Consecutive Graft Copolymerization of Glycidyl Methacrylate and

(PTFE) film by UV-induced graft copolymerization with glycidyl methacrylate (GMA), .... A New Formation Route to form High-adhesion Polytetrafluor...
0 downloads 0 Views 126KB Size
Download PDF
9666

Langmuir 2000, 16, 9666-9672

Consecutive Graft Copolymerization of Glycidyl Methacrylate and Aniline on Poly(Tetrafluoroethylene) Films M. C. Zhang, 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 April 14, 2000. In Final Form: July 3, 2000 Chemical modification of Ar plasma-pretreated poly(tetrafluoroethylene) (PTFE) film by UV-induced graft copolymerization with glycidyl methacrylate (GMA), followed by oxidative graft copolymerization of aniline and reactive immobilization of polyaniline (PANI) chains have been carried out to render the PTFE surface conductive. The surface compositions and microstructures of the graft-copolymerized PTFE films were studied by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), respectively. The PANI chains grafted onto PTFE film surface were similar to the PANI homopolymer. The surface resistance of the aniline graft-copolymerized PTFE film could be reduced to the order of about 104 Ω/sq, compared to the order of 1016 Ω/sq for the pristine PTFE film. Cohesive failure occurred inside the bulk of PTFE film when an epoxy adhesive was applied to peel off the grafted PANI layer from the GMA graft-copolymerized PTFE substrate. The strong adhesion arose from the fact that the PANI chains were covalently bonded onto the GMA graft-copolymeried PTFE film through the curing of epoxide groups of the grafted GMA polymer by the amine groups of the aniline molecules during oxidative graft copolymerization and the amine groups of PANI after the oxidative polymerization of aniline.

Introduction The deposition of electroactive polymers onto solid surfaces has attracted the attention of many researchers due to its important applications in areas of electromagnetic shielding, electromagnetics interference (EMI), anticorrosion coatings, and microelectronics device fabrication.1-5 During the past two decades, several methods, such as oxidative chemical or electrochemical deposition,10 spin-coating11 and dip-coating,12 have been developed to deposit electroactive polymers onto solid substrates, including textile materials,6 polymer films,7-9 and glass sheets.3 More recently, plasma-polymerization and deposition of polyaniline (PANI) film on FEP substrate has been reported.13 Generally, the lack of adhesion is a major problem associated with the deposition of conductive polymer on solid surface. To improve the adhesion of * To whom correspondence should be addressed. Fax: (65) 7791936. E-mail: [email protected] (1) Joo, J.; Epstein, A. J. Appl. Phys. Lett. 1994, 65, 2278. (2) Jeon, N. L.; Nuzzo, R. G.; Xia, Y.; Mrksich, M.; Whitesides, G. M. Langmuir 1995, 11, 3024. (3) Huang, Z.; Wang, P. C.; Feng, J.; MacDiarmid, A. G. Synth. Met. 1997, 85, 1375. (4) Wei, Y.; Wang, J.; Yeh, J. M.; Spellane, P. Polymer, 1995, 36, 4535. (5) Olmedo, L.; Hourquebie, P.; Jousse, F. Synth. Met. 1995, 69, 205. (6) Turyan, I.; Mandler, D. J. Am. Chem. Soc. 1998, 120, 10 733. (7) 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. (8) Avlyanov, J. K.; Josefowicz, J. Y.; MacDiarmid, A. G. Synth. Met. 1995, 73, 205. (9) Gregory, R. B.; Kimbrell, W. C.; Kuhn, H. H. Synth. Met. 1989, 28, C-823. (10) Neoh, K. G.; Teo, H. W.; Kang, E. T. Langmuir 1998, 14, 2820. (11) Niwa, O.; Tamamura, T. Synth. Met. 1987, 18, 677. (12) Ojio, T.; Miyata, S. Polym. J. 1986, 18, 95. (13) Gong, X.; Dai, L.; Mau, A. W. H.; Griesser, H. J. J. Polym. Sci. A: Polym. Chem. 1998, 36, 633.

