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Brush-Type Amphiphilic Diblock Copolymers from “Living”/Controlled Radical Polymerizations and Their Aggregation Behavior Zhenping Cheng,†,‡ Xiulin Zhu,‡ E. T. Kang,*,† and K. G. Neoh† Department of Chemical and Biomolecular Engineering, National University of Singapore, Kent Ridge, Singapore 119260, and School of Chemistry and Chemical Engineering, Suzhou University, Suzhou 215006, People’s Republic of China Received April 18, 2005. In Final Form: June 5, 2005 Two brush-type amphiphilic diblock copolymers, poly(poly(ethylene glycol)methyl ether methacrylateblock-polystyrene) (P(PEGMA)-b-PS) and poly(glycidyl methacrylate)-block-poly(poly(ethylene glycol)methyl ether methacrylate) (P(GMA)-b-P(PEGMA)) were synthesized, respectively, via consecutive atomtransfer radical polymerizations (ATRPs) and reversible addition-fragmentation chain-transfer (RAFT) polymerizations. The diblock copolymers were characterized by gel permeation chromatography (GPC), 1H nuclear magnetic resonance (NMR) spectroscopy, and FT-IR spectroscopy. The aggregation behavior of the two amphiphilic diblock copolymers in water was also studied. Scanning electron and transmission electron microscopic images revealed that spherical micelles (40-80 nm in diameter) from self-assembly of the P(PEGMA)-b-PS copolymers and wormlike micelles (60-120 nm in length and 20-30 nm in diameter) from self-assembly of the P(GMA)-b-P(PEGMA) copolymers were prevalent. The spherical P(PEGMA)b-PS micelles could self-assemble gradually into giant aggregates of several micrometers in diameter.

Introduction Over the past decade, controlled polymerizations have evolved rapidly to improve the properties of polymeric materials. Among these, controlled radical polymerizations, atom-transfer radical polymerization (ATRP),1 and reversible addition-fragmentation chain-transfer (RAFT)2 polymerization in particular have become the most popular methods because of their adaptability to a wide range of functional monomers under less stringent experimental conditions. A number of excellent works on ATRP and RAFT polymerization have subsequently been reported.3-13 Amphiphilic block copolymers have attracted considerable attention.14-16 Depending on their architecture and * To whom all correspondence should be addressed. Telephone: +65-6874-2189. Fax: +65-6779-1936. E-mail address: [email protected]. † National University of Singapore. ‡ Soochow University. (1) Matyjaszewski, K.; Xia, J. H. Chem. Rev. 2001, 101, 2921. (2) Chiefari, J.; Chong, Y. K.; Ercole, F.; Kristina, J.; Jeffery, J.; Le, T. P. T.; Mayadunne, R. T. A.; Meijs, G. F.; Moad, C. L.; Moad, G.; Rizzardo, E.; Thang, S. H. Macromolecules 1998, 31, 5559. (3) Wang, A. R.; Zhu, S. P. Macromol. Rapid Commun. 2004, 25 (9), 925. (4) You, Y. Z.; Hong, C. Y.; Wang, W. P.; Lu, W. Q.; Pan, C. Y. Macromolecules 2004, 37, 9761. (5) Zhou, G.; Harruna. I. I. Macromolecules 2004, 37, 7132. (6) Mayadunne, R. T. A.; Jeffery, J.; Moad, G.; Rizzardo, E. Macromolecules 2003, 36, 1505. (7) Loiseau, J.; Doerr, N.; Svau, J. M.; Egraz, J. B.; Llauro M. F.; Ladaviere, C.; Claverie, J. Macromolecules 2003, 36, 3066. (8) Yusa, S.; Shimada, Y.; Mitsukami, Y.; Yamamoto, T.; Morishima, Y. Macromolecules 2003, 36, 4208. (9) Christopher, B.-K.; Davis, T. P.; Heuts, J. P. A.; Stenzel, M. H.; Vana, P.; Whittaker, M. J. Polym. Sci., Part A: Polym. Chem. 2003, 41, 365. (10) Smulders, W.; Monteiro, M. J. Macromolecules 2004, 37, 4474. (11) Pyun, J.; Kowalewski, T.; Matyjaszewski, K. Macromol. Rapid Commun. 2003, 24, 1043. (12) Wang, X.-S.; Malet, F. L. G.; Armes, S. P.; Haddleton, D. M.; Perrier, S. Macromolecules 2001, 34, 162. (13) Wang, X.-S.; Armes, S. P. Macromolecules 2000, 33, 6640. (14) Alexandridis, P., Lindman, B., Eds. Amphiphilic Block Copolymers; Elsevier: Amsterdam, 2000. (15) Moffitt, M.; Khougaz, K.; Eisenberg, A. Acc. Chem. Res. 1996, 29, 95.

