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Langmuir 2008, 24, 688-691
Preparation of Micrometer-Sized, Monodisperse “Janus” Composite Polymer Particles Having Temperature-Sensitive Polymer Brushes at Half of the Surface by Seeded Atom Transfer Radical Polymerization† Hasan Ahmad,‡,§ Naohiko Saito,‡ Yasuyuki Kagawa,‡ and Masayoshi Okubo*,‡ Graduate School of Engineering, Kobe UniVersity, Kobe 657-8501, Japan, and Department of Chemistry, Rajshahi UniVersity, Rajshahi 6205, Bangladesh ReceiVed September 28, 2007. In Final Form: NoVember 9, 2007 Micrometer-sized, monodisperse polystyrene (PS)/poly[methyl methacrylate-(chloromethyl)styrene] [P(MMACMS)] composite particles having hemispherical structure were prepared by solvent evaporation from toluene droplets containing dissolved PS and P(MMA-CMS) dispersed in aqueous solution, which had been prepared using the membrane method. The formation of hemispherical (“Janus”) morphology by phase separation between the PS and the P(MMA-CMS) was confirmed by both optical and electron microscopy. Atom transfer radical polymerization (ATRP) of 2-(dimethylamino)ethyl methacrylate (DM) was subsequently carried out in the presence of hemispherical PS/P(MMA-CMS) composite particles in an aqueous dispersed system. After polymerization, the morphology of the particles changed from spherical to “mushroom” shape as observed by scanning electron microscopy, indicating that DM polymerized inside or on the surface of half [P(MMA-CMS) phase] of the particles. 1H NMR spectra were consistent with chloromethyl functional groups in P(MMA-CMS) operating as ATRP initiators in the DM polymerization.
Introduction Micrometer-sized, monodisperse composite polymer particles have been extensively studied for advanced industrial applications such as in the microelectronics and biomedical fields.1-9 The performance and properties of these composite polymer particles are largely influenced by their morphology. Over the past few decades extensive research has been aimed at controlling the particlemorphologyintheprocessofmultistagepolymerizations.10-23 Particle morphology is mostly governed by a competition between †
Part CCCIV of the series “Studies on Suspension and Emulsion”. * To whom correspondence should be addressed. Phone/fax: +81-78803-6161. E-mail:
[email protected]. ‡ Kobe University. § Rajshahi University. (1) Okubo, M.; Izumi, J.; Hosotani, T.; Yamashita, T. Colloid Polym. Sci. 1997, 275, 797-801. (2) Okubo, M.; Ise, E.; Yamashita, T. J. Appl. Polym. Sci. 1999, 74, 278-285. (3) Khan, M. A.; Armes, S. P. Langmuir 1999, 15, 3469-3475. (4) Yildiz, U.; Hazer, B. Polymer 2000, 41, 539-544. (5) Hora`k, D. J. Polym. Sci., Polym. Chem. Ed. 2001, 39, 3707-3715. (6) Wang, D.; Dimonie, V. L.; Sudol, E. D.; El-aasser, M. S. J. Appl. Polym. Sci. 2002, 84, 2721-2732. (7) Camli, T.; Tuncel, M.; S¸ enel, S.; Tuncel, A. J. Appl. Polym. Sci. 2002, 84, 414-429. (8) Ahmad, H.; Miah, M. A. J.; Rahman, M. M. Colloid Polym. Sci. 2003, 281, 988-992. (9) He´rault, D.; Saluzzo, C.; Lemaire, M. React. Funct. Polym. 2006, 66, 567-577. (10) Okubo, M.; Yamada, A.; Matsumoto, T. J. Polym. Sci., Polym. Chem. Ed. 1980, 16, 3219-3228. (11) Okubo, M.; Katsuta, Y.; Matsumoto, T. J. Polym. Sci., Polym. Chem. Ed. 1980, 18, 481-486. (12) Okubo, M.; Ando, M.; Yamada, A.; Katsuta, Y.; Matsumoto, T. J. Polym. Sci., Polym. Lett. Ed. 1981, 19, 143-147. (13) Min, T. I.; Klein, A.; El-aasser, M. S.; Vanderhoff, J. W. J. Polym. Sci., Polym. Lett. 1983, 21, 2845-2861. (14) Cho, I.; Lee, K. W. J. Appl. Polym. Sci. 1985, 30 (5), 1903-1926. (15) Okubo, M. Makromol. Chem. Macromol. Symp. 1990, 35/36, 307-325. (16) Sheu, H. R.; El-aasser, M. S.; Vanderhoff, J. W. J. Polym. Sci., Polym. Chem. Ed. 1990, 28, 629-651. (17) Sundberg, D. C.; Casassa, A. P.; Pantazopoulos, J.; Muscato, M. R. J. Appl. Polym. Sci. 1990, 41, 1425-1442. (18) Chen, Y. C.; Dimonie, V. L.; El-aasser, M. S. J. Appl. Polym. Sci. 1991, 42, 1049-1063. (19) Chen, Y. C.; Dimonie, V. L.; El-aasser, M. S. J. Appl. Polym. Sci. 1992, 45, 487-499. (20) Winzor, C. L.; Sundberg, D. C. Polymer 1992, 33, 3797-3810.
