Preparation of “Mushroom-like” Janus Particles by Site-Selective

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Preparation of “Mushroom-like” Janus Particles by Site-Selective Surface-Initiated Atom Transfer Radical Polymerization in Aqueous Dispersed Systems† Takuya Tanaka, Masaru Okayama, Yukiya Kitayama, Yasuyuki Kagawa, and Masayoshi Okubo* Graduate School of Engineering, Kobe University, Kobe 657-8501, Japan Received December 14, 2009. Revised Manuscript Received February 3, 2010 A versatile approach for the preparation of micrometer-sized, monodisperse, “mushroom-like” Janus polymer particles in aqueous dispersed systems is proposed. The synthetic methodology of the Janus particles consists of the following two steps. The first step is the preparation of spherical poly(methyl methacrylate) (PMMA)/poly(styrene-2-(2bromoisobutyryloxy)ethyl methacrylate) (P(S-BIEM)) Janus particles based on the internal phase separation induced by solvent evaporation from the solvent droplets dissolving the polymers. The second step is surface-initiated atom transfer radical polymerization (ATRP) of 2-(dimethylamino)ethyl methacrylate (DM) using the Janus particles with ATRP initiator groups at one side of the surface as macroinitiator. As a consequence, mushroom-like PMMA/P(SBIEM)-graft-poly(DM) Janus particles were prepared, which had pH-responsive property.

Introduction Morphology control of polymer particles is of substantial importance for the creation of functional colloidal materials. Extensive research for the morphology control has thus been carried out based on thermodynamic and kinetic aspects.1-3 As a result, polymer particles with various morphologies have been designed and fabricated. In recent years, polymer particles comprising two surfaces of different chemistries and/or polarities (so-called “Janus” particles) have attracted much attention from the viewpoint of industrial applications as well as for academic curiosities because of their potential for colloidal surfactants,4-8 chemical and biological sensors,9-11 display materials of the electronic paper,12,13 and self-motile colloidal particles.14,15 Since a pioneering work of Casagrande et al.,4 various sophisticated techniques for the synthesis of † Part CCCXXXVII of the series “Studies on Suspension and Emulsion”. *To whom correspondence should be addressed: Tel þ81-78-803-6161; e-mail [email protected].

(1) Okubo, M. Makromol. Chem., Macromol. Symp. 1990, 35/36, 307–325. (2) Dimonie, V. L.; Daniels, E. S.; Shaffer, O. L.; El-Aasser, M. S. In Emulsion Polymerization and Emulsion Polymers; Lovell, P. A., El-Aasser, M. S., Eds.; John Wiley & Sons: New York, 1997; Chapter 9, pp 293-326. (3) Sundberg, D. C.; Durant, Y. G. Polym. React. Eng. 2003, 11, 379–432. (4) Casagrande, C.; Fabre, P.; Rapha€el, E.; Veyssie, M. Europhys. Lett. 1989, 9, 251–255. (5) Binks, B. P.; Fletcher, P. D. I. Langmuir 2001, 17, 4708–4710. (6) Glaser, N.; Adams, D. J.; B€oker, A.; Krausch, G. Langmuir 2006, 22, 5227– 5229. (7) Walther, A.; Hoffmann, M.; M€uller, A. H. E. Angew. Chem., Int. Ed. 2008, 120, 723–726. (8) Kim, J.-W.; Lee, D.; Shum, H. C.; Weitz, D. A. Adv. Mater. 2008, 20, 3239– 3243. (9) Takei, H.; Shimizu, N. Langmuir 1997, 13, 1865–1868. (10) Choi, J.; Zhao, Y.; Zhang, D.; Chien, S.; Lo, Y.-H. Nano Lett. 2003, 3, 995– 1000. (11) Behrend, C. J.; Anker, J. N.; McNaughton, B. H.; Brasuel, M.; Philbert, M. A. J. Phys. Chem. B 2004, 108, 10408–10414. (12) Nisisako, T.; Torii, T.; Takahashi, T.; Takizawa, Y. Adv. Mater. 2006, 18, 1152–1156. (13) Kim, S.-H.; Jeon, S.-J.; Jeong, W. C.; Park, H. S.; Yang, S.-M. Adv. Mater. 2008, 20, 4129–4134. (14) Golestanian, R.; Liverpool, T. B.; Ajdari, A. Phys. Rev. Lett. 2005, 94, 220801. (15) Howse, J. R.; Jones, R. A.; Ryan, A. J.; Gough, T.; Vafabakhsh, R.; Golestanian, R. Phys. Rev. Lett. 2007, 99, 048102.

