Oxidative Graft Polymerization of Aniline on PTFE Films Modified by

Oct 9, 2002 - solution. A high extent of APTS chemisorption, in turn, led to a high graft concentration of ... control of electromagnetic radiation (E...
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Langmuir 2002, 18, 9035-9040

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Oxidative Graft Polymerization of Aniline on PTFE Films Modified by Surface Hydroxylation and Silanization L. Y. Ji, E. T. Kang,* and K. G. Neoh Department of Chemical Engineering, National University of Singapore, Kent Ridge, Singapore 119260

K. L. Tan Department of Physics, National University of Singapore, Kent Ridge, Singapore 119260 Received June 8, 2002. In Final Form: August 21, 2002 Chemical modification of argon plasma-pretreated poly(tetrafluoroethylene) (PTFE) films by UV-induced graft copolymerization with 2-hydroxyethyl methacrylate (HEMA), followed by silanization with (paminophenyl)trimethoxysilane (APTS), and finally oxidative graft polymerization of aniline had been carried out to render the PTFE surfaces electrically conductive. The chemical composition of the PTFE surface after each step of surface modification was analyzed by X-ray photoelectron spectroscopy (XPS). The graft concentration of the HEMA polymer and the aniline polymer increased with increasing concentration of the respective monomer used for graft polymerization. The extent of APTS chemisorption increased with the graft concentration of the HEMA polymer, as well as the concentration of the APTS solution. A high extent of APTS chemisorption, in turn, led to a high graft concentration of the aniline polymer and thus a low surface resistance of the resulting PTFE film upon acid treatment (reprotonation) of the grafted aniline polymer. The grafted aniline polymer exhibited similar intrinsic oxidation states and reprotonation-deprotonation behavior as those of the aniline homopolymer. The grafted aniline polymer was found to be more effectively protonated by 10-camphorsulfonic acid (CSA) than by HClO4. The optimum surface resistance of the aniline polymer-grafted PTFE film was on the order of 103 Ω/sq.

Introduction Surface modification of polymers via molecular design is one of the most versatile means of incorporating new functionalities into the existing polymers.1,2 These new functionalities can lead to the improvement of surface hydrophilicity,3 hydrophobicity,4 biocompatibility,5 metal adhesion,6 and electrical conductivity.7 Among the synthetic polymers, poly(tetrafluoroethylene) (PTFE) is potentially an ideal candidate for biomaterials8 and microelectronics packaging9 applications because of its chemical inertness and excellent thermal and dielectric properties. In this connection, a large number of excellent studies on the surface modification of PTFE have been carried out to enhance its surface functionalities.8-12 An alternative approach to improving the surface conductivity of insulating polymers is through the * To whom correspondence should be addressed. Telephone: (65) 6874-2189. Fax: (65) 6779-1936. E-mail: [email protected]. (1) Uyama, Y.; Kato, K.; Ikada, Y. Adv. Polym. Sci. 1998, 137, 1. (2) Kang, E. T.; Zhang, Y. Adv. Mater. 2000, 12, 1481. (3) Inagaki, N.; Tasaka, S.; Umehara, T. J. Appl. Polym. Sci. 1999, 71, 2191. (4) Hehergen, A.; Uyama, Y.; Okada, T.; Ikada, Y. J. Appl. Polym. Sci. 1993, 48, 1825. (5) Ikada, Y. Biomaterials 1994, 15, 725. (6) Charbonnier, M.; Romand, M.; Kogelschatz, U.; Esrom, H.; Seebock, R. In Metallized Plastics 7: Fundamental and Applied Aspects; Mittal, K. L., Ed.; VSP: Utrecht, The Netherlands, 2001; p 3. (7) Kang, E. T.; Tan, K. L.; Kato, K.; Uyama, Y.; Ikada, Y. Macromolecules 1996, 29, 6872. (8) Pu, F. R.; Williams, R. L.; Markkula, T. K.; Hunt, J. A. Biomaterials 2002, 23, 2411. (9) Sacher, E. Prog. Surf. Sci. 1994, 47, 273. (10) Shi, M. K.; Lamontagne, B.; Selmani, A.; Martinu, L.; Sacher, E.; Wertheimer, M. R.; Yelon, A. J. Vac. Sci. Technol., A 1994, 12, 29. (11) Konig, U.; Nitschke, M.; Menning, A.; Eberth, G.; Pilz, M.; Arnhold, C.; Simon, F.; Adam, G.; Werner, C. Colloid. Surf. B: Biointerfaces 2002, 24, 63. (12) Inagaki, N.; Tasaka, S.; Narushima, K.; Teranishi, K. J. Appl. Polym. Sci. 2002, 83, 340.

