Controlled Grafting of Comb Copolymer Brushes on Poly

Surface modification of poly(tetrafluoroethylene) (PTFE) films by well-defined comb copolymer brushes was carried out. Peroxide initiators were genera...
0 downloads 0 Views 137KB Size
Download PDF
450

Langmuir 2005, 21, 450-456

Controlled Grafting of Comb Copolymer Brushes on Poly(tetrafluoroethylene) Films by Surface-Initiated Living Radical Polymerizations W. H. Yu, E. T. Kang,* and K. G. Neoh Department of Chemical & Environmental Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore Received June 11, 2004. In Final Form: October 13, 2004 Surface modification of poly(tetrafluoroethylene) (PTFE) films by well-defined comb copolymer brushes was carried out. Peroxide initiators were generated directly on the PTFE film surface via radio frequency Ar plasma pretreatment, followed by air exposure. Poly(glycidyl methacrylate) (PGMA) brushes were first prepared by surface-initiated reversible addition-fragmentation chain transfer polymerization from the peroxide initiators on the PTFE surface in the presence of a chain transfer agent. Kinetics study revealed a linear increase in the graft concentration of PGMA with the reaction time, indicating that the chain growth from the surface was consistent with a “controlled” or “living” process. R-Bromoester moieties were attached to the grafted PGMA by reaction of the epoxide groups with 2-bromo-2-methylpropionic acid. The comb copolymer brushes were subsequently prepared via surface-initiated atom transfer radical polymerization of two hydrophilic vinyl monomers, including poly(ethylene glycol) methyl ether methacrylate and sodium salt of 4-styrenesulfonic acid. The chemical composition of the modified PTFE surfaces was characterized by X-ray photoelectron spectroscopy.

1. Introduction Surface modification of polymers via molecular design is one of the most versatile approaches to imparting new functionalities, such as improved hydrophilicity, biocompatibility, conductivity, and lubricative and adhesive properties, to the existing polymers.1-3 Surface modification of fluoropolymers, for example, poly(tetrafluoroethylene) (PTFE), has been of particular interest, because the fluoropolymers are one of the most important families of engineering polymers. They are well-known for their physical and chemical inertness.4-6 A large amount of work had been devoted to the surface modification of fluoropolymers by chemical,7 plasma,8-10 irradiation,11 corona discharge,12 flame,13 and ozone treatments.14 These approaches, though very useful, offer a somewhat limited opportunity for molecular engineering and design of the fluoropolymer surfaces. Recently, much attention has been centered on the modification of fluoropolymers via surface graft copolymerization or surface-initiated polymeriza* To whom all correspondence should be addressed. Tel.: +65-6874-2189. Fax: +65-6779-1936. E-mail address: cheket@ nus.edu.sg. (1) Zhao, B.; Brittain, W. J. Prog. Polym. Sci. 2000, 25, 667. (2) Kato, K.; Uchida, E.; Kang, E. T.; Uyama, Y.; Ikada, Y. Prog. Polym. Sci. 2003, 28, 209. (3) Uyama, Y.; Kato, K.; Ikada, Y. Adv. Polym. Sci. 1998, 137, 1. (4) Souzy, R.; Ameduri, B.; Boutevin, B. Prog. Polym. Sci. 2004, 29, 75. (5) Kang, E. T.; Zhang, Y. Adv. Mater. 2000, 12, 1481. (6) Sacher, E. Prog. Surf. Sci. 1994, 47, 273. (7) Costello, C. A.; McCarthy, T. J. Macromolecules 1987, 20, 2819. (8) Chan, C. M.; Ko, T. M.; Hiraoka, H. Surf. Sci. Rep. 1996, 24, 1. (9) Griesser, H. J.; Da, Y.; Hughes, A. E.; Gengenbach, T. R.; Mau, A. W. H. Langmuir 1991, 7, 2484. (10) Golub, M. A.; Lopata, F. S.; Finney, L. S. Langmuir 1994, 10, 3629. (11) Tian, J.; Xue, Q. J. J. Appl. Polym. Sci. 1998, 69, 435. (12) Vasilets, V. N.; Hirata, I.; Iwata, H.; Ikada, Y. J. Polym. Sci., Part A: Polym. Chem. 1998, 36, 2215. (13) Mathieson, I.; Brewis, D. M.; Sutherland, I.; Cayless, R. A. J. Adhes. 1994, 46, 49. (14) Boutevin, B.; Robin, J. J.; Serdani, A. Eur. Polym. J. 1992, 28, 1507.

