Adsorption of Well-Defined Fluorine-Containing Polymers onto Poly

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Langmuir 2008, 24, 13075-13083

13075

Adsorption of Well-Defined Fluorine-Containing Polymers onto Poly(tetrafluoroethylene) Shuko Suzuki,†,‡ Michael R. Whittaker,§ Edeline Wentrup-Byrne,*,† Michael J. Monteiro,*,§ and Lisbeth Grøndahl*,‡ Tissue Repair and Regeneration Program, Institute of Health and Biomedical InnoVation and the School of Physical and Chemical Sciences, Queensland UniVersity of Technology, Brisbane QLD 4001, Australia, School of Molecular and Microbial Sciences, UniVersity of Queensland, Brisbane QLD 4072, Australia, and Australian Institute of Bioengineering and Nanotechnology, UniVersity of Queensland, Brisbane QLD 4072, Australia ReceiVed July 18, 2008. ReVised Manuscript ReceiVed September 10, 2008 Adsorption of well-defined fluorinated polymers onto clinically relevant poly(tetrafluoroethylene) (PTFE) substrates offers an attractive method for modifying the surface properties of chemically inert PTFE. Reversible additionfragmentation chain transfer (RAFT) was successfully used for synthesis of the polymers in this study: the homopolymers poly(2,3,4,5,6-pentafluorostyrene) (PFS), poly(2,2,3,3-tetrafluoropropyl acrylate) (PTFPA), and poly(2,2,3,3-tetrafluoropropyl methacrylate) (PTFPMA) as well as their block copolymers with tert-butyl acrylate (tBA). Watersoluble blocks were synthesized through the hydrolysis of the t-butyl side groups of P(tBA) to the corresponding carboxylic acid. Adsorption of selected polymers onto PTFE from a series of solvents (methyl ethyl ketone (MEK), dimethylformamide (DMF), fluorobenzene (FB), dichloromethane (DCM)) was investigated using X-ray photoelectron spectroscopy (XPS) and sessile water drop measurements. The three homopolymers studied all adsorbed irreversibly (i.e., were not removed by washing) from organic solvents at ambient temperature. PFS displayed the highest adsorption, and was attributed to strong hydrophobic interactions. From angle-resolved XPS it was concluded that PFS became impregnated into the PTFE substrate down to depths of 100 Å when using FB as a solvent. The carboxylic acidcontaining block copolymers adsorbed more effectively from DMF (a good solvent for the poly(acrylic acid) block) compared to MEK. The resulting modified PTFE substrates displayed high stability with respect to desorption in aqueous solution, yet conformational changes of the adsorbed polymer resulted in a switchable hydrophobic-hydrophilic surface (in air or water, respectively). These results highlight the success of a facile and simple approach to irreversibly adsorb functional polymers to a nonfunctional fluorinated surface.

Introduction Fluoropolymers are well-known for their unique characteristics resulting from their chemical resistance1 to acids, bases and other solvents,2,3 low dielectric constants,3 and good thermal stability.1 This results in their usefulness in a broad range of applications, including low energy surface coatings,1,3,4 biomedical and drug delivery applications,5 and separation and microfiltration of proteins.6 The inert nature of fluoropolymers in a biological setting was originally considered an advantage. However, over the last * Corresponding authors. E-mail: [email protected]; telephone: (617) 3365 3671; fax: (617) 3365 4299; address: Chemistry building, School of Molecular and Microbial Sciences, University of Queensland, Brisbane QLD 4072, Australia (L.G.). E-mail: [email protected]; telephone: (617) 3346 4164; fax: (617) 3346 3973; address: Australian Institute of Bioengineering and Nanotechnology, University of Queensland, Brisbane QLD 4072, Australia (M.J.M). E-mail: [email protected]; telephone: (617) 3138 1226; fax: (617) 31381804; address: Tissue Repair and Regeneration Program, Institute of Health and Biomedical, Institute of Health and Biomedical Innovation and the School of Physical and Chemical Sciences, Queensland University of Technology, Brisbane QLD 4001, Australia (E.W.-B.). † Queensland University of Technology. ‡ School of Molecular and Microbial Sciences, University of Queensland. § Australian Institute of Bioengineering and Nanotechnology, University of Queensland. (1) Meussdoerffer, J. N.; Niederpriim, H. Chem. Zeitung 1980, 104, 45–48. (2) Fu, G. D.; Yuan, Z.; Kang, E. T.; Neoh, K. G.; Lai, D. M.; Huan, A. C. H. AdV. Funct. Mater. 2005, 15(2), 315–322. (3) Zang, Z.-B.; Ying, S.-K.; Shi, Z.-Q. Polymer 1999, 40, 5439–5444. (4) Li, K.; Wu, P.; Han, Z. Polymer 2002, 43, 4079–4086. (5) Zhang, Z.; Ying, S.; Zhang, Q.; Xu, X. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 2670–2676. (6) Saito, K. Sep. Sci. Technol. 2002, 37, 535–538.

two decades, an increasing emphasis has been on designing biocompatible surfaces through the adsorption and adhesion of bioactive molecules (e.g., heparin7 or cell adhesive peptides or proteins8). The chemical inertness of fluoropolymers makes chemical modification difficult, and often harsh surface modification techniques including ion-implantation,9 plasma treatment,10 radiation-induced grafting using either gamma-irradiation or plasma-activation are required.11 More recently, polymers have been grown from chemically modified surfaces using “living” radical polymerization techniques such as atom transfer radical polymerization (ATRP)12 or reversible additionfragmentation chain transfer (RAFT).13 (7) Hoffman, J.; Larm, O.; Scholander, E. Carbohydr. Res. 1983, 117, 328– 331. (8) Dekker, A.; Reitsma, K.; Beugeling, T.; Bantjes, A.; Feijen, J.; Vanaken, W. G. Biomaterials 1991, 12, 130–138. (9) Colwell, J. M.; Wentrup-Byrne, E.; Bell, J. M.; Wielunski, L. S. Surf. Coat. Technol. 2003, 168(2-3), 216–222. (10) (a) Chen, J.-R.; Wakida, T. J. Appl. Polym. Sci. 1997, 63, 1733–1739. (b) Ryan, M. E.; Badyal, J. P. S. Macromolecules 1995, 28, 1377–1382. (c) Wilson, D. J.; Eccles, A. J.; Steel, T. A.; Williams, R. L.; Pond, R. C. Surf. Interface Anal. 2000, 30, 36–39. (d) Yamada, Y.; Yamada, T.; Tasaka, S.; Inagaki, N. Macromolecules 1996, 29, 4331–4339. (e) Biederman, H.; Zeuner, M.; Zalman, J.; Bilkova, P.; Slavinska, D.; Stelmasuk, V.; Boldyreva, A. Thin Solid Films 2001, 392, 208–213. (11) (a) Dargaville, T. R.; George, G. A.; Hill, D. J. T.; Whittaker, A. K. Prog. Polym. Sci. 2003, 28, 1355–1376. (b) Grøndahl, L.; Cardona, F.; Chiem, K.; Wentrup-Byrne, E. J. Appl. Polym. Sci. 2002, 86, 2550–2556. (c) WentrupByrne, E.; Grøndahl, L.; Suzuki, S. Polym. Int. 2005, 54, 1581–1588. (12) Liu, Y. L.; Luo, M. T.; Lai, J. Y. Macromol. Rapid Commun. 2007, 28, 329–333. (13) Yoshikawa, C.; Goto, A.; Tsujii, Y.; Fukuda, T.; Yamamoto, K.; Kishida, A. Macromolecules 2005, 38, 4604–4610.