incorporation of “synthetic metals”, such as polyaniline (PAn), polypyrrole (PPy), and polythiophene (PTH).13 Thin films of these electroactive polymers on insulating substrates are potentially important to the control of electromagnetic radiation (EMR) and the dissipation of electrostatic charges.14 Modification of fluoropolymer surfaces, after wet chemical treatment, hydrogen plasma treatment, UV laser irradiation, and electron beam exposure, with electroactive polymers has been carried out.15 Coating of the emeraldine (EM) base form of PAn on a polymer substrate modified by graft copolymerization with a polymeric acid can give rise to a conductive polymer surface.16 These surface modification techniques are aimed at improving the adhesion of the electroactive polymers to the polymer substrates. More recently, self-assembly of pyrrole,17,18 thiophene,19,20 and aniline21,22 monomer derivatives, containing thiol and silane functional groups, on electrodes has also been of great research interest. The assembled monomer units can be polymerized chemically or electrochemically within (13) Handbook of Organic Conductive Molecules and Polymers; Nalwa, H. S. Ed.; John Wiley & Sons: Chichester, 1997. (14) Kulkarni, V. G. In Handbook of Conducting Polymers, 2nd ed.; Skotheim, T. A., Elsenbaumer, R. L., Reynolds, J. R., Eds.; Marcel Dekker: New York, 1998; p 1059. (15) Van Dyke, L. S.; Brumlik, C. J.; Liang, W.; Lei, J.; Martin, C. R.; Yu. Z.; Li, L.; Collins, G. J. Synth. Met. 1994, 62, 75. (16) Pun, M. Y.; Neoh, K. G.; Kang, E. T.; Loh, F. C.; Tan, K. L. J. Appl. Polym. Sci. 1995, 56, 355. (17) Willicut, R. J.; McCarley, R. M. Langmuir 1995, 11, 296. (18) Simon, R. A.; Ricco, A. J.; Wrighton, M. S. J. Am. Chem. Soc. 1982, 104, 2031. (19) Ng, S. C.; Miao, P.; Chen, Z. K.; Chan, H. S. O. Adv. Mater. 1998, 10, 782. (20) Zotti, G.; Schiavon, G.; Zecchin, S.; Berlin, A.; Pagani, G.; Canavesi, A. Langmuir 1997, 13, 2694. (21) Sato, N.; Nonaka, T. Chem. Lett. 1995, 805. (22) Rubinstein, I.; Rishpon, J.; Sabatani, E.; Redondo, A.; Gottesfeld, S. J. Am. Chem. Soc. 1990, 112, 6135.

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the monolayer.23,24 The substrates used are mostly inorganic materials, such as gold,17,19,21 platinum,23 and indium tin oxides (ITO).20,22,24 In the present work, surface modification of argon plasma-pretreated PTFE film was carried out via UVinduced graft copolymerization with 2-hydroxyethyl methacrylate (HEMA), followed by silanization of (p-aminophenyl)trimethoxysilane (APTS), and finally oxidative graft polymerization with aniline. The chemisorption of the APTS on the hydroxylated PTFE surface is shown to be an effective approach for introducting the aniline units, which are used for the covalent bonding of the aniline polymer chains. The chemical composition and structure of the surface-modified PTFE films are characterized by X-ray photoelectron spectroscopy (XPS) and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, respectively. Experimental Section Materials. PTFE film having a thickness of about 0.1 mm and a density of 2.18 g/cm3 was used in this study and was obtained from Goodfellow Inc. of Cambridge, UK. The surface of the film was cleaned with acetone in an ultrasonic water bath for 0.5 h before use. The monomers, 2-hydroxyethyl methacrylate (HEMA) and aniline, and the solvent, N-methylpyrrolidinone (NMP), used for surface graft copolymerization were obtained from the Aldrich Chemical Co. of Milwaukee, WI. The aminosilane, APTS, used for silanization of the HEMA graftcopolymerized PTFE film surfaces was purchased from Gelest Inc. of Tullytown, PA, and was used as received. The chemical structures of HEMA and APTS are shown below.