thermodynamic and kinetic factors. Recently our group reported the preparation and formation mechanism of polystyrene (PS)/ poly(methyl methacrylate) (PMMA) composite particles having hemispherical structure by solvent evaporation from solvent droplets containing dissolved PS and PMMA dispersed in an aqueous solution of sodium dodecyl sulfate (SDS).24,25 Reactive composite polymer particles bearing chloromethyl groups are currently of particular interest especially as a carrier matrix in biotechnology and biomedical applications. In one of the early attempts, our group reported the incorporation of (chloromethyl)styrene (CMS) at the surface of the PS particles by seeded dispersion polymerization.26 Further improvement of those polymer particles with an amine group was also investigated.27 Margel and co-workers performed direct synthesis of poly(CMS) (PCMS) particles by dispersion polymerization in a dispersion medium comprised of dimethyl sulfoxide and ethanol.28 Tuncel et al. studied the effect of initiator and stabilizer concentrations and the monomer/dispersion feed ratio on the preparation of monodisperse PCMS microbeads by dispersion polymerization.29 The present paper is an extension of our early work,24,25 emphasizing the selective surface modification of hemispherical composite particles to diversify their applications. In this paper we report the preparation of CMS-incorporated micrometersized composite polymer particles having hemispherical structure by the solvent evaporation method. The dormant chlorine atoms (21) Lee, S.; Rudin, A. J. Polym. Sci., Polym. Chem. Ed. 1992, 30, 22112216. (22) Jonsson, J.-E.; Hassander, H.; Tornell, B. Macromolecules 1994, 27, 19321937. (23) Karlsson, O.; Hassander, H.; Wesslen, B. J. Appl. Polym. Sci. 1997, 63, 1543-1555. (24) Okubo, M.; Saito, N.; Fujibayashi, T. Colloid Polym. Sci. 2005, 283, 691-698. (25) Saito, N.; Kagari, Y.; Okubo, M. Langmuir 2006, 22, 9397-9402. (26) Okubo, M.; Ikegami, K.; Yamamoto, Y. Colloid Polym. Sci. 1989, 267, 193-198. (27) Okubo, M.; Iwasaki, Y.; Yamamoto, Y. Colloid Polym. Sci. 1992, 270, 733-737. (28) Margel, S.; Nov, E.; Fisher, I. J. Polym. Sci., Polym. Chem. Ed. 1991, 29, 347. (29) Bahar, T.; Tuncel, A. Polym. Eng. Sci. 1999, 39, 1849-1855.
10.1021/la702994u CCC: $40.75 © 2008 American Chemical Society Published on Web 01/11/2008
Preparation of “Janus” Composite Polymer Particles
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Table 1. Preparation of PS and P(MMA-CMS) by Solution Polymerization styrene mass (g)
MMA mass (g)
CMS mass (g)
PSa 10 (9.6 × 10-2 mol) P(MMA-CMS)b (0.5 mol %) 9.93 (9.93 × 10-2 mol) 0.0761 (5.0 × 10-4 mol) P(MMA-CMS)b (5 mol %) 9.3 (9.3 × 10-2 mol) 0.746 (4.9 × 10-3 mol) d
toluene vol (mL)
AIBN mass (g)
11.5 17 17
0.015 (9.1 × 10-5 mol) 0.03 (1.8 × 10-4 mol) 0.03 (1.8 × 10-4 mol)
Mwc × 10-4 Mw/Mnd 10.1 2.13 1.82
2.82 3.28 2.92
a Conditions: N2, 70 °C, 24 h. b Conditions: N2, 70 °C, 30 h. c Weight-average molecular weight, measured by gel permeation chromatography. Polydispersity index, measured by gel permeation chromatography.
