Synthesis of Golf-ball-like Polystyrene Microspheres ... - CSJ Journals

doi:10.1246/cl.130297 Published on the web June 1, 2013

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Synthesis of Golf-ball-like Polystyrene Microspheres from a Pickering Emulsion Stabilized by Amphiphilic Janus Microspheres Yongfei Xu,1 Xueping Ge,2 Xiang Ji,1 Mozhen Wang,*1 and Xuewu Ge1 CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China 2 Bioengineering Research Center, School of Engineering, University of Kansas, Lawrence, Kansas 66045, USA 1

(Received April 9, 2013; CL-130297; E-mail: [email protected]) Novel golf-ball-like polystyrene (PS) microspheres were obtained through £-ray-radiation-initiated polymerization of styrene (St) monomers in a Pickering emulsion stabilized by amphiphilic snowman-like PS Janus microspheres, which were synthesized via seed emulsion polymerization of St swollen in poly(acrylic acid)-functionalized crosslinked PS (PA-CPS) seed microspheres. It was found that the stability of the Pickering emulsion and the hole size of the golf-ball-like PS microspheres depend on the geometry of the amphiphilic snowman-like PS Janus microspheres, which can be tuned by the weight ratio of the styrene monomer to PA-CPS seed microspheres (wS-CPS). This work provides a simple strategy to synthesize amphiphilic Janus microspheres and opens a new way to prepare polymer microspheres with holes on the surface. In recent years, there has been great interest in the preparation of Janus particles with anisotropic shapes or surface chemistry14 because such particles may be very useful in molecular recognition,5 self-assembling processes,6 and formation of Pickering emulsions.79 The protect-and-release method was proposed for the first time.10 A monoparticulate layer was formed on a planar solid substrate so that one side of the particles was protected by the substrate in the following modification process, while the other side was modified chemically. Recently, chemical modification of particles at liquidliquid interfaces has been reported.11 The entire surface of the particle is divided into two parts by the oilwater interface. In order to prevent rotation of the particles during the chemical modification, wax was employed as the oil phase. When the emulsion formed above the melting point of wax is cooled to room temperature, the particles are locked at the surface of the solidified wax phase and cannot rotate during the subsequent chemical modification. Liu et al. reported the preparation of Janus particles via biphasic atomic transfer radical graft polymerization at the Pickering emulsion interface.12 Another method to synthesize nonspherical Janus particles is seed emulsion polymerization.13,14 The phase separation induced by the polymerization of the monomer swollen in seed latex particles caused the newly formed polymer to extrude out of the seed particles, leading to the formation of two-phase nonspherical particles. Generally, the surfaces of the Janus particles prepared through most of the methods mentioned above have homogenous hydrophilic hydrophobic property, i.e., either hydrophilic or hydrophobic. If anisotropic surface properties are required, additional chemical treatment should be carried out to incorporate the required groups to one of the bulbs of the nonspherical particles containing reactive sites.8,14,15