Hydroxylation of PTFE Surface by UV-induced Graft Copolymerization with HEMA: The HEMA-g-PTFE Surface. The PTFE films were cut into strips of about 2 cm × 3 cm in size. They were pretreated with Ar plasma before being subjected to UV-induced graft copolymerization with HEMA. A cylindrical type glow discharge cell, Model SP100, manufactured by Anatech Ltd. of Springfield, VA, was used for the plasma pretreatment. The plasma power applied was kept at 36 W at a radio frequency of 40 kHz. The film was placed between the two parallel electrodes (separation 2.5 cm) and subjected to the glow discharge for a predetermined period of time at an Ar pressure of about 0.5 Torr. The Ar plasma-pretreated polymer films were then exposed to the atmosphere to affect the formation of surface peroxide and hydroperoxide species25 before graft copolymerization. The UV-induced surface graft copolymerization with HEMA was carried out in a Riko model RH 400-10 W rotary photochemical reactor, manufactured by Riko Denki Kogyo of Chiba, Japan. The reactor was equipped with a 1000 W high-pressure Hg lamp and a constant-temperature water bath (28 °C). The Ar plasma-pretreated PTFE films were immersed in 30 mL of aqueous solution of HEMA in a Pyrex tube. The HEMA monomer concentration was varied from 5 to 20 wt %. Each reaction mixture was thoroughly degassed and sealed under a nitrogen atmosphere. It was then subjected to UV irradiation for 30 min. After each grafting experiment, the PTFE film was washed thoroughly with copious amounts of doubly distilled water to remove the physically absorbed homopolymer and unreacted monomer. Silanization of the HEMA-g-PTFE Surface by APTS: The APTS-HEMA-g-PTFE Surface. The silanization of the hy(23) Oh, S. Y.; Yun, Y. J.; Kim, D. Y.; Han, S. H. Langmuir 1999, 15, 4690. (24) Inaoka, S.; Collard, D. M. Langmuir 1999, 15, 3752. (25) Suzuki, M.; Kishida, A.; Iwata, H.; Ikada, Y. Macromolecules 1986, 19, 1804.

Ji et al. droxylated PTFE film surface from graft copolymerization with HEMA was carried out in a dry toluene solution of APTS for 1 min. The concentrations of the APTS were varied from 0.5 to 10 wt %. After the silane treatment, the PTFE film was rinsed with copious amounts of toluene to remove the physically adsorbed silane molecules. The APTS became coupled to the HEMA-gPTFE surface via the formation of a Si-O bond.26 The resulting surface is referred to as the APTS-HEMA-g-PTFE surface. Oxidative Graft Polymerization of Aniline: The PAnAPTS-HEMA-g-PTFE Surface. After silanization, the PTFE film was immersed in a 0.5 M aqueous solution of HCl containing 0.02-0.2 M aniline and the corresponding amount of (NH4)S2O8 oxidant to achieve a monomer-to-oxidant molar ratio of 1. The reaction was allowed to proceed for 5 h in an ice bath or in a water bath at an elevated temperature between 30 and 50 °C. Aniline monomer was graft-polymerized onto the PTFE film surface via the aniline functional groups of the chemically coupled silane molecules. The as-prepared emeraldine (EM) salt of polyaniline (PAn) on the PTFE surface was converted to the neutral EM base by equilibrating in copious amounts of doubly distilled (DI) water for 24 h under stirring. The regular base treatment with NaOH or NH4OH was not adopted in order to avoid the possible etching of the chemically coupled APTS molecules via the dissociation of the Si-O bonds. The PTFE film was subsequently immersed in a large volume of NMP (a good solvent for EM base) to remove the physically absorbed aniline homopolymer. The PTFE film so obtained was referred to as the PAn-APTS-HEMA-g-PTFE film. Reprotonation of the PAn-APTSHEMA-g-PTFE film was carried out in a 1 M aqueous solution of 10-camphorsulfonic acid (CSA) or HClO4 for 10 min. Surface Characterization. The chemical composition of the PTFE surface after each step of surface modification was analyzed by X-ray photoelectron spectroscopy (XPS). The XPS measurements were made on a Kratos Analytical AXIS HSi spectrometer with a monochromatized Al KR X-ray source (1486.6 eV photons) at a constant dwell time of 100 ms and a pass energy of 40 eV. The core-level signals were obtained at a photoelectron takeoff angle of 90° (with respect to the sample surface). The PTFE substrates were mounted on the standard sample studs by means of double-sided adhesive tapes. The X-ray source was run at a reduced power of 150 W. The pressure in the analysis chamber was maintained at 7.5 × 10-9 Torr or lower during each measurement. All binding energies (BEs) were referenced to the C1s hydrocarbon peak at 284.6 eV or the CF2 peak at 291.8 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 peak-area ratios, after correcting with the experimentally determined sensitivity factors, and were reliable to (10%. The elemental sensitivity factors were determined using stable binary compounds of wellestablished stoichiometries. Static water contact angles of the pristine and the HEMA-g-PTFE film surfaces were measured at 25 °C and 60% relative humidity by the sessile drop method, using a telescopic goniometer (Rame-hart, Model 100-00-(230), Mountain Lakes, NJ). The telescope with a magnification power of 23× was equipped with a protractor of 1° graduation. For each sample, at least five measurements on different surface locations were averaged. The attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra of the surface graft-copolymerized PTFE films were measured on a Shimadzu Model 8101 FTIR. The surface resistance was measured by the two-probe method, using a Hioki Model 3265 digital electrometer.