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Janus particles, for example, macrophase separation induced by seeded polymerization,1-3,8 selective cross-linking and subsequent dissolution of microphase-separated triblock terpolymer,7,16 microfluidic technique,12,13,17-20 electrohydrodynamic jetting process,21 controlled surface nucleation,6,22-24 and toposelective surface modifications based on the particle monolayer,4,9-11,15,25-28 Pickering emulsion,29-33 and partially protected particle,34 have so far been proposed and reviewed.35-38 (16) Erhardt, R.; Zhang, M.; B€oker, A.; Zettl, H.; Abetz, C.; Frederik, P.; Krausch, G.; Abetz, V.; M€uller, A. H. E. J. Am. Chem. Soc. 2003, 125, 3260–3267. (17) Millman, J. R.; Bhatt, K. H.; Prevo, B. G.; Velev, O. D. Nat. Mater. 2004, 4, 98–102. (18) Nie, Z.; Li, W.; Seo, M.; Xu, S.; Kumacheva, E. J. Am. Chem. Soc. 2006, 128, 9408–9412. (19) Shepherd, R. F.; Conrad, J. C.; Rhodes, S. K.; Link, D. R.; Marquez, M.; Weitz, D. A.; Lewis, J. A. Langmuir 2006, 22, 8618–8622. (20) Dendukuri, D.; Hatton, T. A.; Doyle, P. S. Langmuir 2007, 23, 4669–4674. (21) Roh, K.-H.; Martin, D. C.; Lahann, J. Nat. Mater. 2005, 4, 759–763. (22) Teranishi, T.; Inoue, Y.; Nakaya, M.; Oumi, Y.; Sano, T. J. Am. Chem. Soc. 2004, 126, 9914–9915. (23) Reculusa, S.; Poncet-Legrand, C.; Perro, A.; Duguet, E.; Bourgeat-Lami, E.; Mingotaud, C.; Ravaine, S. Chem. Mater. 2005, 17, 3338–3344. (24) Yu, H.; Chen, M.; Rice, P. M.; Wang, S. X.; White, R. L.; Sun, S. Nano Lett. 2005, 5, 379–382. (25) Fujimoto, K.; Nakahama, K.; Shidara, M.; Kawaguchi, H. Langmuir 1999, 15, 4630–4635. (26) Lu, Y.; Xiong, H.; Jiang, X.; Xia, Y.; Prentiss, M.; Whitesides, G. M. J. Am. Chem. Soc. 2003, 125, 12724–12725. (27) Cayre, O. J.; Paunov, V. N.; Velev, O. D. Chem. Commun. 2003, 2296–2297. (28) Wang, B.; Li, B.; Zhao, B.; Li, C. Y. J. Am. Chem. Soc. 2008, 130, 11594– 11595. (29) Takahara, Y. K.; Ikeda, S.; Ishino, S.; Tachi, K.; Ikeue, K.; Sakata, T.; Hasegawa, T.; Mori, H.; Matsumura, M.; Ohtani, B. J. Am. Chem. Soc. 2005, 127, 6271–6275. (30) Hong, L.; Jiang, S.; Granick, S. Langmuir 2006, 22, 9495–9499. (31) Suzuki, D.; Tsuji, S.; Kawaguchi, H. J. Am. Chem. Soc. 2007, 129, 8088– 8089. (32) Liu, B.; Wei, W.; Qu, X.; Yang, Z. Angew. Chem. 2008, 120, 4037–4039. (33) Berger, S.; Synytska, A.; Ionov, L.; Eichhorn, K.-J.; Stamm, M. Macromolecules 2008, 41, 9669–9676. (34) Perro, A.; Reculusa, S.; Pereira, F.; Delville, M.-H.; Mingotaud, C.; Duguet, E.; Bourgeat-Lami, E.; Ravaine, S. Chem. Commun. 2005, 5542–5543. (35) Perro, A.; Reculusa, S.; Ravaine, S.; Bourgeat-Lami, E.; Duguet, E. J. Mater. Chem. 2005, 15, 3745–3760. (36) Walther, A.; M€uller, A. H. E. Soft Matter 2008, 4, 663–668. (37) Yang, S.-M.; Kim, S.-H.; Yi, G.-R. J. Mater. Chem. 2008, 18, 2177–2190. (38) Wurm, F.; Kilbinger, A. F. M. Angew. Chem., Int. Ed. 2009, 48, 8412–8421.

