDS molecules are adsorbed, the swelling rate of seeds is enhanced with lower interfacial tension, and the intraparticle viscosity is decreased accordingly, which is convenient for the radicals to be transported to the interior of the particles to initiate the polymerization.

´ lvarez, R. (37) Martı´n-Rodrı´guez, A.; Cabrerizo-Vı´lchez, M. A.; Hidalgo-A Colloids Surf., A: Physiochem. Eng. Asp. 1996, 108, 263–271.

In conclusion, we have demonstrated a modified seeded emulsion polymerization for the fabrication of monodisperse

Conclusions

Fabrication of Monodisperse Polymer Hollow Spheres

polymer hollow spheres. The method is facile and effective. The morphologies of hollow spheres are mainly governed by the DVB content and the HEMA amount in monomers, which are responsible for the “dissolution” of seeds and phase separation of the constituent polymers, respectively. Under the optimized conditions, hollow spheres with different cavity size and crosslinked shell thickness can be obtained by changing the monomer/ seed weight ratio. In addition, hollow spheres with large void can be prepared by increasing the size of seeds and their zeta potential. Because of decreased interfacial tension in the system, SDS will induce to form solid particles. We anticipate that this method would provide a platform for the synthesis of functional

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polymer hollow spheres using linear PS seeds with optical or magnetic properties, which would have potential applications in the field of medicine. Acknowledgment. This work is financially supported by the National Natural Science Foundation of China (Nos. 20534040, 20674026) and the Special Funds for Major State Basic Research Projects (No. 2007CB936402). Supporting Information Available: TGA analysis and TEM images. This material is available free of charge via the Internet at http://pubs.acs.org. LA802782W