Results and Discussion The processes of surface modification of the Ar plasmapretreated PTFE film via UV-induced graft copolymerization with HEMA, silanization of the hydroxylated surface with APTS, oxidative graft polymerization of aniline, and the interconversion between the insulating and conducting states of the grafted aniline polymer by (26) Vargo, T. G., Jr.; Gardella, J. A.; Litwiler, K. S.; Broght, F. V. Langmuir 1991, 7, 142.

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Figure 1. Schematic diagram illustrating the processes of surface modification of Ar plasma-pretreated PTFE films via graft copolymerization with HEMA, silanization of APTS, and oxidative graft polymerization with aniline.

deprotonation/reprotonation are shown schematically in Figure 1. Each process is described and discussed in detail below. Surface Modification of PTFE Film via Ar Plasma Pretreatment and Graft Copolymerization with HEMA: The HEMA-g-PTFE Surface. The changes in surface [O]/[F] atomic ratio of the Ar plasma-pretreated PTFE film, after the air exposure, as a function of the Ar plasma treatment time were first evaluated. The [O]/[F] ratios are determined from the sensitivity factor-corrected O1s and F1s core-level spectral area ratios. An increase in the [O]/[F] ratios is observed upon increasing the Ar plasma treatment time, in agreement with the results generally reported in the literature.27,28 The Ar plasma treatment causes the breakage of some C-F bonds. The subsequent exposure of the activated surface to air causes oxygen to be incorporated onto the PTFE surface, leading to surface oxidation and formation of peroxide and hydroperoxide species.25 The peroxides formed on the PTFE surface under similar conditions have been quantified by the reaction with SO2.29 The optimum concentration of peroxide species achieved corresponds to an Ar plasma treatment time of about 60 s. Thus, an Ar plasma pretreatment time of 60 s is selected in the present work for the PTFE substrates. Figure 2 shows the respective C1s core-level spectra of the pristine PTFE film and the 60-s Ar plasma-pretreated PTFE films after having been subjected to UV-induced graft copolymerization in two different concentrations (8 and 20 wt %) of HEMA solution for 30 min. The C1s corelevel spectrum of the pristine PTFE surface consists of a main peak component at the binding energy (BE) of about 291.8 eV, attributable to the substrate CF2 species.27 The presence of the surface-grafted HEMA polymer can be deduced from the appearance of the C1s peak components with BEs at 284.6 eV for the CH species, 286.2 eV for the CO species, and 288.7 eV for the COO species.30 The [CH]: [CO]:[COO] peak component area ratios for the PTFE (27) Badey, J. P.; Espuche, E. U.; Jugnet, Y.; Sage. D.; Duc, T. M.; Chabert, B. Polymer 1994, 35, 2472. (28) Da, Y. X.; Griesser, H. J.; Mau, A. W. H.; Schmidt, R.; Liesegang, J. Polymer 1991, 32, 1126. (29) Wu, S.; Kang, E. T.; Neoh, K. G.; Tan, K. L. Langmuir 2000, 16, 5192. (30) Wu, S.; Kang, E. T.; Neoh, K. G.; Cui, C. Q.; Lim, T. B. IEEE Trans. Adv. Pack. 2000, 23, 538.