10.1021/la802300q CCC: $40.75  2008 American Chemical Society Published on Web 10/17/2008

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The irreversible adsorption of homo- and block copolymers onto bulk fluoropolymer films offers an alternative approach to modify surfaces. Marchant and co-workers14 investigated the physisorption of fluorocarbon surfactant polymers onto poly(tetrafluoroethylene) (PTFE) in water, and found that increased length of the fluorocarbon moiety in the polymer backbone showed a monotonic increase in the amount of adsorbed surfactant. Low surface energy fluorinated polymers can adsorb polymers in water as long as there is a reduction in the interfacial free energy between the polar solvent and the fluorinated surface. The driving force for preferential adsorption is through longrange hydrophobic attractive forces,15 which can extend to 100 nm.16 For example, poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) was surface-modified through the spontaneous adsorption of poly(L-lysine)17 from an aqueous solution. “Living” radical polymerization has provided a synthetic route to homo- and block copolymers with well-defined chain lengths and chain length distributions. ATRP has produced many welldefined fluorinated polymers18 ranging from pentafluorostyrene19,20 to a series of acrylates with fluorinated side chains.3,21 The molecular weight distributions were in most cases narrow, with polydispersity indexes ranging between 1.1 and 1.3. Surprisingly, there are only a few corresponding examples using RAFT polymerizations.18 Pai et al.22 modified poly(dimethyl siloxane) (PDMS) to form a di(trithiocarbonate) functional endgroup, and chain extended it with N,N-dimethyl acrylamide (DMA) and 2-(N-butyl perfluorooctanefluoro-sulfonamido) ethyl acrylate (BFA) using the RAFT process to form A-B-A triblock copolymers. In another study, the block copolymer poly(ethylene oxide)-b-poly(1,1,2,2-tetrahydroperfluorodecyl acrylate) was synthesized using a poly(ethylene oxide) macro-RAFT agent.23 Yu et al.24 copolymerized 2,3,4,5,6-pentafluorostyrene with poly(4-vinylbenzyl chloride), which was grafted from a hydrogenterminated silicon surface using RAFT. There are, however, no reports on the polymerization of 2,2,3,3-tetrafluoropropyl acrylate (TFPA, Scheme 1), and 2,2,3,3-tetrafluoropropyl methacrylate (TFPMA, Scheme 1) by RAFT. In this paper we describe the first reported synthesis of the homopolymers of the three monomers in Scheme 1 and their block formation with tert-butyl acrylate (tBA) using the RAFT process. Hydrolysis of t-butyl groups led to the production of diblocks in which the second block consisted of water-soluble poly(acrylic acid) (PAA). Adsorption studies were carried out on these fluorinated homo- and block copolymers onto a PTFE substrate in organic solvents. The chemistry of the surface after (14) Wang, S.; Marchant, R. E. Macromolecules 2004, 37, 3353–3359. (15) (a) Franks, F., Water: A. ComprehensiVe Treatise; Plenum: New York, 1973; Vol. 4, Chapter 1. (b) Israelachvili, J.; Pashley, R. Nature (London, U.K.) 1982, 300(5890), 341. (16) (a) Craig, V. S. J.; Ninham, B. W.; Pashley, R. M. Langmuir 1999, 15(4), 1562–1569. (b) Hartmann, J.; Urbani, C.; Whittaker, M. R.; Monteiro, M. J. Macromolecules 2006, 39(3), 904–907. (17) Shoichet, M. S.; McCarthy, T. J. Macromolecules 1991, 24, 1441. (18) Hansen, N. M. L.; Jankova, K.; Hvilsted, S. Eur. Polym. J. 2007, 43, 255–293. (19) Borkar, S.; Jankova, K.; Siesler, H. W.; Hvilsted, S. Macromolecules 2004, 37(3), 788–794. (20) (a) Jankova, K.; Hvilsted, S. Macromolecules 2003, 36(5), 1753–1758. (b) Jankova, K.; Jannasch, P.; Hvilsted, S. J. Mater. Chem. 2004, 14, 2902–2908. (21) (a) Li, H.; Zhang, Z. B.; Hu, C. P.; Ying, S. K.; Wu, S. S.; Xu, X. D. React. Funct. Polym. 2003, 56, 189–197. (b) Perrier, S.; Jackson, S. G.; Haddleton, D. M.; Ame´duri, B.; Boutevin, B. Tetrahedron 2002, 58, 4053–4059. (c) Perrier, S.; Jackson, S. G.; Haddleton, D. M.; Ame´duri, B.; Boutevin, B. Macromolecules 2003, 36, 9042–9049. (22) Pai, T. S. C.; Barner-Kowollik, C.; Davis, T. P.; Stenzel, M. H. Polymer 2004, 45, 4383–4389. (23) Ma, Z.; Lacroix-Desmazes, P. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 2405–2415. (24) Yu, W. H.; Kang, E. T.; Neoh, K. G. Ind. Eng. Chem. Res. 2004, 43, 5194–5202.

Suzuki et al. Scheme 1. Fluorine-Containing Monomers (A) 2,3,4,5,6-Pentafluorostyrene (FS) (B) 2,2,3,3-Tetrafluoropropyl Acrylate (TFPA), and (C) 2,2,3,3-Tetrafluoropropyl Methacrylate (TFPMA)

adsorption was quantified down to depths of 100 Å using X-ray photoelectron spectroscopy (XPS) and angle-resolved XPS (ARXPS). Adsorption of the PAA diblock copolymers was also examined using sessile water drop measurements.

