pubs.acs.org/Langmuir © 2010 American Chemical Society
Facile Fabrication of Hollow Polymer Microspheres through the Phase-Inversion Method Yichen Cao, Bo You,* and Limin Wu* Department of Materials Science and the Advanced Coatings Research Center of the China Educational Ministry, Fudan University, Shanghai 200433, PR China Received January 29, 2010. Revised Manuscript Received March 10, 2010 This letter reports a novel, facile method of fabricating hollow polymer microspheres based on the phase-inversion method. In this approach, when hydrophobic chlorinated polypropylene was grafted with methyl methacrylate, butyl acrylate, and acrylic acid via free-radical polymerization and then neutralized by triethylamine and gradually diluted with deionized water, phase inversion happened, directly yielding hollow polymer microspheres. SEM, TEM, and optical images confirmed the hollow structure. A formation mechanism of the hollow polymer microspheres was proposed.
Introduction Recently, hollow polymer microspheres have attracted considerable attention because they have unique properties such as low density and excellent thermal insulation and optical scattering properties and therefore have potential applications in many fields such as microcapsules, pigments, catalyst loading, and drug delivery.1-8 Several approaches have been developed to fabricate hollow polymer microspheres, such as emulsion polymerization including an acid/alkali swelling9 and solvent swelling10,11 method, a template method,12-14 solvent evaporation and phase separation,15-18 self-assembly,19-21 and so forth. Among these, seeded emulsion is one of the most widely used methods of generating hollow microspheres because of the perfect monodispersity. On the basis of this method, a succedent curing-evaporation or swelling-etching treatment is usually used to maintain the skeleton of microspheres and generate hollow interior structures. *Corresponding authors. E-mail:
[email protected] (L.W.); youbo@ fudan.edu.cn (B.Y.).
(1) Wei, B.; Wang, S.; Song, H.; Liu, H.; Li, J.; Liu, N. Pet. Sci. 2009, 6, 306–312. (2) Zhao, Y.; Jiang, L. Adv. Mater. 2009, 21, 1–18. (3) Li, G.; Yang, X.; Wang, B.; Wang, J.; Yang, X. Polymer 2008, 49, 3436–3443. (4) Yang, X.; Chen, L.; Huang, B.; Bai, F.; Yang, X. Polymer 2009, 50, 3556– 3563. (5) Langer, R. Nature 1998, 392(suppl.), 5–10. (6) Bergbreiter, D. E. Angew. Chem., Int. Ed. 1999, 38, 2870–2872. (7) Miao, S.; Zhang, C.; Liu, Z.; Han, B.; Xie, Y.; Ding, S.; Yang, Z. J. Phys. Chem. C 2008, 112, 774–780. (8) Sukhorukov, G.; Fery, A. Prog. Polym. Sci. 2005, 30, 885–897. (9) Okubo, M.; Ichikawa, K.; Fujimura, M. Colloid Polym. Sci. 1991, 269, 1257–1262. (10) Im, S. H.; Jeong, U. Y.; Xia, Y. Nat. Mater. 2005, 9, 671–675. (11) Jeong, U. Y.; Im, S. H.; Camargo, P. H. C.; Kim, J. H.; Xia, Y. Langmuir 2007, 23, 10968–10975. (12) Donath, E.; Sukhorukov, G. B.; Caruso, F.; Davis, S. A.; M€ohwald, H. Angew. Chem., Int. Ed. 1998, 37, 2201–2205. (13) Poortinga, A. T. Langmuir 2008, 24, 1644–1647. (14) Minami, H.; Kobayashi, H.; Okubo, M. Langmuir 2005, 21, 5655–5658. (15) Li, M.; Rouaud, O.; Poncelet, D. Int. J. Pharm. 2008, 363, 26–39. (16) Gao, F.; Su, Z.; Wang, P.; Ma, G. Langmuir 2009, 25, 3832–3838. (17) Hsieh, S. J.; Wang, C.; Chen, C. Macromolecules 2009, 42, 4787–4794. (18) Hao, D.; Gong, F.; Hu, G.; Lei, J.; Ma, G.; Su, Z. Polymer 2009, 50, 3188– 3195. (19) Huang, H. Y.; Remsen, E. E.; Kowalewski, T.; Wooley, L. K. J. Am. Chem. Soc. 1999, 121, 3805–3806. (20) Wong, M. S.; Cha, J. N.; Choi, K. S.; Deming, T. J.; Stucky, G. D. Nano Lett. 2002, 2, 583–587. (21) Pochan, D. J.; Chen, Z.; Cui, H.; Hales, K.; Qi, K.; Wooley, K. L. Science 2004, 306, 94–97.