Figure 2. C1s core-level spectra of (a) the pristine PTFE film and the HEMA-g-PTFE films from UV-induced graft copolymerization of the 60-s Ar plasma-pretreated PTFE films in (b) 8 wt % and (c) 20 wt % HEMA monomer solutions (UV grafting time ) 30 min)

surfaces with a low graft concentration (Figure 2b) and a high graft concentration (Figure 2c) are on the order of 3.6:2.1:1.0 and 3.2:2.1:1.0, respectively, in fairly good agreement with the theoretical ratios of 3:2:1, dictated by the chemical structure of HEMA. Full coverage of the PTFE surface by the HEMA polymer to beyond the probing depth of the XPS technique (about 7.5 nm in an organic matrix31) is indicated by the disappearance of the CF2 peak component in the C1s core-level spectrum in Figure 2c, obtained at the HEMA monomer concentration of 20 wt %. The concentration of the surface-grafted HEMA polymer, as well as the water contact angle, as a function of the HEMA monomer concentration used for graft copolymerization is shown in Figure 3. The graft concentration within the probing depth of the XPS technique is expressed as the [COO]/[F] ratio, since one repeating unit (31) Tan, K. L.; Woon, L. L.; Wong, H. K.; Kang, E. T.; Noeh, K. G. Macromolecules 1993, 29, 2832.

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Figure 5. C1s, N1s, and Si2p core-level spectra of the HEMAg-PTFE film with [COO]/[F] ratios of (a-c) 1.2 and (d-f) 5.3 after silanization in 5 wt % APTS solution for 1 min. Figure 3. Effect of HEMA monomer concentration on the graft concentration ([COO]/[F] ratio) and water contact angle of the 60-s Ar plasma-pretreated PTFE films.

Figure 4. ATR-FTIR spectrum of (a) the pristine PTFE film, (b) the HEMA-g-PTFE film ([COO]/[F] ) 5.3), and (c) the PAnAPTS-HEMA-g-PTFE film ([N]/[C] ) 0.13).

of HEMA polymer contains one COO group. The graft concentration increases with the monomer concentration. At the monomer concentration of about 20 wt %, the PTFE surface is almost fully covered by the HEMA polymer ([COO]/[F] ) 5.3). The water contact angle of the graftmodified PTFE surface decreases with increasing monomer concentration, and thus with increasing HEMA polymer graft concentration. The water contact angle of the HEMA-g-PTFE surface with a [COO]/[F] ratio of 5.3 is on the order of 45°, compared to about 115° for the pristine PTFE surface. This observation is consistent with the hydrophilic nature of the HEMA polymer.32 The presence of the grafted HEMA polymer on the Ar plasmapretreated PTFE film under the present reaction conditions is also confirmed by the attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectrum, as shown in Figure 4. The appearance of the absorption peak at 1720 cm-1, attributable to the CdO group of the HEMA polymer,32 confirms the presence of the surface-grafted HEMA polymer. The PTFE substrate gives rise to a characteristic absorption band at about 1140 cm-1, attributable to the CF2 stretching vibration.33 (32) Lee, S. D.; Hsiue, G. H.; Wang, C. C. J. Appl. Polym. Sci. 1994, 54, 1279. (33) Vargo, T. G., Jr; Gardella, J. A.; Meyer, A. E.; Baier, R. E. J. Polym. Sci, Part A: Chem. 1991, 29, 555.