conductive polymers on polymer substrates, chemical modification of polymer surfaces prior to deposition has also been reported.14-16 On the other hand, carboxylized thiophene trimer has been successfully assemblied on silicon wafers by means of alkylaminosilane coupling agent.17 Covalent bonding involving chemical reactions between the conductive polymer layer and the surface molecules of the substrate appears to be an attractive approach. Among the techniques available, surface modification of the substrates via graft copolymerization is one of the most widely studied techniques, as it allows for the molecular redesign of most substrate surfaces to impart new and specific functionalities.18-20 Numerous conductive polymers have been “rediscovered”, synthesized, and characterized during the last two decades.21 Among these polymers, the century-old PANI has been of particular interest because of its environmental stability, controllable electrical conductivity, and interesting redox properties associated with the chain nitrogen.22,23 On the other hand, fluoropolymers, such as poly(tetrafluoroethylene) (PTFE), have many desirable (14) Clark, D. T.; Dilks, A.; Shuttleworth, D. In Polymer Surfaces; Clark, D. T., Feast, W. J., Eds; Wiley: New York, 1978, Ch. 9. (15) Pun, M. Y.; Neoh, K. G.; Kang, E. T.; Loh, F. C.; Tan, K. L. J. Appl. Polym. Sci. 1995, 56, 355. (16) Neoh, K. G.; Teo, H. W.; Kang, E. T. Langmuir 1998, 14, 2820. (17) Cao, C.; Wang, C.; Cao, Y.; Xie, T. et al. J. Photochem. Photobiology A: Chemistry 1999, 127, 101. (18) Uyama, Y.; Kato, K.; Ikada, Y. Adv. Polym. Sci. 1998, 137, 1. (19) Ra˚nby, B. J. Adhesion Sci. Technol. 1995, 9, 599. (20) Kang, E. T.; Neoh, K. G.; Tan, K. L.; Uyama, Y.; Morikawa, N.; Ikada, Y. Macromolecules 1992, 25, 1959. (21) Nalwa, H. S., Ed. Handbook of Organic Conductive Molecules and Polymers; John Wiley & Sons: Chichester, 1997, Vols. I-IV. (22) Kang, E. T.; Neoh, K. G.; Tan, K. L. Prog. Polym. Sci. 1998, 23, 277. (23) Gospodinova, N.; Terlemezyan, L. Prog. Polym. Sci. 1998, 23, 1443.

10.1021/la000568l CCC: $19.00 © 2000 American Chemical Society Published on Web 10/14/2000

Consecutive Graft Copolymerization

properties, such as thermal stability, a low dielectric constant, and chemical inertness, that make them ideal dielectric materials for microelectronics applications.24-26 As to the practical applications, extensive work has been done to overcome the surface inertness of PTFE and other fluoropolymers. These efforts have included the modification of the fluoropolymer surfaces by wet chemical, hydrogen plasma, UV laser, and electron beam treatments.27 In the present work, a novel method for the covalent immobilization of the PANI chains onto the surfaces of PTFE films is described. The method initially involves surface modification of the Ar plasma-pretreated PTFE film via UV-induced graft copolymerization with glycidyl methacrylate (GMA). The GMA graft-copolymerized PTFE surface is then subjected to oxidative graft copolymerization with aniline. Thermal curing of the epoxide functional groups of the graft GMA chains with the amine groups of the PANI homopolymer readily resulted in further covalent bonding of the PANI chains on the insulating PTFE substrate. Experimental Section Materials. The poly(tetrafluoroethylene) (PTFE) film of 0.1 mm thickness was purchased from Goodfellow Ltd. of Cambridge, UK. The surfaces of the PTFE films were cleaned ultrasonically with acetone and then soaked in acetone for 24 h before use. The glycidyl methacrylate (GMA) and aniline (An) monomers were obtained from Aldrich Chemical Co., USA and were used as received. The solvents, such as 1,4-dioxane, acetone, ethanol, N-methylpyrrolidinone (NMP), and other chemicals, were of reagent grade. They were also obtained from Aldrich Chem. Co. and were used as received. Plasma Pretreatment and Surface Graft Copolymerization. A cylindrical quartz glow discharge cell, Model SP-100, manufactured by Anatech Ltd, of USA, was used for the plasma pretreatment of the PTFE substrate. The glow discharge was generated at a frequency of 40 kHz and a plasma power of 35 W. The pressure in the quartz cell was maintained at ∼0.58 Torr of argon, whereas the polymer films were subjected to the glow discharge for 80 s. The Ar plasma-pretreated PTFE films were exposed to air for about 10 min to facilitate the formation of surface peroxides and hydroperoxides species for the subsequent UV-induced surface graft copolymerization process.28 The surface modification of PTFE film by graft copolymerization with GMA and aniline was carried out in two steps. The first step involved the UV-induced graft copolymerization of GMA on the Ar plasma-pretreated PTFE film (the GMA-g-PTFE surface). The second step involved the oxidative graft copolymerization of aniline on the GMA-g-PTFE surface and the thermal curing of the PANI chains. The surfaces of the PTFE films were first activated by Ar plasma treatment and atmospheric exposure. They were then immersed individually in about 30 mL of 1,4-dioxane solution of GMA monomer in a Pyrex tube. The GMA monomer concentration was 20 vol %. The dissolved air in the reaction mixture was removed by purging with purified nitrogen for about 1 h before the tube was sealed off with a silicone stopper. The reaction mixture was then subjected to UV-induced graft polymerization in a Riko Rotary (Model RH 400-10W) 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. All of the UV-induced graft polymerization experiments were carried out at a constant temperature of 28 °C. The PTFE films after graft copolymerization (24) Park, J. M.; Matienzo, L. J.; Spencer, D. F. J. Adhes. Sci. Technol. 1991, 5, 153. (25) Granneman, E. H. A. Thin Solid Films 1993, 228, 1. (26) Silvain, J. F.; Ehrhardt, J. J.; Picco, A.; Lutgen, P. ACS Symp. Ser. 1990, 440, 453. (27) Sacher, E. Prog. Surf. Sci. 1994, 47, 273. (28) Suzuki, M.; Kishida, A.; Iwata, H.; Ikada, Y. Macromolecules 1986, 19, 1804.