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The application of Janus particles as Pickering emulsion stabilizers has been reported in the literature. Weitz et al. synthesized silane-functionalized crosslinked PS Janus particles with tunable geometries. The Janus particles, which were called colloid surfactants, can stabilize the hexadecane/water emulsion.8 Liu’s group synthesized snowman-like magnetic and nonmagnetic nanocomposite asymmetric particles (SMNAPs) via seeded emulsion polymerization initiated by £-ray radiation.16 The as-synthesized SMNAPs served as magnetically controllable solid surfactants to stabilize O/W mixtures. However, there is no report on what would happen if the polymerizable oil phase, e.g., styrene, stabilized with the Janus particles is initiated to polymerize into microspheres. In this work, we present a direct synthesis of amphiphilic snowman-like polystyrene microspheres, i.e., one bulb of the microspheres is hydrophilic and the other is hydrophobic, by radiation seed emulsion polymerization at room temperature. The prepared amphiphilic snowman-like microspheres were used as colloidal surfactants to form a styrene/water Pickering emulsion, and novel golf-ball-like PS microspheres were finally obtained. Styrene (St) and divinylbenzene (DVB) were purified by passing them through a basic alumina column to remove the inhibitor before use. Acrylic acid (AA) was distilled and stored at ¹20 °C prior to use. Reagent-grade potassium persulfate (KPS) was purified by recrystallization in methanol. Deionized water was used in all experiments. Monodisperse poly(acrylic acid)-functionalized crosslinked PS (PA-CPS) seed microspheres were first synthesized via emulsifier-free polymerization of styrene in the presence of AA. The detailed process is described in Supporting Information.17 Then, 0.1 g of the prepared PA-CPS seed microspheres was ultrasonically dispersed into 15 mL of deionized water. A certain amount of St was added to the solution under mechanical stirring. The weight ratios of St to PA-CPS seed microspheres (wS-CPS) were set as 1/1, 2/1, and 3/1. The emulsion was continually stirred for 24 h. Then, the system was exposed to 60 Co £-ray radiation field (7.4 © 1014 Bq, located in USTC, China) after 10 min of degassing by bubbling N2. The absorbed dose rate was 15 Gy min¹1, and the total absorbed dose was 58 kGy. The resulting products were collected by centrifugation (6000 rpm, 5 min) and then washed with ethanol thrice. 0.01 g of snowman-like Janus microspheres prepared at wS-CPS of 3/1 was dispersed ultrasonically in 15 mL of deionized water. Then, 0.4 g of styrene was added to the solution under stirring. The resulting emulsion was irradiated by 60Co £-rays at a dose rate of 50 Gy min¹1 and a total absorbed dose of 60 kGy after removing the dissolved oxygen by bubbling N2 for 10 min. The products were collected by centrifugation (6000 rpm, 5 min) and then washed with ethanol three times.

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Scheme 1. (A) Synthesis of amphiphilic snowman-like Janus microspheres by seed emulsion polymerization. (B) Synthesis of golf-ball-like microspheres from a Pickering emulsion polymerization stabilized by amphiphilic snowman-like Janus microspheres. The morphology of the prepared latex particles was studied by SEM (JEOL JSM6700, 5.0 kV) and TEM (H-7650, JEOL2011, 100 kV). All the samples were prepared by dispersing one drop of the ethanol dispersion on copper grids and drying in air. The number-average diameter (Dn), weightaverage diameter (Dw), and polydispersity index (PDI) of the microspheres were calculated by the following equations, with the diameters of at least 100 particles measured in the SEM and TEM images: X X ni Di ni Di 4 Dw Dn ¼ X ; Dw ¼ X ; PDI ¼ ð1Þ Dn ni ni Di 3 where ni is the number of particles with a diameter of Di. The optical photographs of the Pickering emulsions were recorded by an optical microscope (Leica DM 1000) with a Lecia EC3 high-resolution digital charge-coupled device (CCD) camera. Fourier transform infrared (FTIR) spectra were obtained on a Bruker VECTOR22 FTIR spectrometer to characterize the PA-CPS microspheres via the KBr method. The synthesis of amphiphilic snowman-like Janus microspheres is illustrated in Scheme 1A. First, PA-CPS microspheres were prepared by an emulsifier-free polymerization in a dispersion system consisting of St, DVB, and water, as well as a small amount of AA. The as-prepared PA-CPS microspheres have an average diameter of 270 nm and a narrow PDI of 1.01, as shown in Figure S1 (Left).17 The FTIR spectrum of the PA-CPS microspheres (Figure S1 (Right))17 exhibits the characteristic peaks of the AA units. It can be expected that the AA units are prone to be located on the surface of the PS microspheres since AA is soluble in water. Second, the PA-CPS microspheres were redispersed in water and swollen by a certain amount of St monomers. Then, the polymerization of St was initiated by £-ray radiation at room temperature. The TEM and SEM images of the final products are displayed in Figure 1. Evidently, snowman-like microspheres, which consist of one bulb with a size close to the original PACPS seed microspheres and another newly formed smaller bulb, were obtained. The formation of the new PS bulb should be attributed to the phase separation initially driven by the elastic stress since the seed PA-CPS microspheres are crosslinked, as illustrated in the literature.1820 During the polymerization, this phase separation is enhanced due to the difference in free volume between the seed polymers and the newly generated

