N-Alkylation of Polyaniline with Simultaneous Surface Graft

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N-Alkylation of Polyaniline with Simultaneous Surface Graft Copolymerization for Inducing and Maintaining a Conductive State Baozong Zhao, K. G. Neoh,* F. T. Liu, E. T. Kang, and K. L. Tan Department of Chemical and Environmental Engineering, National University of Singapore, Kent Ridge, Singapore 119260, and Department of Physics, National University of Singapore, Kent Ridge, Singapore 119260 Received July 10, 2000. In Final Form: September 29, 2000 Surface modification of the freestanding polyaniline (PANI) base film and coating on low-density polyethylene film was carried out by graft copolymerization with vinyl benzyl chloride (VBzCl) using UV-induced and heat-induced methods. These samples were characterized by X-ray photoelectron spectroscopy, UV-visible absorption spectroscopy, atomic force microscopy, and conductivity measurements. The reaction of PANI with VBzCl results in the alkylation of the imine nitrogen. The chloride ions (Cl-) formed during the alkylation then serve as the counterions to the N+ components of PANI resulting in a doped and conductive state. At the same time, the polymerization of the VBzCl via the vinyl groups results in the formation of a hydrophobic layer on the PANI surface. The hydrophobic polymer layer acts as the barrier for preventing the undoping of the graft copolymerized samples, and these grafted samples can maintain their conductive state even when exposed to aqueous solutions with pH as high as 12.

Introduction As one of the most well-known conductive polymers, polyaniline (PANI) has been extensively investigated for its special characteristics and potential applications.1 However, some of the factors currently limiting the applications of such materials include the problems of processability of the polymers, low mechanical strength, electrical stability, and environmental compatibility.2 PANI in the emeraldine base form (Scheme 1(I)) contains alternating amine and imine repeat units.3,4 The emeraldine base (EB) can be easily obtained by the polymerization of aniline under fairly mild conditions, followed by treatment with base solutions. When the EB is doped by protonic acids, protonation occurs at the imine nitrogen sites to yield the conductive polysemiquinone (Scheme 1(II)), in which the polarons delocalize along the chain.3,5 The conductivity of PANI is very dependent on the protonation level, with the conductivity changing by a factor of 1010 between the undoped and doped states. PANI doped by mobile inorganic anions such as Cl- or ClO4readily undergoes partial undoping when immersed in water or exposed to simulated weathering owing to the migration of anions out of the polymer matrix.6,7 In basic solutions, the undoping process becomes even more rapid. Various methods have been employed to immobilize the counterions in the polymer matrix with the aim of * To whom correspondence should be addressed. Tel: +65 8742186. Fax: +65 7791936. Email: [email protected]. (1) Kang, E. T.; Neoh, K. G.; Tan, K. L. Prog. Polym. Sci. 1998, 23, 277. (2) MacDiarmid, A. G.; Chiang, J. C.; Richter, A. F.; Epstein, A. J. Synth. Met. 1987, 18, 285. (3) Green, A. G.; Woodhead, A. E. J. Chem. Soc. Trans. 1912, 101, 1117. (4) MacDiarmid, A. G.; Epstein, A. J. Faraday Discuss. Chem. Soc. 1989, 88, 317. (5) Asturias, G. E.; MacDiarmid, A. G.; McCall, R. P.; Epstein, A. J. Synth. Met. 1989, 29, E157. (6) Neoh, K. G.; Kang, E. T.; Tan, K. L. Polym. Degrad. Stab. 1994, 27, 107. (7) Liu, F. T.; Neoh, K. G.; Kang, E. T.; Li, S.; Han, H. S.; Tan, K. L. Polymer 1999, 40, 5285.

Scheme 1

increasing their stability in aqueous media. These methods include the following: (i) the sulfonation of PANI via treatment with fuming sulfuric acid,8,9 (ii) copolymerization of aniline and metanilic acid,10 and (iii) use of organic or polymeric anions.11-13 None of these methods results in a product that can maintain its electrical stability in aqueous media of pH > 7. Recently, we developed a method to prevent the loss of counterions from the PANI by surface graft copolymerization with hydrophobic monomers such as pentafluorostyrene or styrene.14 This method is effective in maintaining the electrical stability of PANI films and coatings even in mildly basic solutions. In this paper, we report a new method for inducing electrical conductivity in PANI by UV or thermally induced alkylation of PANI by vinyl benzyl chloride (VBzCl). Concurrent with the alkylation reaction, the VBzCl also undergoes polymerization forming a hydrophobic graft (8) Kang, E. T.; Neoh, K. G.; Woo, Y. L.; Tan, K. L. Polym. Commun. 1990, 32, 412. (9) Yue, J.; Wang, Z. H.; Cromack, K. R.; Epstein, A. J.; MacDiarmid, A. G. J. Am. Chem. Soc. 1991, 113, 2665. (10) Neoh, K. G.; Kang, E. T.; Tan K. L. Synth. Met. 1993, 60, 13. (11) Neoh, K. G.; Pun, M. Y., Kang, E. T.; Tan K. L. Synth. Met. 1995, 73, 209. (12) Chen, S. A.; Lee H. T. Macromolecules 1995, 28, 2858. (13) Neoh, K. G.; Kang, E. T.; Tan, K. L. J. Phys. Chem. 1997, 101, 726. (14) Zhao, B. Z.; Neoh, K. G.; Liu, F. T.; Kang, E. T.; Tan, K. L. Langmuir 1999, 15, 8259.