Langmuir, Vol. 16, No. 24, 2000 9667 with GMA (the GMA-g-PTFE surfaces) were washed ultrasonically with copious amounts of acetone to remove the residual monomer and adsorbed homopolymer before being dried under reduced pressure. The oxidative graft copolymerization of aniline on the GMAg-PTFE surface was carried out in 0.5 M aqueous solution of H2SO4, containing the GMA-g-PTFE films, 0.2 M of aniline, and 0.2 M of (NH4)2S2O8 oxidant. The reaction mixture was stirred rigorously for about 5 h. Except for the presence of the GMAg-PTFE film, the oxidative polymerization method was thus similar to that commonly employed for the oxidative homopolymerization of aniline to produce the emeraldine (EM) salt.29 The GMA-g-PTFE film with both the covalently grafted EM salt and the physically coated (adhered) EM salt on the surface was then transferred to a vacuum oven for thermal treatment at 100 °C for 6 h, unless stated otherwise. The thermal treatment allowed for the curing of the physically adhered EM homopolymer on the GMA-g-PTFE surface. The EM salt on the PTFE surface was converted to the neutral EM base form by treatment with a large excess of 0.5 M NaOH for 1 h. The surface neutralized film was cleaned ultrasonically in NMP for 5 min and then immersed in a large volume of NMP for at least 48 h, with continuous stirring, to remove the residual amount of the physically adsorbed EM base polymer before been dried under reduced pressure (the PANI-GMA-g-PTFE film). The PANI-GMA-g-PTFE film could be reprotonated in 1 M H2SO4 to regenerate the conductive EM salt surface layer. It could also be treated with hydrazine, according to the method reported by Green and Woodhead,30 to generate a fully reduced leucoemeraldine (LM) surface layer. Adhesion Measurement. For adhesion strength measurements, the PANI-GMA-g-PTFE surface was coated with a commercial epoxy adhesive (Aldralite Stand, from Ciba-Geigy Chem. Co. of Switzerland). The epoxy adhesive was cured at 120 °C for 5 h before each T-peel adhesion strength test. The measurement of the T-peel adhesion strengths was carried out on an Instron 5540 Tensile Tester from the Instron Corp. of USA. All of the measurements were performed at a crosshead speed of 0.5 cm/min. Each adhesion strength reported was the average of at least three sample measurements. Surface Characterization. X-ray photoelectron spectroscopy (XPS) was used to determine the surface compositions of the samples. The XPS measurements were made on a VG ESCALAB MkII spectrometer with a MgKR X-ray source (1253.6 eV photons). The X-ray source was run at a reduced power of 120 W (12 kV and 10 mA). The samples were mounted on the standard VG sample studs by means of double-sided adhesive tapes. The core-level spectra were obtained at a photoelectron takeoff angle (R, measured with respect to the sample surface) of 75 °C. The pressure in the analysis chamber was maintained at 10-9 Torr or lower during each measurement. To compensate for surface charging effects, all of the binding energies 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 of the 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 sensitivity factors were determined using stable binary compounds of well-defined stoichiometries. The surface morphology of the samples was characterized by atomic force microscopy (AFM), using a Nanoscope IIIa scanning-force microscope. All of the images were obtained in air using the tapping mode under a constant force (scan size: 10 µm; set point: 3.34 µV; scan rate: 2.0 Hz).