Silanization of Hydroxylated PTFE Film with APTS: The APTS-HEMA-g-PTFE Surface. The HEMA-g-PTFE surface can be further functionalized through the coupling of the OH groups of the grafted HEMA chains with a silane coupling agent (containing Si-O group).26,34 Figure 5 shows the respective C1s, N1s, and Si2p core-level spectra of two HEMA-g-PTFE films with [COO]/[F] ratios of 1.2 (Figure 5a-c) and 5.3 (Figure 5d-f) after silanization in 5 wt % toluene solution of APTS for 1 min. The changes in the C1s core-level line shape and the appearance of the N1s and Si2p core-level signals after silanization suggest that the APTS must have been successfully chemisorbed onto the HEMA-g-PTFE surfaces (the APTS-HEMA-g-PTFE surfaces). The coupling of APTS has also resulted in a further decrease in the relative intensity of the CF2 component (compare Figure 5a to Figure 2b). Thus, the combined thickness of the APTS and HEMA layers on the surface with low HEMA graft concentration ([COOH]/[F] ) 1.2) is also approaching the XPS probing depth of about 7.5 nm.31 The N1s core-level spectra comprise only a single peak component at the BE of about 399.4 eV, attributable to the amino groups of the chemisorbed APTS silane.35 The effect of the graft concentration of HEMA polymer on the amount of coupled silane on the PTFE surface, expressed as the [Si]/[F] atomic ratio and determined from the Si2p and F1s core-level spectral area ratio, is shown in Figure 6. The [Si]/[F] ratios increase with the increase in graft concentration of the HEMA polymer, suggesting that the amount of the coupled APTS depends on the surface concentration of the hydroxyl groups associated with the grafted HEMA polymer. The effect of the APTS solution concentration on the amount of coupled silane on the HEMA-g-PTFE surface ([COO]/[F] ratio of 5.3) is also shown in Figure 6. The [Si]/[F] ratio increases sharply with the APTS solution concentration at low APTS concentration and reaches an asymptotic value for APTS concentration above 2 wt %. Thus, the APTS-HEMA-g-PTFE films from silanization in 5 wt % APTS solution, having a [Si]/[F] ratio of 1.68 and a [N]/[C] ratio of 0.076, are used for the subsequent oxidative graft polymerization with aniline. The high surface concentration of the amino groups from the silanization reaction is expected to promote the subsequent graft polymerization reaction of aniline. For comparison purpose, APTS silane treatment was also carried out on the 60-s Ar plasma-treated PTFE film after air exposure. The Ar plasma-pretreated PTFE surface was not subjected (34) Plueddemann, E. P. Silane Coupling Agents; Plenum Press: New York and London, 1982. p 33. (35) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. In Handbook of X-ray Photoelectron Spectroscopy; Chastine, J., Ed.; PerkinElmer Corporation: Eden Prairie, MN, 1992; p 43.

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Figure 7. C1s and N1s core-level spectra of the APTS-HEMAg-PTFE film ([COOH]/[F] ) 5.3, [Si]/[F] ) 1.68, and [N]/[C] ) 0.076) after oxidative graft polymerization in (a, b) 0.02 M and (c, d) 0.1 M aniline solution, followed by deprotonation in water.

Figure 6. Effect of the graft concentration of the HEMA polymer on the chemisorption of APTS on the HEMA-g-PTFE surfaces. Effect of APTS silane concentration on the chemisorption of the HEMA-g-PTFE film ([COO]/[F] ) 5.3) is also shown.

to UV-induced graft copolymerization with HEMA prior to silanization. The XPS results revealed that the APTS was also chemisorbed onto the Ar plasma-treated PTFE surface to give rise to a [Si]/[F] ratio of about 0.06. This [Si]/[F] ratio, however, is substantially lower than those achievable in the presence of surface-grafted HEMA chains (see Figure 6). Thus, the contribution of the grafted HEMA chains to the surface silanization reaction is ascertained. It should be noted, however, that optimization of the hydroxyl group concentration from plasma treatment alone could provide an alternative approach to HEMA graft copolymerization. It has been reported that fluoropolymer surfaces can be homogeneously hydroxylated by exposure to a RF plasma composed of a vapor mixture of hydrogen and methanol.36 Oxidative Graft Polymerization of Aniline onto the APTS-HEMA-g-PTFE Surface: The PAn-APTSHEMA-g-PTFE Surface. The aniline groups on the APTS-HEMA-g-PTFE surface can serve as the anchoring and starting sites for the oxidative graft polymerization of aniline in the presence of an oxidant.37 Figure 7 shows the C1s and N1s core-level spectra of the APTS-HEMAg-PTFE film, with a [Si]/[F] ratio of 1.68, after oxidative graft polymerization in 0.5 M HCl solution containing 0.02 M (parts a and b) and 0.1 M (parts c and d) aniline, followed by deprotonation of the surface-grafted polyaniline (PAn in its emeraldine or EM salt form) in doubly distilled water for 24 h to convert the EM salt to its neutral base form. The physically adsorbed EM homopolymer, on the other hand, has been removed by repeated extraction with NMP. The presence of the grafted aniline polymer is indicated by the broadening of the N1s core-level line shape and the appearance of a new peak component at a BE of about 398.2 eV, attributable to the quinonoid imine (dN-) unit of the grafted PAn (in the EM base form).37 The peak component at BE ) 399.4 eV is attributable to both the benzenoid amine (-NH-) units of the EM base and the amine groups of the initially coupled APTS silane. The extent of the aniline graft polymerization can be (36) Vargo, T. G.; Gardella, J. A.; Calvert, J. A.; Chen, M. S. Science 1993, 262, 1711. (37) Kang, E. T.; Neoh, K. G.; Tan, K. L. Prog. Polym. Sci. 1998, 23, 277.