Table 2. Preparation of PS/P(MMA-CMS)-b-PDM Composite Particles by ATRPa DM mass (g) PS/P(MMA-CMS)-b-PDM 0.3846 (2.45 × 10-3 mol) a
PS/ P(MMA-CMS)b mass (g) 0.1
CuBr2 mass (g)
PMDETA mass (g)
ascorbic acid mass (g)
0.011 (4.93 × 10-5 mol) 0.0085 (5.74 × 10-5 mol) 0.0043 (2.44 × 10-5 mol)
water mass (g) 10
Conditions: 5 °C, N2, 80 cycles/min, 48 h. b Dispersed in TTAB aqueous solution (5 g L-1).
localized on half of the particle surface were then utilized to form polymer brushes comprising 2-(dimethylamino)ethyl methacrylate (DM) by atom transfer radical polymerization (ATRP).30,31 Poly(DM) (PDM) is a thermally responsible water-soluble polymer. An aqueous solution of the polymer exhibits a sharp phase transition at about 34 °C,32 and the temperature characteristic of this event is termed the lower critical solution temperature. Such amphiphilic “Janus” particles have been fascinating objects in stabilization of emulsions33,34 and for biomedical applications due to the interesting characteristics of the temperature-sensitive swelling-deswelling.34-43 Experimental Section Materials. Styrene (S) and methyl methacrylate (MMA) were distilled under reduced pressure in a nitrogen atmosphere and stored in the refrigerator before use. CMS, 98% purity, was washed with 1% NaOH aqueous solution to remove any inhibitor. DM of reagent grade from Nacalai Tesque, Japan, was used as received. Reagent grade 2,2′-azobis(isobutyronitrile) was purified by recrystallization in methanol. CuBr2 and ascorbic acid from Nacalai Tesque, Japan, N,N,N′,N′,N′′-pentamethyldiethylenetriamine (PMDETA) from Aldrich, SDS from Wako, Japan, and tetradecyltrimethylammonium bromide (TTAB) from Tokyo Kasei Kogyo Co. Ltd. were all of reagent grade and used without any purification. Shirasu porous glass (SPG) membrane was obtained from SPG Technology Co. Ltd., Japan. Deionized water with a specific resistance of 5 × 106 Ω‚cm was used after distillation. Other chemicals were used as received. Preparation of PS/P(MMA-CMS) Composite Particles. PS and P(MMA-CMS) with different proportions of CMS (0.5 and 5 mol %) were prepared by solution polymerization under the conditions listed in Table 1. The 1H NMR analysis revealed that the initial CMS was incorporated into the polymer during the copolymerization with (30) Matyjaszewski, K.; Xia, J. H. Chem. ReV. 2001, 101, 2921-2990. (31) Kamigaito, M.; Ando, T.; Sawamoto, M. Chem. ReV. 2001, 101, 36893745. (32) Matsumoto, T.; Nakamae, K.; Okubo, M.; Sue, M.; Simao, M.; Komura, M. Kobunshi Ronbunshu 1974, 31, 669-675. (33) Binks, B. P.; Fletcher, D. I. Langmuir 2001, 17, 4708-4710. (34) Glaser, N.; Adams, D. J.; Bo¨ker, A.; Krausch, G. Langmuir 2006, 22, 5227-5229. (35) Okubo, M.; Ahmad, H. Colloid Polym. Sci. 1995, 273, 817-821. (36) Okubo, M.; Ahmad, H. Colloid Polym. Sci. 1996, 274, 112-116. (37) Okubo, M.; Ahmad, H. J. Polym. Sci., Polym. Chem. Ed. 1996, 34, 31473153. (38) Okubo, M.; Ahmad, H.; Komura, M. Colloid Polym. Sci. 1996, 274, 1188-1191. (39) Okubo, M.; Ahmad, H. J. Polym. Sci., Polym. Chem. Ed. 1998, 36, 883888. (40) Okubo, M.; Ahmad, H.; Suzuki, T. Colloid Polym. Sci. 1998, 276, 470475. (41) Okubo, M.; Ahmad, H. Colloids Surf., A 1999, 153, 429-433. (42) Ahmad, H.; Okubo, M.; Kamatari, Y. O.; Minami, H. Colloid Polym. Sci. 2002, 280, 310-315. (43) Alam, M. A.; Miah, M. A. J.; Ahmad, H. Colloid Polym. Sci. 2007, 285, 715-720.
