Branched Fluoropolymer−Si Hybrids via Surface-Initiated ATRP of

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Branched Fluoropolymer-Si Hybrids via Surface-Initiated ATRP of Pentafluorostyrene on Hydrogen-Terminated Si(100) Surfaces F. J. Xu, Z. L. Yuan, E. T. Kang,* and K. G. Neoh Department of Chemical and Biomolecular Engineering, National University of Singapore, Kent Ridge, Singapore 119260 Received May 26, 2004. In Final Form: June 28, 2004 Linear, branched, and arborescent fluoropolymer-Si hybrids were prepared via surface-initiated atom transfer radical polymerization (ATRP) from the 4-vinylbenzyl chloride (VBC) inimer and ClSO3H-modified VBC that were immobilized on hydrogen-terminated Si(100), or Si-H, surfaces. The simple approach of UV-induced coupling of VBC with the Si-H surface provided a stable, Si-C bonded monolayer of “monofunctional” ATRP initiators (the Si-VBC surface). The aromatic rings of the Si-VBC surface were then sulfonated by ClSO3H to introduce sulfonyl chloride (-SO2Cl) groups and to give rise to a monolayer of “bifunctional” ATRP initiators. Kinetics study indicated that the chain growth of poly(pentafluorostyrene) from the functionalized silicon surfaces was consistent with a “controlled” or “living” process. The chemical composition and functionality of the silicon surface were tailored by the well-defined linear and branched fluoropolymer brushes. Atomic force microscopy images revealed that the surface-initiated ATRP of pentafluorostyrene (PFS) had proceeded uniformly on the Si-VBC surface to give rise to a dense and molecularly flat surface coverage of the linear brushes. The uniformity of surfaces with branched brushes was controlled by varying the feed ratio of the monomer and inimer (VBC in the present case). The living chain ends on the functionalized silicon surfaces were used as the macroinitiators for the synthesis of diblock copolymer brushes, consisting of the PFS and methyl methacrylate polymer blocks.

1. Introduction Polymer brushes could be described as polymer chains tethered to a surface or interface with a sufficiently high grafting density.1 Tethering of polymer brushes on a solid substrate is an effective method of modifying the surface properties of the substrate.1,2 Recently, considerable attention has been paid to the manipulation and control of the physicochemical properties of single-crystal silicon surfaces because of their crucial importance to the modern microelectronics industry.2-6 Hybrid structures, consisting of dense polymer brushes immobilized on single-crystal silicon surfaces, are of importance to applications in microelectronics, sensors, and photovoltaics.3-8 In many cases, the dense polymer brushes can serve as an effective etching barrier in the microlithographic process, provide excellent mechanical and chemical protection of the substrate, alter the electrochemical interface characteristics of the substrate, and provide new pathways to the functionalization of silicon surfaces for molecular recognition and sensing.3,7-13 Well-defined hybrid structures and * To whom correspondence should be addressed. Tel: +65-68742189. Fax: +65-6779-1936. E-mail: [email protected]. (1) Milner, S. T. Science 1991, 251, 905. (2) Halperin, A.; Tirrell, M.; Lodge, T. P. Adv. Polym. Sci. 1992, 100, 31. (3) Buriak, J. M. Chem. Rev. 2002, 102, 1272. (4) Husemann, M.; Morrison, M.; Benoit, D.; Frommer, J.; Mate, C. M.; Hinsberg, W. D.; Hedrick, J. L.; Hawker, C. J. J. Am. Chem. Soc. 2000, 122, 1844. (5) Shah, R. R.; Mecerreyes, D.; Husemann, M.; Rees, I.; Abbott, N. L.; Hawker, C. J.; Hedrick, J. L. Macromolecules 2000, 33, 597. (6) Kong, X.; Kawai, T.; Abe, J.; Iyoda, T. Macromolecules 2001, 34, 1837. (7) Waltenburg, H. N.; Yates, J. T. Chem. Rev. 1995, 95, 1589. (8) Royea, W. J.; Juang, A.; Lewis, N. S. Appl. Phys. Lett. 2000, 77, 1988. (9) Sieval, A. B.; Demirel, A. L.; Nissink, J. W. M.; Linford, M. R.; Van der Maas, J. H.; Dejeu, W. H.; Zuilhof, H.; Sudholter, E. J. R. Langmuir 1998, 14, 1759. (10) Letant, S. E.; Sailor, M. J. Adv. Mater. 2001, 13, 335. (11) Sailor, M. J.; Lee, E. J. Adv. Mater. 1997, 9, 783. (12) Kwok, K. S.; Ellenbogen, J. C. Mater. Today 2002, 5, 28.

interfaces are also crucial to the development of nanolevel and molecular electronics.12 Branched polymers are of increasingly importance in surface and interface science because of their distinctive chemical and physical properties, such as low intrinsic viscosity, high solubility, good miscibility, and polyfunctionality.14 Because of these intrinsic properties, branched brushes attached to flat substrate surfaces are useful in many applications, including data storage, nanolithography, corrosion inhibition, chemical sensing, cellular engineering, surface passivation, and micrometer-scale patterning.14-19 With the progress in polymerization methods, it is possible to prepare well-defined graft polymer chains on various substrate surfaces by cationic polymerization,20 anionic polymerization,21 radical polymerization,22 nitroxide-mediated radical polymerization,23 reversible addition-fragmentation chain transfer polymerization,16 and atom transfer radical polymerization (ATRP).24-27 (13) Kato, K.; Uchida, E.; Kang, E. T.; Uyama, Y.; Ikada, Y. Prog. Polym. Sci. 2003, 28, 209. (14) Mori, H.; Mu¨ller, A. H. E. Top. Curr. Chem. 2003, 228, 1. (15) Hierlemann, A.; Campbell, J. K.; Baker, L. A.; Crooks, R. M.; Ricco, A. J. J. Am. Chem. Soc. 1998, 120, 5323. (16) Tully, D. C.; Trimble, A. R.; Fre´chet, J. M. J.; Wilder, K.; Quate, C. F. Chem. Mater. 1999, 11, 2892. (17) Tully, D. C.; Fre´chet, J. M. J. Chem. Commun. 2001, 1229. (18) Li, J.; Piehler, L. T.; Qin, D.; Baker, J. R.; Tomalia, D. A.; Meier, D. J. Langmuir 2000, 16, 5613. (19) Zhou, Y.; Bruening, M. L.; Bergbreiter, D. E.; Crooks, R. M.; Wells, M. J. Am. Chem. Soc. 1996, 118, 3773. (20) Jordan, R.; UIman, A. J. Am. Chem. Soc. 1998, 120, 243. (21) Ingall, M. D. K.; Honeyman, C. H.; Mercure, J. V.; Bianconi, P. A.; Kunz, R. R. J. Am. Chem. Soc. 1999, 121, 3607. (22) Prucker, O.; Ruehe, J. Macromolecules 1998, 31, 602. (23) 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. (24) Baum, M.; Brittain, W. J. Macromolecules 2002, 35, 610. (25) Ejaz, M.; Yamamoto, S.; Ohno, K.; Tsujii, Y.; Fukuda, T. Macromolecules 1998, 31, 5934. (26) Zhao, B.; Brittain, W. J. J. Am. Chem. Soc. 1999, 121, 3557.