chemical composition, they can self-assemble into micelles, vesicles, and a variety of other morphologies.17-21 Because of these interesting morphologies and properties, block copolymer micelle systems have played an important role in biology, colloidal science, drug and gene delivery, and synthesis of advanced materials.22-26 Block copolymers can be synthesized by a number of methods, including living ionic polymerization,27-30 RAFT polymerization,31-33 and ATRP.1,34 Poly(ethylene glycol)methyl ether methacrylate (PEGMA) is a useful macromonomer for biomedical applications due to its nonadhesive nature to (16) Hawmleg, I. W., Ed. The Physics of Block Copolymers; Oxford Science: Oxford, U.K., 1998. (17) Fo¨rster, S.; Plantenberg, T. Angew. Chem., Int. Ed. 2002, 41, 688. (18) Gao, Z.; Varshney, S. K.; Wong, S.; Eisenberg, A. Macromolecules 1994, 27, 7923. (19) Nakano, M.; Matsuoka, H.; Yamaoka, H.; Poppe, A.; Richter, D. Macromolecules 1999, 32, 697. (20) Zhang, L.; Eisenberg, A. Science 1995, 268, 1728. (21) Xu, R.; Winnik, M. A.; Riess, G.; Chu, B.; Croucher, M. D. Macromolecules 1992, 25, 644. (22) Massey, J.; Power, K. N.; Manners, I.; Winnik, M. A. J. Am. Chem. Soc. 1998, 120, 9533. (23) Savic, R.; Luo, L. B.; Eisenberg, A.; Maysinger, D. Science 2003, 300, 615. (24) Rosler, A.; Vandermeuler, G. W. M.; Klok, H. A. Adv. Drug Delivery Rev. 2001, 53, 95. (25) Jenekhe, S. A.; Chen, L. D. Science 1998, 279, 1903. (26) Yasugi, K.; Nagasaki, Y.; Kato, M.; Kataoka, K. J. Controlled Release 1999, 62, 89. (27) Hillmyer, M. A.; Schmuhl, N. W.; Lodge, T. P. Macromol. Symp. 2004, 15, 51. (28) Raghunadh, V.; Baskaran, D.; Sivaram, S. J.Polym. Sci., Part A: Polym. Chem. 2004, 42, 875. (29) Hirao, A.; Hayashi, M. Acta Polym. 1999, 50 (7), 219. (30) Aoshima, S.; Sugihara, S.; Shibayama, M.; Kanaoka, S. Macromol. Symp. 2004, 215, 151. (31) Schilli, C. M.; Zhang, M.; Rizzardo, E.; Thang, S. H.; Chong, Y. K.; Edwards, K.; Karlsson, G.; Mu¨ller, A. H. E. Macromolecules 2004, 37, 7861. (32) Albertin, L.; Stenzel, M.; Barner-Kowollik, C.; Fo¨ster, L. J. R.; Davis, T. P. Macromolecules 2004, 37, 7530. (33) Pai, T. S. C.; Barner-Kowollik, C.; Davis, T. P.; Stenzel, M. H. Polymer 2004, 5 (13), 4383. (34) Kamigaito, M.; Ando, T.; Sawamoto, M. Chem. Rev. 2001, 101, 3689.