tion.5,15-21 Surface graft copolymerization requires the generation of active species, such as peroxide and hydroperoxide, on the surface. These active species can be generated effectively by plasma, Ar+ beam, γ-ray, and electron beam treatments.6 Through the intelligent choice of monomers with appropriate functional groups, new molecular functionalities can be incorporated onto the activated fluoropolymer surfaces under relatively mild conditions.5 On the other hand, progress in polymerization has made it possible to produce polymer chains or brushes on a surface with controlled length and structure.1,22 Until recently, polymers of various architectures (block, comb, graft, hyperbranched, star, etc.) have been synthesized mainly by ionic polymerizations. However, ionic polymerizations usually require more stringent experimental conditions and have less tolerance for functional groups. Controlled or living radical polymerizations combine the virtues of living ionic polymerization with the versatility and convenience of free-radical polymerization.23,24 Successful examples of the living radical polymerization include nitroxide-mediated radical polymerization,25 atom transfer radical polymerization (ATRP),26,27 and reversible addition-fragmentation chain transfer (RAFT) polymer(15) Tan, K. L.; Woon, L. L.; Wong, H. K.; Kang, E. T.; Neoh, K. G. Macromolecules 1993, 26, 2832. (16) Inagaki, N.; Tasaka, S.; Goto, Y. J. Appl. Polym. Sci. 1997, 66, 77. (17) Yang, M. R.; Chen, K. S. Mater. Chem. Phys. 1997, 50, 11. (18) Becker, W.; Bothe, M.; Schmidt-Naake, G. Angew. Makromol. Chem. 1999, 273, 57. (19) Akinay, E.; Tincer, T. J. Appl. Polym. Sci. 2001, 79, 816. (20) Konig, U.; Nitschke, M.; Menning, A.; Eberth, G.; Pilz, M.; Arnhold, C.; Simon, F.; Adam, G.; Werner, C. Colloids Surf., B 2002, 24, 63. (21) Li, J. Y.; Sato, K.; Ichiduri, S.; Asano, S.; Ikeda, S.; Iida, M.; Oshima, A.; Tabata, Y.; Washio, M. Eur. Polym. J. 2004, 40, 775. (22) Edmondson, S.; Osborne, V. L.; Huck, W. T. S. Chem. Soc. Rev. 2004, 33, 14. (23) Coessens, V.; Pintauer, T.; Matyjaszewski, K. Prog. Polym. Sci. 2001, 26, 337. (24) Pyun, J.; Matyjaszewski, K. Chem. Mater. 2001, 13, 3436. (25) Hawker, C. J.; Bosman, A. W.; Harth, E. Chem. Rev. 2001, 101, 3661. (26) Matyjaszewski, K.; Xia, J. Chem. Rev. 2001, 101, 2921.

10.1021/la0485531 CCC: $30.25 © 2005 American Chemical Society Published on Web 12/07/2004