10.1021/la048706k CCC: $27.50 © 2004 American Chemical Society Published on Web 08/14/2004

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ATRP is a recently developed “living” radical polymerization method, involving a copper halide/nitrogen-based ligand catalyst.28 The method does not require stringent experimental conditions, as in the case of cationic and anionic polymerization. This controlled radical polymerization technique allows for the polymerization and block copolymerization of a wide range of functional monomers, such as styrene,29,30 acrylates,31 and methacrylates32 in a controlled fashion, yielding polymers with narrowly dispersed molecular weights, predetermined by the concentration ratio of the consumed monomer to initiator.33 There are a number of reports in the literature dealing with the modification of silicon surfaces by ATRP.6,34-42 The synthetic concept for preparing branched polymers on a silicon wafer has been demonstrated in self-condensing vinyl polymerization via surface-initiated ATRP.42 For the surface-initiated ATRP, surface initiators were immobilized on an oxidized silicon surface, a hydrogenterminated silicon surface, or a monolayer-modified silicon wafer, usually in multistep processes.34-39 More recently, highly branched cationic polyelectrolytes have been prepared by self-condensing atom transfer copolymerization.43 In this work, linear, branched, and highly branched (arborescent) poly(pentafluorostyrene) (PPFS)-silicon (PPFS-Si) hybrids were prepared via surface-initiated ATRP from the 4-vinylbenzyl chloride (VBC) inimer and ClSO3H-modified VBC. VBC was covalently immobilized on a single silicon substrate by the one-step UV-induced coupling of VBC with a hydrogen-terminated Si(100) surface (Si-H surface) to give rise to the Si-VBC surface. The Si-VBC surface was then sulfonated by ClSO3H to introduce the sulfonyl chloride (-SO2Cl) group onto the aromatic rings of VBC to give rise to the Si-VBC-SO2Cl surface. Surface-initiated ATRPs of pentafluorostyrene (PFS), in the presence and absence of VBC, on the SiVBC and the Si-VBC-SO2Cl surfaces were carried out to give rise to linear, branched, and arborescent PPFSSi hybrids. The VBC inimer molecules in the reaction mixture acted both as the free initiators at the beginning of the ATRP process and as the “branching units” in the arborescent polymer (Figure 1). The chemical composition and topography of the modified silicon surfaces were characterized by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), respectively. (27) Yamamoto, S.; Ejaz, M.; Tsujii, Y.; Fukuda, T. Macromolecules 2000, 33, 5608. (28) Wang, J. M.; Matayjaszewski, K. J. Am. Chem. Soc. 1995, 117, 5614. (29) Matayjaszewski, K.; Patten, T. E.; Xia, J. J. Am. Chem. Soc. 1997, 119, 674. (30) Qiu, J.; Matayjaszewski, K. Macromolecules 1997, 30, 5643. (31) Davis, K. A.; Paik, H. K.; Matayjaszewski, K. Macromolecules 1999, 32, 1767. (32) Wang, J. L.; Grimaud, T.; Matayjaszewski, K. Macromolecules 1997, 30, 6507. (33) Coessens, V.; Pintauer, T.; Matayjaszewski, K. Prog. Polym. Sci. 2001, 26, 337. (34) Granville, A. M.; Boyes, S. G.; Akgun, B.; Foster, M. D.; Brittain, W. J. Macromolecules 2004, 37, 2790. (35) Ejaz, M.; Yamamoto, S.; Tsujii, Y.; Fukuda, T. Macromolecules 2002, 35, 1412. (36) Zhao, B.; Brittain, W. J. Macromolecules 2000, 33, 8813. (37) Yu, W. H.; Kang, E. T.; Neoh, K. G. J. Phys. Chem. B 2003, 107, 10198. (38) Zhao, B.; He, T. Macromolecules 2003, 36, 8599. (39) Boyes, S. G.; Brittain, W. J.; Weng, X.; Cheng, Z. D. Macromolecules 2002, 35, 4960. (40) Wang, J. Y.; Chen, W.; Liu, A. H.; Lu, G.; Zhang, G.; Zhang, J. H.; Yang, B. J. Am. Chem. Soc. 2002, 124, 13358. (41) Ejaz, M.; Ohno, K.; Tsujii, Y.; Fukuda, T. Macromolecules 2000, 33, 2870. (42) Matayjaszewski, K.; Xia, J. H. Chem. Rev. 2001, 101, 2921. (43) Mori, H.; Boker, A.; Krausch, G.; Mu¨ller, A. H. E. Macromolecules 2001, 34, 6871.

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Figure 1. Schematic diagram illustrating the processes of UVinduced coupling of VBC on the Si-H surface to give rise to the Si-VBC surface, the sulfonation of the Si-VBC surface by ClSO3H to give rise to the Si-VBC-SO2Cl surface, and the surface-initiated ATRP of PFS on the Si-VBC and Si-VBCSO2Cl substrates.

2. Experimental Section 2.1. Materials. (100)-Oriented single-crystal silicon, or Si(100), wafers, with a thickness of about 1.5 mm and a diameter of 150 mm, were purchased from Unisil Co. of Santa Clara, CA. The as-received wafers were polished on one side and doped lightly as n-type. The silicon wafers were cut into square chips of about 1.2 cm × 1.2 cm in size. To remove the organic residues on the surface, the silicon substrate was washed with “piranha” solution, consisting of 98 wt % concentrated sulfuric acid (70 vol %) and hydrogen peroxide (30 vol %). Caution: Piranha solution reacts violently with organic materials and should be handled very carefully! After rinsing with copious amounts of doubly distilled water, the silicon chips were dried at 70 °C in a vacuum oven for 2 h. Hydrofluoric acid (HF) (37 wt %), VBC, chlorosulfonic acid (C1SO3H), PFS, methyl methacrylate (MMA), 2,2′-byridine (Bpy), copper(I) chloride, copper(II) chloride, and anhydrous-grade dimethylsulfoxide (DMSO) were obtained from Aldrich Chemical Co. of Milwaukee, WI. PFS and MMA were distilled under reduced pressure and stored under an argon atmosphere at -10 °C. 2.2. Immobilization of the ATRP Initiators on the Si-H Surface. The pristine (oxide-covered) silicon chips were immersed in 10 vol % HF in individual Teflon vials for 2 min to remove the oxide film and to leave behind a uniform H-terminated Si(100) surface (the Si-H surface). In this work, two different types of initiators, viz., the “monofunctional” VBC (Scheme 1 in Figure 1) and the “‘bifunctional” ClSO3H-modified VBC (Scheme 2 in Figure 1), were employed for the surface-initiated ATRP. The monofunctional initiator was immobilized first via UVinduced coupling of the vinyl group of VBC with the Si-H surface to give rise to a covalently bonded (Si-C bonded) VBC monolayer (the Si-VBC surface). The bifunctional initiator was obtained via the reaction of the Si-VBC surface with ClSO3H to introduce the sulfonyl chloride (-SO2Cl) group onto the aromatic ring of VBC (the Si-VBC-SO2Cl surface). For the UV-induced coupling of VBC with the Si-H surface, a small amount of VBC was introduced onto the freshly prepared Si-H surface. The Si-H chip was sandwiched between two quartz plates, and a uniform thin liquid film of VBC formed on the Si-H surface. The assembly was placed in a Pyrex tube and subjected to UV irradiation for 30 min in a Riko RH400-10W rotary photochemical reactor (manufactured by Riko Denki Kogyo of Chiba, Japan). The reactor was equipped with a 1000 W highpressure Hg lamp and a constant-temperature bath. All UVinduced reactions were carried out at a constant temperature of