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Figure 1. Schematic diagram illustrating the consecutive ATRP and consecutive RAFT polymerization processes for preparing the brush-type amphiphilic diblock copolymers.

proteins.35-42 Recently, brush-type copolymers based on PEGMA have been of growing interest because of their unique properties.43-49 Thus, the morphologies of their micelles from self-assembly in water should also be of interest. The interesting morphology of micelles from (35) Nakayama, Y.; Miyamura, M.; Hirano, Y.; Goto, K.; Matsuda, T. Biomaterials 1999, 20, 963. (36) Jo, S.; Park, K. Biomaterials 2000, 21, 605. (37) Wang, P.; Tan, K. L.; Kang, E. T. J. Biomater. Sci., Polym. Ed. 2000, 11, 169. (38) Zhang, F.; Kang, E. T.; Neoh, K. G.; Wang, P.; Tan, K. L. Biomaterials 2001, 22, 1541. (39) Qiu, Y. X.; Klee, D.; Plu¨ster, W.; Severich, B.; Ho¨cker, H. J. Appl. Polym. Sci. 1996, 61, 2373. (40) Cheo, S. H. Y.; Wang, P.; Tan, K. L.; Ho, C. C.; Kang, E. T. J. Mater. Sci.: Mater. Med. 2001, 12, 377. (41) Belfer, S.; Fainshtain, R.; Purinson, Y.; Gilron, J.; Nystro¨m, M.; Ma¨ntta¨ri, M. J. Membr. Sci. 2004, 239, 55. (42) Liu, Y.; Lee, J. Y.; Kang, E. T.; Wang, P.; Tan, K. L. React. Funct. Polym. 2001, 47, 201. (43) Holder, S. J.; Rossi, N. A. A.; Yeoh, C.-T.; Durand, G. G.; Boerakkerb, M. J.; Sommerdijk, N. A. J. M. J. Mater. Chem. 2003, 13, 2771. (44) Neugebauer, D.; Zhang, Y.; Pakula, T.; Sheiko, S. S.; Matyjaszewski, K. Macromolecules 2003, 36, 6746. (45) Neugebauer, D.; Zhang, Y.; Pakula, T.; Matyjaszewski, K. Polymer 2003, 44, 6863. (46) Ali, M. M.; Sto¨ver, H. D. H. Macromolecules 2004, 37, 5219. (47) Bes, L.; Angot, S.; Limer, A.; Haddleton, D. M. Macromolecules 2003, 36, 2493. (48) Han, S.; Hagiwara, M.; Ishizone, T. Macromolecules 2003, 36, 8312. (49) Ishizone, T.; Han, S.; Okuyama, S.; Nakahama, S. Macromolecules 2003, 36, 42.

brush-type amphiphilic diblock copolymers of PEGMA and benzyl methacrylate has been reported.47 In the present work, we report on the preparation of two brush-type amphiphilic diblock copolymers with P(PEGMA) as the hydrophilic brush block, viz., poly(poly(ethylene glycol)methyl ether methacrylate-block-polystyrene) (P(PEGMA)b-PS) and poly(glycidyl methacrylate)-block-poly(poly(ethylene glycol)methyl ether methacrylate) (P(GMA)-bP(PEGMA), via consecutive ATRPs and consecutive RAFT polymerizations, respectively. The morphologies of their micelles from self-assembly in water were characterized by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). Experimental Section Materials. The monomers, glycidyl methacrylate (GMA, 97%) and styrene (St, 99+%), were obtained from Aldrich Chemical Co. (Milwaukee, WI). After passing through inhibitor-removing columns (Aldrich Chemical Co.), they were stored under an argon atmosphere at -10 °C. N,N-Dimethylformamide (DMF, analytical reagent) and tetrahydrofuran (THF, analytical reagent) were obtained from Fisher Scientific Co. (Leics, U.K.). Poly(ethylene glycol)methyl ether methacrylate (PEGMA, 97%, Mn ) 1100 g‚mol-1, containing 23 ethylene glycol units (n ) 23)) was obtained from Aldrich. It was dissolved in THF and passed through an inhibitor-removing column, concentrated in a rotary evaporator, and then dried under reduced pressure. The RAFT agent, 2-cyanoprop-2-yl 1-dithionaphthalate (CPDN, structure shown in Figure 1), was synthesized according to procedures reported