Grafting of Comb Copolymer Brushes on PTFE Films

Langmuir, Vol. 21, No. 1, 2005 451

ization.28 Preparation of well-defined polymer brushes via surface-initiated living radical polymerization has also received a considerable amount of attention in recent years.29-39 In the present work, we report on the surface modification of the PTFE film with well-defined comb copolymer brushes from a combination of surface-initiated RAFT polymerization and ATRP. Glycidyl methacrylate (GMA) was first graft-copolymerized on the Ar plasma-treated PTFE surface, via the surface-initiated RAFT polymerization technique. Ring-opening reaction of the epoxide groups of the GMA polymer [poly(glycidyl methacrylate); PGMA] brushes with 2-bromo-2-methylpropionic acid (BMPA) resulted in the immobilization of initiators for surface-initiated ATRP. Two kinds of well-defined comb copolymer brushes were prepared on the PTFE surface. 2. Experimental Section 2.1. Materials. A PTFE film having a thickness of about 0.01 cm and a density of 2.18 g/cm3 was used in this study and was obtained from Goodfellow, Inc., of Cambridge, U.K. All the chemical reagents were purchased from Aldrich Chemical Co. of Milwaukee, WI. GMA was distilled under a reduced pressure and stored in an argon atmosphere at -10 °C. Poly(ethylene glycol) methyl ether methacrylate (PEGMA) macromonomer (Mn ∼ 300) was passed through the silica gel column to remove the inhibitor. It was also stored under an argon atmosphere at -10 °C. Diethyl ether was distilled over LiAlH4 before use. Cumyl dithiobenzoate, used as the chain transfer agent (CTA), was prepared according to the published procedures.28,40 Copper(I) bromide was purified according to procedures described in the literature.31 2,2′-Bipyridine (Bpy) and other chemical reagents were used without further purification. 2.2. Argon Plasma Pretreament. PTFE films were cut into strips of about 2 cm × 4 cm in size. The surface of the PTFE film was thoroughly cleaned with acetone in an ultrasonic water bath before use. They were pretreated with Ar plasma before graft copolymerization. Argon plasma pretreatment of the cleaned PTFE films was performed between two parallel-plate aluminum electrodes of 7 cm × 13 cm in area in a glow-discharge quartz reaction chamber (model SP100) manufactured by Anatech Co. of Springfield, VA. The plasma power supply was set at 35 W at a radio frequency (RF) of 40 kHz. The strips were placed on the bottom electrode and exposed to the glow discharge at an argon flow rate of 50 sccm and a pressure of about 0.5 Torr for 90 s. This set of glow-discharge conditions had been found to be optimum for activating the PTFE surface.41,42 The plasmapretreated PTFE films were subsequently exposed to the (27) Kamigaito, M.; Ando, T.; Sawamoto, M. Chem. Rev. 2001, 101, 3689. (28) Chiefari, J.; Chong, Y. K.; Ercole, F.; Krstina, 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. (29) Ejaz, M.; Yamamoto, S.; Ohno, K.; Tsujii, Y.; Fukuda, T. Macromolecules 1998, 31, 5934. (30) Husseman, M.; Malmstro¨m, E. E.; McNamara, M.; Mate, M.; Mecerreyes, D.; Benoit, D. G.; Hedrick, J. L.; Mansky, P.; Huang, E.; Russell, T. P.; Hawker, C. J. Macromolecules 1999, 32, 1424. (31) Matyjaszewski, K.; Miller, P. J.; Shukla, N.; Immaraporn, B.; Gelman, A.; Luokala, B. B.; Siclovan, T. M.; Kickelbick, G.; Vallant, T.; Hoffmann, H.; Pakula, T. Macromolecules 1999, 32, 8716. (32) Mori, H.; Boker, A.; Krausch, G.; Mu¨ller, A. H. E. Macromolecules 2001, 34, 6874. (33) Tsujii, Y.; Ejaz, M.; Sato, K.; Goto, A.; Fukuda, T. Macromolecules 2001, 34, 8872. (34) Baum, M.; Brittain, W. J. Macromolecules 2002, 35, 610. (35) Zheng, G. D.; Sto¨ver, H. D. H. Macromolecules 2003, 36, 1808. (36) Yu, W. H.; Kang, E. T.; Neoh, K. G.; Zhu, S. J. Phys. Chem. B 2003, 107, 10198. (37) Khan, M.; Huck, W. T. S. Macromolecules 2003, 36, 5088. (38) Edmondson, S.; Huck, W. T. S. J. Mater. Chem. 2004, 14, 730. (39) Granville, A. M.; Boyes, S. G.; Akgun, B.; Foster, M. D.; Brittain, W. J. Macromolecules 2004, 37, 2790. (40) Moad, G.; Chiefari, J.; Chong, Y. K.; Krstina, J.; Mayadunne, R. T. A.; Postma, A.; Rizzardo, E.; Thang, S. H. Polym. Int. 2000, 49, 993. (41) Kang, E. T.; Tan, K. L.; Kato, K.; Uyama, Y.; Ikada, Y. Macromolecules 1996, 29, 6872.

Figure 1. Schematic diagram illustrating the processes of Ar plasma pretreatment of the PTFE surface, surface-initiated RAFT polymerization of GMA, attachment of the ATRP initiators to the PGMA brushes, and preparation of the comb copolymer brushes on the PTFE surface. atmosphere for about 20 min to affect the formation of peroxide and hydroperoxide species,16,43 which were used for the subsequent surface-initiated RAFT polymerization. The peroxides formed on the PTFE films were determined quantitatively by reaction with 2,2-diphenyl-1-picrylhydrazyl (DPPH). Three pieces of the argon plasma-treated PTFE films of 3 cm2 each were immersed in 5 mL of a 0.1 mM deaerated toluene solution of DPPH. The reaction mixture was kept at 85 °C for 6 h to decompose the peroxides formed on the PTFE film surface. The DPPH molecules consumed during reaction with the peroxides were deduced from the difference in transmittance of the reaction mixture at 520 nm, before and after the thermal decomposition of the peroxides, and the absorption calibration curve of the DPPH solutions. The calibration curve was obtained by measurement of the transmittance of six DPPH solutions in the concentration range of 0.05-0.1 mM. The peroxide concentration on the 90-s argon plasma-treated PTFE film surface was estimated to be about 0.3 units/nm2. 2.3. Surface-Initiated RAFT Polymerization on the Ar Plasma-Pretreated PTFE Substrate. For the preparation of PGMA brushes on the PTFE surface, GMA (3.13 mL, 23 mmol), and CTA (15.5 mg, 0.046 mmol) were added to 6.25 mL of dimethylformamide (DMF). The solution was stirred and degassed with argon for 20 min. One piece of the Ar plasmapretreated PTFE film and benzoyl peroxide (BPO; 2.5 mg, 0.011 (42) Yang, G. H.; Kang, E. T.; Neoh, K. G.; Zhang, Y.; Tan, K. L. Langmuir 2001, 17, 211. (43) Kuzuya, M.; Ito, H.; Kondo, S.; Noda, N.; Noguchi, A. Macromolecules 1991, 24, 6612.