Published on Web 02/16/2010

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In the surface modification technique for particles, the “grafting from” method based on surface-initiated controlled/living radical polymerization, in particular, atom transfer radical polymerization (ATRP), is a powerful tool.39-42 Polymer molecules are generated from only initiator (e.g., bromine, chlorine) groups at the surface of the particles. In other words, the locus of polymerization can be confined by geometry control of the initiator groups on the particle. Liu et al.32 have recently reported the synthesis of Janus particles by surface-initiated ATRP from initiator groups on the one side surface after masking them on the other side surface of particles having their homogeneous distribution at the whole surface by fixing the particles at Pickering emulsion interface. Wang et al.28 and Berger et al.33 have also synthesized Janus particles by fixing bare particles on the solid substrate or at the Pickering emulsion interface to modify initiator groups on the one side of the surface, followed by surface-initiated ATRP. Recently, we have proposed an efficient synthetic approach of Janus particles in an aqueous dispersed system, which does not require any interfaces for fixing particles. Biphasic-separated polystyrene (PS)/poly(methyl methacrylate) (PMMA) Janus particles were prepared by release of toluene as a common good solvent from homogeneous PS/PMMA/toluene droplets dispersed in an aqueous medium (solvent evaporation method).43-45 The phase-separated morphology of PS/PMMA Janus particles minimizing the total interfacial free energy was controlled by tuning interfacial tensions between each polymer phase and aqueous medium using emulsifier, resulting in spherical and “snowman-like” Janus particles. Moreover, to provide amphiphilicity to such Janus particles, we tried to prepare PS/poly(methyl methacrylate-chloromethylstyrene) (P(MMA-CMS))graft-poly(2-(dimethylamino)ethyl methacrylate) (PDM) Janus particles by surface-initiated activator generated by electron transfer for ATRP (AGET ATRP) of 2-(dimethylamino)ethyl methacrylate (DM) in an aqueous medium using spherical PS/ P(MMA-CMS) Janus particles with ATRP initiator (chlorine) groups at one side of the surface as macroinitiator obtained by the solvent evaporation method.46 As presumed, obtained particles had a “mushroom-like” shape; however, they did not exhibit pH-responsive properties in water based on the nature of PDM.47 After that, it was clarified that this shape is unexpectedly attributed to the dissolution of P(MMA-CMS) [and/ or P(MMA-CMS)-g-PDM] surface layers into the medium of DM aqueous solution during the prolonged polymerization process (45 °C for 48 h).48 In this article, we challenged again and succeeded to synthesize mushroom-like Janus polymer particles having pH-responsive PDM moiety at one side of the surface by surface-initiated AGET ATRP of DM in aqueous dispersed systems using spherical PMMA/poly(styrene-2-(2-bromoisobutyryloxy)ethyl methacrylate) (P(S-BIEM)) macroinitiator Janus particles with highly active initiator groups at one side of the surface prepared by the (39) von Werne, T.; Patten, T. E. J. Am. Chem. Soc. 1999, 121, 7409–7410. (40) Pyun, J.; Matyjaszewski, K. Chem. Mater. 2001, 13, 3436–3448. (41) Ohno, K.; Koh, K.; Tsujii, Y.; Fukuda, T. Angew. Chem., Int. Ed. 2003, 42, 2751–2754. (42) Chen, X.; Randall, D. P.; Perruchot, C.; Watts, J. F.; Patten, T. E.; von Werne, T.; Armes, S. P. J. Colloid Interface Sci. 2003, 257, 56–64. (43) Saito, N.; Kagari, Y.; Okubo, M. Langmuir 2006, 22, 9397–9402. (44) Saito, N.; Nakatsuru, R.; Kagari, Y.; Okubo, M. Langmuir 2007, 23, 11506–11512. (45) Tanaka, T.; Nakatsuru, R.; Kagari, Y.; Saito, N.; Okubo, M. Langmuir 2008, 24, 12267–12271. (46) Ahmad, H.; Saito, N.; Kagawa, Y.; Okubo, M. Langmuir 2008, 24, 688– 691. (47) Okubo, M.; Ahmad, H. Colloid Polym. Sci. 1995, 273, 817–821. (48) The details of this result will be submitted soon.