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earlier.50 The purity of CPDN was greater than 96% (Waters 515 HPLC; 1H NMR (CDCl3, (ppm): δ 1.95 (s, 6H), 7.42 (m, 2H), 7.51(m, 2H), 7.85 (m, 2H), and 8.10 (m, 1H)). The initiator, R,R′azobisisobutyronitrile (AIBN, 97%), was obtained from Kanto Chemical Co. (Tokyo, Japan) and was recrystallized in anhydrous ethanol. Methyl 2-bromopropionate (MBP, 98%) and 4,4′-dinoyl2,2′-bipyridyl (dNbpy, 97%) were supplied by Aldrich and were used as received. 2,2′-Bipyridyl (bpy, 99+%) was obtained from Aldrich and was recrystallized in acetone. Copper(I) chloride (CuCl, 98+%) was supplied by Aldrich and was dissolved in hydrochloric acid, precipitated into a large amount of deionized water, filtered, washed with anhydrous ethanol, and dried under vacuum at room temperature. The dialysis tubing cellulose membrane (cellulose tubing with cutoff molecular weight of 12 400) was obtained from Sigma-Aldrich Co. (St. Louis, MO). All other solvents (reagent or HPLC grade) were obtained from Fisher Scientific or Aldrich and were used as received. ATRP of PEGMA Using MBP as the Initiator. The reaction mixture, containing 45 mg of CuCl, 213 mg of bpy, 5.0 g of PEGMA, 5.0 mL of toluene, and 51 µL of MBP, were introduced into a 10 mL dry glass tube. The mixture was purged with argon for approximately 20 min to remove the dissolved oxygen. The tube was then sealed. Polymerization was carried out at 90 °C under continuous stirring for 12 h. At the end of the polymerization reaction, the glass tube was quenched in cold water and opened, diluted with 5 mL of THF, passed through Al2O3, precipitated into 100 mL of diethyl ether, centrifuged at 5000 rpm for 10 min, and filtered. The homopolymer P(PEGMA) was dried under reduced pressure at room temperature for at least 24 h until a constant weight was obtained. The conversion of PEGMA was about 60%. Mn of P(PEGMA) ∼ 17 700, as determined from gel permeation chromatography (GPC) results. ATRP of St Using P(PEGMA) as the Macroinitiator. The procedures used for the brush-type diblock copolymerization of St were similar to those used for the ATRP of PEGMA. CuCl (10 mg, 0.100 mmol), dNbpy (82 mg, 0.200 mmol), St (3.5 mL, 4.545 mmol), and PPEGMA (1.0 g, 0.057 mmol obtained above) were dissolved in 5.0 mL of p-xylene in a 20 mL dry glass tube under stirring. The homogeneous solution was purged with argon for approximately 20 min. The glass tube was then sealed. Polymerization was carried out at 120 °C for 5 days. At the end of the polymerization reaction, the glass tube was quenched in cold water and opened, diluted with 5 mL of THF, passed through Al2O3, and precipitated into 100 mL of methanol. The copolymer, P(PEGMA)-b-PS, was dried under reduced pressure at room temperature for at least 24 h until a constant weight was obtained. The conversion of St was about 59%. Mn of P(PEGMA)-b-PS ∼ 35 600, as determined from GPC results. RAFT Polymerization of GMA Using CPDN as the RAFT Agent. For the RAFT polymerization of GMA, 5 mL (36.7 mmol) of GMA, 31.5 mg (0.183 mmol) of AIBN, and 159.7 mg (0.592 mmol) of CPDN were introduced into a 10 mL dry glass tube. The light red homogeneous solution was purged with argon for approximately 20 min. The glass tube was then sealed. Polymerization was carried out at 60 °C for 4.5 h. At the end of the polymerization reaction, the glass tube was quenched in cold water and opened, diluted with 5 mL of THF, and precipitated into 100 mL of methanol. The homopolymer P(GMA) was dried under reduced pressure at room temperature for at least 24 h until a constant weight was obtained. The conversion of GMA was about 89%. Mn of P(GMA) ∼ 6940, as determined from GPC results. RAFT Polymerization of PEGMA Using P(GMA) as the Macro-RAFT Agent. The procedures used for the block copolymerization of PEGMA were similar to those used for the RAFT polymerization of GMA. A 0.5 g (0.45 mmol) amount of PEGMA, 0.8 mg (0.005 mmol) of AIBN, and 0.1 g (0.014 mmol) of P(GMA) obtained from above were dissolved in 2.0 mL of DMF in a 10 mL dry glass tube under stirring. The homogeneous solution was purged with argon for approximately 20 min. The glass tube was then sealed. Polymerization was carried out at 85 °C for 5 days. The color of solution changed gradually from light red to light yellow. At the end of the polymerization reaction, the glass (50) Zhu, J.; Zhu, X. L.; Cheng, Z. P.; Liu, F.; Lu, J. M. Polymer 2002, 43, 7037.