452

Langmuir, Vol. 21, No. 1, 2005

mmol) were then added to the solution. The reaction flask was sealed under an argon atmosphere and kept in an 85 °C oil bath for a predetermined period of time. After the reaction, the PTFE film with surface-grafted PGMA was removed from the reaction mixture and washed thoroughly with an excess volume of acetone (PTFE-g-PGMA surface, Figure 1). The “free” PGMA formed in the solution by the free initiator was recovered by precipitating in excess methanol. 2.4. Preparation of the Comb Copolymer Brushes on the PTFE-g-PGMA Surface. ATRP initiator was attached to the PGMA brushes by reaction of the epoxy groups with BMPA. The PTFE-g-PGMA film was immersed in 10 mL of 0.6 M tetrahydrofuran (THF) solution of BMPA in a conical flask. The reaction mixture was kept at 70 °C overnight. The film was then washed thoroughly with excess amounts of acetone, prior to being dried under a reduced pressure. For the preparation of PEGMA comb copolymer brushes on the PTFE-g-PGMA-Br surfaces, PEGMA (4 mL, 14 mmol), CuBr (10 mg, 0.07 mmol), CuBr2 (3.9 mg, 0.0175 mmol), and Bpy (27.5 mg, 0.175 mmol) were added to 8 mL of doubly distilled H2O. The mixture was stirred and purged with argon for 20 min. The PTFEg-PGMA-Br substrate was then introduced into the solution. The reaction flask was sealed and kept at room temperature for 6 h. After the reaction, the PTFE film with surface grafted PGMAcb-PPEGMA copolymer brushes (the PTFE-g-PGMA-cb-PPEGMA film) was removed from the reaction mixture and washed thoroughly with an excess volume of doubly distilled water. For the preparation of poly(styrene sulfonate) comb copolymer brushes on the PTFE-g-PGMA-Br surfaces, sodium salt of 4-styrenesulfonic acid (NaSS; 2.9 g, 14 mmol), CuBr (10 mg, 0.07 mmol), CuBr2 (3.9 mg, 0.0175 mmol), and Bpy (27.5 mg, 0.175 mmol) were added to 6.4 mL of H2O. The mixture was stirred and purged with argon for 20 min. The PTFE-g-PGMA-Br substrate was introduced into the solution. The reaction flask was sealed and kept at room temperature for 6 h. After the reaction, the PTFE film with surface grafted PGMA-cb-PNaSS copolymer brushes (the PTFE-g-PGMA-cb-PNaSS surface) was removed from the reaction mixture and washed thoroughly with excess doubly distilled water. 2.5. Materials Characterization. The chemical composition of the pristine and the modified PTFE surfaces was determined by X-ray photoelectron spectroscopy (XPS). The XPS measurements were performed on a Kratos AXIS HSi spectrometer using a monochromatized Al KR X-ray source (1486.6 eV photons) at a constant dwell time of 100 ms and a pass energy of 40 eV. The samples were mounted on the standard sample studs by means of double-sided adhesive tapes. The core-level signals were obtained at a photoelectron takeoff angle (R, measured with respect to the sample surface) of 90°. The X-ray source was run at a reduced power of 150 W (15 kV and 10 mA). The pressure in the analysis chamber was maintained at 10-8 Torr or lower during each measurement. All binding energies (BEs) were referenced to the C(1s) hydrocarbon peak at 284.6 eV. Surface elemental stoichiometries were determined from the spectral area ratios, after correcting with the experimentally determined sensitivity factors, and were reliable to within (10%. The elemental sensitivity factors were calibrated using stable binary compounds of well-established stoichiometries. Monomer conversion was determined gravimetrically. The molecular weight and molecular weight distribution of the PGMA homopolymer were determined by gel permeation chromatography (GPC). GPC measurements were carried out using an HP 1100 HPLC equipped with a PLgel 5 µm MIXED-C column and a HP 1047A refractive index detector. THF was used as the mobile phase. Monodispersed polystyrene standards (Polymer Lab, Agilent Co.) with M h n values of 684 500, 48 200, 2840, and 162 were used to generate the calibration curve. Static water contact angles of the pristine and functionalized PTFE surfaces were measured at 25 °C and 60% relative humidity by the sessile drop method, using a 3-µL water droplet in a telescopic goniometer (Rame-Hart, model 100-00-(230), manufactured by the Rame-Hart, Inc., of Mountain Lakes, NJ). The telescope with a magnification power of 23× was equipped with a protractor of 1° graduation. For each sample, at least three measurements from different surface locations were averaged. Each angle reported was reliable to (3°.

Yu et al.

Figure 2. XPS C(1s) core-level spectra of (a) the pristine PTFE surface, (b) the 90-s Ar plasma-pretreated PTFE surface after exposure to air, and (c) the PTFE-g-PGMA surface from 6 h of surface-initiated RAFT polymerization.