Experimental Section Materials. 2,3,4,5,6-Pentafluorostyrene (FS, Aldrich, 99%), TFPMA (Aldrich, 99%), TFPA (Matrix Scientific, 97%), and tBA (Aldrich, 98%) were passed through a column of basic alumina to remove the inhibitor. 2,2-Azobis(isobutyronitrile) (AIBN, Fluka, 98%) and 1,1′-azobis(cyclohexanecarbonitrile) (Vazo 88, DuPont, 98%) were recrystallized twice from methanol prior to use. The RAFT agents, 1-phenylethyl phenyl dithioacetate (PEPDTA) and cumyl dithiobenzoate (CDB) were synthesized according to the literature.25 Two methods were employed to purify CDB. In method 1, CDB was purified by passing it once through aluminum oxide (neutral activity), and then twice through silica using hexane as the eluent. In method 2, CDB was purified by passing it twice through aluminum oxide (neutral activity), and then twice through silica using hexane as the eluent. AR-grade ethyl acetate (EA, 99.5%), methanol (99.8%), n-hexane (95%), dimethyl sulfoxide (DMSO, 99.9%), dichloromethane (DCM, 99.8%), fluorobenzene (FB, 99.8%), methyl ethyl ketone (MEK, 99.5%), and dimethylformamide (DMF, 99.8%) and ACS reagent acetone (99.5%) were used without further purification. HPLC-grade tetrahydrofuran (THF, 99.8%) was used in most cases, except for the polymerization, where inhibitor-free anhydrous THF (Aldrich, 99.9%) was used. Trifluoroacetic acid (TFA, 98%, Aldrich) and sodium cyanoborohydride (95%, Aldrich) were used as received. PTFE virgin tape (1.5 mm thickness) was obtained from E-Plas, Victoria, Australia. The thickness was measured to be 1.54-1.58 mm. Methods. Typical RAFT Polymerization of FS. A 21.8 mg portion of PEPDTA (8.00 × 10-5 mol, 2.81 × 10-2 M), and 1.95 mg of Vazo 88 (8.00 × 10-6 mol, 2.81 × 10-3 M) were dissolved in 4.00 g of polystyrene (PS; 2.06 × 10-2 mol, 7.25 M). Aliquots of 0.5 mL were transferred to six individual ampoules, which were degassed by four freeze-evacuate-thaw cycles and sealed. These samples were placed in an oil bath at 80 °C and removed after the required time, such that six different time points (i.e., conversions) were reached for each experiment. For the synthesis, quenching was with liquid nitrogen, exposure to air, and dilution with THF. The polymer was purified by precipitation into methanol, vacuum filtration, and exhaustive drying under vacuum at room temperature. The polymerization reaction of the PFS polymers used in the adsorption studies were quenched after 26.2 h at a conversion of 87%. Typical RAFT Polymerization of TFPMA. A 19.6 mg portion of CDB (7.20 × 10-5 mol, 2.50 × 10-2 M), and 1.18 mg of AIBN (7.20 × 10-6 mol, 2.50 × 10-3 M) were dissolved in 3.60 g of TFPMA (1.80 × 10-2 mol, 6.25 M). Aliquots of 0.5 mL were transferred to sixindividualampoules,whichweredegassedbyfourfreeze-evacuate-thaw cycles and sealed. These samples were placed in an oil bath at 60 °C and removed after the required time, such that six different time points were reached for each experiment. In the synthesis, quenching was with liquid nitrogen, exposure to air, and dilution with THF. (25) (a) Oae, S.; Yagihara, T.; Okabe, T. Tetrahedron 1972, 28, 3203. (b) Quinn, J. F.; Barner, L.; Rizzardo, E.; Davis, T. P. J. Polym. Sci., Part A: Polym. Chem. 2001, 40(1), 19–25.

Adsorption of Fluoropolymers onto PTFE The polymer was purified by precipitation into n-hexane, vacuum filtration, and exhaustive drying under vacuum at room temperature. For the adsorption studies, the polymerization reaction was stopped after 15 h at a conversion of 76%. Typical RAFT Polymerization of TFPA. A 21.8 mg portion of PEPDTA (8.00 × 10-5 mol, 2.63 × 10-2 M), and 1.31 mg of AIBN (8.00 × 10-6 mol, 2.63 × 10-3 M) were dissolved in 4.00 g of TFPA (2.15 × 10-2 mol, 7.08 M). Six different conversions were obtained as for the TFPMA polymerization. After quenching, exposure to air, and dilution with ethyl acetate, the polymer was purified as for PTFPMA. PTFPA synthesis was quenched after 3 h at a conversion of 90%. Synthesis of Poly(TFPMA-b-tBA). A 0.19 g portion of PTFPMA (Mn ) 38 000, PDI ) 1.11, 5.01 × 10-6 mol, 8.46 × 10-3 M) was dissolved into a solution of tBA (0.26 g, 2.03 × 10-3 mol, 3.43 M), AIBN (0.16 mg, 9.74 × 10-7 mol, 1.65 × 10-3 M), and THF (0.26 g, 3.63 × 10-3 mol, 6.13 M). The mixture was placed in a glass ampule, deoxygenated by five freeze-thaw-pump cycles, and sealed, and the sample was polymerized at 60 °C. The polymerization was stopped after 20.1 h corresponding to 57% conversion by quenching with liquid nitrogen, exposure to air, and dilution with THF. The polymer was purified as above. Synthesis of Poly(FS-b-tBA) in THF. A 0.18 g portion of PFS (Mn ) 29 200, PDI ) 1.06, 6.03 × 10-6 mol, 6.86 × 10-3 M) was dissolved into a solution of tBA (0.406 g, 3.17 × 10-3 mol, 3.45M), AIBN (0.29 mg, 1.77 × 10-6 mol, 1.92 × 10-3 M), and THF (0.403 g, 5.59 × 10-3 mol, 6.09M). The mixture was placed in a glass ampule, deoxygenated by five freeze-thaw-pump cycles, and sealed. The sample was polymerized at 60 °C. The polymerization was stopped after 1.7 h at a conversion of 40% or after 2.8 h at a conversion of 72%. Quenching and purification were as above. Synthesis of Poly(FS-b-tBA) in EA. A 79.90 mg portion of PFS (Mn ) 20 100, PDI ) 1.06, 3.97 × 10-6 mol, 8.60 × 10-3 M) was dissolved into a solution of tBA (0.20 g, 1.58 × 10-3 mol, 3.42M), AIBN (0.07 mg, 4.26 × 10-7 mol, 9.23 × 10-4 M), and EA (0.20 g, 2.27 × 10-3 mol, 4.92 M). The mixture was placed in a glass ampule, deoxygenated by five freeze-thaw-pump cycles, and sealed. The sample was polymerized at 60 °C. The polymerization was stopped after 1.8 h at a conversion of 46%. Dilution was with ethyl acetate. The polymer was purified by precipitation into n-hexane, vacuum filtration, and exhaustive drying under vacuum at room temperature. Synthesis of Poly(TFPA-b-tBA). A 0.16 g portion of PTFPA (Mn ) 39 900, PDI ) 1.06, 3.89 × 10-6 mol, 8.45 × 10-3 M) was dissolved into a solution of tBA (0.20 g, 1.57 × 10-3 mol, 3.41 M), AIBN (0.07 mg, 4.26 × 10-7 mol, 9.26 × 10-4 M), and EA (0.20 g, 2.27 × 10-3 mol, 4.93 M). The mixture was placed in a glass ampule, deoxygenated by five freeze-thaw-pump cycles, and sealed. The sample was polymerized at 60 °C, and conversion was monitored by Raman spectroscopy. The polymerization was stopped after 1.9 h at a conversion of 59% by quenching with liquid nitrogen, exposure to air, and dilution with ethyl acetate. The polymer was purified by precipitation into methanol/water (1:1), centrifugation, and exhaustive drying under vacuum at room temperature. Preparation of PAA-Containing Fluorinated Block Polymers. The hydrolysis of tBA side groups on the block copolymers to acrylic acid was carried out according to a literature procedure.26 An example is the synthesis of poly(TFPA-b-AA). A 5-fold molar excess of TFA (32.5 mg, 2.85 × 10-4 mol, 5.18 × 10-1 M) with respect to the tBA groups of the block copolymer was added dropwise to a solution of 28 mg poly(TFPA-b-tBA) dissolved in 0.5 mL of DCM. The reaction was carried out at room temperature by stirring for 24 h. The polymer was then precipitated into n-hexane, and dried exhaustively under vacuum at room temperature. Stability of the PFS RAFT End-Groups. A 1 mg/mL solution of fluorinated PS prepared from the RAFT polymerization using PEPDTA in THF or ethyl acetate was left at room temperature without stirring. The changes in the UV-vis absorption at 310 nm, (26) Ma, Q.; Wooley, K. L. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 4805–4820.