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Other popular approaches are based on the micelle self-assembly of amphiphilic block copolymers or the layer-by-layer encapsulating technology of polyelectrolytes for biological carriers and sustained release.22 The self-assembly method focuses on the precise synthesis control of block copolymer chains, and layerby-layer encapsulating technology relies on a template strategy, followed by the removal of template cores to gain hollow structures. Of course, people can also prepare polymer microsphere materials with hollow core/porous shell structures through a combination of more than two methods.23 Unfortunately, however, a majority of these approaches require complex processes, such as the removal of templates, selected solvent/monomer swelling or cross-linking treatment, and dialysis, and the yield of hollow microspheres is usually very low. In this letter, we report a novel, feasible method for preparing hollow polymer microspheres via a phase-inversion process. The phase-inversion method is commonly employed to disperse hydrophobic polymers or solvent-based resins into water-based systems for waterborne coatings, adhesives, and inks in order to lower the VOC. In this process, hydrophilic groups such as carboxylic acid or amino groups are first introduced onto the hydrophobic polymer chains and then neutralized by basic or acidic groups, respectively. The modified polymer solution in organic solvent is afterwards diluted by continuously adding water under a high shear speed to form an aqueous polymer particle dispersion.24 Very interestingly, our study shows that this process can directly produce hollow polymer microspheres without any other treatment.
Experimental Section Methyl methacrylate (MMA), n-butyl acrylate (BA), acrylic acid (AA), butanone, xylene, triethylamine, and azoisobutyronitrile (AIBN) were all purchased from Sinopharm Chemical Reagent Company (China). The solubility of butanone in water is 290 g/L at 20 °C whereas xylene is insoluble in water. Chlorinated polypropylene (CPP) was purchased from Toyo Kasei. Ethylene acrylic acid copolymer (EAA, Primacor 5980I, 20.5% (22) Peyratout, S. C.; D€ahne, L. Angew. Chem., Int. Ed. 2004, 43, 3762–3783. (23) He, X.; Ge, X.; Liu, H.; Wang, M.; Zhang, Z. Chem. Mater. 2005, 17, 5891– 5892. (24) Qiu, D.; Li, J.; Yang, Z. Acta Polym. Sin. 2002, 5, 699–702.
Published on Web 04/09/2010
DOI: 10.1021/la100450y
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acrylic acid comonomer) was purchased from Dow Chemical Company. The CPP/poly(MMA-BA-AA) graft copolymer was synthesized by the free-radical polymerization of MMA, BA, and AA in the presence of CPP in butanone using AIBN as the initiator. CPP was first dissolved in butanone at 60 °C in a 250 mL round-bottomed flask equipped with a mechanical stirrer, a temperature controller, a nitrogen inlet, and a condenser and then mixed with monomers. This mixture was heated to 78 °C under stirring and then added via AIBN solution in butanone and kept at that temperature for 6 h under a slow stream of nitrogen to obtain CPP/poly(MMA-BA-AA) composite polymers. The synthesis formulations are listed in Table S1 in Supporting Information. A certain amount of the synthesized composite polymers was neutralized with triethylamine and then gradually diluted with deionized water. As the addition of water continued, phase inversion happened whereby polymers were evenly dispersed in a water continuous matrix, directly generating hollow microsphere dispersions. All dispersions were centrifuged and washed with water two times to obtain the final products. Transmission electron microscopy (TEM) images were obtained with a Hitachi H600 (Hitachi Corporation). The samples were diluted and dried onto carbon-coated copper grids for TEM observation. Scanning electron microscopy (SEM) images were obtained with a Cambridge (U.K.) S-360 scanning electron microscope. All SEM samples were sputter coated with gold before observation. Because poly(MMA-BA-AA) could be dissolved in methanol but CPP could not, all of the acrylic copolymers that were not grafted onto CPP could be extracted into methanol by a Soxhlet apparatus at 75 °C for 48 h. The graft ratio (Rg) was calculated using following equation Rg ¼
MT - ME 100% MT
ð1Þ
where ME is the theoretical CPP mass contained in the sample according to the polymerization formulation and MT is the remaining grafted CPP weight after extraction. Nile red was mixed with the as-synthesized CPP/poly(MMABA-AA) composite polymers and then neutralized with triethylamine and diluted with water to form a water-based dispersion. This dispersion was filtered with a 200 mesh (ASTM) filter cloth and observed with an Olympus LX71 (magnification 640) fluorescence microscope. The fluorescence property of the dispersion was also detected with a fluorescence spectrophotometer (RF-5301PC, Shimadz) at an excitation wavelength of 480 nm, and emission was collected from 500 to 900 nm. GPC (Waters 1515/Waters 2414, Milford, MA) was run using an RI detector at 35 °C with tetrahydrofuran as the eluent and GPC standard polystyrene as the calibration standard. DSC was performed on a DuPont 2000-910 differential scanning calorimeter. The samples were run between 0 and 130 °C under N2 flow at a heating rate of 20 °C min-1.