expressed as the [N]/[C] ratio and determined from the N1s and C1s core-level spectral area ratio. However, it should be noted that some of the nitrogen atoms are contributed by the aniline units of the initially coupled APTS silane molecules. As shown in Figure 7, the [N]/[C] ratio increases substantially (from the initial value of 0.076 for the APTSHEMA-g-PTFE surface) after the oxidative graft polymerization with aniline, providing further support to the presence of surface-grafted PAn. The [dN-]/[-NH-] ratio is somewhat less than 1 for the surface with low PAn graft concentration (Figure 7b), suggesting that the excess amount of the amine unit must have resulted from the initially coupled APTS silane. At the high PAn graft concentration, however, the [dN-]/[-NH-] ratio approaches 1 (Figure 7). A quinonoid imine to benzenoid amine ratio ([dN-]/[-NH-] ratio) of unity is characteristic of that of the 50% intrinsically oxidized EM base form of PAn.37 The grafting of PAn on the APTS-HEMAg-PTFE surface is also confirmed by the ATR-FTIR spectra, as shown in Figure 4. The appearance of the two distinct absorption bands at about 1510 and 1600 cm-1, attributable to benzenoid unit and quinonoid unit,38 respectively, suggests the presence of PAn in its 50% intrinsically oxidized EM base form. Thus, the aniline units of the initially coupled APTS molecules must have served as the “anchoring sites” for the subsequent graft polymerization of aniline. The effect of aniline monomer concentration on the extent of aniline graft polymerization on the APTS-HEMA-g-PTFE surface (initial [COOH]/[F] ) 5.3, [Si]/[F] ) 1.68, and [N]/[C] ) 0.076) is shown in Figure 8. A high concentration of aniline monomer leads to a high graft concentration of PAn on the APTS-HEMAg-PTFE surface. In the present work, the oxidative graft polymerization of aniline is carried out in 0.1 M aniline solution. To investigate the surface conductivity of the PAnAPTS-HEMA-g-PTFE film, acid treatment (reprotonation) was carried out to convert the insulating EM base to its conducting EM salt form. Two types of acid, an organic acid, camphorsulfonic acid (CSA), and an inorganic acid, HClO4,37 were used. Reprotonation occurs readily in a 1 M aqueous solution of either acid. Figure 9 shows the N1s and S2p core-level spectra of the PAn-APTS-HEMA-gPTFE films with [N]/[C] ratios of 0.10 (parts a and b) and 0.13 (parts c and d), after reprotonation in 1 M CSA. Comparison of the N1s core-level spectra in parts a and c of Figure 9 to those of the corresponding undoped PAn(38) Tang, J.; Jing, X.; Wang, B.; Wang, F. Synth. Met. 1988, 24, 231.

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Figure 8. Effect of aniline monomer concentration on the graft concentration of PAn (expressed as the [N]/[C] ratio) and on the surface resistance of the PAn-APTS-HEMA-g-PTFE films after reprotonation in 1 M CSA and 1 M HClO4.

Ji et al.

and the bulk conductivity of 1-100 S/cm for the protonated PAn homopolymer film.40 In all cases, the surface resistance of the PAn-APTS-HEMA-g-PTFE film reprotonated by 1 M CSA acid is found to be lower than that of the corresponding surface reprotonated by 1 M HClO4. The result is similar to some extent to that of the PAn/CSA film cast from m-cresol.41 The relationship between surface resistance and bulk resistivity (F) is given by Rs ) F/t, where t is the thickness of the film.42 The thickness of the grafted PAn layer can be approximated from the sampling depth of the XPS technique (about 7.5 nm for an organic matrix31), since the Si signals arising from the underlying APTS silane are barely discernible. Thus, the bulk resistivity of the PAn layer on the surface-modified PTFE film is on the order of 10-2 Ω cm, which is equivalent to a bulk conductivity (σ) of about 102 S/cm, as σ ) 1/F. Thus, the electrical conductivity of the aniline polymer is preserved in the grafted PAn layer, as is also suggested by the N1s line shape of the reprotonated PAn-APTSHEMA-g-PTFE surface (Figure 9c). The so-obtained PAnAPTS-HEMA-g-PTFE films are potentially useful to control electromagnetic radiation and in applications for electrostatic charge dissipation, which require a surface resistance on the order of 103-106 Ω/sq.14 For applications requiring higher surface conductivities, the grafted aniline polymer can be coupled to the palladium reduction process in an acid Pd(II) ion solution. The PAn-APTS-HEMAPTFE surface with the electrolessly deposited palladium can then be used to catalyze the electroless plating of copper,43 resulting in a copper-metallized PTFE surface. Conclusion