Langmuir, Vol. 24, No. 22, 2008 13077 indicating hydrolysis of the dithioester RAFT end-groups, were monitored over an 18 day period. Fluorinated Homo and Copolymer Adsorption onto PTFE. PTFE films (approximately 1.0 × 0.5 cm2) were washed in a series of solvents (2 h each in chloroform and n-hexane and 16 h in methanol with stirring at room temperature) and dried before use. Each film was then immersed in the relevant polymer solution (1 mg/mL) in different solvents (e.g., DCM, FB, MEK or DMF) in a glass tube for 16 h. The sample was then washed three times (∼10 s each) with the same solvent that had been used for immersion and subsequently dried. Stability of the Adsorbed Amphiphilic Block Copolymers. The simulated body fluid (SBF) was prepared according to the method described by Kim et al.27 The substrate was soaked in 10 mL of SBF solution at 36.5 ( 0.2 °C for a period of two weeks with a regular solution change. The materials were then washed extensively with Milli-Q water and subsequently dried in a vacuum oven at 40 °C. XPS was used to assess the adsorbed polymer stability. Analytical Techniques. FT-Raman Spectroscopy. To obtain the degree of conversion, FT-Raman spectra (PE Spectrum 2000 NIR FTIR, 64 scans, 8 cm-1 resolution, wavenumber range 4000-360 cm-1) were recorded at various time points. Spectral information was extracted by means of spectral analysis software (GRAMS/32, Galactic Industries Corp., Salem, NH). The area under the vinyl peak at 1640 cm-1 that was normalized to the nonchanging signal was used for the conversion calculation. In the case of FS, the vinyl peak appeared at 1620 cm-1. In addition, one sample for each reaction protocol was polymerized in situ in an FT-Raman spectrometer (32 scans, 8 cm-1 resolution, wavenumber range 4000 - 360 cm-1) at 60 °C to obtain a conversion/time curve. Size Exclusion Chromatography. Size exclusion chromatography (SEC) measurements were performed using a Waters Alliance 2690 Separations Module equipped with three 7.8 × 300 mm Waters Styragel GPC columns (2 linear Ultrastyragel and one Styragel HR3 columns), an autosampler, column heater, differential refractive index detector and a photodiode array (PDA) connected in series. HPLCgrade THF was used as eluent at a flow rate of 1 mL min-1. PS standards ranging from 2000000 - 517 g/mol were used for calibration. Molecular weights of all polymers are reported relative to PS standards. The use of PS standards as well as the choice of SEC conditions were based on previous studies for PFS synthesized via the ATRP technique, where good agreement was found between the calculated Mn and the experimental data (Mn range of ∼1000 to ∼20 000).19 However, it should be noted that the PS standards were also used for PTFPA and PTFPMA without additional verification. Hence, these Mn values are not absolute but rather relative to PS. UV-Vis Spectroscopy. UV spectra were recorded on a Hitachi U-3000 UV-vis spectrometer in the wavelength range from 190 to 700 nm. Nuclear Magnetic Resonance. 1H NMR spectra were recorded using a 300.14 MHz spectrometer (Avance Bruker). Software used was TOPSPIN 1.3. Chemical shifts are given in parts per million (ppm) relative to the residual solvent peak. X-ray Photoelectron Spectroscopy. XPS spectra were recorded using a Kratos Axis Ultra X-ray photoelectron spectrometer with monochromated Al KR X-ray source (1486.6 eV) running at 150 W (15 kV, 10 mA emission current). The survey scans were collected at 1200-0 eV with 1.0 eV steps at a pass energy of 160 eV; the narrow scans were collected at 0.1 eV steps at a pass energy of 20 eV. Vision 2 software was used for data acquisition. High-resolution spectra were resolved into individual Gaussian-Lorentzian peaks using a least-squares fitting program (CasaXPS, Casa Software, Ltd.). The binding energies were charge-corrected using a C-F2 of PTFE (292.5 eV).28 Component energies, number of peaks and peak widths (full width at half-maximum (fwhm) of 1.0 for Cs) were fixed initially and refinement was done only for peak heights. In a final refinement cycle, component energies and peak widths were also refined, and (27) Kim, H. M.; Miyazaki, T.; Kokubo, T.; Nakata, T. Bioceramics 2000, 13, 47–50. (28) Beamson, G.; Briggs, D. High Resolution XPS of Organic Polymers: The Scienta ESCA300 Database; John Wiley & Sons, Ltd: West Sussex, U.K., 1992.