Cheng et al. tube was quenched in cold water and opened, diluted with 2 mL of acetone, and precipitated into 50 mL of diethyl ether. The block copolymer, P(GMA)-b-P(PEGMA), was dried under reduced pressure at room temperature for at least 24 h until a constant weight was obtained. The conversion of PEGMA was about 91%. Mn of P(GMA)-b-P(PEGMA) ∼ 18 800, as determined from GPC results. Preparation of Micelle Solutions. (a) Brush-Type Amphiphilic Diblock Copolymer of P(PEGMA)-b-PS. The diblock copolymer (60 mg) was first dissolved in THF (3 mL) to give a 20 mg‚mL-1 polymer solution. The polymer solution (0.90 mL) was added dropwise into deionized water (19.1 mL) under stirring. The micelle solution was divided into two parts. One part of the micelle solution was dialyzed against deionized water for 4 days to remove THF. The dialysate was exchanged every 8 h. The micelle so-obtained (after dialysis) was referred to as Micelle-A. The micelle from the other part of the solution (without dialysis) was referred to as Micelle-B. The polymer concentration for both Micelle-A and Micelle-B solutions was about 0.9 mg‚mL-1. The morphology of Micelle-A and Micelle-B, and their selfassemblies, were studied by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). (b) Brush-Type Amphiphilic Diblock Copolymer of P(GMA)-b-P(PEGMA). A 2.0 mg amount of the diblock copolymer was directly dispersed in 1 mL of deionized water under agitation in an ultrasonic bath. The micelle solution with a polymer concentration of about 2.0 mg‚mL-1 was used for the subsequent morphological studies by FESEM. Characterizations. Conversion of the monomers was determined gravimetrically. Molecular weights and molecular weight distributions were measured on an HP 1100 HPLC system (GenTech Scientific, Inc., Arcade, NY), equipped with a PLgel 5 µm MIXED-C column and a HP 1047A refractive index detector. THF was used as the mobile phase at a flow rate of 1 mL‚min-1 and at 35 °C. Monodispersed polystyrene standards (Polymer Laboratories Inc., Amherst, MA) were used to generate the calibration curve. Calibration of the instrument was performed with polystyrene standards of molecular weights between 1.27 × 103 and 5 × 106 g‚mol-1. 1H nuclear magnetic resonance (NMR) spectra were measured in CDCl3 on an Bruker AMX 500 MHz spectrometer at ambient temperature. FT-IR spectra of polymers were obtained from a Bio-Rad FTS 135 FT-IR spectrophotometer (Bio-Rad Laboratories, Inc., Cambridge, MA). Each spectrum was collected by accumulating 100 scans at a resolution of 8 cm-1. FESEM images were recorded on a JEOL JSM-6700F FESEM at an accelerating voltage of 5 kV. The samples were prepared by mounting a drop (∼20 µL) of the micelle solution on a clean sheet of copper and allowing the samples to dry under reduced pressure. A thin layer of platinum was sputter-coated on the sample for charge dissipation during FESEM imaging. TEM image was recorded on a JEOL JEL2010 transmission electron microscope at 200 kV. The sample was prepared by mounting a drop (∼20 µL) of the micelle solution on the carboncoated Cu grids and allowing the samples to dry in air.

Results and Discussion Synthesis of the Brush-Type Amphiphilic Diblock Copolymers. A number of excellent works on the ATRP of PEGMA with molecular weights in the range of 300475 g‚mol-1 have been reported.11-13 In the present work, PEGMA with an average molecular weight of 1100 g‚mol-1 was selected as the macromonomer. Two controlled free radical polymerization techniques, ATRP and RAFT polymerization, were employed to synthesize two separate brush-type amphiphilic diblock copolymers containing the P(PEGMA) hydrophilic brushes. The two synthetic pathways are shown schematically in Figure 1. The brushtype amphiphilic diblock copolymer, P(PEGMA)-b-PS, was prepared by two consecutive ATRPs (Figure 1a): (i) synthesis of well-defined P(PEGMA) via ATRP of PEGMA, using MBP as the initiator and CuCl/bpy as the catalyst system, and (ii) synthesis of the well-defined diblock copolymer, P(PEGMA)-b-PS, via ATRP of St, using the P(PEGMA) obtained above as the macroinitiator and CuCl/

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Figure 2. GPC traces of (a) P(PEGMA), (b) P(PEGMA)-b-PS, (c) P(GMA), and (d) P(GMA)-b-(P(PEGMA).