Figure 9. N1s and S2p core-level spectra of the PAn-APTSHEMA-g-PTFE film with [N]/[C] ratios of (a, b) 0.10 and (c, d) 0.13 after reprotonation in 1 M CSA.

APTS-HEMA-g-PTFE surfaces in parts b and d of Figure 7 suggests that the surface-grafted PAn on the PTFE film can be effectively reprotonated (redoped) by CSA, as indicated by the disappearance of the imine (dN-) peak component and the appearance of the corresponding proportion of the positively charged nitrogen (N+) in each spectrum. Similar results are observed for the treatment with 1.0 M HClO4. Thus, the reprotonation behavior of the PAn-APTS-HEMA-g-PTFE surface is not unlike that of the PAn (EM base) homopolymer, in which protonation occurs preferentially at the imine units.37 The successful reprotonation of the PAn-APTS-HEMA-g-PTFE surface is also indicated by the changes in color of the film surface from blue for the insulating EM base form to green for the conducting EM salt. This deprotonation-reprotonation behavior of the PAn-APTS-HEMA-g-PTFE surfaces further confirms the presence of the grafted PAn chains on the PTFE surfaces. The surface resistance of the reprotonated PAn-APTSHEMA-g-PTFE films as a function of the aniline monomer concentration used for graft polymerization is shown in Figure 8. Generally, the surface resistance decreases with increasing aniline concentration used in the oxidative polymerization. The surface resistance is on the order of 103 Ω/sq at the high PAn graft concentration ([N]/[C] ) 0.13), compared to 1015 Ω/sq for the pristine PTFE film39

Ar plasma-pretreated PTFE films were subjected to surface modification, in sequence, by UV-induced graft copolymerization with HEMA, silanization of APTS, and oxidative graft polymerization with aniline (the PAnAPTS-HEMA-g-PTFE surfaces). The graft concentration of the HEMA polymer increased with increasing HEMA monomer concentration. The extent of the APTS silanization was affected by both the graft concentration of the HEMA polymer and the APTS concentration in the silanization solution. The aniline groups of the surfacecoupled APTS molecules were activated in the presence of an oxidant to induce the graft polymerization of aniline. The graft concentration of PAn was enhanced by the increase in APTS surface concentration on the PTFE surfaces, as well as by the increase in aniline monomer concentration used for the oxidative graft polymerization. The intrinsic oxidation state and the deprotonationreprotonation behavior of the grafted PAn were not unlike those of the aniline homopolymer. The surface resistance of the PAn-APTS-HEMA-g-PTFE film reprotonated by CSA was lower than that of the corresponding film reprotonated by HClO4. The optimum surface resistance was on the order of 103 Ω/ sq. LA0260483 (39) Goodfellow Catalogue (Metals, Alloys, Compounds, Ceramics, Polymers, Composites); Goodfellow, Ltd.: Cambridge, U. K., 1996/1997; p 447. (40) Trivedi, D. C. Polyaniline. In Handbook of Organic Conductive Molecules and Polymers; Nalwa, H. S. Ed.; John Wiley & Sons: Chichester, 1997; Vol. 2, p 505. (41) Abell, L.; Adams, P. N.; Monkman, A. P. Polymer 1996, 37, 5927. (42) Jaeger, R. C. In Modular Series on Solid State Devices; Neudeck, G. W., Pierret, R. F., Eds.; Addison-Wesley: Reading, MA, 1993; Vol. 5, p 66. (43) Zhang, M. C.; Kang, E. T.; Neoh, K. G.; Tan, K. L. J. Electochem. Soc. 2001, 148, C71.