13078 Langmuir, Vol. 24, No. 22, 2008 these changed by less than 1.0%. Peak fit results were imported into Excel for final illustrations. AR-XPS was performed using stage orientations of φ ) 0° and 30° from normal, which allow analysis to depths of approximately 100 and 60 Å, respectively. Sessile Drop Contact Angle Measurements. A custom built apparatus fitted with a Kodak Digital Science DC120 camera linked to a Kodak Digital Science Picture Postcard Software imaging program was used to measure the contact angles. The measurements were performed manually at room temperature. For advancing contact angle, a drop of Milli-Q water (5 µL) was placed into contact with the flat surface using a microsyringe. An image was recorded immediately. The syringe tip was then placed in contact with the drop, and another 5 µL was added to advance the drop edge slowly. This addition was repeated twice to create a total of 20 µL; images were recorded each time. The images for the 5, 10, and 15 µL drops were used for the advancing angle measurements. Receding contact angles were measured following the same procedure by withdrawing water from the drop. Only the last 5 µL drop image was used. The following equation was used to calculate the average contact angle (θ) using height (h) and distance (d):29

h ) d ⁄ 2 tan(θ ⁄ 2) The reported values are the average measurement values for two samples from three different locations for each sample. The angle value errors are the standard deviations.

Results and Discussion RAFT-Mediated Homopolymerizations of FS, TFPMA, and TFPA. The bulk polymerization of FS was carried out in the presence of PEPDTA and 1,1′-azobis(cyclohexanecarbonitrile) (Vazo 88) at 80 °C. This RAFT-agent has been shown to provide good control for styrene and acrylates without the deleterious effects of rate retardation such as is found when using CDB.30-32 Figure 1A shows the conversion profiles at two concentrations of PEPDTA (where [PEPDTA]/[Vazo 88] was kept constant at a ratio of 10) targeting 25 and 50 K at full conversion. The conversion versus time was relatively linear until approximately 50% conversion, after which a gel effect was observed by the significant rate increase. High conversions (>90%) were found for the 25 and 50 K polymerizations after 1000 and 1500 min, respectively. The number-average molecular weight (Mn) for both polymerizations increased linearly with conversion, and was in good agreement with theory up to 50% conversion (Figure 1B). At higher conversions, the Mn found experimentally was lower than the Mn calculated (eq 1). This suggests that much greater amounts of bimolecular termination occur in these RAFT-mediated polymerizations. Because of the high reaction temperature (80 °C), additional radicals derived through thermal self-initiation of pentafluorstyrene coupled with the long polymerization times, lasting beyond 1000 min, would explain the lower than expected Mn values at higher conversions. Inclusion of the thermal initiation term results in eq 2.33 (29) Birdi, K. S. Surface tension and interfacial tension of liquids. In Handbook of Surface and Colloid Chemistry; Birdi, K. S., Ed.; CRC Press: Boca Raton, FL, 2003. (30) (a) Monteiro, M. J.; de Brouwer, H. Macromolecules 2001, 34, 349–351. (b) Barner-Kowollik, C.; Quinn, J. F.; Nguyen, T. L. U.; Heuts, J. P. A.; Davis, T. P. Macromolecules 2001, 34, 7849–7857. (c) Goh, Y.-K.; Whittaker, M. R.; Monteiro, M. J. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 5232–5245. (d) Barner-Kowollik, C.; Buback, M.; Charleux, B.; Coote, M. L.; Drache, M.; Fukuda, T.; Goto, A.; Klumperman, B.; Lowe, A. B.; Mcleary, J. B.; Moad, G.; Monteiro, M. J.; Sanderson, R. D.; Tonge, M. P.; Vana, P. J. Polym. Sci. A: Polym. Chem. 2006, 44(20), 5809–5831. (31) Plummer, R.; Goh, Y.-K.; Whittaker, A. K.; Monteiro, M. J. Macromolecules 2005, 38, 5352–5355. (32) Monteiro, M. J. J. Polym. Sci. A: Polym. Chem. 2005, 43, 3189–3204. (33) (a) Johnston-Hall, G.; Monteiro, M. J. Macromolecules 2008, 41(3), 727– 736. (b) Johnston-Hall, G.; Monteiro, M. J. J. Polym. Sci., Part A: Polym. Chem. 2008, 46(10), 3155–3173.

Suzuki et al.

Pn ) Pn )

[M]ox [RAFT]0 + af([I]0 - [I]t) [M]ox

[RAFT]0 + af([I]0 - [I]t) + a

∫0t Rthdt

(1) (2)

where Pn () Mn/mwmon) is the average chain length, [RAFT]0 is the initial RAFT agent concentration, a is the mode of termination and is set to 1 (a ) 1 for termination exclusively by combination, and 2 exclusively by disproportionation), x represents the fractional conversion of monomer to polymer, [I]0 is the initial initiator concentration, [I]t is the initiator concentration at time t calculated from the integrated rate loss equation ([I]t ) [I]0 exp(-kdt)), where kd is the rate coefficient for loss of initiator to radicals, and f is the initiator efficiency factor. The rate of thermal initiation can then be calculated by substituting Pn,exp found from experiment into eq 2 and solving for Rth. The value determined for Rth was close to 5 × 10-7 M-1s-1, which is close to the literature value of 5.7 × 10-7 M-1s-1 at 60 °C.34 The polydispersity index (PDI) values of the PFS polymers synthesized were all below 1.1, and at full conversion were below 1.06 (Figure 1B). This showed that the polymer chains were of near uniform chain length, and the polymerization of FS with PEPDTA was well controlled. The bulk homopolymerization of the fluorinated acrylate (TFPA) was carried out in the presence of PEPDTA and 2,2azobis(isobutyronitrile) (AIBN) at 60 °C. Figure 1C shows the conversion profiles at two concentrations of PEPDTA and reveals the presence of an inhibition period. With increased PEPDTA concentration the inhibition time increased from 20 to 40 min despite the concomitant increase in the initiator concentration. The reason for this is attributed to termination of the leaving radical group on PEPDTA (i.e., CH2(CH3)Ph) with all other radicals in the system - termed primary radical termination.35 Therefore, the greater the concentration of primary radicals (from a greater concentration of PEPDTA) the longer the inhibition period.32 After this initial inhibition period the rates were similar and rapid, reaching high conversions (>80%) in under 160 min (Figure 1C). The molecular weight data showed that the Mn of the polymers was in excellent agreement with theory and the PDI for all conversions were below 1.08 (Figures 1D). The RAFT agent CDB was chosen to mediate the fluorinated methacrylate monomer, as PEPDTA has been shown to be a poor agent for most methacrylates.36 The bulk homopolymerization of TFPMA in the presence of CDB and AIBN at 60 °C also gave polymers with controlled molecular weights. It was observed that the rate of polymerization for TFPMA (Figure 1E) was significantly increased when using CDB that had been purified by passing it through an aluminum oxide column twice and then twice through a silica column. These results are in agreement with previous findings.31 The Mn evolution of the polymer with conversion showed excellent agreement between experiment and theory (Figure 1F). The PDIs were found to be slightly higher than those observed for both the PFS and PTFPA polymers ranging from 1.3 (at ∼ 10% conversion) to 1.13 (at ∼ 90% conversion). (34) Pryor, W. A.; Huang, T.-L. Macromolecules 1969, 2(1), 70–77. (35) (a) Pham, B. T. T.; Tonge, M. P.; Monteiro, M. J.; Gilbert, R. G. Macromolecules 2000, 33, 2383–2390. (b) Lonsdale, D. E.; Johnston-Hall, G.; Fawcett, A.; Bell, C. A.; Urbani, C. N.; Whittaker, M. R.; Monteiro, M. J. J. Polym. Sci., Part A: Polym. Chem. 2007, 45(16), 3620–3625. (36) Quinn, J. F.; Barner, L.; Barner-Kowollik, C.; Rizzardo, E.; Davis, T. P. Macromolecules 2002, 35, 7620–7627.