Results and Discussion Figure 1 shows the electron microscopy images of typical polymer microspheres prepared with different contents of carboxyl groups on the basis of the phase-inversion method. It can be seen that some pores from about 120 to 500 nm in diameter are observed on the surfaces of polymer microspheres when 16 wt % AA based on the total mass of monomers is contained (Figure 1a). These pores should be attributed to incomplete phase inversion under our experimental conditions.18 When more AA is used (e.g., 24 and 30 wt %), complete hollow microspheres are obviously observed despite a wide size distribution (Figure 1b,c) and almost all microspheres are hollow as seen from the SEM image of a larger sample (Figure S1 in Supporting Information). Because of the thin shell and high vacuum in the electron microscope, these 6116 DOI: 10.1021/la100450y
Figure 1. SEM images of hollow spheres prepared with different weight ratios of AA in the total monomer mass: (a) 16, (b) 24, and (c) 30 wt %.
Figure 2. (a) Fluorescence spectra of a microsphere dispersion loaded with Nile red and a pure microsphere dispersion. (b) Optical image of dried microspheres, where the inset is a captured image of microspheres in a water solution. The scale bar is 1 μm.
Figure 3. SEM images of microspheres prepared with different ratios of MMA/BA: (a) 6:1, (b) 4.5:2.5, and (c) 3:4.
hollow polymer microspheres have collapsed and show a bowllike structure in the SEM images,25 although they are really spherical under the optical microscope (Figure S2). The mean shell thickness of hollow microspheres decreases from about 150 to 60 nm, but the average size of the microspheres increases when the AA content increases from 24 to 30 wt %. This suggests that microspheres with more carboxylic acid salt groups could carry more hydrophilic solvent and water, increasing the total interface, which would enlarge the microsphere sizes and therefore thin the shells. These hollow microspheres were not aggregated and could be redispersed in water according to the light-scattering results (Figure S3). Figure 2 further illustrates the fluorescence spectrum of hollow microspheres by loading lipid-soluble fluorochrome Nile red. Nile red was chosen because it can enter only the hydrophobic phase but cannot be dissolved in water. From Figure 2a, it can be seen that the dispersion of microspheres loaded with Nile red has a strong fluorescence emission whereas the pure microsphere dispersion does not. The optical image of the microspheres dried at room temperature, as shown in Figure 2b, shows that all of these microspheres are spherical and the strong contrast between the orange color of the shell and the relatively darker core further confirms the hollow structure. When the composition of grafting copolymer poly(MMABA-AA) varies, hollow microspheres can still be obtained but too much soft monomer would easily cause conglutination among the hollow microspheres, as demonstrated in Figure 3. Figure 4 reveals the influence of CPP content on the microsphere morphology. Without CPP, no hollow structures are observed (Figure 4a). When CPP is used, the obtained polymer (25) Cheng, X.; Chen, M.; Wu, L.; Gu, G. Langmuir 2006, 22, 3858–3863.
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Letter Scheme 1. Formation Mechanism of Hollow Microspheres via the Phase-Inversion Method
Figure 4. SEM images of microspheres prepared with different CPP contents on the basis of the total weight: (a) 0, (b) 7.5, (c) 8.6, and (d) 9.8 wt %.