Results and Discussion The processes of surface modification of the PTFE film by Ar plasma-pretreatment, UV-induced graft copolymerization with GMA, oxidative graft copolymerization of aniline, and reactive immobilization of PANI chains are shown schematically in Figure 1. The details for each process are discussed below. (29) Ray, A.; Asturias, G. E.; Kershner, D. L.; Richter, A. F.; MacDiarmid, A. G.; Epstein, A. J. Synth. Met. 1989, 29, E141.

9668

Langmuir, Vol. 16, No. 24, 2000

Zhang et al.

Figure 1. Schematic representations of the surface modification processes and the surface structures of the PANI-GMA-g-PTFE films.

Surface Graft Copolymerzation of the Ar PlasmaPretreated PTFE Film with GMA: The GMA-g-PTFE Surface. The surface modification of PTFE film via UVinduced graft copolymerization with GMA has been welldocumented.31,32 Figure 2, parts (a) to (d), shows the respective C 1s core-level and wide scan spectra of a pristine PTFE film, a 80 s Ar plasma-pretreated PTFE film, and the two 80-s Ar plasma-pretreated PTFE films after having been subjected to UV-induced graft copolymerization in 20 vol % GMA solution for 20 and 120 min, respectively. The XPS wide-scan spectrum of the pristine PTFE surface used in this study shows only peaks due to carbon and fluorine. The C 1s core-level spectrum of this surface consists of a main peak component at the binding energy (BE) of about 291.7 eV, attributable to the -CF2- species,32 and a broad minor peak component of about 8 to 10 eV lower in BE. The area of the minor component is about 12% of the total area of the C 1s core-level spectrum. It is attributable to the combined contribution of the X-ray satellite peaks arising from the Mg KR3 and KR4 radiations of the CF2 species (at the BE’s of 283.3 and 281.6 eV, respectively) and the adventitious hydrocarbon present on the film surface.33,34 After 80 s of Ar plasma treatment, an additional peak due to oxygen is discernible in the wide-scan spectrum. A small ridge is also discernible in the BE region of about 286 eV in the C 1s core-level spectrum. Ar plasma treatment causes the breakage of some C-F bonds in the surface region of the PTFE film, (30) Green, A. G.; Woodhead, A. E. J. Chem. Soc. 1910, 2388, 97. (31) Wu, S. Y.; Kang, E. T.; Neoh, K. G.; Tan, K. L. Macromolecules 1999, 32, 186. (32) Wang, T.; Kang, E. T.; Neoh, K. G.; Tan, K. L.; Cui, C. Q.; Lim, T. B. J. Adhesion Sci. Technol. 1997, 11, 679. (33) Crowe, R.; Badyal, J. P. S. J. Chem. Soc., Chem. Commun. 1991, 958. (34) G. E. Muilenberg, Ed. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corporation: Eden Prairie, MN, 1997, p 17.

resulting in the defluorination of the film surface and the formation of active radical species and carbon singly bonded to fluorine. 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.28 On the other hand, the presence of surface-grafted GMA polymer can be deduced from the three C 1s peak components with binding energies similar to those of the GMA homopolymer, viz., at 284.6 eV for the C-H species, 286.2 eV for the CO species and 288.7 eV for the COO species32 (Figure, parts (c) and (d)). The graft concentrations can be defined in this case as the [epoxide]/[F] ratios and derived from the equivalent COO to F 1s spectral component area ratios (as each GMA molecule has one COO species and one epoxide unit). From Figure 2, it can be deduced that the graft concentration increases with increasing UV graft copolymerization time. For PTFE surface with the UV graft copolymerization time of 20 min, the underlying PTFE substrate is still discernible, as suggested by the persistent of the CF2 signal at the BE of 291.7 eV (Figure 2(c)). At the UV graft copolymerization time of 120 min, the PTFE surface is completely covered by the graftted GMA polymer, as shown in Figure 2(d), to beyond the probing depth of the XPS technique (∼7.5 nm in an organic matrix35). The effect of UV graft copolymerization time on the graft concentration of the GMA polymer is summarized in Figure 3. On the other hand, the AFM studies suggest that the average surface roughness (Ra) values of the pristine PTFE film, the 80-s Ar plasma-treatment PTFE film, the 80-s Ar plasma-treatment PTFE film after complete coverage by the grafted GMA polymer are in the order of 80 ( 10, 87 ( 10, and 141 ( 10 nm, respectively. (35) Tan, K. L.; Woon, L. L.; Wong, H. K.; Kang, E. T.; Neoh, K. G. Macromolecules 1993, 26, 2832.