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solvent evaporation method, which did not dissolve into the medium of DM aqueous solution.

Experimental Section Materials. Styrene (S) and methyl methacrylate (MMA) were distilled under reduced pressure in a nitrogen atmosphere. 2-(2Bromoisobutyryloxy)ethyl methacrylate (BIEM) was synthesized by the reaction of hydroxyethyl methacrylate (HEMA) and 2-bromoisobutyryl bromide according to a previous report.49 Reagent-grade 2,20 -azobis(isobutyronitrile) (AIBN) was purified by recrystallization with methanol. DM (Tokyo Kasei Kogyo Co. Ltd., Japan), HEMA, toluene, CuBr2, ascorbic acid (Nacalai Tesque Inc., Japan), N,N,N0 ,N0 ,N00 -pentamethyldiethylenetriamine (PMDETA), 2-bromoisobutyryl bromide (Aldrich Chemical Co. Ltd.), sodium dodecyl sulfate (SDS) (Wako Pure Chemical Industries, Ltd., Japan), and polyoxyethylene nonylphenyl ether with an average ethylene oxide chain length of 30.7 units (Emulgen 931; Kao Co., Japan) were used as received. Water was purified using an Elix UV system (Nihon Millipore K.K., Japan).

Preparation of PMMA/P(S-BIEM) Macroinitiator Janus Particles. PMMA and P(S-BIEM) were respectively synthesized by solution (co)polymerizations of MMA and S/BIEM (95/5, mol/mol) in toluene at 70 °C for 24 h employing AIBN as initiators. Number-average molecular weights (Mn, g mol-1) and polydispersity indices (PDI) of PMMA and P(S-BIEM) measured with gel permeation chromatography (GPC) were respectively 3.9  104 and 2.57 and 4.4  104 and 2.07. Polymer composition of P(S-BIEM) measured with proton nuclear magnetic resonance (1H NMR) was molar ratio of 99.6:0.4 (see Figure S1 in the Supporting Information). A homogeneous toluene solution (3.3 g) of PMMA and P(S-BIEM) (polymer/ toluene = 1/12, w/w) was stored in a Teflon storage tank and permeated through the Shirasu Porous Glass (SPG) membrane with an average pore size of 3.9 μm (SPG Technology Co., Ltd., Japan) into 57.8 mM SDS aqueous solution under an appropriate pressure (ca. 0.03 MPa). Toluene was slowly released by evaporation from the aqueous dispersion under gentle stirring with a magnetic stirrer at room temperature for 48 h in an uncovered beaker (22 cm2 surface area between the dispersion and air).

Preparation of PMMA/P(S-BIEM)-g-PDM Janus Particles. PMMA/P(S-BIEM) macroinitiator Janus particles dispersed in the aqueous solution of SDS were repeatedly washed by serum replacement with 10 g L-1 Emulgen 931 aqueous solution. Aqueous solution (4.1 g) of DM (1.62 mmol), CuBr2 (22 μmol), and PMDETA (22 μmol) was mixed with the purified PMMA/P(S-BIEM) dispersion (5.0 g, 2 wt % solid content); subsequently, the mixture was transferred to a glass tube and degassed using several N2/vacuum cycles, followed by the addition of an aqueous solution (1.0 g) of ascorbic acid (8.8 μmol, 0.8 equiv ratio to CuBr2 including in this system). The glass tube containing the dispersion was once again degassed with several N2/vacuum cycles and sealed off under vacuum. Surface-initiated AGET ATRP of DM with PMMA/P(S-BIEM) macroinitiator Janus particles was carried out at 60 °C under a N2 atmosphere in a sealed glass tube. Monomer conversion was determined by gravimetry. Measurements. Molecular weight distributions of PMMA and P(S-BIEM) were measured by GPC with two S/DVB gel columns (TOSOH Corp., TSK gel GMHHR-H, 7.8 mm i.d.  30 cm) using tetrahydrofuran as eluent at 40 °C at a flow rate of 1.0 mL min-1 employing refractive index (TOSOH RI-8020/21) and ultraviolet detectors (TOYO SODA UV-8II). The interfacial tension between polymer-toluene (1/5, w/w) phase and water was measured by the pendant drop method with a Kyowa Interface Science Drop Master 500. The density of polymer-toluene (49) Matyjaszewski, K.; Gaynor, S. G.; Kulfan, A.; Podwika, M. Macromolecules 1997, 30, 5192–5194.