Figure 5. (a) GPC traces of typical CPP/poly(MMA-BA-AA) composite polymers before phase inversion, hollow microspheres, and pure CPP. (b) DSC thermograms of CPP and hollow microspheres.
microspheres are hollow, as shown in Figure 4b,c. However, if too much CPP is used, some small solid particles are also shown besides hollow microspheres (Figure 4d). These small solid particles should be from superfluous ungrafted CPP. Typical hollow polymer microspheres were further analyzed by GPC and DSC. Figure 5a shows the typical GPC curves of solvent-based CPP/poly(MMA-BA-AA) composite polymers, hollow microspheres, and pure CPP. The hollow microspheres have almost the same GPC retention time as pure CPP, suggesting that the microspheres are mainly composed of CPP. The second GPC peak of composite polymers before phase inversion almost disappears in the case of hollow microspheres, indicating that hydrophilic and ungrafted acrylic copolymers have been dissolved in water after phase inversion occurred and were washed away. In Figure 5b, the Tg of hollow microspheres is about 53 °C, higher than 35.3 °C for pure CPP, further confirming that the hollow microspheres are composed of acrylic copolymer-grafted CPP. The typical graft ratio is about 4.7% based on the Soxhlet measurement, thus it can be estimated that around 99.4% of acrylic copolymers were removed after purification. Optimizing the polymerization parameters could possibly decrease the loss of acrylic copolymers. To understand further the formation mechanism of hollow polymer microspheres in this approach, we performed two control experiments: one was from the physical blending of CPP and the poly(MMA-BA-AA) copolymer solution in butanone, where obvious macrophase separation can be seen whereas the grafted copolymer solution forms a uniform unstratified solution (Figure S4). This physical mixture did not form a hollow structure Langmuir 2010, 26(9), 6115–6118
via the phase-inversion method because the phase-inversion attempt did not succeed. In another experiment, we used xylene to replace butanone; however, no hollow structures could be observed from electron microscopy (Figure S5). On the basis of the experimental results and discussion, the formation mechanism of hollow polymer microspheres through the phase-inversion method can be explained as follows (Scheme 1): when the carboxyl groups were neutralized by amine and a small quantity of water was added, some water was absorbed into hydrophilic domains that were dispersed in the continuous solvent phase. As more water was added with sustained stirring, phase inversion happened at the critical point. The hydrophobic CPP swelled because some butanone formed dispersed domains that were surrounded by hydrophilic acrylic polymer chains in the water continuous phase. However, because butanone is semimiscible with water, microsphere formation by the coagulation of polymer during phase inversion is possible, leading to the generation of water compartments due to the presence of the swelled water-butanone mixture trapped in the polymer, which becomes phase separated as the butanone fraction is reduced by selective diffusion through the polymer membrane into the now-diluted water-butanone continuous phase. The diffusion of water is instead prevented by the hydrophobic character of the coagulated polymer. The driving force of diffusion for butanone is the result of partitioning between the diluted butanone-water mixture and the polymer-swelling compartment. When butanone and water evaporated, hollow structures could be directly obtained. If carboxyl groups were not enough, then incomplete phase separation occurred, which caused multiporous microspheres. To verify this mechanism, CPP was replaced by ethylene acrylic acid copolymer (EAA) with other parameters being equal, and hollow polymer microspheres could also be obtained (Figure S6).
Conclusions It was shown that hollow CPP/poly(MMA-BA-AA) copolymer microspheres could be directly prepared via a phase-inversion method. In this approach, when hydrophobic CPP was grafted with MMA, BA, and AA via free-radical polymerization and then neutralized by triethylamine and gradually diluted by deionized water, phase-inversion happened, directly yielding hollow polymer microspheres. The semimiscibility of butanone with water and the carboxyl group content are vital to forming this hollow structure. The former condition favors the generation of aqueous compartments in hydrophobic CPP upon reaching the upper miscibility threshold. The latter plays a key role in reducing the interfacial tension, thus favoring the generation of both inner- and DOI: 10.1021/la100450y
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outer-sphere interfaces. We would anticipate that this method can provide a paradigm for the synthesis of hollow polymer spheres, on the basis of which some other polymer hollow microspheres could be prepared and used in coatings, inks, catalyst loading, nanoparticle dispersion, and so forth. Acknowledgment. Financial support of this research from the National “863” Foundation, the Science & Technology Foundation of Shanghai (0952 nm01000), the Shanghai-Unilever Research and Development Fund (07SU07001), the Shanghai
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Semiconductor Lighting Project (09DZ1141000), and the Shanghai Leading Academic Discipline Project (B113) is appreciated. Supporting Information Available: Synthesis formulations, SEM image of a larger sample, optical images of spherical microspheres, DLS curve, optical images of a physical blend, SEM image of the microspheres using xylene as a solvent and of the hollow microspheres with EAA replacing CPP. This material is available free of charge via the Internet at http://pubs.acs.org.
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