Consecutive Graft Copolymerization

Figure 2. Wide scan and C 1s core-level spectra of: (a) a pristine PTFE film, (b) a PTFE film with an Ar plasma treatment time of 80 s, and PTFE films after UV-induced graft copolymerization with GMA for (c) 20 and (d) 120 min.

Figure 3. Effect of the UV-induced graft copolymerization time on the GMA polymer graft concentration, defined as the [epoxide]/[F] ratio, and the corresponding concentration of the grafted PANI, defined as the [N]/[C] ratio.

Thus, the Ar plasma treatment only causes a slight increase in surface roughness of the PTFE film. However, the Ra value increases from about 87 to 141 nm after the PTFE surface has been graft copolymerized with GMA. The significant increase in the average Ra value is consistent with the presence of the grafted GMA polymer on the PTFE surface. Graft Copolymerization of Aniline on the GMAg-PTFE Surface: The PANI-GMA-g-PTFE Surface.

Langmuir, Vol. 16, No. 24, 2000 9669

Oxidative graft copolymerization of aniline, with simultaneous coating of the PANI homopolymer, was carried out on the GMA-g-PTFE films with different graft concentrations of the GMA polymer. Figure 4 shows the wide scan, C 1s and N 1s core-level spectra of three GMAg-PTFE films after the oxidative graft copolymerization in 1 N H2SO4 containing 0.2 M aniline, followed by deprotonation of the grafted PANI salt with a base and homopolymer extraction by NMP. Prior to oxidative graft copolymerization with aniline, the surface composition of the GMA-g-PTFE substrate in Figure 4(a) is the same as that of the GMA-g-PTFE film shown in Figure 2(c) ([epoxide]/[F] ) 0.13), whereas the surface composition of the GMA-g-PTFE substrates in Figure 4, parts (b) and (c), is the same as that of the GMA-g-PTFE film shown in Figure 2(d) ([epoxide]/[F]) ) ∞). The PANI-GMA-g-PTFE samples, with the surface compositions shown in Figure 4(a) and 4(b), have been treated at 100 °C for 6 h in a vacuum oven prior to being converted to EM base and subjected to NMP extraction. On the other hand, no thermal pretreatment was carried out on the PANI-GMAg-PTFE film shown in Figure 4(c). In comparison with the spectra shown in Figure 2(c) and 2(d), a new N 1s peak at the BE of about 399 eV is observed in the wide scan spectra of Figure 4(a) to 4(c). The N 1s core-level spectra in Figure 4(a) to 4(c) are dominated by the quinonoid imine (dN- structure) and benzenoid amine (-NH- structure) species, which correspond, respectively, to peak components with BE’s at about 398.2 and 399.4 eV.22,36 The presence of about equal amounts of the imine and amine nitrogen in the N 1s core-level spectra is consistent with the intrinsic redox state of the EM base of PANI ([dN-]/ [-NH-] ) 1). The residual amount of the high BE component has probably originated from the surface oxidation products or weakly charge-transfer complexed oxygen.37 The XPS results, the N 1s core-level line shapes in particalar, thus confirm that PANI has been incorporated onto the GMA-g-PTFE surface. Significant changes in the line shape of the C 1s corelevel spectra, after the incorporation of PANI, are also observed. The intensities of the peak components associated with the CF2, CO, and COO species have decreased substantially. In the present case, the amount of PANI incorporated can be expressed as the [N]/[C] ratio and determined from the corresponding N 1s and C 1s corelevel spectral area ratio. If the GMA-g-PTFE surface was completely covered by PANI to beyond the probing depth of the XPS technique, the [N]/[C] ratio should approach about 0.17 because every aniline unit has one nitrogen atom and six carbon atoms. For the GMA-g-PTFE surfaces with GMA polymer graft concentrations ([epoxide]/[F] ratios) of 0.13 and ∞ (complete coverage), the [N]/[C] ratios of the corresponding PANI-GMA-g-PTFE surfaces (Figure 4(a) and 4(b)) are about 0.08 and 0.16. On the other hand, the [N]/[C] ratio of the PANI-GMA-g-PTFE surface with an [Epoxide]/[F] ratio of ∞ but without the thermal treatment (Figure 4(c)) is only about 0.09. Thus, the amount of the incorporated PANI depends on the graft concentration of the GMA polymer on the PTFE surface, as well as on the extent of thermal treatment or thermal curing of the PANI-GMA-g-PTFE surface to incorporate the aniline homopolymer. Finally, the observation of a fairly strong O1s signal in the wide scan spectrum of the PANI-GMA-g-PTFE surface with an almost complete coverage of the aniline polymer (36) Kang, E. T.; Neoh, K. G.; Tan, T. C.; Khor, S. H.; Tan, K. L. Macromolecules 1990, 23, 2918. (37) Kang, E. T.; Neoh, K. G.; Khor, S. H.; Tan, K. L.; Tan, B. T. G. J. Chem. Soc., Chem. Commun. 1989, 696.