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28 °C. After UV irradiation, the silicon substrate was rinsed thoroughly with an excess amount of acetone. The VBC-coupled Si-H (Si-VBC) substrate was then dried by pumping under reduced pressure. Sulfonyl chloride, as an ATRP initiator, gives a much higher rate of initiation than the rate of monomer propagation.44 A characteristic feature of sulfonyl halides as initiators is that although they are easily generated, they dimerize to form disulfones and disproportionate only slowly. Thus, they can react with monomers and initiate the polymerization process efficiently. Well-controlled polymerizations of a large number of monomers have been reported in copper-catalyzed ATRP.41-44 The SO2Cl groups could be introduced onto the aromatic rings45-49 of VBC via sulfonation with chlorosulfonic acid (ClSO3H). The Si-VBC silicon chip was immersed in ClSO3H for about 2 min at room temperature. Caution: the reaction is very violent and should be handled very carefully. The chip was removed from ClSO3H, washed thoroughly with chloroform, and dried for about 10 h in a vacuum oven at 50 °C. The -SO2Cl group became covalently attached to the aromatic ring of VBC to give rise to the SiVBC-SO2Cl surface. 2.3. Surface-Initiated ATRP of PFS. For the preparation of the linear PFS polymer (PPFS) brushes on the Si-VBC surface or the Si-VBC-SO2Cl surface, PFS (2 mL, 14.5 mmol), CuCl (14.36 mg, 0.145 mmol), CuCl2 (3.9 mg, 0.029 mmol), and Bpy ligand (45.3 mg, 0.29 mmol) were added to 4 mL of DMSO in a Pyrex tube. The reaction mixture was stirred and degassed with argon for 30 min. The Si-VBC or Si-VBC-SO2Cl substrate was then introduced into the reaction mixture. The reaction tube was sealed and kept in an 80 °C water bath for a predetermined period of time. After the reaction, the silicon substrate with surface-grafted PPFS (the Si-g-PPFS1.1 or Si-g-PPFS2.1 surface, as shown in Figure 1) was removed from the reaction mixture and washed thoroughly with excess DMSO and acetone, in that order. The substrate was dried by pumping under reduced pressure. For the preparation of branched PPFS brushes on the SiVBC surface or the Si-VBC-SO2Cl surface, PFS (2 mL, 14.5 mmol), CuCl (14.36 mg, 0.145 mmol), Bpy ligand (45.3 mg, 0.29 mmol), and a predetermined amount of VBC were added to 4 mL of DMSO in a Pyrex tube. The reaction mixture was stirred and degassed with argon for 30 min. The Si-VBC or Si-VBC-SO2Cl substrate was introduced into the reaction mixture. The reaction tube was sealed and kept in an 80 °C water bath for a predetermined period of time. After the reaction, the silicon substrate with surface-grafted PPFS (the Si-g-PPFS1.2 or Si-gPPFS2.2 surface, as shown in Figure 1) was removed from the solution and washed thoroughly with excess DMSO and acetone, in that order. The substrate was dried by pumping under reduced pressure. One of the unique characteristics of the polymers synthesized by ATRP is the preservation of the active or living end groups throughout the polymerization reaction. To confirm the presence of active chain ends in the grafted PPFS, diblock copolymers consisting of PPFS and MMA polymer (PMMA) blocks were prepared by using the PPFS grafted on the silicon surfaces (Figure 2) as the macroinitators for the ATRP of the second monomer MMA. The PPFS-b-PMMA copolymers on the silicon surfaces with grafted PPFS were prepared under the following reaction conditions: MMA (1.6 mL, 14.5 mmol), CuCl (14.36 mg, 0.145 mmol), CuCl2 (3.9 mg, 0.029 mmol), and Bpy ligand (45.3 mg, 0.29 mmol) in 4 mL of DMSO at 80 °C for 2 h. 2.4. Surface Characterization. The chemical composition of the modified silicon surfaces was determined by XPS. The XPS measurements were performed on a Kratos AXIS HSi spectrometer using a monochromatized Al KR X-ray source (44) Matayjaszewski, K.; Xia, J. H. Chem. Rev. 2001, 101, 2921. (45) Chamoulaud, G.; Be´langer, D. Langmuir 2004, 20, 4989. (46) Holmberg, S.; Holmlund, P.; Wilen, C. E.; Kallio, T.; Sundholm, G.; Sundholm, F. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 591. (47) Fu, R. W.; Wang, F.; Tang, L. Y.; Lei, Y. Q.; Liu, N.; Lang, M. L. J. Appl. Polym. Sci. 2004, 92, 418. (48) Dale, D. J.; Dunn, P. J.; Gologhtly, C.; Hughes, M. L.; Levett, P. C.; Pearce, A. K.; Searle, P. M.; Ward, G.; Wood, A. S. Org. Proc. Res. Dev. 2000, 4, 17. (49) Yue, J.; Gordon, G.; Epstein, A. J. Polymer 1992, 33, 4410.

Xu et al.