3. Results and Discussion Well-defined comb copolymer brushes on PTFE films were synthesized according to the reaction sequence shown schematically in Figure 1: (i) the PTFE film was subjected to 90 s of RF Ar plasma pretreatment, followed by air exposure, to form the peroxide and hydroperoxide species on the surface, (ii) RAFT-mediated graft copolymerization of GMA with the Ar plasma-pretreated PTFE surface was carried out to obtain the PTFE-g-PGMA surface, (iii) attachment of the ATRP initiators to the grafted PGMA chains by reaction of the epoxy groups with BMPA (the PTFE-g-PGMA-Br surface), and (iv) surface-initiated ATRP of a hydrophilic monomer on the PTFE-g-PGMABr surface. The details involved in each reaction step are discussed below. 3.1. Surface Modification of PTFE Films by Ar Plasma Pretreatment and Subsequent RAFT-Mediated Graft Copolymerization. To prepare the polymer brushes on the PTFE surface, a uniform layer of initiators immobilized on the surface is indispensable. The C(1s) core-level spectra of the pristine PTFE film and the 90-s Ar plasma-pretreated PTFE film after air exposure are shown in Figure 2a,b, respectively. For the pristine PTFE surface, the C(1s) core-level spectrum comprises predominately a single peak component at the BE of about 291.7 eV, attributable to the CF2 species.44 The effect of Ar plasma treatment on the PTFE film under similar conditions has been reported earlier.41,42 Argon plasma treatment causes the breakage of CsF bonds, resulting in defluorination of the PTFE surface. Subsequent exposure of the activated surface to air causes oxygen to be incorporated on the PTFE surfaces, leading to surface oxidation and formation of peroxide and hydroperoxide species.16,43 The formation of peroxide species on the argon plasma-treated and air-exposed PTFE film was indicated by reaction with DPPH in toluene at 85 °C, or above the decomposition temperature of the peroxides, for 6 h (see (44) Muilenberg, G. E., Ed. Handbook of X-Ray Photoelectron Spectroscopy; Perkin-Elmer: Eden Prairie, MN, 1978; pp 38 and 94.

Grafting of Comb Copolymer Brushes on PTFE Films

Langmuir, Vol. 21, No. 1, 2005 453

Table 1. Chemical Composition and Contact Angle of the Pristine and Modified PTFE Surfaces chemical composition sample

[O]/[C]a

[F]/[C]a

[CsH]/[CsO]/[OdCsO]/[CF2]b

static water contact angle ((3°)

pristine PTFE surface 90-s Ar plasma-pretreated PTFE surface PTFE-g-PGMA surfacec PTFE-g-PGMA+ surfaced PTFE-g-PGMA-Br surface PTFE-g-PGMA-cb-PPEGMA surfacee PTFE-g-PGMA-cb-PNaSS surfaceg

0.01 0.13 0.37 0.42 0.44 0.47 0.3

1.9 1.1 0.18 0.0 0.13 0.0 0.01

0.04:0.08:0.11:1 3.6:3.0:1.0:0.7 3.1:3.0:1.0:0 4.1:2.8:1.0:1.1 5.1:10.7:1.0:0 11.8:3.0:1.0:0.1

115 65 60 60 58 35 25

a Determined from the XPS core-level spectral area ratio. b Determined from the curve-fitted C(1s) core-level spectra. c Surface-initiated RAFT polymerization from the 90-s Ar plasma-pretreated PTFE substrate: [GMA]/[CTA]/[BPO] ) 500:1:0.25 and [GMA] ) 2.5 M. Solvent, DMF; 85 °C; 6 h. d Surface-initiated RAFT polymerization from the PTFE-g-PGMA substrate ([epoxide]/[F] ) 0.7): [GMA]/[CTA]/[BPO] ) 500:1:0.25 and [GMA] ) 2.5 M. Solvent, DMF; 85 °C; 6 h. e Surface-initiated ATRP from the PTFE-g-PGMA-Br surface: [PEGMA]/ [CuBr]/[CuBr2]/[Bpy] ) 200:1:0.25:2.5 and [PEGMA] ) 1.2 M. Solvent, H2O; room temperature; 6 h. f Surface-initiated ATRP from the PTFE-g-PGMA-Br surface: [NaSS]/[CuBr]/[CuBr2]/[Bpy] ) 200:1:0.25:2.5 and [NaSS] ) 1.5 M. Solvent, H2O; room temperature; 6 h.