MMA. Two different techniques were used to prepare micrometersized PS/P(MMA-CMS) composite particles as outlined below. (i) A homogeneous toluene (0.7 mL) solution of PS (25 mg) and P(MMA-CMS) (25 mg) was mixed with aqueous solutions (15 g) of SDS (5 g‚L-1). The mixtures were stirred vigorously using an NISSEI ABM-2 homogenizer at 2000 rpm for 2 min in 50 mL glass vials. The toluene was allowed to evaporate slowly from the magnetically stirred dispersions kept in uncovered glass vials (the surface area in contact with air was 8 cm2) at room temperature for 24 h. (ii) A homogeneous toluene solution of PS and P(MMA-CMS) (2.0 g) was prepared with the same polymer content as described above. The copolymer used in this experiment contained 5 mol % CMS. The polymer solution was pushed through an SPG membrane having a porosity of 3.0 µm under a nitrogen pressure of 0.030 MPa into 50 mL of SDS aqueous solution (5 g‚L-1) to prepare comparatively monodispersed droplets. Toluene was subsequently evaporated slowly from the droplets under the same conditions as described for technique i. The complete evaporation of toluene under similar conditions has previously been confirmed by gas chromatography.25 Preparation of PS/P(MMA-CMS)-b-PDM by Seeded ATRP. PS/P(MMA-CMS) composite particles containing 2.5 mol % CMS were first washed repeatedly by serum replacement with an aqueous solution of TTAB (5 g‚L-1) to remove SDS, which is not suitable for ATRP as it may react with the copper complex.44,45 Seeded ATRP of DM with PS/P(MMA-CMS) particles dispersed in TTAB aqueous solution was then carried out to form PDM brushes at the surfaces in a sealed glass tube under the conditions listed in Table 2. Part of the samples were kept in an oven at 80 °C until a constant weight was reached. The percentage of conversion was calculated from the solids content by correcting for the amounts of auxiliary materials such as PS/P(MMA-CMS) and TTAB. Characterization. Composite polymer particles were observed with a MICROPHOT-FXA optical microscope, a Hitachi S-2460 scanning electron microscope at a voltage of 20 kV, and a Hitachi 7500 transmission electron microscope at a voltage of 100 kV. The 1H NMR spectrum was recorded with a Bruker ADVANCE 500 NMR spectrometer. Each sample was dissolved in CDCl3 prior to the measurement. Molecular weight distributions were measured by gel permeation chromatography (GPC) with two S/DVB gel columns (TOSOH Co., TSKgel GMHHR-H, 7.8 mm (i.d.) × 30 cm) using THF as an eluent at 40 °C at a flow rate of 1.0 mL min-1 employing refractive index (TOSOH RI-8020/21) and ultraviolet (TOYO SODA UV-8II) detectors. The columns were calibrated with six standard PS samples (1.05 × 103 to 5.48 × 106, Mw/Mn ) 1.01-1.15).
Results and Discussion Figure 1a shows an optical micrograph of PS/P(MMA-CMS) (MMA/CMS ) 95/5, mol/mol) composite particles prepared by (44) Makino, T.; Tokunaga, E. Polym. Prepr. (Am. Chem. Soc., DiV. Polym. Chem.) 1998, 39 (1), 288. (45) Gaynor, S. G.; Qui, J.; Matyjaszewski, K. Macromolecules 1998, 31, 5951.
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Figure 1. Optical micrographs of PS/P(MMA-CMS) composite particles containing 5 mol % CMS prepared by toluene evaporation from PS/P(MMA-CMS)/toluene droplets (1/1/24, w/w/w) dispersed in 5 g‚L-1 SDS aqueous solution using a homogenizer (a) and an SPG membrane (b).
Figure 2. Optical micrograph of PS/(PMMA-CMS)-b-PDM composite particles prepared by seeded ATRP at 45 °C using PS/ P(MMA-CMS) particles containing 5 mol % CMS.
toluene evaporation from the polymer/toluene droplets, emulsified by homogenization, and dispersed in an aqueous solution of SDS (5 g‚L-1). The particle morphology remained the same irrespective of the CMS content in the range of 0-5.0 mol % (data not shown). Phase separation between the PS and the P(MMA-CMS) occurred within polymer/toluene droplets, resulting in hemisphere morphology during toluene evaporation. The hemisphere morphology remained until the complete removal of toluene. Beyond the SDS concentration of 2.0 g‚L-1, the hemispherical morphology of the PS/P(MMA-CMS)/toluene droplets would be thermodynamically stable. The reason for this is discussed in detail from the viewpoint of interfacial free energy in our previous study.25 When using the SPG membrane method, the size distribution of the PS/P(MMA-CMS) composite particles was narrow relative to that obtained by a homogenizer as shown in Figure 1b. Hemispherical morphology was obtained irrespective of the emulsification technique (SPG or homogenizer). PS/ P(MMA-CMS) composite particles were then used as the seed and macroinitiator in seeded ATRP for surface modification with temperature-sensitive PDM components. Because almost all chlorine groups are located at half of the composite particles, half [P(MMA-CMS) phase] of the particles should be selectively modified under these conditions. Figure 2 shows an optical micrograph of PS/P(MMA-CMS)b-PDM composite particles prepared by seeded ATRP at 45 °C (Table 2). The hemispherical morphology remained after seeded ATRP, and the particles were dispersed without coagulation. The conversion was approximately 60% by gravimetry. In addition to this, polymerizations were also carried out at 30 and 60 °C. At 30 °C, the conversion was very low (