9670

Langmuir, Vol. 16, No. 24, 2000

Zhang et al.

Figure 4. Wide scan, C 1s and N 1s core-level spectra of the base-treated PANI-GMA-g-PTFE films, prepared from the GMAg-PTFE films having a GMA polymer graft concentration of (a) 0.13, and (b) and (c) ∞. Before been converted to the EM base form, samples in (a) and (b) had been treated at 100 °C for 6 h in a vacuum oven, whereas no such treatment was carried out for the sample in (c).

(Figure 4(b), [N]/[C] ) 0.16) suggests the presence a significant amount of surface adsorbed and/or chargetransfer complexed oxygen. The phenomenon has been widely observed in PANI and is consistent with the reactive nature of the conjugated polymer surfaces.22 Mechanisms of PANI Incorporation on the GMAg-PTFE Surface. Previous study31 has shown that the GMA polymer grafted on the PTFE surface can react with ethylenediamine. The reaction proceeds via the curing of the epoxide groups of the GMA polymer with the amine groups of the ethylenediamine. In the present case of PANI incorporation on the GMA-g-PTFE surface, two reaction mechanisms are probably operative. The amine groups of the PANI homopolymer chains can react with the epoxide groups of the grafted GMA polymer at elevated temperatures. The first mechanism is borne out by the difference in surface composition, the [N]/[C] ratios in particular, of the PANI-GMA-g-PTFE surfaces in the presence and absence of the thermal curing or fixation step prior to deprotonation and NMP extraction (compare Figure 4(b) and 4(c)). The other mechanism involves the reaction of the aniline monomer with the epoxide group of the grafted GMA polymer initially. The immobilized aniline subsequently promotes the oxidative graft copolymerization with aniline. To verify the second mechanism, GMA-gPTFE films having a complete coverage of the GMA polymer on the PTFE surfaces ([epoxide]/[F])∞) were immersed in 0.5 M aqueous solution of H2SO4 containing 0.2 M of aniline, but in the absence of the oxidant, for 2 and 6 h, respectively. The samples were then rinsed with water, immersed in a NMP bath with continuous stirring for 24 h, and finally washed with acetone and distilled

water before been dried under reduced pressure. Figure 5 shows the wide scan, C 1s and N 1s core-level spectra of these two samples. In comparison with the wide scan spectrum shown in Figure 2(d) for the GMA-g-PTFE substrate prior to immersing in the aniline solution, a new N 1s peak component at the BE of about 399.4 eV, arising from the amine species,36 is observed in the wide scan spectra in Figure 5(a) and 5(b). A change in the line shape of the C 1s core-level spectrum is also observed. The relative intensities of the CO/CN and COO species have decreased. These results indicate that the aniline monomer has been incorporated onto the GMA-g-PTFE surface. The concentration of the incorporated aniline, defined as the [N]/[C] ratio, increases with increasing reaction time. Thus, chemical reaction has occurred between the amine group of aniline and the epoxide group of the grafted GMA polymer. Furthermore, the N 1s corelevel spectrum, which is centered at about 399.4 eV, can be assigned to the amine species of the incorporated aniline.36 The immobilized aniline, thus can initiate the subsequent oxidative graft copolymerization with free aniline. The effect of the graft concentration of GMA polymer on the PTFE surface on the extent of PANI incorporation is summarized in Figure 3. Thus, with a high concentration of the grafted GMA polymer on the PTFE surface, complete coverage of the PTFE surface (to beyond the probing depth of the XPS technique) by PANI can be achieved, as indicated by the fact that the [N]/[C] ratio approaches 0.17 as an asymptotic value. After the reactive immobilization of PANI, the average roughness (Ra) value of the PANI-GMA-g-PTFE surface, as derived