Figure 2. Si 2p core-level spectra of (a) the pristine (oxidecovered) Si(100) surface and (b) the Si-H surface, (c) Cl 2p and (d) C 1s core-level spectra of the Si-VBC surface, and (e) C 1s and (f) S 2p core-level spectra of the Si-VBC-SO2Cl surface. (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 corelevel 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. In peak synthesis, the line width (full width at half-maximum, or fwhm) for the Gaussian peaks was maintained constant for all components in a particular spectrum. Surface elemental stoichiometries were determined from XPS spectral area ratios and were reliable to within (5%. The elemental sensitivity factors were calibrated using stable binary compounds of well-established stoichiometries. The static water contact angles of the pristine and functionalized Si-H surfaces were measured at 25 °C and 60% relative humidity, using the sessile drop method with a 3 µL water droplet, in a telescopic goniometer (Rame-Hart model 100-00-(230), manufactured by the Rame-Hart, Inc., Mountain Lakes, NJ.). The telescope with a magnification power of 23× was equipped with a protractor of 1° graduation. For each angle reported, at least three measurements from different surface locations were averaged. The angle reported was accurate to (3°. The thickness of the polymer brushes grafted on the silicon substrate was determined by ellipsometry. The measurements were carried out on a variable angle spectroscopic ellipsometer (model VASE, J.A. Woollam Inc., Lincoln, NE) at incident angles of 70° and 75° in the wavelength range of 200-1000 nm. For each sample, the thickness measurements were made on at least four different surface locations. Each thickness value reported

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Table 1. Chemical Composition and Static Water Contact Angle of the Functionalized Silicon Surfaces sample

[C-Cl]/[C]a

[S]/[C]b

[Cl]/[C]c

static water contact angle ((3°)d

Si-VBC Si-VBC-SO2Cl

1.1:9.0 (1.0:9.0) 0.99:9.0 (1.0:9.0)

0.0:0.0 0.81:9.0 (1.0:9.0)

0.98:9.0 (1.0:9.0) 1.5:9.0 (1.8:9.0)

86

a Determined from the XPS curve-fitted C 1s core-level spectra. Values in parentheses are the theoretical ratios. b Determined from the sensitivity factor corrected S 2p and C 1s core-level spectral area ratio. The value in parentheses is the theoretical ratio. c Determined from the sensitivity factor corrected XPS core-level spectral area ratios. The values in parentheses are the theoretical ratios. The value in parentheses for the Si-VBC-SO2Cl surface is based on 80% sulfonation. d Static water contact angles for the pristine (oxide-covered) Si(100) surface and the Si-H surface were about 20° and 72°, respectively. It is difficult to obtain a reliable static water contact angle for the Si-VBC-SO2Cl surface because of the hydrolysis reaction.

was accurate to (1 nm. Data were recorded and processed using the WVASE32 software package. The topography of the modified silicon surfaces was studied by AFM, using a Nanoscope IIIa AFM from Digital Instruments Inc. In each case, an area of 5 × 5 µm square was scanned using the tapping mode. The drive frequency of the equipment, with the voltage between 3 and 4.0 V, was 330 ( 50 kHz. The drive amplitude was about 300 mV, and the scan rate was 0.5-1.0 Hz. The arithmetic mean of the surface roughness (Ra) reported was calculated from the roughness profile determined by AFM.

3. Results and Discussion 3.1. Immobilization of VBC on the Si-H Surface. The chemical composition of the silicon surfaces at various stages of surface modification was determined by XPS. Two peak components at the BEs of about 99 and 103 eV, attributable to the Si-Si and Si-O species, respectively, are observed in the Si 2p core-level spectrum of the pristine (oxide-covered) silicon surface (Figure 2a). To obtain the hydrogen-terminated silicon (Si-H) surface, the pristine silicon chip (Si(100) substrate) was treated with dilute hydrofluoric acid solution to remove the native oxide layer. No oxidized silicon species in the BE region of 101-103 eV was detected in the Si 2p spectrum after the HF treatment (Figure 2b). This result confirms that the Si-H surface is predominantly hydrogen-terminated.37,50,51 For the preparation of polymer brushes on the silicon surface, a uniform and dense monolayer of initiators immobilized on the silicon surface is indispensable. In this work, both monofunctional and bifunctional ATRP initiators, based on VBC and sulfonated VBC, respectively, were immobilized on the Si-H surface for the surface-initiated ATRP (Figure 1). VBC was immobilized via UV-induced coupling on the Si-H surface to give rise to a monofunctional initiator monolayer (the Si-VBC surface). The Si-H group on the surface can be homolytically dissociated by UV to form a radical site, which reacts readily with an alkene to give rise to a surface-tethered alkyl radical on the β-carbon. The radical subsequently abstracts an H atom from the adjacent Si-H bond. The abstraction creates a new reactive silicon radical to allow the above reaction to propagate as a chain reaction on the Si-H surface.3,52 The well-established hydrosilylation of unsaturated compounds due to UV-induced homolytic cleavage of Si-H bonds dictates the formation of covalently bonded monolayers instead of surface-initiated radical polymerization. A range of alkenes and alkynes have been successfully coupled with the Si-H surface to yield alkyl and alkenyl monolayers.3 In a “hypothetical scenario” that VBC oligomers are also grafted on the silicon surface during the initiator layer deposition, only branched fluoropolymers are expected in the subsequent surface-initiated ATRP. Figure 2c,d shows, respectively, the Cl 2p and C 1s core-level spectra of the VBC-coupled Si-H surface (the Si-VBC surface). The Cl 2p core-level spectrum consists of the Cl 2p3/2 and Cl 2p1/2 peak components at the BEs of about 200.1 and 202.2 eV, respectively,53 attributable to the covalently bonded chlorine species. The

C 1s core-level spectrum of the Si-VBC surface can be curve-fitted with three peak components having BEs at about 283.9, 284.6, and 286.2 eV, attributable to the C-Si, C-H, and C-Cl species, respectively.54,55 The π-π* shakeup satellite associated with the aromatic ring of VBC is also discernible at the BE of about 291 eV. The appearance of the C-Si and C-Cl species, as well as the π-π* shakeup satellite, confirms the presence of a reactively coupled VBC layer on the Si-H surface. Table 1 summarizes the chemical compositions of the functionalized silicon surfaces. The [C-Cl]/[C] ratio, as determined from the curvefitted C 1s core-level spectrum, is about 1.1:9.0. The [Cl]/ [C] ratio, as determined from the sensitivity factor corrected XPS Cl 2p and C 1s core-level spectral area ratio, is about 0.98:9.0. These ratios are in agreement with their theoretical ratio of 1.0:9.0. Thus, UV-induced cleavage of the carbon-chloride bond of VBC probably has not occurred to a significant extent and the benzyl chloride groups have been successfully immobilized on the Si-H surface to cater for the subsequent ATRP process on the silicon surface. The initiator concentration for the surface-initiated ATRP is relatively low, compared to that for ATRP carried out in the bulk or solution. Sulfonyl chloride, as an ATRP initiator, can give a much higher rate of initiation than the rate of monomer propagation.44 To obtain a higher surface initiator concentration, a surface with bifunctional initiators was prepared by sulfonation of the Si-VBC surface with ClSO3H to introduce the sulfonyl chloride (-SO2Cl) group onto the aromatic ring46-49 of VBC (the Si-VBC-SO2Cl surface). For electrophilic substitution on the aromatic rings having both the deactivating and meta-directing substituent (-CH2Cl) and the weakly activating and ortho/para-directing substituent (-CH2CH2-),56 sulfonation will probably occur to a larger extent at the position meta to the -CH2Cl substituent. Figure 2e,f shows the respective C 1s (consisting of C-Si, C-H, C-SO2Cl, and C-Cl peak components at the BEs of about 283.9, 284.6, 285.3, and 286.2 eV)54,55 and S 2p (consisting of S 2p3/2 and S 2p1/2 peak components at the BEs of about 167.1 and 168.2 eV, respectively)56 core-level spectra of the Si-VBC-SO2Cl surface, confirming that the -SO2Cl group has been successfully immobilized on the Si-VBC surface. As shown in Table 1, the [C-Cl]/[C] ratio, as (50) Higashi, G. S.; Becker, R. S.; Chabal, Y. J. Appl. Phys. Lett. 1991, 58, 1656. (51) Higashi, G. S.; Chabal, Y. J.; Trucks, G. W.; Raghavachari, K. Appl. Phys. Lett. 1990, 56, 656. (52) Boukherroub, R.; Benseba, S. M. F.; Wayner, D. D. M. Langmuir 1999, 15, 3831. (53) Beamson, G.; Briggs, D. High-Resolution XPS of Organic Polymers: The Scienta ESCA300 Database; John Wiley: Chichester, U.K., 1992; p 278. (54) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. In X-ray Photoelectron Spectroscopy; Chastain, J., Ed.; Perkin-Elmer: Eden Prairie, MN, 1992; p 43. (55) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. In X-ray Photoelectron Spectroscopy; Chastain, J., Ed.; Perkin-Elmer: Eden Prairie, MN, 1992; p 40. (56) Morrison, R. T.; Boyd, R. E. Organic Chemistry; Allyn & Bacon: Boston, 1973; p 342.