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solution was measured with a pycnometer (volumetric flask type). All measurements were performed at room temperature (ca. 20 °C). The accuracy of the interfacial tensions reported was (0.1 mN m-1. The columns were calibrated with six standard PS samples (1.05  103-5.48  106, Mw/Mn = 1.01-1.15). The decomposition rate of DM in water was determined with a Shimadzu GC-2014 gas chromatography (GC) using helium as the carrier gas, dimethyl sulfoxide as a solvent, and methanol as an internal standard. Surface Analysis of Composite Polymer Particles. X-ray photoelectron spectroscopy (XPS) data of PMMA/PS, PMMA/ P(S-BIEM), and PMMA/P(S-BIEM)-g-PDM Janus particles were obtained with a Shimadzu ESCA-3400 apparatus using magnesium KR radiation (1253.6 eV) at a potential of 10 kV and an X-ray current of 20 mA. The pressure in the measurement chamber was ca. 8.0  10-7 Pa. Dried particles were pressed on an indium plate and stored under reduced pressure by continuous operation of an oil rotary pump just before the measurement. The zeta-potential of PMMA/P(S-BIEM)-g-PDM Janus particles dispersed in 1.0 g L-1 Emulgen 931 aqueous solution was measured at 25 °C with an Otsuka ELSZ-2 zeta potential and particle size analyzer. Observation of Composite Polymer Particles. Composite polymer particles were observed with a Nikon Eclipse 80i optical microscope and a Hitachi S-2460 scanning electron microscope (SEM) at an acceleration voltage of 15 kV. The dried particles were stained with ruthenium tetraoxide (RuO4) vapor at room temperature for 30 min in the presence of 1.0 wt % RuO4 aqueous solution, embedded in an epoxy matrix, cured at room temperature, and subsequently microtomed. Ultrathin cross sections of 100 nm thickness were observed with a JEOL JEM-1230 transmission electron microscope (TEM) at an acceleration voltage of 100 kV. Number-average diameter (Dn) and its coefficient of variation (Cv) of the particles were determined by measuring the diameters of more than 200 particles in TEM photographs (not cross-sectional images) with image analysis software (WinROOF, Mitani Co., Ltd.).

Results and Discussion Figure 1 shows the two-step synthetic approach to obtain mushroom-like Janus particles having PDM moiety at the one side of the surface in aqueous dispersed systems. The first step is macrophase separation between PMMA and P(S-BIEM) in toluene droplets induced by evaporation of toluene as a common good solvent, resulting in spherical PMMA/P(S-BIEM) Janus particles. The second step is site-selective surface-initiated AGET ATRP of DM using the Janus particles as macroinitiator. BIEM as initiator moiety was selected as substitute for CMS used in the previous work46 because of its high activity derived from bromine group bonded to tertiary carbon.50 First, spherical PMMA/P(SBIEM) (1/1, w/w) macroinitiator Janus particles with almost the same interfacial area between each polymer phase and the aqueous medium, so-called hemispherical morphology, were prepared using SDS as emulsifier as follows. Step I: Preparation of Spherical Janus Particles by Phase Separation in Toluene Droplets Dissolving PMMA and P(S-BIEM) with Evaporation. Homogeneous PMMA/P(SBIEM)/toluene droplets internally phase-separated into PMMAtoluene and P(S-BIEM)-toluene phases as toluene evaporated, resulting in the formation of thermodynamically equilibrium morphology minimizing the total interfacial free energy. To achieve the hemispherical morphology of PMMA/P(S-BIEM)/ toluene droplets, almost the same values of interfacial tensions between toluene solutions dissolving PMMA and P(S-BIEM) and (50) Goto, A.; Fukuda, T. Macromol. Rapid Commun. 1999, 20, 633–636.