Figure 3. 1H NMR spectra of (a) P(PEGMA)-b-PS and (b) P(GMA)-b-P(PEGMA).

dNbpy as the catalyst system. The other brush-type amphiphilic diblock copolymer, P(GMA)-b-P(PEGMA), was prepared by two consecutive RAFT polymerizations (Figure 1b): (i) synthesis of well-defined P(GMA) via RAFT polymerization of GMA, using CPDN as the RAFT agent, and (ii) synthesis of the well-defined diblock copolymer, P(GMA)-b-P(PEGMA), via RAFT polymerization of PEGMA, using the P(GMA) obtained above as the macro-RAFT agent. The size, structure, and composition of the polymers and copolymers were studied by GPC, FT-IR, and 1H NMR spectroscopy. Figure 2 shows the GPC traces of (a) P(PEGMA), (b) P(PEGMA)-b-PS, (c) P(GMA), and (d) P(GMA)-b-(P(PEGMA). The GPC traces of the four samples show a monomodal distribution. Parts a and b of Figure 3 show the respective 1H NMR spectra of the P(PEGMA)-b-PS and P(GMA)-b-(P(PEGMA) copolymers. The chemical shifts at 3.38 ppm (a) and 3.60-4.20 ppm (b) in Figure 3a can be assigned, respectively, to the protons of methoxyl group (-OCH3) and methylene protons (-OCH2-CH2-) of the pendant poly(ethylene glycol) (PEG) brushes of P(PEGMA).43,47,51,52 The chemical shifts at 6.45-7.09 ppm can be assigned to the aromatic protons (c in Figure 3a) of the PS block. The chemical shifts at δ ) 3.23 ppm (d) and δ ) 2.63 and 2.84 ppm (e) in Figure 3b can be assigned to protons of the oxirane ring of the P(GMA) block. The two protons labeled e are in different chemical environments and consequently give rise to two different resonances.53 These results indicate that the epoxide groups remain intact in the diblock copolymer throughout the RAFT polymerizations. The chemical shifts at 3.38 ppm (f in Figure 3b) can also be assigned to the protons of (51) Hester, J. F.; Banerjee, P.; Won, Y.-Y.; Akthakul, A.; Acar, M. H.; Mayes, A. M. Macromolecules 2002, 35, 7652. (52) Chen, Y. W.; Wang, W. C.; Yu, W. H.; Kang, E. T.; Neoh, K. G.; Vora, R. H.; Ong, C. K.; Chen, L. F. J. Mater. Chem. 2004, 14, 1406. (53) Zhu, J.; Zhou, D.; Zhu, X. L.; Chen, G. J. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 2558.

Figure 4. IR spectra of (a) P(PEGMA)-b-PS, (b) PS, (c) P(GMA)b-P(PEGMA), (d) P(GMA), and (e) P(PEGMA).

methoxyl group (-OCH3) of the pendant PEG brushes of P(PEGMA).47 Figure 4 shows the FT-IR spectra of (a) P(PEGMA)-bPS, (b) PS, (c) P(GMA)-b-P(PEGMA), (d) P(GMA), and (e) P(PEGMA). Comparison of the spectra in Figure 4a, b,e suggests that the absorption bands at 759 and 699 cm-1 (characteristic of CH out-of-plane vibration of the benzene ring)54 in Figure 4a are consistent with the presence of PS block in the brush-type amphiphilic diblock copolymer. The characteristic absorption band at 1731 cm-1 (attributable to CdO stretching vibration) in Figure 4a, on the other hand, is associated with the P(PEGMA) brushes in the P(PEGMA)-b-PS copolymer. Similarly, comparison of the spectra in Figure 4c-e suggests that the characteristic absorption bands at 1731 cm-1 (CdO stretching vibration) and 1100 cm-1 (C-O-C stretching vibration)38,55 in Figure 4c are consistent with the presence of P(PEGMA) brushes and P(GMA) block in the (P(GMA)b-P(PEGMA) copolymer. Thus, the chemical shifts in the 1 H NMR spectra and the characteristic FT-IR absorption bands are consistent with the respective structures of the brush-type amphiphilic diblock copolymers, P(PEGMA)b-PS and P(GMA)-b-P(PEGMA). The number-average molecular weights (Mn’s) and the polydispersity indices (PDI’s) of the corresponding polymers and copolymers, deduced from both GPC and 1H NMR spectroscopy results, are listed in Table 1. The Mn, deduced from GPC results, of the P(PEGMA)-b-PS diblock copolymer increases to 35 600, from 17 700 for the corresponding P(PEGMA) homopolymer. The Mn of the P(GMA)-b-P(PEGMA) diblock copolymer increases to 18 800, from 6940 for the corresponding P(GMA) homopolymer. In addition, entry 3 in Table 1 shows that the Mn’s of 6940 and 6700 g‚mol-1 for the P(GMA), deduced, (54) Zou, M.; Zhang, Z.; He, W.; Ge, X.; Fan, F. Polym. Int. 2004, 53, 1033. (55) Dean, J. A. Analytical Chemistry Handbook; McGraw-Hill: New York, 1995; p 61.