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determined from the XPS curve-fitted C 1s core-level spectra, is about 0.99:9.0, in agreement with its corresponding theoretical ratio of 1.0:9.0. The [S]/[C] ratio, as determined from the sensitivity factor corrected S 2p and C 1s core-level spectral area ratio, is about 0.81:9.0, in comparison to the theoretical [S]/[C] ratio of 1.0:9.0 for the fully sulfonated VBC. Thus, about 80% of VBC has been sulfonated by ClSO3H. Steric effect arising from the spatial distribution of the covalently bonded VBC monolayer on the silicon surface may have limited the complete sulfonation of the monolayer by ClSO3H. The [Cl]/[C] ratio, based on a sulfonation rate of 80%, should be about 1.8: 9.0 (Table 1). However, the [Cl]/[C] ratio, as determined from the sensitivity factor corrected XPS Cl 2p and C 1s core-level spectral area ratio, is about 1.5:9.0. The deviation was probably caused by the conversion (hydrolysis) of close to 40% of the hydrophobic -SO2C1 groups into the hydrophilic -SO3H groups during sample preparation for the XPS measurement, as -SO2C1 reacts readily with moisture in air or water.46-49 Thus, for the Si-VBC-SO2Cl surface, the ratio of the aromatic rings with two initiators (-CH2Cl and -SO2Cl) to those with a single initiator (-CH2Cl) is about 1:1 to result in a surface [Cl]/ [C] ratio of 1.5:9.0. The presence of both bifunctional and monofunctional initiators has given rise to a mixture of linear and branched polymers on the silicon surface, as shown in Scheme 2 of Figure 1. 3.2. Surface-Initiated ATRP on the Si-VBC and Si-VBC-SO2Cl Surfaces. The physicochemical properties of the silicon surface can be tuned by the choice of a variety of vinyl monomers. Ultralow dielectric constant interlays are required to reduce the resistance-capacitance (RC) time delay, cross talk, and power dissipation in the new generation of high-density integrated circuits.58,59 Fluoropolymers are promising materials for interlayer dielectric application because of their low dielectric constants and good chemical and thermal stability.58-61 Most of studies have involved fluoropolymer films with ultrahydrophobic properties and low dielectric constants from plasma polymerization and deposition.58-64 In this work, fluoropolymer-silicon hybrids, consisting of linear and highly branched PFS polymers covalently attached on the silicon surface, were prepared. To prepare the hyperbranched brushes65 of PFS on the Si-VBC and Si-VBC-SO2Cl surfaces, the inimer (VBC) is added during the surface-initiated ATRP of PFS. The free VBC molecules added in the reaction mixture act both as the free initiators to rapidly establish an equilibrium between the dormant and active chains at the beginning of the ATRP (see also below) and as the branching units in the hyperbranched brushes of PFS (Scheme 1.2 and Scheme 2.2 in Figure 1). For the surfaceinitiated ATRP, the low surface initiator concentration can lead to a low concentration of the deactivating species (Cu(II) complex) being formed at the beginning of the reaction. Thus, a sufficient concentration of the deactivat(57) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. In X-ray Photoelectron Spectroscopy; Chastain, J., Ed.; Perkin-Elmer: Eden Prairie, MN, 1992; p 60. (58) Fu, G. D.; Kang, E. T.; Neoh, K. G. Adv. Mater. 2004, 16, 839. (59) Clark, D. T.; Brennan, W. J. J. Fluorine Chem. 1998, 40, 419. (60) Yu, Z. J.; Kang, E. T.; Neoh, K. G. Langmuir 2002, 18, 10221. (61) Sacher, E. Prog. Surf. Sci. 1994, 47, 273. (62) Silverstein, M. S.; Sandrin, L.; Sacher, E. Polymer 2001, 42, 4299. (63) Zhang, L.; Chin, W. S.; Huang, W.; Wang, J. Q. Surf. Interface Anal. 1999, 28, 16. (64) Zhang, Y.; Yang, G. H.; Kang, E. T.; Neoh, K. G.; Huang, W.; Huan, A. C. H.; Wu, S. Y. Langmuir 2002, 18, 6373. (65) Yan, D.; Mu¨ller, A. H. E.; Matyjaszewski, K. Macromolecules 1997, 30, 7024.

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Figure 3. C 1s and Cl 2p core-level spectra of (a,b) the Si-gPPFS1.1 surface ([PFS]/[CuCl]/[CuCl2]/[Bpy] ) 100:1:0.2:2 in DMSO at 80 °C for 2 h), (c,d) the Si-g-PPFS2.1 surface ([PFS]/ [CuCl]/[CuCl2]/[Bpy] ) 100:1:0.2:2 in DMSO at 80 °C for 2 h), (e,f) the Si-g-PPFS1.2 surface ([PFS]/[CuCl]/[VBC]/[Bpy] ) 100: 1:10:2 in DMSO at 80 °C for 2 h), and (g,h) the Si-g-PPFS2.2 surface ([PFS]/[CuCl]/[VBC]/[Bpy] ) 100:1:10:2 in DMSO at 80 °C for 2 h).

ing Cu(II) complex is necessary to quickly establish an equilibrium between the dormant and active chains at the beginning of the reaction. If this equilibrium is not controlled properly, the process resembles that of the conventional redox-initiated radical polymerization.24 The Cu(II) complex can be obtained by the reaction of Cu(I) complex with the initiator or by addition at the beginning of the reaction.66 In this work, for the preparation of the linear polymer brushes on the Si-VBC and Si-VBCSO2Cl surfaces (Scheme 1.1 and Scheme 2.1 in Figure 1), CuCl2 (the Cu(II) complex) was chosen to control the surface-initiated ATRP. For the preparation of the branched polymer brushes on the Si-VBC and Si-VBCSO2Cl surfaces (Scheme 1.2 and Scheme 2.2 in Figure 1), the free VBC added in the reaction mixture can act as the free initiator to control the equilibrium. The presence of grafted PFS polymer (PPFS) on the Si-VBC and Si-VBC-SO2Cl surfaces was confirmed by XPS analysis. Figure 3 shows the respective C 1s and Cl 2p core-level spectra of (a,b) the Si-g-PPFS1.1 surface, (c,d) the Si-g-PPFS2.1 surface, (e,f) the Si-g-PPFS1.2 surface, and (g,h) the Si-g-PPFS2.2 surface. The C 1s core-level spectra can be curve-fitted with four peak components with BEs at about 284.6, 285.4, 286.2, and 288.3 eV, attributable to the C-H, C-C6F5, C-CF/C-Cl, and C-F (66) 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.