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Figure 1. Kinetic plots of the bulk polymerization of PFS initiated with Vazo 88 in the presence of PEPDTA at 80 °C. (A) conversion vs time. (B) Mn and PDI vs conversion. Curve (a): [PEPDTA] ) 56 mM, and [Vazo-88] ) 5.6 mM; curve (b): [PEPDTA] ) 28 mM and [Vazo-88] ) 2.8 mM. Kinetic plots of the bulk polymerization of TFPA initiated with AIBN in the presence of PEPDTA at 60 °C. (C) conversion vs time, (D) Mn and PDI vs conversion. Curve (a): [PEPDTA] ) 56 mM, and [AIBN] ) 5.6 mM; curve (b): [PEPDTA] ) 28 mM and [AIBN] ) 2.8 mM. Kinetic plots of the bulk polymerization of TFPMA initiated with AIBN in the presence of CDB at 60 °C. (E) Conversion vs time, (F) Mn and PDI vs conversion. Curve (a): [CDB] ) 50 mM and [AIBN] ) 5.0 mM; curves (b) and (c): [CDB] ) 25 mM and [AIBN] ) 2.5 mM. Effect of CDB purity: Using hexane as eluent, CDB was passed through a neutral activity aluminum oxide column followed by a silica column (a and b) once and twice, respectively (method 1), and (c) twice each (method 2). Table 1. Experimental Results from the Chain Extension of Fluorinated MacroRAFT Agents with tBA polymera

solvent

polym. time (h)

conv. (%)

Mn theory

Mn SEC

PDI SEC

PFS PFS PFS PTFPMA PTFPA

THF THF EA THF EA

1.7 2.8 1.8 20.1 1.9

40 72 46 57 59

54 400 73 900 43 500 64 000 70 200

49 900b 79 800b 62 500 69 200 81 700

1.23 1.31 1.15 1.18 1.11

a The Mn and PDI values for the macro-RFT agents are listed in the Experimental Section. b SEC chromatogram was bimodal.

Block Copolymerizations of Fluorinated Polymers with The fluorinated homopolymers were chain extended with tBA, allowing amphiphilic block copolymers to be prepared by simple acid hydrolysis. The molecular weight data for the macroRAFT agents are given in the Experimental Section, while those of the resulting bock copolymers are given in Table 1. In general, the Mn values of the block copolymers are close to theory with PDIs below 1.2, showing that well-defined block polymers can tBA.

Figure 2. Degradation of the RAFT end-groups of PFS, stored in THF (curve a) and EA (curve b) monitored by UV-vis absorbance spectroscopy at 310 nm.

be successfully synthesized using RAFT.37 However, very poor control was observed in the block synthesis using the macro RAFT agent PFS in THF where a bimodal Mn distribution was

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Figure 3. C1s narrow scans of PTFE films (A) untreated PTFE and (B) PFS adsorbed onto PTFE from MEK.

Figure 4. AR-XPS spectra of C1s PFS-adsorbed PTFE (A) from MEK and (B) from FB. Table 2. Chemical Characteristics of Polymers Used in Adsorption Studies polymer

Mn

Pn

PDI

no. Fa

PFS PTFPMA PTFPA P(FS101-b-tBA237)b P(FS101-b-tBA141)b P(FS101-b-AA237)b P(FS101-b-AA141)b

27 400 38 900 39 900 50 200 38 000 36 900c 30 000c

140 193 213 101 + 237 101 + 141 101 + 237 101 + 141

1.06 1.13 1.06 1.11 1.07

700 773 853 505 505 505 505

a Number of fluorine atoms on the polymer chain calculated from Mn and structure of repeating units. b MacroRAFT PFS Mn ) 19869, Pn ) 101, PDI ) 1.04. c Calculated assuming 100% cleavage of tBA segments.

Figure 5. Relationship between molecular weight of PFS and absorbed amounts of PFS onto PTFE films.

observed by SEC (Figure S1). This was obviated by changing the solvent to EA. The stability of the PFS RAFT end-group in THF and EA at room temperature over an 18 day period was followed using UV-vis spectroscopy. The decrease in absorbance at 310 nm corresponding to a loss of dithioester (Figure 2) indicates

that PFS stored in THF undergoes significant loss of RAFT endgroups. This presumably occurs as a result of their oxidation by residual amounts of peroxides in the THF. In contrast, there was little or no end-group loss when stored in ethyl acetate. PAA-containing block copolymers containing carboxylic acid functional groups were prepared by the hydrolysis of the t-butyl groups in the PtBA block of PTFPMA-b-PtBA, PTFPA-b-PtBA and PFS-b-PtBA (prepared in EA; Table 1). Completion of this

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Table 3. Atomic % of Elements from XPS Survey Scans and Atomic C% of Different C Elements from the High-Resolution C1s Scans atomic % from atom % from C1s survey XPS scan narrow XPS scan polymer

solvent

a

PTFE PFS (Pn ) 140)

DCM FB MEK PTFPMA (Pn ) 193) DCM FB MEK PTFPA (Pn ) 213) DCM FB MEK a

F 69 59 54 57 56 60 62.5 55 60 64

O

C

31 41 46 43 1 43 1 39 0.0 37.5 1 44 1 39 0 36

C-F2 C-others 99.3 78 72 ( 3 72 ( 6 85 97.5 99.2 87 98 99.6

0.7 22 29 ( 3 28 ( 6 15 2.5 0.8 13 2 0.4

Untreated PTFE.