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Figure 1. Schematic representation of preparation of “mushroom-like” PMMA/P(S-BIEM)-g-PDM Janus particle by siteselective surface-initiated AGET ATRP of DM in the aqueous medium using biphasic-separated spherical PMMA/P(S-BIEM) Janus particle with bromine groups at one side of the surface as macroinitiator. The macroinitiator Janus particle was obtained by slow release of toluene from homogeneous PMMA/P(S-BIEM)/ toluene droplet dispersed in the aqueous medium.

SDS aqueous solution are required. According to our previous study,51 the interfacial tensions between toluene solutions dissolving PS and PMMA, which were respectively obtained from the upper and lower layers of phase-separated PS/PMMA/ toluene solution, and SDS aqueous solution above critical micelle concentration (cmc: ca. 8 mM) indicates the almost same values. In the two layers, each polymer phase contained a small amount of the other polymers (i.e., PS solution contained PMMA and vice versa). As a result, PS/PMMA composite particles prepared by the solvent evaporation method in SDS aqueous solutions above cmc exhibited the hemispherical morphology. On the basis of this knowledge, we adopted the SDS aqueous solution above cmc (57.8 mM) as a medium to prepare PMMA/P(S-BIEM)/ toluene droplets. Moreover, the interfacial tension (9.7 mN/m) between toluene solution dissolving P(S-BIEM) and water was significantly lower than that (34.4 mN/m)43 between toluene solution dissolving PS (without BIEM) and water. This result indicates that BIEM units in P(S-BIEM) preferentially locate at the surface of the droplet to reduce the interfacial tension. Consequently, bromine groups in the BIEM units are expected to effectively perform as initiating points for surface-initiated AGET ATRP. The SPG membrane emulsification technique was employed for the preparation of micrometer-sized, monodisperse PMMA/ P(S-BIEM)/toluene droplets.52,53 Toluene was slowly released by evaporation from the homogeneous droplets so as to achieve thermodynamic equilibrium morphology. After complete evaporation of toluene, biphasic-separated PMMA/P(S-BIEM) particles reflecting the thermodynamically equilibrium morphology of the droplets are eventually obtained. Figure 2 show optical micrograph and SEM photograph of PMMA/P(S-BIEM) particles and TEM photograph of ultrathin cross sections of the RuO4-stained particles, where P(S-BIEM) is preferentially stained by RuO4. Relatively monodisperse spherical particles exhibiting the hemispherical (i.e., Janus) morphology were formed without coagulation. The Dn (μm) and Cv (%) of the particles were 4.8 and 9.8, respectively. Figure 3 shows Br3d X-ray photoelectron spectra of PMMA/ PS composite particles as a blank and the PMMA/P(S-BIEM) Janus particles. It was confirmed that the bromine groups existed at the surface of the Janus particles although the bromine distribution at the surface is not clear. The Janus particles dispersed in the SDS aqueous solution were repeatedly washed by serum replacement with 10 g L-1 Emulgen 931 (nonionic (51) Saito, N.; Kagari, Y.; Okubo, M. Langmuir 2007, 23, 5914–5919. (52) Ma, G.-H.; Omi, S.; Dimonie, V. L.; Sudol, E. D.; El-aasser, M. S. J. Appl. Polym. Sci. 2002, 85, 1530–1543. (53) Chaiyasat, P.; Ogino, Y.; Suzuki, T.; Minami, H.; Okubo, M. Colloid Polym. Sci. 2008, 286, 217–223.

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Figure 2. (a) Optical micrograph and (b) SEM photograph of PMMA/P(S-BIEM) (1/1, w/w) macroinitiator particles obtained by release of toluene from polymer/toluene droplets (1/12, w/w) dispersed in 57.8 mM SDS aqueous solution, which were prepared by the SPG membrane emulsification technique. (c) TEM photograph of ultrathin cross sections of the RuO4-stained particles, where PMMA and P(S-BIEM) phases respectively appear as bright and dark.