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Table 1. Molecular Weights and Polydispersity Indices of the Polymers and Copolymers DPd Entry 1 2 3 4

Sample P(PEGMA) P(PEGMA)-b-PS P(GMA) P(GMA)-b-P(PEGMA)

a

Mn

17 700 35 600 6 940 18 840

b

Mn -(1)

60 000(2) 6 700(3) 39 700(4)

c

Mn

6 560 50 600 7 750 37 200

PDIa

FBe

1.27 1.66 1.34 1.42

16(5) 16(5) 47(6) 47(6)

SBf 407(7) 30(7)

a From GPC results, calibrated with PS standards. b From 1H NMR spectroscopy results. (1)Cannot be determined because of the overlap of chemical shifts of the initiator (MBP) and P(PEGMA) polymer. (2)Calculated from the ratio of protons of the methoxyl group of the P(PEGMA) brushes to the aromatic protons of the PS block. (3)Calculated from the ratio of aromatic protons of the RAFT agent (CPDN) to protons of the oxirane ring in the P(GMA) homopolymer. (4)Calculated from the ratio of protons of the oxirane ring in the P(GMA) block to protons of the methoxyl group in the P(PEGMA) brushes. c Theoretical value calculated from the molar ratio of monomer to initiator for the ATRP process or from the molar ratio of the monomer to RAFT agent for the RAFT process. d Degree of polymerization. e DP of the first block (FB) or the initial homopolymer. (5)Calculated from GPC results. (6)Calculated from NMR spectroscopy results. f DP of the second block (SB). (7)Calculated from NMR spectroscopy results.