Experimental Section). The peroxide concentration on the 90-s Ar plasma-treated PTFE film surface was estimated at about 0.3 units/nm2. When the Ar plasma-pretreated PTFE film was kept in the same DPPH solution at room temperature, or below the decomposition temperature of the peroxides, in a control experiment, no significant change in DPPH concentration was observed after the same period of time. These results, together with the appearance of oxidized carbon species on the PTFE surface (see XPS data below), confirm the formation of peroxides on the Ar plasma-treated PTFE film after air exposure and their subsequent decomposition into peroxide radicals at an elevated temperature. Surface oxidation is also ascertained by the appearance of three new peak components at the BEs of about 286.2, 287.5, and 288.7 eV, attributable to the CsO, the CdO, and OdCsO species,44,45 in the C(1s) core-level spectrum of the Ar plasma-pretreated PTFE surface. The presence of a trace amount of sCsF species, arising from partial defluorination of the sCF2s moiety in PTFE, cannot be resolved unambiguously in the C(1s) core-level spectrum. The BE of the sCsFs species (∼288.5 eV)45 overlaps that of the OdCsO species. A peroxide-initiated mechanism for surface graft copolymerization with a vinyl monomer under UV irradiation has been suggested for the Ar plasma-pretreated PTFE surface.5,16,41 In this work, the generated peroxide initiators on the PTFE surface were used to initiate the growth of GMA polymer brushes in the presence of a RAFT CTA. Arising from the very low concentration of initiating sites on the surface, addition of a free or sacrificial initiator to the polymerization system can decrease the monomer/initiator ratio and facilitate the growth of brushes. The free initiator can act as a scavenger for the finite level of impurities present in the reaction mixture.33,34 The presence of graft-polymerized GMA on the PTFE surface (the PTFE-g-PGMA surface) is ascertained by the changes in the C(1s) core-level line shape. The C(1s) corelevel spectrum of the PTFE-g-PGMA surface, from 6 h of surface-initiated RAFT polymerization, can be curve-fitted with four peak components at the BEs of 284.6 eV for the CsH species, 286.4 eV for the CsO species, 288.7 eV for the OdCsO species, and 291.7 eV for the CF2 species,44,45 as shown in Figure 2c. The presence of a small amount of the CsS species,45 associated with the CTA at the chain end, cannot be resolved unambiguously from the dominant CsH peak component at the BE of 284.6 eV. Table 1 summarizes the chemical composition of the pristine and modified PTFE surfaces. For the GMA homopolymer, the (45) Beamson, G.; Briggs, D. High-Resolution XPS of Organic Polymers: The Scienta ESCA300 Database; John Wiley: Chichester, U.K., 1992; p 278.

theoretical [O]/[C] ratio and [CsH]/[CsO]/[OdCsO] ratio are 0.43 and 3:3:1, respectively. The corresponding ratios for the PTFE-g-PGMA surface, obtained from XPS analysis, are 0.37 and 3.6:3.0:1.0. In addition, the CF2 peak component associated with the PTFE substrate persists in the curve-fitted C(1s) core-level spectra of the PTFEg-PGMA surface. This result suggests that the thickness of the grafted PGMA layer is below the probing depth of the XPS technique (about 7.5 nm in an organic matrix).15 The carbon signal from the underlying PTFE substrates, thus, has contributed to the lowering of the [O]/[C] ratio for the PTFE-g-PGMA surface, in comparison to that of the PGMA homopolymer. The graft concentration of the PGMA brushes grown on the PTFE surface can be defined simply as the [epoxide]/[F] ratio and can be derived from the OdCsO peak component to the F(1s) spectral area ratio, because each GMA molecule has one OdCsO species and one epoxide unit. A graft concentration of about 0.7 is obtained for the GMA polymer grafted on the PTFE film via the present surface-initiated RAFT polymerization for 6 h. The water contact angle of the GMA graftcopolymerized PTFE surface is shown in Table 1. In comparison with the pristine PTFE surface, the hydrophobicity of the PTFE-g-PGMA surface has decreased substantially. Controlled experiments, under conditions similar to those for the surface-initiated RAFT polymerization on the Ar plasma-pretreated PTFE film, confirmed that RAFT-mediated graft copolymerization of GMA cannot be carried on the pristine PTFE surface. The kinetics of surface-initiated RAFT polymerization of GMA was studied by monitoring the changes in the [epoxide]/[F] ratio (the GMA graft concentration) and the variation in molecular weight (Mn) of the free polymer formed in solution as a function of time. Figure 3a shows the relationship between the graft concentration of GMA polymer brushes and the polymerization time. An approximately linear increase in the [epoxide]/[F] ratio of the PTFE-g-PGMA surface with polymerization time was observed. In addition, an approximately linear relationship between the PGMA graft concentration and the molecular weight of the “free” polymer formed in the solution was also observed when cumyl dithiobenzoate was used as the CTA (Figure 3b). Additional evidence on the controlled polymerization is provided by the “free” PGMA formed in solution from the free initiator. Figure 4a shows the linear relationship between ln([M0]/[M]) and polymerization time, where [M0] is the initial monomer concentration and [M] is the monomer concentration at time t. The result indicates that the concentration of the growing species remains constant and first-order kinetics is operative. Figure 4b shows the relationship between the number-average

454

Langmuir, Vol. 21, No. 1, 2005

Figure 3. Dependence of the graft concentration of PGMA on the PTFE film ([epoxide]/[F] ratio), from surface-initiated RAFT polymerization, on (a) the polymerization time and (b) the molecular weight (Mn) of the free PGMA formed in the solution. Reaction conditions, [GMA]/[CTA]/[BPO] ) 500:1:0.25 and [GMA] ) 2.5 M; solvent, DMF; temperature, 85 °C.