Figure 3. Br3d X-ray photoelectron spectra of (a) PMMA/PS composite particles and (b) PMMA/P(S-BIEM) macroinitiator Janus particles prepared by release of toluene from PMMA/PS/ toluene or PMMA/P(S-BIEM)/toluene droplets dispersed in an SDS aqueous solution.

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Figure 5. (a-c) Optical micrographs and (c0 ) SEM photograph of PMMA/P(S-BIEM)-g-PDM Janus particles prepared by surfaceinitiated AGET ATRP of DM in the aqueous medium using spherical PMMA/P(S-BIEM) macroinitiator Janus particles at 60 °C for 5 min (a), 10 min (b), and 60 min (c, c0 , c00 ) and (b) TEM photograph of ultrathin cross sections of the RuO4stained particles, where P(S-BIEM) phase appears as dark and PMMA and PDM phases appear bright. PDM content in the particles (wt %): (a) 30; (b) 41; (c, c0 , c00 ) 50.

Figure 6. X-ray photoelectron spectra of (a) PMMA/P(S-BIEM) macroinitiator Janus particles and (b) PMMA/P(S-BIEM)-gPDM Janus particles (PDM content: 50 wt %).

surfactant) aqueous solution to remove SDS, which is unsuitable for ATRP due to the formation of copper complex.54 Step II: Surface-Initiated AGET ATRP from the Macroinitiator Janus Particle. PMMA/P(S-BIEM) Janus particles dispersed in the Emulgen 931 aqueous solution were subsequently employed as macroinitiator for surface-initiated AGET ATRP of DM. To avoid Cu(I) oxidation by dissolved oxygen in the

dispersion, Cu(I) was generated in situ by Cu(II) reduction with ascorbic acid after degassing process using N2/vacuum cycles.55 Figure 4 shows conversion-time plot at 60 °C. The polymerization smoothly proceeded without coagulation and reached ∼40% conversion for 30 min. The reason why the conversion plateaued at the 40% will be discussed later. Figure 5a-c shows optical micrographs of the particles at various conversions of the surface-initiated AGET ATRP of DM at 60 °C. A new layer (“mushroom cap”) was clearly generated from one side of the surface and grew in thickness with increasing conversion. SEM photograph indicates relatively monodisperse mushroom-like particles were formed (Figure 5c0 ). As seen in TEM photograph (Figure 5c00 ), the mushroom cap was only generated from the P(S-BIEM) phase. X-ray photoelectron spectra of the spherical macroinitiator particles and the mushroom-like particles indicate that a N1s peak due to PDM was only observed in the latter particles (Figure 6). These results indicate that the mushroom cap was PDM formed from one side of the surface. Our approach would achieve the large-scale production of Janus particles because of needlessness of the infinite interface where homogeneous particles are fixed for the surface modification and be applied to other water-soluble monomer systems. Furthermore, the hydrophilic and hydrophobic balance of the

(54) Gaynor, S. G.; Qiu, J.; Matyjaszewski, K. Macromolecules 1998, 31, 5951– 5954.

(55) Min, K.; Gao, H.; Matyjaszewski, K. J. Am. Chem. Soc. 2005, 127, 3825– 3830.

Figure 4. Conversion-time plot for surface-initiated AGET ATRP of DM in an Emulgen 931 aqueous solution using PMMA/ P(S-BIEM) macroinitiator Janus particles at 60 °C.

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Figure 7. Zeta-potential of the mushroom-like PMMA/P(SBIEM)-g-PDM Janus particles dispersed in the aqueous medium at 25 °C as a function of pH.

Figure 8. Residual DM (O) and formed MAA (0) and DMAE (Δ) in DM aqueous solution in the absence of initiator at 60 °C as functions of time. Their amounts were independently estimated with GC. The initial concentration of DM aqueous solution was 0.16 M.