process was verified by 1H NMR with the complete loss of the proton resonance at 1.4 ppm corresponding to the t-butyl group (Figure S2). Mn data for these block-copolymers were calculated based on the 100% conversion of the tBA groups to AA groups. Adsorption Studies of Fluorinated Homopolymers onto PTFE. The first adsorption studies were carried out by immersing a prewashed PTFE film into a solution of PFS (Pn ) 140, 1 mg/mL) in MEK for 16 h. XPS was used to quantify the relative amount of PFS adsorbed onto PTFE using high-resolution C1s XPS spectra (Figure 3). The peak at 292.5 eV corresponds to the C-F2 from PTFE. Untreated PTFE showed trace amounts of aliphatic carbon (0.7%) at 285.3 eV, indicating the presence of a very small hydrocarbon impurity. The C1s spectrum of the PFS-adsorbed PTFE surface can be curve-fitted using five peak components with binding energies of 286.1, 286.7, 286.8, 288.9, and 292.5 eV. These are attributed to the C*-H (peak 1), C*-C6F5 (peak 2), C*-CF (aromatic, peak 3), C*-F (aromatic, peak 4), and C*-F2 (PTFE, peak 5) species, respectively.38 From such high-resolution spectra the atomic % carbon arising from the PTFE substrate (C-F2) and carbon arising from adsorbed homopolymers (C-others) give an indication of the relative amounts of homopolymer adsorbed onto PTFE. A value of 100% C-other (i.e., signal arising only from the adsorbed PFS) would correspond to a surface layer consisting exclusively of PFS with a thickness of at least 100 Å. Values below 100% C-other can arise either from thinner PFS layers on top of the PTFE substrate or from PFS impregnated into the PTFE substrate. Adsorption of PFS onto PTFE yielded a C-other of 77%, which decreased to 28% after washing three times with MEK. We propose that the PFS that is readily removed in the washing step was present as a surface layer (possibly on top of the PFTE substrate), while the PFS that remained after washing is bound to the PTFE substrate by becoming impregnated into the PTFE substrate. AR-XPS was performed on the washed sample at two angles of 0° and 30° corresponding to penetration depths of 100 and 60 Å, respectively (Figure 4A). There is a small but significant difference in the relative amounts of PFS detected at these depths with a smaller amount for a probe depth for 100 Å, indicating that, in MEK, PFS is preferentially distributed at more shallow depths. The effect of the molecular weight of the PFS polymer on adsorption onto PTFE from MEK after washing was examined using three different degrees of polymerization (Pn ) 32, 61, 140). Figure 5 shows that adsorption increased with the number of FS units in the polymer chain from 10% C-others at 32 units (37) Monteiro, M. J. J. Polym. Sci., Part A: Polym. Chem. 2005, 43(22), 5643–5651. (38) Zhang, F.; Xu, J.; Kang, E. T.; Neoh, K. G. Ind. Eng. Chem. Res. 2006, 45, 3067–3073.

to 28% at 140 units. This indicates that increasing molecular weight of the PFS homopolymers increased the amount of PFS bound to the PTFE substrate. The adsorption of PFS (Pn ) 140), PTFPMA (Pn ) 193), and PTFPA (Pn ) 213) onto PTFE in various solvents was investigated by XPS survey and high-resolution scans (Table 3). The XPS survey scans of washed PTFE and PFS-adsorbed films showed only F and C, whereas a small amount of O was observed on the PTFPMA- and PTFPA-adsorbed PTFE films, in agreement with the chemical structures of the polymers. The relative amount of adsorbed polymer based on the C-others region in the high resolution C1s spectra revealed that, regardless of the solvent, PFS adsorbs more efficiently than either PTFPMA or PTFPA, which both had similar adsorption values. It appears, therefore, that the chemical structure of the homopolymer and not the number of fluorine atoms in the polymer chain (Table 2) affects adsorption to the PFTE film. PFS is much less polar39 than PTFPMA and PTFPA, suggesting that the strong hydrophobic interactions between PFS and the PTFE substrate are the major contributor to the increased adsorption compared to the other two F-polymers. Solvent plays a major role in the adsorption of PTFPMA and PTFPA homopolymers onto PTFE. In contrast, there is no solvent effect on the amount of adsorbed PFS (Table 3), and only a small variation in the distribution of PFS in that it appears that a more homogeneous distribution of the polymer exists to a depth of 100 Å when adsorbed from FB (Figure 4A,B). For the PTFPMA and PTFPA homopolymers, it was found that the amount adsorbed remained virtually unchanged by washing. In addition, the amount adsorbed decreased in the solvents in the order: DCM . FB > MEK. The inert nature of PTFE is well-known, its swelling behavior in solvents is considered negligible,40 and only minimal swelling in fully chlorinated or fluorinated solvents not containing hydrogen is reported.41 Thus it is not possible to explain the difference in the amount of adsorbed PTFPMA or PTFPA in terms of PTFE swelling. However, results clearly show that the nature of the interactions between the homopolymer and the solvent as well with the PTFE substrate is very different for the PFS homopolymer compared to the PTFPMA or PTFPA homopolymers. Adsorption of Fluorinated Block Copolymers onto PTFE. Since the aim of this study was to adsorb functional block copolymers in order to change the surface properties of PTFE, the PFS-based block copolymers were chosen for further studies as the PFS homopolymer showed the highest adsorption onto PTFE in all solvents investigated. Two well-defined block copolymers were synthesized from the same PFS macro-RAFT (Pn ) 101), and contained Pn’s of the second PtBA block of 141 and 237. The adsorption of either P(FS101-b-tBA141) or P(FS101b-tBA237) in an MEK solution onto PTFE yielded a value for C-other of approximately 21% after washing (Table 4), which is similar to that found for the homopolymer (PFS, Pn ) 140, C-other ) 28%). The homopolymer PtBA showed no adsorption to PTFE, and therefore the PTFE film should only be impregnated with PFS and the PtBA block resides at the interface. The contact angles after adsorption of these block copolymers onto PTFE showed a decrease in both advancing and receding angle values compared to the unmodified PTFE substrate (Table 4). The advancing contact angle for P(FS101-b-tBA237) adsorbed onto PTFE was identical to that of PtBA (θA ) 88°),42 whereas the (39) Bucholz, T. L.; Loo, Y.-L. Macromolecules 2006, 39(18), 6075–6080. (40) Brandrup, J., Immergut, E. H., Grulke, E. A., Eds., Polymer Handbook, 4th ed.; Wiley-Interscience: New York, 1999; Vol. 1. (41) Sperati, C. A. In History of High Performance Polymers; Seymour, R. B., Kirshenbaum, G. S., Eds.; Marcel Dekker:, 1986; pp 267-278. (42) Walters, K. B.; Hirt, D. E. Polymer 2006, 47, 6567–6574.