Chem. Lett. 2013, 42, 963965

Figure 1. TEM (A1A3) and SEM (B1B3) photographs of the amphiphilic snowman-like Janus microspheres prepared with different wS-CPS: 1/1 (for A1 and B1); 2/1 (for A2 and B2); 3/1 (for A3 and B3). (C) The average diameters of the hydrophobic bulb (d1, ) and the hydrophilic bulb (d2, ) of the amphiphilic Janus microspheres prepared with wS-CPS. polymers so that a new noncrosslinked PS bulb would be extruded out of the swollen PA-CPS seed microspheres. As a result, the prepared snowman-like microspheres are amphiphilic Janus particles, i.e., they have a hydrophobic PS bulb and another hydrophilic crosslinked PS bulb. According to the above-mentioned formation mechanism, the hydrophobic PS bulb extruded from the hydrophilic PA-CPS seed microspheres should be sharply enlarged with the increase in wS-CPS, as shown in Figure 1C. The average diameter of the newly formed bulb is 175, 251, and 334 nm, respectively, when wS-CPS is set as 1/1, 2/1, and 3/1. At the same time, the average diameter of the corresponding larger bulb shows only a slight increase, i.e., 350, 357, and 402 nm. The surface activity of the Janus particles has been studied previously at an oilwater interface by Binks et al.21 In our work, the amphiphilicity of the prepared snowman-like microspheres has also been verified when the microspheres are dispersed in a heptane/water system, as shown in Figure S2.17 The prepared snowman-like microspheres aggregate at the interface between heptane and water, which indicates that they are amphiphilic and can be used as colloid surfactants. Figure 2 shows the optical images of the emulsion stabilized by the amphiphilic snowman-like Janus microspheres prepared with different wS-CPS when a hydrophobic St monomer instead of heptane is mixed with water. Obviously, the geometry of the amphiphilic Janus microspheres plays important roles in the formation of the Pickering emulsion, judging from the results in Figure 1C. The smaller the difference in the size of the two bulbs of the snowman-like Janus microspheres, the easier is the formation of the Pickering emulsion droplets. Numerous particle-stabilized styrene droplets varying from 5 to 20 ¯m could be observed in the emulsion containing the snowman-like Janus microspheres prepared with a wS-CPS of 3/1 (Figure 2-A3), i.e., the difference in the size of the two bulbs is only 68 nm, the smallest in the three prepared samples (The differences in the



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Figure 2. Optical images of the emulsion stabilized by the amphiphilic snowman-like Janus microspheres prepared with different wS-CPS: 1/1 (for A1); 2/1 (for A2); 3/1 (for A3).

Figure 3. TEM and SEM images of golf-ball-like PS microspheres prepared by Pickering emulsion polymerization stabilized by amphiphilic Janus microspheres prepared under a wS-CPS of 3/1. sizes of the two bulbs are 106 and 175 nm, respectively, when wS-CPS is 2/1 and 1/1). The results indicate that symmetric snowman-like microspheres easily assemble around the droplets to form a stable Pickering emulsion. When the stable Pickering emulsion (Figure 2-A3) is radiated by £-rays at room temperature, microsized polymer microspheres with many holes on the surface, i.e., golf-ball-like microspheres, were obtained, as displayed in Figure 3A. The average diameter of the holes is 135 nm, with a PDI of 1.40. The TEM image in Figure 3B proves that the holes distribute only on the surface individually. At the same time, nearly no snowmanlike Janus microspheres could be observed in the system. Instead, only spherical microspheres with diameters of about 200 nm (close to the size of the original PA-CPS microspheres, 270 nm) appear. The FTIR spectrum of these spherical microspheres is nearly the same as that of the PA-CPS microspheres displayed in Figure S1 (Right).17 Scheme 1B illustrates the formation mechanism of the golf-ball-like microspheres. The key step is the formation of St droplets stabilized by the snowman-like Janus microspheres. The hydrophobic noncrosslinked PS bulb of the Janus microspheres will be immersed in the droplet, while the hydrophilic PA-CPS bulb stays outside in the water phase. The hydrophobic noncrosslinked PS bulb of the Janus microspheres could be dissolved in St monomers during the radiation-induced polymerization of St (the radiation time is 20 h). Therefore, the hydrophilic crosslinked PS seed microspheres will be detached from the droplet due to the dissolution of the noncrosslinked PS molecules and the combined effect of volume shrinkage resulting from the polymerization of the St droplets and the phase separation in the swollen PA-CPS bulbs. The size of the detached crosslinked PS microspheres would be slightly smaller than that of the original PA-CPS microspheres since a part of the noncrosslinked PS molecules may dissolve in the St monomers. In this work, poly(acrylic acid)-functionalized crosslinked polystyrene (PA-CPS) microspheres were first prepared as seed Chem. Lett. 2013, 42, 963965