mushroom-like Janus particles is easily tunable by controlling the morphology of the macroinitiator Janus particles by varying each interfacial tension. Figure 7 shows the zeta-potential of the mushroom-like Janus particles dispersed in the aqueous medium at 25 °C as a function of pH. The zeta-potential value of the mushroom-like Janus particles exhibited ∼28 mV at a low pH, decreased on raising the pH to around 7, and eventually indicated zero charge at a high pH. This is attributed to the fact that the tertiary amino groups in the PDM in the mushroom-like Janus particles are protonated at the low pH and deprotonated at the pH above around pKa (ca. 6.8).56 This result indicates that the mushroom-like Janus particles exhibit the pH-responsive property in aqueous dispersed systems. The stimulus-responsive property of the mushroom-like Janus particles will be discussed elsewhere in detail. It is noted that DM is autocatalytically hydrolyzed in water to methacrylic acid (MAA) and 2-(dimethylamino)ethanol (DMAE);57 the half-life period is 17 h at pH 7.4 and 37 °C.58 The hydrolysis rate was accelerated by increasing temperature. Figure 8 indicates the rate of decomposition of DM attributed to the hydrolysis and following formation of MAA and DMAE in water in the absence of initiator at 60 °C, measured with GC. 73% DM was hydrolyzed to MAA and DMAE for 30 min. The hydrolysis reaction of DM competitively occurs with the propagation reaction during the polymerization. Moreover, MAA cannot be polymerized with (56) Matsumoto, T.; Okubo, M.; Onoe, S. Kobunshi Ronbunshu 1975, 32, 333–337. (57) Matsumoto, T.; Okubo, M.; Onoe, S. Kobunshi Ronbunshu 1975, 32, 162– 167. (58) van de Wetering, P.; Zuidam, N. J.; van Steenbergen, M. J.; van der Houwen, O. A. G. J.; Underberg, W. J. M.; Hennink, W. E. Macromolecules 1998, 31, 8063–8068.

Langmuir 2010, 26(11), 7843–7847

Article

Figure 9. Optical micrographs of mushroom-like Janus particles (a) before and (b) after further chain extension by surface-reinitiated AGET ATRP of DM in the aqueous medium using the mushroom-like PMMA/P(S-BIEM)-g-PDM Janus particles as macroinitiator at 60 °C for 1 h.

ATRP because of irreversible reaction with copper complex to form metal carboxylates.59 The hydrolysis of DM must be the reason why the polymerization did not proceed above the conversion of 40% (Figure 4). Considering the above results, PDM end-groups in the mushroom-like PMMA/P(S-BIEM)-g-PDM Janus particles should still be active attributed to capping with bromine and would thus be reinitiated by addition of extra monomer. The mushroom-like Janus particles were once dried after centrifugal washing with methanol and subsequently redispersed in the aqueous medium to be employed as the macroinitiator for surface-reinitiated AGET ATRP of DM (chain extension) under the same conditions as previously performed. Figure 9 shows optical micrographs of mushroom-like Janus particles before and after the chain extension at 60 °C for 1 h. The chain extension led to further expand the PDM layers although the conversion of DM leveled off at about 40% as well as in Figure 4. These results suggest that our approach can also allow block copolymer to be provided at one side of the particles.

Conclusions In the present study, a versatile synthetic approach of micrometer-sized, monodisperse, mushroom-like Janus polymer particles, which had pH-responsive property, in aqueous dispersed systems was demonstrated. Spherical PMMA/P(S-BIEM) macroinitiator Janus particles having bromine groups at one side of the surface were designed from thermodynamic viewpoint and prepared by release of toluene from homogeneous PMMA/P(SBIEM)/toluene droplets dispersed in the SDS aqueous solution above the cmc. Subsequently, mushroom-like PMMA/P(SBIEM)-g-PDM Janus particles were successfully obtained by siteselective surface-initiated AGET ATRP of DM with the spherical macroinitiator Janus particles in an aqueous dispersed system. The details of the stimulus responsiveness of the mushroom-like Janus particle will be demonstrated soon. Acknowledgment. The authors thank Associate Professor Minoru Mizuhata (Kobe Univ.) for zeta-potential measurement. This work was supported by Grant-in-Aid for Scientific Research (Grant 21245050) from the Japan Society for the Promotion of Science (JSPS) and by Research Fellowships of the JSPS for Young Scientists (given to Y.K.). Supporting Information Available: Experimental procedure for 1H NMR spectrum of P(S-BIEM). This material is available free of charge via the Internet at http://pubs.acs.org. (59) Patten, T. E.; Matyjaszewski, K. Adv. Mater. 1998, 10, 901–915.

DOI: 10.1021/la904701r

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