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Table 4. Atomic C% of Different C Elements from the High-Resolution XPS C1s Scans and Water Contact Angle Data for Block Copolymers Adsorbed onto PTFE polymer PTFE P(FS-b-tBA237) P(FS-b-tBA141) P(FS-b-AA237) P(FS-b-AA141) P(FS-b-AA141)

solvent MEK MEK DMF DMF MEK

C-F2 (%)

C-others (%)

θA (°)

θR (°)

hysteresis (°)

21 22 56 51 37

109 ( 6 88 ( 2 98 ( 3 91 ( 6 88 ( 5 101 ( 4

111 ( 6 86 ( 5 90 ( 6 31 ( 3 28 ( 3 49 ( 3

2(5 8(7 60 ( 7 60 ( 6 52 ( 5

79 78 44 49 63

advancing contact angle for P(FS101-b-tBA141) adsorbed onto PTFE (θA ) 98°) lay between that for PTFE/PFS (θA ) 108°)19 and PtBA. No significant contact angle hysteresis was observed in these systems. Both the XPS and contact angle results support the conclusion that PFS blocks are impregnated into the PTFE substrate and that the PtBA blocks remain at the interface and possibly extend into the solvent. The data obtained suggests that the copolymer with the long PtBA segment provides a better coverage of the surface than the short PtBA segment, which in this case results in exposure of some of the PTFE/PFS regions on the surface. The conversion of the second PtBA block to PAA was carried out through hydrolysis with TFA, resulting in PAA that is nonionized and therefore hydrophobic. It was found that MEK was not a good solvent for P(FS-b-AA237), and therefore DMF was used as the solvent for the PAA-containing block copolymers. Interestingly, these block copolymers showed higher adsorption onto PTFE compared to their respective P(FS-b-tBA) blocks (Table 4), even when the adsorption study was carried out in a MEK solution. In comparison, the relatively higher contact angles (both advancing and receding) of the block copolymer adsorbed from MEK correlates well with the significantly lower amount adsorbed from this solvent (Table 4). These findings correlate well with those of Cho et al.43 who found that P(S-b-DMS) block copolymers adsorbed efficiently onto PS in supercritical CO2 in which the PS block is insoluble but the PDMS block is soluble. More efficient adsorption was found when the solvation of the PDMS block was optimal. Thus, in our system, the higher solubility of PAA in DMF compared to MEK results in a more

efficient adsorption process. This results in the formation of a surface where the PFS block is impregnated into the substrate and the PAA block extends from the surface. It is clear that, although there appears to be a small effect on the amount of adsorbed block copolymer from DMF, the observed contact angles are within experimental error. The P(FS-b-AA)adsorbed PTFE samples formed from DMF displayed advancing contact angles of ∼90° corresponding to the hydrocarbon backbone of the PAA segment. This indicates that, after adsorption, washing, and drying, the functional groups are pointing away from the surface as a result of the propensity to reduce surface tension. The receding contact angles were very low (θR ∼ 30°), reaching a value similar to that for PAA (θR ∼ 25°),44 indicating a fast deprotonation of the carboxylic acid groups and reorientation of the polymer chains during the wetting process (Figure 6). The structures proposed for these adsorbed block copolymers in air and water are based on work by Koberstein45 and illustrated in Figure 6. These results highlight an interesting outcome of this particular surface modification: a switchable hydrophobic-hydrophilic surface (in air or water, respectively). To examine the stability of the adsorbed block copolymers, the modified films were immersed in SBF solution for 2 weeks. The XPS C1s narrow scans showed no significant loss of adsorbed block copolymers, indicating the presence of stable films (data not shown). This surface modification of PTFE thus provides a more stable system than PAA adsorbed to FEP from water46 and demonstrates that these block copolymers are very suitable for changing the surface properties of PTFE. In addition, the introduced functional carboxylic acid groups can

Figure 6. Water droplet (5 µL) profiles on the surfaces of P(FS-b-AA141)-adsorbed PTFE (A) advancing and (B) receding. Schematic representation of adsorbed block copolymer reorganization (C) in air and (D) in water.

Adsorption of Fluoropolymers onto PTFE

be useful for the attachment of bioactive molecules to the PTFE surface expanding its biomedical applications.

Conclusion We have developed a facile alternate route for the synthesis of well-defined fluorinated polymers using the RAFT process. We have demonstrated the importance of the purity of the CBD RAFT agent for maintaining fast polymerization rates. It was also found that storing PFS in THF resulted in the loss of the RAFT end-groups, presumably due to oxidation by residual peroxides. Block copolymers consisting of a fluorinated polymer first block and a second block containing protecting (t-butyl) groups were successfully synthesized. Block copolymer formation via hydrolysis of the t-butyl groups was facile. The adsorption of the homopolymers onto PTFE showed that hydrophobic interactions play an important role with PFS adsorbing in significantly larger amounts than the less hydrophobic PTFPMA and PTFPA. The block copolymers proved a judicious choice for creating highly hydrophilic surfaces in water after adsorption onto the PTFE substrate. The fact that the surface modification of the PTFE is achieved in a nondestructive manner means that

Langmuir, Vol. 24, No. 22, 2008 13083

this process has many potential applications, not least in the field of biomaterials science. Acknowledgment. The authors thank Drs. Llew Rintoul and Barry Wood for their technical support with Raman spectroscopy and XPS, respectively. M.J.M. acknowledges financial support from the Australian Research Council (ARC) Discovery grant and receipt of a QEII Fellowship (ARC fellowship). Supporting Information Available: Two figures are available as Supporting Information. Figure S1 includes the SEC chromatograms of the chain extension of PFS with tBA carried out in tetrahydrofuran and ethyl acetate, and Figure S2 gives the 1H NMR spectrum of P(TFPAb-tBA) as the neat polymer and after hydrolysis with TFA. This information is available free of charge via the Internet at http://pubs.acs.org. LA802300Q (43) Cho, D.; Kim, Y. J.; Erkey, C.; Koberstein, J. T. Macromolecules 2005, 38, 1829–1836. (44) Pan, F.; Wang, P.; Lee, K.; Wu, A.; Turro, N. J.; Koberstein, J. T. Langmuir 2005, 21, 3605–3612. (45) Koberstein, J. T. J. Polym. Sci., Part B: Polym. Phys. 2004, 42, 2942– 2956. (46) Coupe, B.; Evangelista, M. E.; Yeung, R. M.; Chen, W. Langmuir 2001, 17, 1956–1960.