Consecutive Graft Copolymerization

Langmuir, Vol. 16, No. 24, 2000 9671

Figure 5. Wide scan, C 1s and N 1s core-level spectra of the GMA-g-PTFE films ([epoxide]/[F] ) ∞) after immersed in 0.2 M aniline solution for (a) 2 h and (b) 6 h.

from AFM image, increases further from 141 ( 10 nm for the original GMA-g-PTFE substrate to 163 ( 10 nm. Surface Conductivity of the Protonated PANIGMA-g-PTFE Film. To investigate the surface conductivity of the aniline graft-copolymerized GMA-g-PTFE surface, reprotonation of the PANI-GMA-g-PTFE films by H2SO4 has been carried out to covert the insulating EM base surface layer into the conductive EM salt form of PANI. Figure 6(a) shows the respective C 1s and N 1s core-level spectra of a PANI-GMA-g-PTFE surface ([N]/ [C] ) 0.16) after reprotonation for 1 h in 1 M H2SO4 solutions. The N 1s core-level spectrum of the reprotonated PANI-GMA-g-PTFE surface is curve-fitted with two peak components, with BEs at 399.4 and 402.0 eV for NH and N+ species,37 respectively. The disappearance of imine peak component and the appearance of an equivalent amount of the N+ component is consistent with the fact that protonation in EM occurs preferentially at the imine units.22,29 The EM base of the PANI-GMA-g-PTFE surface can also be converted readily to the fully reduced leucoemeraldine (LM) form by treatment with hydrazine. The N 1s core-level spectrum of the fully reduced PANIGMA-g-PTFE surface is dominated by the -NH- peak component at the BE of about 399.4 eV,37 as shown in Figure 6(b). Thus, the deprotonation-reprotonation behavior and the intrinsic redox states of the incorporated PANI on the GMA-g-PTFE surfaces are grossly similar to those of the aniline homopolymer. Figure 7 shows the effect of PANI concentration ([N]/ [C] ratio) on the surface resistance of the reprotonated PANI-GMA-g-PTFE films. The surface resistance decreases with increasing [N]/[C] ratio. At the [N]/[C] ratio of 0.16, the surface resistance is about 7 × 103 Ω/sq, in comparison to the surface resistance of 1016 Ω/sq for the pristine PTFE film38 and the bulk conductivity of about 10 S/cm for the protonated PANI powders.29 The relation

Figure 6. C 1s and N 1s core-level spectra of the base-treated PANI-GMA-g-PTFE films with a [N]/[C] ratio of 0.16 after: (a) reprotonation in 1 M H2SO4 for 1 h and (b) reduction by hydrazine.

between surface resistance (Rs) and bulk resistivity (F) is given by Rs ) F/t, where t is the thickness of the grafted PANI film. As the [N]/[C] ratio for the present surface with a high PANI graft concentration approaches the value of 0.17 expected if there was complete coverage by PANI (38) Goodfellow Catalogue, (Metals, Alloys, Compounds, Ceramics, Polymers, Composites), 1996/1997, p 447.

9672

Langmuir, Vol. 16, No. 24, 2000

Zhang et al.

Table 1: T-peel Adhesion Strengths of the Epoxy Adhesive with the Surface-modified PTFE Films

a

No.

laminate

1 2 3 4

epoxy/PANI-GMA-g-PTFE epoxy/PANI-GMA-g-PTFEa epoxy/PANI-GMA-g-PTFE epoxy/PTFE (80 s plasma)

GMA polymer concentration [epoxide]/[F]

PANI concentration [N]/[C]

T-peel adhesion strength (N/cm)

0.13 ∞ ∞

0.08 0.09 0.16

5.1 ( 0.5 12.7 ( 1.0 12.3 ( 1.0 2.5 ( 0.5

The PANI-GMA-g-PTFE sample was not subjected to thermal curing/fixation prior to deprotonation and NMP extraction.