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Table 2. Chemical Composition, Layer Thickness, and Static Water Contact Angle of the Graft-Polymerized Silicon Surfaces surface compositionsb sample d

Si-g-PPFS1.1 Si-g-PPFS2.1d Si-g-PPFS1.2e Si-g-PPFS2.2e

[Cl]/[C]a

[F]/[C]a

VBC (mol %)

PFS (mol %)

layer thicknessc ((1 nm)

static water contact angle ((3°)

0.01 0.03 0.06 0.07

4.8:8.0 5.1:8.0 4.6:8.0 4.5:8.0

3.6 0.0 7.2 9.0

96.4 100.0 92.8 91.0

6 8 24 30

102 105 98 96

a Determined from the sensitivity factor corrected XPS core-level spectral area ratios. b Obtained from the relationships [F]/[C] ) 5/8 for PFS, [F]/[C] ) 0/9 for VBC, and [F]/[C] ) 5[PFS]/(8[PFS] + 9[VBC]) for the copolymer. Thus, [VBC]/[PFS] ratios could be obtained from the [F]/[C] ratio, and surface compositions were calculated from the relationships VBC (mol %) ) [VBC]/([VBC] + [PFS]) and PFS (mol %) ) [PFS]/([VBC] + [PFS]). c The thickness of the VBC monolayer and VBC-SO2Cl monolayer on the silicon surface was about 0.3 nm. d Reaction conditions: [PFS]/[CuCl]/[CuCl ]/[Bpy] ) 100:1:0.2:2 in DMSO at 80 °C for 2 h. e Reaction conditions: [PFS]/[CuCl]/[VBC]/[Bpy] 2 ) 100:1:10:2 in DMSO at 80 °C for 2 h.

species, respectively.54,55,58-60 The Cl 2p core-level spectrum consists of the Cl 2p3/2 and Cl 2p1/2 peak components at the BEs of about 200.1 and 202.2 eV, respectively.53 The above results indicate that PPFS has been successfully grafted on the Si-VBC and Si-VBC-SO2Cl surface via surface-initiated ATRP. As shown in Figure 3, the C 1s line shape and the [Cl]/ [C] ratio of the four functionalized silicon surfaces show subtle differences, reflecting differences in the surface chemical composition. Table 2 summarizes the analysis results of the Si-g-PPFS1.1, Si-g-PPFS2.1, Si-g-PPFS1.2, and Si-g-PPFS2.2 surfaces. The [Cl]/[C] ratios of the linear PPFS brushes on the Si-g-PPFS1.1 surface, initiated by the monofunctional initiator, and on the Si-g-PPFS2.1 surface, initiated by the bifunctional initiator, are 0.01 and 0.03, respectively. For the corresponding branched PPFS brushes on the Si-g-PPFS1.2 and Si-g-PPFS2.2 surfaces, with added VBC acting as the branching unit and free initiator, the [Cl]/[C] ratios are higher (0.06 and 0.07, respectively). The substantial increase in the Cl content for the latter surfaces is consistent with the presence of highly branched or arborescent PPFS with active end groups in the side chains. The [F]/[C] ratios of the Si-gPPFS surfaces were determined from the sensitivity factor corrected F 1s and C 1s core-level spectral area ratios. The results are also shown in Table 2. For the Si-g-PPFS1.1 surface, the VBC and PFS molar contents in the graft chains are 3.6% and 96.4%, respectively. The thickness of the grafted PPFS layer at the ATRP time of 2 h is about 6 nm, which is less than the sampling depth of the XPS technique (about 7.5 nm in a fluorocarbon polymer matrix67). Thus, the underlying VBC monolayer on the silicon surface remains discernible. For the Si-g-PPFS2.1 surface, the thickness of the grafted PPFS layer at the ATRP time of 2 h is about 8 nm, which exceeds the sampling depth of the XPS technique. The surface composition is dominated completely by PPFS. The [F]/[C] ratio is about 5.1:8.0, in fairly good agreement with the corresponding theoretical ratio of 5.0:8.0 for PPFS. For the branched PPFS brushes of the Si-g-PPFS1.2 and Sig-PPFS2.2 surfaces, the corresponding values of the layer thickness, with a [VBC]/[PFS] feed ratio of 1.0:10 and at the ATRP time of 2 h, are about 24 and 30 nm, respectively. The copolymer compositions ([VBC]/[PFS] ratios) of 7.2%: 92.8% for the Si-g-PPFS1.2 and 9.0%:91.0% for the Si-gPPFS2.2 are fairly close to the monomer feed composition. The data in Table 2 also indicate that the branched brushes can give rise to a much thicker PPFS graft layer and a higher density of the functional groups (active Cl chain ends) at the surface. As shown in Table 1 and Table 2, the variation in static water contact angles for the functionalized silicon surfaces (67) Tan, K. L.; Woon, L. L.; Wong, H. K.; Kang, E. T.; Neoh, K. G. Macromolecules 1993, 26, 2832.

Figure 4. Dependence of the thickness of the grafted PPFS layer on the surface-initiated ATRP time for (a) the Si-g-PPFS1.1 surface ([PFS]/[CuCl]/[CuCl2]/[Bpy] ) 100:1:0.2:2 in DMSO at 80 °C), (b) the Si-g-PPFS2.1 surface ([PFS]/[CuCl]/[CuCl2]/[Bpy] ) 100:1:0.2:2 in DMSO at 80 °C), (c) the Si-g-PPFS1.2 surface ([PFS]/[CuCl]/[VBC]/[Bpy] ) 100:1:10:2 in DMSO at 80 °C), and (d) the Si-g-PPFS2.2 surface ([PFS]/[CuCl]/[VBC]/[Bpy] ) 100:1:10:2 in DMSO at 80 °C).