respectively, from GPC and 1H NMR spectroscopy results, are in fairly good agreement with its theoretical value of 7750. For entries 2 and 4 in Table 1, the Mn’s of 60 000 for the P(PEGMA)-b-PS copolymer and 39 700 for the P(GMA)-b-P(PEGMA) copolymer, deduced from 1H NMR spectroscopy results, are also in fairly good agreement with the corresponding theoretical values of 50 600 and 37 200. However, they are much larger than the corresponding Mn’s of 35 600 and 18 800, deduced from GPC results. The deviations were probably caused by the fact that the hydrodynamic volumes of the brush-type copolymers probably differ substantially from those of the linear polystyrene standards. Based on the Mn’s estimated from GPC and 1H NMR spectroscopy results, the degrees of polymerization (DP’s) of the two amphiphilic diblock copolymers can also be calculated. The results are listed in Table 1. The PDI’s from GPC results for P(PEGMA)16, P(GMA)47, and P(GMA)47-b-P(PEGMA)30 remain less than 1.5. However, the PDI has broadened to 1.66 for the P(PEGMA)16-b-PS407 copolymer (Table 1). Aggregation Behavior of the Brush-Type Amphiphilic Diblock Copolymers in Water. A number of linear block copolymer amphiphiles have been reported previously. Depending on their molecular architecture and chemical composition, they can self-assemble into micelles, vesicles, and a variety of other morphologies.17-21,56,57 As for the brush-type amphiphilic block copolymers, a few excellent studies on the morphologies of micelles from self-assembly in water have been reported.43,47,58-60 Figure 5 shows the FESEM and TEM images of the morphologies of self-assembled micelles in water from the two brushtype amphiphilic diblock copolymers, P(PEGMA)16-b-PS407 and P(GMA)47-b-P(PEGMA)30. Nanosized spherical micelles (Micelle-A, after dialysis to remove THF, Figure 5a) are discernible for the P(PEGMA)16-b-PS407 diblock copolymer, while nanosized wormlike (cylindrical) micelles58 (Figure 5b) are observed for the P(GMA)47-bP(PEGMA)30 diblock copolymer. The diameter of the spherical micelles from the self-assembled P(PEGMA)16b-PS407 copolymers in water varied from about 40 to 80 nm, while the sizes of the wormlike micelles self-assembled from the P(GMA)47-b-P(PEGMA)30 copolymers was about 60-120 nm in length and 20-30 nm in diameter. P(PEGMA) brushes are water-soluble, while PS and P(GMA) blocks are hydrophobic. Thus, the self-assembled spherical micelles probably contain a hydrophobic PS core (56) Zhang, L.; Eisenberg, A. Polym. Adv. Technol. 1998, 9, 677. (57) Soo, P. L.; Eisenberg, A. J. Polym. Sci., Part B: Polym. Phys. 2004, 42, 923. (58) Dean, J. M.; Verghese, N. E.; Pham, H. Q.; Bates, F. S. Macromolecules 2003, 36, 9267. (59) Vriezema, D. M.; Kros, A.; de Gelder, R.; Cornelissen, J. J. L. M.; Rowan, A.E.; Nolt, R. J. M. Macromolecules 2004, 37, 4736. (60) Cloucair, A.; Eisenberg, A. Eur. Phys. J. E 2003, 10, 37.

Figure 5. FESEM images of the self-assembled micelles in water from (a) the P(PEGMA)16-b-PS407 copolymer after dialysis to remove the organic solvent (THF) and from (b) the P(GMA)47b-P(PEGMA)30 copolymer; TEM images of the self-assembled micelles in water from the P(PEGMA)16-b-PS407 copolymer (c) with and (d) without dialysis; FESEM images of the aggregated micelles from the P(PEGMA)16-b-PS407 copolymer without dialysis after the micelle solution was allowed to stand for (e) 86 and (f) 147 days.

surrounded by a hydrophilic P(PEGMA) corona, while the wormlike micelles are probably composed of long, thin tubes of P(GMA) stabilized by a corona of the P(PEGMA) brushes. Micelle formation from amphiphilic block copolymers in aqueous medium probably involves the stretching or deformation of the core-forming blocks in the core, the surface tension between the micelle core and

Brush-Type Amphiphilic Diblock Copolymers

the solvent outside the core, and the intercorona chain interactions.20 Thus, arising from the relatively short hydrophobic P(GMA) block and longer hydrophilic P(PEGMA) block in the P(GMA)47-b-P(PEGMA)30 copolymer, in comparison with the sizes of the corresponding hydrophobic and hydrophilic block in the P(PEGMA)16-b-PS407 copolymer, the stretched or less entangled hydrophobic P(GMA) blocks in the former copolymer will tend to assemble into a cylindrical (wormlike) core stabilized by a water-miscible corona of longer P(PEGMA) chains. A similar transformation from spherical micelle to wormlike micelle has also been observed for the diblock copolymer modifiers in epoxy resin matrixes, when the ratio of the epoxy-immiscible block to the epoxy-miscible block in the block copolymer modifiers was decreased.58 The P(PEGMA)16-b-PS407 spherical micelles in the presence (Micelle-A, Figure 5c) and absence (Micelle-B, Figure 5d) of dialysis to remove THF were studied by TEM. Fairly uniform nanosized spheres are discernible. The relatively long hydrophobic PS block in the present copolymer probably does not favor the observation of the micelles in the form of vesicles. The morphology of micelles from the self-assembled amphiphilic block copolymers is dependent on a number of experimental conditions, such as molecular weight and block length, the presence of the organic cosolvent of the block copolymer, and the storage time of the micelles.59,60 It has been reported that immediately after addition of the THF solution of polystyrene-b-poly(L-isocyanoalanine-(2-thiophen-3-ylethyl)amide) (PS40-b-PIAT50) into water, very small vesicles (