Figure 4. Relationship between (a) ln([M0]/[M]) and polymerization time and (b) Mn and monomer conversion (reaction conditions are similar to those stated in the Figure 3 caption).

molecular weight, Mn, of the “free” PGMA and the extent of conversion of the GMA monomer. The Mn of the “free” PGMA increases linearly with the increase in monomer conversion. The deviation of the linear relationship from the origin suggests that the polymerization rate decreases with reaction time. Some of the active chain ends probably

Yu et al.

have been consumed by a finite extent of chain termination via bimolecular recombination or disproportionation reaction. The lack of a sufficient number of polystyrene standards (see Experimental Section) in the molecular weight range of the synthesized polymers for calibration is another probable cause for the deviation of the kinetic curves from the expected linearity. The polydispersity index (Mw/Mn) of the free PGMA remains at around 1.21.3 throughout the reaction. Although the exact molecular weight of the polymer grafted on the PTFE surface is not known, its molecular weight is expected to be proportional to that of the homopolymer formed in the solution.34 These kinetics results indicate that the process of surfaceinitiated RAFT polymerization of GMA is consistent with a “controlled” process. According to the mechanism of RAFT polymerization, the chain ends of the polymer brushes should be terminated by the dithiobenzoate groups (see Figure 1). These chain ends remain active or “living” to mediate the RAFT polymerization again.33,34 To test the controlled behavior of the surface-initiated RAFT polymerization, a second round of RAFT polymerization was carried out. The PTFEg-PGMA substrate, with a PGMA graft concentration of about 0.7, was returned to a fresh GMA monomer solution at 85 °C for 6 h. An increase in graft concentration of PGMA was observed by XPS, as indicated by the disappearance of the CF2 species, after the second round of RAFT-mediated graft copolymerization of GMA (PTFEg-PGMA+, Table 1). 3.2. Preparation of Well-Defined Comb Copolymer Brushes on the PTFE-g-PGMA-Br by Surface-Initiated ATRP. Matyjaszewski et al. showed that the R-bromoester group is a good initiator for ATRP of styrene, methacrylate, acrylate, and some other vinyl monomers.26 In principle, all nucleophilic groups, such as sNH2, sSH, sOH, and sCOOH groups, will react readily and irreversibly with the epoxy groups. Thus, the reaction between the epoxy group of PGMA and the carboxyl functionality of BMPA was used to introduce the R-bromoester species on the PGMA brushes. Attempts were made to synthesize comb copolymer brushes on PTFE surfaces via surface-initiated ATRP of two hydrophilic vinyl monomers from the R-bromoester-functionalized PTFE-g-PGMA surface. The process is expected to substantially enhance the hydrophilicity and the density of the surface-functional groups on the PTFE films. Figure 5 shows the respective C(1s) and Br(3d) corelevel spectra of the PTFE-g-PGMA surface after reaction with BMPA for 12 h (the PTFE-g-PGMA-Br surface). A new Br(3d) core-level spectrum, with the Br(3d5/2) peak component at the BE of about 69.2 eV,44 is observed for the PTFE-g-PGMA-Br surface, indicating that the bromoester species have been successfully attached to the PGMA chains. In comparison with the C(1s) core-level spectrum of the corresponding PTFE-g-PGMA surface (Figure 2c), a slight decrease in the CsO/CsBr and OdCsO peak component areas, as well as their component area ratio, is observed in the curve-fitted C(1s) core-level spectrum of the PTFE-g-PGMA-Br surface, consistent with the coupling of BMPA with the GMA unit. The presence of the CsBr species at the BE of about 286.2 eV, associated with the BMPA coupled to the GMA units, is obscured by the CsO component. The rates of ATRP of hydrophilic monomers are known to be greatly accelerated in aqueous media.45,46 In this study, surface-initiated ATRP of two hydrophilic mono(46) Huang, W. X.; Kim, J. B.; Bruening, M. L.; Baker, G. L. Macromolecules 2002, 35, 1175.

Grafting of Comb Copolymer Brushes on PTFE Films

Langmuir, Vol. 21, No. 1, 2005 455

Figure 5. XPS C(1s) and Br(3d) core-level spectra of the PTFEg-PGMA-Br surface.