Figure 7. Effect of the incorporated PANI concentration, defined as the [N]/[C] ratio, on the surface resistance of the 1 M H2SO4 protonated PANI-GMA-g-PTFE films.

to a thickness exceeding the XPS probing depth, the thickness of the PANI layer can be estimated from the XPS probing depth. Thus, using the XPS probing depth of 7.5 nm as the approximate thickness35 of the grafted aniline polymer layer, the bulk resistivity is about 10-2 ohm‚cm, which is equivalent to a bulk conductivity value (σ) of the order of 102 S/cm, as σ ) 1/F. The preservation of the equivalent bulk conductivity by the surface grafted PANI chains makes the surface-modified PTFE film in this study potentially valuable for antistatic applications and in the shielding of electromagnetic interference (EMI). Furthermore, the surface resistance of the PANI-GMAg-PTFE film can be changed by varying the protonation level because the grafted PANI is not unlike that of the PANI homopolymer. Adhesion Strength of the Grafted PANI on the PTFE Surface. Previous studies31,39 have shown that the GMA polymer from the UV-induced graft copolymerization is covalently bonded on the PTFE surface. In the present case, to investigate the adhesion between the incorporated PANI and the grafted GMA polymer, an epoxy adhesive was applied and cured on the PANI-GMAg-PTFE surface in order to peel off the PANI layer from the PTFE substrate. The compositions of the delaminated surfaces were analyzed by XPS. Table 1 shows the T-peel adhesion strengths of the three epoxy/PANI-GMA-g-PTFE laminates. For the purpose of comparison, the adhesion strength of the epoxy adhesive with a 80-s Ar plasma(39) Zhang, M. C.; Kang, E. T.; Neoh, K. G.; Tan, K. L. J. Adheson Sci. Technol. 1999, 13, 819.

pretreated PTFE film is also shown in Table 1. The T-peel adhesion strength increases after the plasma-pretreated PTFE surface has been further modified with the GMA polymer and PANI. The adhesion strength is strongly dependent on the graft concentration of the GMA polymer. The incorporated PANI has almost no effect on the observed adhesion strength. The wide scan and C 1s corelevel spectra of the two delaminated surfaces of the epoxy/ PANI-GMA-g-PTFE laminate with a T-peel adhesion strength of about 5.1 N/cm (Sample 1 in Table 1) are almost identical and are very similar to the respective wide scan and C 1s core-level spectra of the pristine PTFE surface shown in Figure 2(a). It is obvious that the failure has occurred inside the bulk of the PTFE film. In fact, a similar result has also been observed for the two delaminated surfaces of the epoxy/PANI-GMA-g-PTFE laminate having a T-peel adhesion strength of 12.3 N/cm (Sample 3 in Table 1). Thus, the surface graft-copolymerized and thermally cured PANI chains must have been covalently bonded on the GMA-g-PTFE surface to form a covalent network structure with the grafted GMA polymer. The covalent bonding of the aniline polymer from surface graft copolymerization and thermal curing to render the PTFE surface conductive makes the present surface modification method a promising approach to the preparation of surface conductive molecular or polymeric composites. Conclusion Ar plasma-pretreated PTFE films were subjected to further surface modification via UV-induced graft copolymerization with GMA, followed by oxidative graft copolymerization with aniline. The microstructures and compositions of the graft-copolymerized surfaces were characterized by XPS and AFM. Two mechanisms were proposed for the incorporation of PANI onto the GMAg-PTFE surface. During the oxidative polymerization of aniline in solution, aniline coupled with the epoxide group of the grafted GMA polymer to promote the subsequent oxidative graft copolymerization with the free aniline in the reaction mixture. On the other hand, the physically adsorbed PANI chains underwent thermal curing with the grafted GMA polymer via the reaction of the amine and the epoxide groups. After deprotonation and NMP extraction, the grafted PANI chains were similar to the emeraldine base homopolymer, as they were readily susceptible to reprotonation by an acid and reduction by hydrazine. The surface resistance of the PANI-GMA-gPTFE could be reduced to the order of 104 Ω/sq when reprotonated, in comparison to that of 1016 Ω/sq for the pristine PTFE surface. Failure occurred inside the bulk of the PTFE substrate when an epoxy adhesive was applied on the PANI-GMA-g-PTFE surface to peel off the incorporated PANI layer, suggesting that the grafted PANI and GMA chains formed a network structure on the PTFE surface. LA000568L