microspheres. Then, amphiphilic snowman-like PS microspheres could be successfully synthesized via the radiationinitiated polymerization of the St monomers swollen in the PACPS microspheres. The geometry of the snowman-like Janus microspheres could be controlled by simply varying the weight ratio of the St monomers to PA-CPS seed microspheres (wS-CPS). The amphiphilic snowman-like Janus microspheres having two bulbs with similar diameters can be used as colloid surfactants to form stable Pickering emulsion droplets of the St monomers in water. During the radiation-induced polymerization of St, the hydrophilic crosslinked PS seed microspheres were detached from the droplet due to the dissolution of the noncrosslinked PS microspheres in St and the volume shrinkage resulting from the polymerization of the St monomers. Finally, novel golf-ball-like microspheres were obtained. This work reveals the potential applications of amphiphilic Janus microspheres in the preparation of polymer microspheres with holes on the surface. This work was financially supported by the National Natural Science Foundation of China (Nos. 51073146, 51103143, 51103039, and 51173175), Specialized Research Fund for the Doctoral Program of Higher Education (No. 20093402110021), and Program for Outstanding Young Teachers of ECUST (No. YK0157134). References and Notes 1 J. Parvole, I. Chaduc, K. Ako, O. Spalla, A. Thill, S. Ravaine, E. Duguet, M. Lansalot, E. Bourgeat-Lami, Macromolecules 2012, 45, 7009. 2 G. Loget, A. Kuhn, J. Mater. Chem. 2012, 22, 15457. 3 Y. Yin, S. Zhou, B. You, L. Wu, J. Polym. Sci., Part A: Polym. Chem. 2011, 49, 3272. 4 S. Weiss, D. Hirsemann, B. Biersack, M. Ziadeh, A. H. E. Müller, J. Breu, Polymer 2013, 54, 1388. 5 S. C. Glotzer, Science 2004, 306, 419. 6 K. Isenbügel, Y. Gehrke, H. Ritter, Macromol. Rapid Commun. 2012, 33, 41. 7 A. Walther, M. Hoffmann, A. H. E. Müller, Angew. Chem., Int. Ed. 2008, 47, 711. 8 J. W. Kim, D. Lee, H. C. Shum, D. A. Weitz, Adv. Mater. 2008, 20, 3239. 9 R. Aveyard, Soft Matter 2012, 8, 5233. 10 X. Y. Ling, I. Y. Phang, C. Acikgoz, M. D. Yilmaz, M. A. Hempenius, G. J. Vancso, J. Huskens, Angew. Chem., Int. Ed. 2009, 48, 7677. 11 S. Jiang, M. J. Schultz, Q. Chen, J. S. Moore, S. Granick, Langmuir 2008, 24, 10073. 12 B. Liu, W. Wei, X. Qu, Z. Yang, Angew. Chem., Int. Ed. 2008, 47, 3973. 13 E. B. Mock, C. F. Zukoski, Langmuir 2010, 26, 13747. 14 C. Tang, C. Zhang, J. Liu, X. Qu, J. Li, Z. Yang, Macromolecules 2010, 43, 5114. 15 X. Meng, Y. Guan, Z. Zhang, D. Qiu, Langmuir 2012, 28, 12472. 16 F. W. Wang, H. R. Liu, J. D. Zhang, X. T. Zhou, X. Y. Zhang, J. Polym. Sci., Part A: Polym. Chem. 2012, 50, 4599. 17 Supporting Information is available electronically on the CSJJournal Web site, http://www.csj.jp/journals/chem-lett/index. html. 18 F. S. Bates, Science 1991, 251, 898. 19 L. P. McMaster, Macromolecules 1973, 6, 760. 20 J. W. Kim, R. J. Larsen, D. A. Weitz, J. Am. Chem. Soc. 2006, 128, 14374. 21 B. P. Binks, P. D. I. Fletcher, Langmuir 2001, 17, 4708.