mers, namely, PEGMA and NaSS, from the PTFE-gPGMA-Br surface was performed in aqueous media at room temperature. The ATRP of PEGMA and NaSS has been widely reported.48-50 The presence of comb copolymer brushes on the PTFE surface was ascertained by surface composition analysis. The C(1s) core-level spectra of the PTFE-g-PGMA-Br surface after being subjected to 6 h of surface-initiated ATRP of PEGMA and NaSS are shown in Figure 6a,b), respectively. In comparison with the C(1s) core-level spectrum of the starting PTFE-g-PGMA-Br surface of Figure 5a, the C(1s) core-level spectra of the comb copolymer-functionalized PTFE surfaces have changed considerably after 6 h of surface-initiated ATRP. The C(1s) core-level spectra of the PTFE-g-PGMA-cb-PPEGMA surface can be curve-fitted with three peaks components having BEs at about 284.6, 286.4, and 288.7, attributable to the CsH, the CsO, the OdCsO species, respectively.44,45 The CF2 peak component has disappeared from the C(1s) core-level spectrum, indicating that the thickness of the PGMA-cb-PPEGMA comb copolymer layer is larger than the sampling depth of the XPS technique (about 7.5 nm in an organic matrix).15 In comparison with the C(1s) core-level spectrum of the PTFE-g-PGMA-cb-PPEGMA surface, a weak CF2 peak component is barely discernible in the C(1s) core-level spectra of the PTFE-g-PGMA-cbPNaSS surfaces. In addition, two new peak components at the BEs of about 168 and 1150 eV, attributable to the S(2p) core-level signal (Figure 6c) and Na(1s) core-level signal (not shown), respectively,44 of the sSO3Na species are also discernible for the PTFE-g-PGMA-cb-PNaSS surface. The minor S(2p) peak component with the S(2p3/2) of BE at about 164.5 eV, attributable to the presence of a trace amount of the sCsS and sCdS species,44 suggests the persistence of the CTAs on the PGMA chain ends. The surface analysis results of the comb copolymer brushes are summarized in Table 1. For the PTFE-gPGMA-cb-PPEGMA surface, the ratio of [CsH]/[CsO]/ [OdCsO] is about 5.1:10.7:1.0. The presence of a prominent CsO peak component is consistent with the fact that the PEGMA macromonomer contains the oligo(ethylene (47) Jones, D. M.; Huck, W. T. S. Adv. Mater. 2001, 16, 1256. (48) Wang, X. S.; Armes, S. P. Macromolecules 2000, 33, 6640. (49) Iddon, P. D.; Robinson, K. L.; Armes, S. P. Polymer 2004, 45, 759. (50) Perrier, S.; Haddleton, D. M. Macromol. Symp. 2002, 182, 261.

Figure 6. (a) C(1s) core-level spectrum of the PTFE-g-PGMAcb-PPEGMA surface and (b) C(1s) and (c) S(2p) core-level spectra of the PTFE-g-PGMA-cb-PNaSS surface. The synthesis conditions for both surfaces are given in Table 1.

Figure 7. Dependence of the graft concentration of PNaSS comb copolymer brushes on the PTFE-g-PGMA-Br surface ([S]/ ([S] + [OdCsO]) ratio) on ATRP time. Surface-initiated ATRP conditions, [NaSS]/[CuBr]/[CuBr2]/[Bpy] ) 200:1:0.25:2.5 and [NaSS] ) 1.5 M; solvent, H2O; room temperature.

glycol) side chain. For the PTFE-g-PGMA-cb-PNaSS surface, the presence of an intense CsH peak component is consistent with the fact that the NaSS monomer contains predominately hydrocarbon species. The variation in the water contact angle of the PTFE surfaces with different comb copolymer brushes indicates that the hydrophilicity of the surface can be easily tuned. After the PGMA chains have been grafted with the well-defined hydrophilic side chains, the hydrophilicity of the modified PTFE surfaces have been greatly improved. The graft concentration of the NaSS comb copolymer can be defined simply as the mole percent of the NaSS units in the total comb copolymer ([NaSS] + [GMA]) and can be calculated from the [NaSS]/([NaSS] + [GMA]) or [S]/([S] + [OdCsO]) ratio. Each GMA molecule has one epoxide ring and one OdCsO species, while each NaSS molecule has one sulfur atom. The S and OdCsO concentrations were derived, respectively, from the sensitivity-factor-corrected S(2p) core-level spectral area and OdCsO peak component area. The kinetics of the surface-

456

Langmuir, Vol. 21, No. 1, 2005

initiated ATRP of NaSS from the PTFE-g-PGMA-Br surface was studied by monitoring the changes in the [S]/([S] + [OdCsO]) ratio as a function of polymerization time. Figure 7 shows the relationship between the graft concentration of the NaSS comb copolymer brushes and the polymerization time. An approximately linear increase in the [S]/([S] + [OdCsO]) ratio with polymerization time was observed, suggesting that the growth of the NaSS side chains was a controlled process. 4. Conclusions Well-defined comb copolymer brushes on PTFE films were successfully synthesized by surface-initiated living radical polymerizations. GMA was first graft-copolymer-

Yu et al.

ized on the Ar plasma-pretreated PTFE film via the surface-initiated RAFT polymerization. The epoxy groups of the PGMA brushes were subsequently used for the coupling of the ATRP initiators via reaction with BMPA. Comb copolymer brushes on PTFE films were subsequently prepared via surface-initiated ATRP of two hydrophilic vinyl monomers, including PEGMA and NaSS. Thus, the present work has illustrated that new surface functionalities and molecular architecture arising from well-defined graft chains can be incorporated onto the inert fluoropolymer films via surface-initiated living radical polymerizations. LA0485531