suggests that the hydrophilicity of the silicon surfaces can be readily tuned. The static water contact angle of the pristine (oxide-covered) Si(100) surface is about 20°. The water contact angles for the Si-H surface and the SiVBC surface are about 72° and 86°, respectively. It is difficult to obtain a reliable static water contact angle for the Si-VBC-SO2Cl surface because the -SO2Cl groups are converted (hydrolyzed) into the hydrophilic -SO3H groups in the presence of water. When the silicon surface is grafted with PPFS, it becomes more hydrophobic. For the Si-g-PPFS1.1 and Si-g-PPFS2.1 surfaces with the linear PPFS brushes, the water contact angle increases to over 100° (Table 2). The water contact angles of the Si-g-PPFS surfaces are thus comparable to that of the homopolymer film of PPFS (about 107°). For the Si-g-PPFS1.2 and Sig-PPFS2.2 surfaces with the branched PPFS brushes, the water contact angles are slightly smaller than those of the Si-g-PPFS1.2 and Si-g-PPFS2.2 surfaces. The presence of VBC branch units in the PPFS, as well as the high concentration of active chain ends, probably has contributed to the slight reduction in water contact angles. The kinetics of PPFS growth from the Si-VBC and SiVBC-SO2Cl surfaces via ATRP was investigated. For the Si-g-PPFS1.1 and Si-g-PPFS2.1 surfaces with the linear PPFS brushes, an approximately linear increase in thickness of the grafted PPFS layer with polymerization time is observed (as shown, respectively, by line a and line b in Figure 4). Based on the stoichiometry and structure of the initiators on the silicon surfaces (Figure

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Figure 5. AFM images of (a) the Si-VBC surface, (b) the Si-VBC-SO2Cl surface, (c,d) the Si-g-PPFS1.1 surfaces obtained at the ATRP time of 2 and 8 h, respectively, (e,f) the Si-g-PPFS2.1 surfaces obtained at the ATRP time of 2 and 8 h, respectively, (g,h) the Si-g-PPFS1.2 surfaces obtained at the ATRP time of 2 h and [PFS]/[VBC] ratios of 10 and 20, respectively, and (i,j) the Si-gPPFS2.2 surfaces obtained at the ATRP time of 2 h and [PFS]/[VBC] ratios of 10 and 20, respectively. Note: Ra of the freshly prepared Si-H surface is about 0.18 nm.

1), the film thickness of the Si-g-PPFS2.1 surface should be at least 50% larger than that of the corresponding Sig-PPFS1.1 surface, as is also suggested by the kinetics data in Figure 4. These results indicate that the chain growth from the functionalized silicon surfaces is consistent with a “controlled” or living process. For the Si-g-PPFS1.2 and Si-g-PPFS2.2 surfaces with the branched PPFS brushes, the thickness of the grafted PPFS layer increases at an even faster rate during the initial 2 h of ATRP, as shown by line c and line d, respectively, in Figure 4. According to the proposed reaction mechanism for the preparation of the branched brushes,14,42,65 the chain growth is initiated from VBC and VBC-SO2Cl immobilized on the silicon surface and in the bulk. The initiator moiety from both types of VBC can add to the double bond of another VBC or PFS to form new propagating centers. Thus, ATRP

proceeds simultaneously and rapidly on the surface and in the bulk. As a result, the increase in thickness of the grafted polymer film is no longer governed by the initial stoichiometry of the initiators on the surface. After the ATRP time of 2 h, the monomer (PFS) concentration has decreased substantially. The initially rapid rate of surfaceinitiated ATRP becomes diffusion-controlled. As a result, the thickness of the grafted PPFS layer increases only slowly after 2 h. Chain termination on the surface, followed by bimolecular coupling or disproportionation reactions that consume the active chains, may also become important with the increasing ATRP time. 3.3. Surface Topography. The changes in topography of the Si-H surface after modification by surface-initiated ATRP of PFS were investigated by AFM. Figure 5 shows the representative AFM images of (a) the Si-VBC surface,

Branched Fluoropolymer-Si Hybrids

(b) the Si-VBC-SO2Cl surface, (c,d) the Si-g-PPFS1.1 surfaces obtained at the ATRP time of 2 and 8 h, respectively, (e,f) the Si-g-PPFS2.1 surfaces obtained at the ATRP time of 2 and 8 h, respectively, (g,h) the Si-gPPFS1.2 surfaces obtained at the ATRP time of 2 h and [PFS]/[VBC] monomer feed ratios of 10 and 20, respectively, and (i,j) the Si-g-PPFS2.2 surfaces obtained at the ATRP time of 2 h and [PFS]/[VBC] monomer feed ratios of 10 and 20, respectively. The freshly prepared Si-H surface is rather uniform and smooth, with a root-meansquare surface roughness value (Ra) of about 0.18 nm. The Si-VBC and Si-VBC-SO2Cl surfaces remain molecularly uniform with Ra values of about 0.23 and 0.43 nm, respectively. After surface graft polymerization of PFS via ATRP for 2 h, the Ra values for the Si-g-PPFS1.1 and Si-g-PPFS2.1 surfaces have increased slightly to about 1.1 and 1.0 nm, respectively. The fact that the Ra values change only slightly indicates that the ATRP-mediated linear graft polymerization of the PFS has proceeded uniformly on the functionalized silicon surfaces. As shown in Figure 5c,e, the grafted PPFS chains on the modified silicon surfaces exist as a distinctive overlayer. The prolonged graft polymerization time of 8 h has given rise to a denser coverage of PPFS on the Si-VBC and SiVBC-SO2Cl surfaces, as shown in Figure 5d,f, respectively. Molecularly smooth surfaces, with the corresponding Ra values of only about 0.72 and 0.83 nm, were obtained for these two modified silicon substrates. For the Si-gPPFS1.2 and Si-g-PPFS2.2 surfaces, obtained at the ATRP time of 2 h and [PFS]/[VBC] ratio of 10, the Ra values have increased substantially to about 10.5 and 11.3 nm, respectively. The relatively large Ra values are expected from the reaction mechanism that gives rise to the “branched” brushes.14,42,65 During the relative short ATRP time of 2 h, the surface graft polymerization has given rise to dense PPFS coverages on the functionalized silicon surfaces, as shown in Figure 5g,i, respectively. The corresponding thickness values for the grafted PPFS layer on the Si-g-PPFS1.2 and Si-g-PPFS2.2 surfaces are about 24 and 30 nm, respectively. When the feed ratio of [PFS]/ [VBC] is increased to 20, the Ra values for the Si-g-PPFS1.2 and Si-g-PPFS2.2 surfaces, obtained at the ATRP time of 2 h, have decreased to about 4.4 and 5.1 nm, respectively. Thus, through the control of the [monomer]/[inimer] feed ratio, it is feasible to control the uniformity of surfaces functionalized by branched brushes from surface-initiated ATRP. The formation of the nanosized islands (“spikes”) on the surfaces probably has resulted from the nanoscale phase aggregation of the grafted polymer chains when the surfaces have been dried.37 3.4. Block Copolymer Brushes. One of the unique characteristics of polymers synthesized by ATRP is the preservation of the active end groups during the polymerization process. Thus, it is possible to synthesize welldefined block copolymers via the ATRP process. To check for the existence of living chain ends on the surface-grafted PPFS, surface-initiated ATRP is again used to synthesize the PPFS-b-PMMA diblock copolymer brushes, using the grafted PPFS as the macroinitiator. The formation of block copolymer brushes was confirmed by XPS and ellipsometry. Figure 6 shows the C 1s corelevel spectra of (a) the Si-g-PPFS1.1-b-PMMA surface, (b) the Si-g-PPFS1.2-b-PMMA surface, (c) the Si-g-PPFS2.1b-PMMA surface, and (d) the Si-g-PPFS2.2-b-PMMA surface. These surfaces were obtained under the reaction conditions of [MMA]/[CuCl]/[CuCl2]/[Bpy] ) 100:1:0.2:2 in DMSO at 80 °C for 2 h. Their starting surface compositions correspond to those described in Figure 3. The C 1s core-level spectra of the Si-g-PPFS1.1-b-PMMA

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Figure 6. C 1s core-level spectra of (a) the Si-g-PPFS1.1-bPMMA surface, (b) the Si-g-PPFS1.2-b-PMMA surface, (c) the Si-g-PPFS2.1-b-PMMA surface, and (d) the Si-g-PPFS2.2-bPMMA surface. Reaction conditions: [MMA]/[CuCl]/[CuCl2]/ [Bpy] ) 100:1:0.2:2 in DMSO at 80 °C for 2 h. The starting surface compositions correspond to those shown in Figure 3.

(Figure 6a) and Si-g-PPFS2.1-b-PMMA (Figure 6c) surfaces can be curve-fitted with four peak components having BEs at about 284.6, 285.4, 286.2, and 288.3 eV, attributable to the C-H, C-C6F5, C-CF/C-O, and C-F/OdC-O species, respectively.54,55,58-60 The thicknesses of the grafted PMMA layers for the Si-g-PPFS1.1-b-PMMA and Si-g-PPFS2.1-b-PMMA surfaces are about 6 and 7 nm, respectively, as shown in Table 3, and are less than the sampling depth of the XPS technique (about 7.5 nm in an organic matrix67). Thus, the C-C6F5, C-CF, and C-F peak components associated with the PPFS are still discernible. The composition of the functionalized silicon surfaces can be obtained from the XPS-derived [F]/[C] ratios. The results are also shown in Table 3. For the Si-g-PPFS1.1-b-PMMA and Si-g-PPFS2.1-bPMMA surfaces, the difference in the MMA and PFS composition indicates the difference in thickness of the grafted PMMA layer. For the Si-g-PPFS1.2-b-PMMA (Figure 6b) and Si-g-PPFS2.2-b-PMMA (Figure 6d) surfaces, the F signal is no longer discernible in the wide scan spectra. The corresponding thickness values of the grafted PMMA layers are about 9 and 11 nm, respectively, which exceed the sampling depth of the XPS technique. Thus, the surface composition is dominated by that of PMMA. The C 1s core-level spectra of the Si-g-PPFS1.2b-PMMA and Si-g-PPFS2.2-b-PMMA surfaces can be curvefitted with three peak components having BEs at about 284.6, 286.2, and 288.4 eV, attributable to the C-H, C-O, and OdC-O species, respectively.54,55 Comparison of the C 1s core-level spectra in Figure 6 to those in Figure 3 indicates that the intensity of the C-H peak component has increased significantly after block copolymerization of MMA. The water contact angle of the surfaces with the PPFS-b-PMMA diblock copolymer brushes has decreased to about 70° (Table 3), which is comparable to that of the Si-g-PMMA surface.37 The above results confirm that MMA has been successfully block-copolymerized on the Si-g-PPFS1.2 and Si-g-PPFS2.2 surfaces. The dormant sites at the end of the grafted PPFS chains were reactivated during the subsequent block polymerization process, resulting in the formation of block copolymer brushes on the functionalized silicon surface. The variations in surface

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Table 3. Chemical Composition, Layer Thickness, and Static Water Contact Angle of the Graft-Polymerized Silicon Surfaces surface compositionsc samplea

[F]/[C]b

MMA (mol %)

PFS (mol %)

increase in layer thicknessd ((1 nm)

static water contact angle ((3°)

Si-g-PPFS1.1-b-PMMA Si-g-PPFS2.1-b-PMMA Si-g-PPFS1.2-b-PMMA Si-g-PPFS2.2-b-PMMA

1.1:8.0 0.24:8.0 0.0:8.0 0.0:8.0

85.0 96.9 100.0 100.0

15.0 3.1 0.0 0.0

6 7 9 11

72 70 67 66

a Reaction conditions: [MMA]/[CuCl]/[CuCl ]/[Bpy] ) 100:1:0.2:2 in DMSO at 80 °C for 2 h. b Determined from the sensitivity factor 2 corrected F 1s and C 1s core-level spectral area ratios. c Obtained from the relationships [F]/[C] ) 5/8 for PFS, [F]/[C] ) 0/5 for MMA, and [F]/[C] ) 5[PFS]/(8[PFS] + 5[MMA]) for the diblock. Thus, [MMA]/[PFS] ratios could be obtained from the [F]/[C] ratio, and surface compositions were calculated from the relationships MMA (mol %) ) [MMA]/([MMA] + [PFS]) and PFS (mol %) ) [PFS]/([MMA] + [PFS]). d Surface-initiated ATRP of MMA from the Si-g-PPFS 1.1 (PPFS thickness ) 6 nm), Si-g-PPFS2.1 (PPFS thickness ) 8 nm), Si-g-PPFS1.2 (PPFS thickness ) 24 nm), and Si-g-PPFS2.2 (PPFS thickness ) 30 nm) surfaces, respectively.

composition and thickness of the grafted PMMA layer for the four silicon surfaces arise from the difference in density of the active chain ends (active Cl content) on the starting surfaces (Table 2 and Figure 3). The higher the Cl content of the starting surface, the easier it is for the surface to be functionalized by a second monomer via the surfaceinitiated ATRP. 4. Conclusions Fluoropolymer-Si hybrids consisting of linear and highly branched (arborescent) brushes of PPFS, prepared via surface-initiated ATRP of PFS from VBC and ClSO3Hmodified VBC that were covalently bonded on the hydrogen-terminated silicon surfaces, could be readily prepared. The VBC inimer added during the surfaceinitiated ATRP process acted both as a free initiator and

as a branching unit in the branched brushes. It is thus feasible to tailor the chemical functionality and molecular architecture of the silicon surface via the choice of covalently bonded initiators (e.g., monofunctional or bifunctional initiator) on the silicon surface and the addition of an inimer during the surface-initiated graft polymerization. The uniformity of surfaces with covalently attached arborescent polymer brushes can be controlled by varying the feed ratio of the monomer and the inimer (VBC in the present case). The living chain ends of the polymers on the hybrid surfaces could be used as the macroinitiators for further molecular design of the hybrid surfaces, for example, via block copolymerization with another functional monomer. LA048706K