10.1021/la000967g CCC: $19.00 © 2000 American Chemical Society Published on Web 12/02/2000

N-Alkylation of Polyaniline

layer over the PANI surface. The pristine and graftmodified samples were characterized by surface as well as bulk analytical techniques. The effectiveness of this method in enhancing the electrical conductive stability of PANI films and coatings in water and basic solutions is also reported. Experimental Section Materials. The monomer, 4-vinyl benzyl chloride, used for

graft copolymerization was obtained from Aldrich Chemical Co. Low-density polyethylene film (LDPE, 0.125 mm in thickness) was obtained from Goodfellow Ltd. (U.K.). The aniline monomer (99.5%) was obtained from Merck Chemical Co. and was redistilled before use. Preparation of PANI Film and Coating. Emeraldine salt was prepared by the oxidative polymerization of aniline with ammonium persulfate in 0.5 M H2SO4 according to the method described in the literature for 1 M HCl.2 It was converted to neutral emeraldine base (EB) by treatment with excess 0.5 M NaOH for 24 h, followed by washing with deionized water until the filtrate was neutral. The base powder was dried under reduced pressure. Freestanding EB films of about 10-20 µm in thickness were prepared by casting from N-methylpyrrolidinone (NMP) solution containing 5 wt % of EB. Before polyaniline was coated on LDPE, the LDPE films were soaked in methanol in an ultrasonic bath for 30 min and then dried under reduced pressure. The cleaned LDPE film was then cut into strips of 2 cm × 4 cm in size and pretreated by O2 plasma for 60 s. The plasma power applied was kept at 30 W at a radio frequency of 40 kHz. A coating of PANI on LDPE was formed by immersing the pretreated LDPE film into a mixture containing 0.10 M aniline and 0.025 M ammonium persulfate in 0.5 M H2SO4 for 1 h. The O2 plasma pretreatment of the LDPE substrate enhances the adhesion of the PANI coating. The PANI coating was converted to the base form (EB coating) by dipping into 0.5 M NaOH for 2 h. It was then washed with deionized water and pumped dry under reduced pressure. Surface Graft Copolymerization. Two methods were employed for the graft copolymerization of VBzCl on polyaniline surface, the UV-induced method and the heat-induced method. In the UV-induced method, drops of VBzCl monomer were placed on the surface of a freestanding EB film or EB coating on LDPE film. The film was then sandwiched between two quartz plates to ensure that the monomer was spread evenly on the film’s surfaces. This assembly was put in a Pyrex tube and exposed to near-UV irradiation for a period of time in a rotary photochemical reactor, RH 400-10 W (manufactured by Riko Denki Kogyo of Chiba, Japan). The reactor was equipped with a 1000 W highpressure mercury lamp and a constant-temperature water bath. All UV-induced graft copolymerization experiments were carried out at a constant temperature of 28 °C. After the reaction, the film was removed and washed with copious amounts of chloroform to remove any unreacted monomer and homopolymer. The film was then dried under reduced pressure. The so-modified freestanding EB film is denoted as EB-UVT, while the EB coating on LDPE film is denoted as PE-UVT, where T represents the UV irradiation time in minutes. In the heat-induced method, the EB freestanding film or EB coating on LDPE film was immersed in a solution of a 20% (by volume) VBzCl in dioxane. The solution was purged with argon for 40 min, after which the tube was sealed and immersed in a water bath at 80 °C. At the end of a specified period of time, the film was taken out and washed with excess chloroform to remove the unreacted monomer and homopolymer. The film was then dried under reduced pressure. The modified EB films and coatings are denoted as EB-HT and PE-HT respectively, where T again represents the reaction time but in hours instead of minutes. Stability Tests. The pristine and modified freestanding EB film and EB coating on LDPE film were immersed in a copious amount of aqueous solutions of different pH values for a period

Langmuir, Vol. 16, No. 26, 2000 10541 of time under ambient conditions. The pH value of the solution was adjusted by adding HCl or NaOH into deionized water. Sample Characterization. The electrical conductivity of the film was measured by the two-probe method and reported as the sheet resistance (Rs) in Ω/sq (conductivity (S/cm) ) 1/(Rs × thickness of the film)).15 The UV-visible absorption spectra of PANI coating on LDPE film were measured using a UV-visnear-IR scanning spectrophotometer (Shimadzu UV-3101PC), with the pristine LDPE film as the reference. A VG ESCALAB MkII spectrometer with a Mg KR X-ray source (1253.6 eV) was used for the XPS measurements. The X-ray source was run at a reduced power of 120 W (12 kV and 10 mA), and the pressure in the analysis chamber was maintained at 10-8 mbar or lower during the measurements. The core-level spectra were obtained at a photoelectron takeoff of 75°, measured with respect to the film surface. Surface elemental stoichiometries were determined from the peak area ratios, after correcting with experimentally determined sensitivity factors, and are accurate to within (10%.13 All binding energies (BEs) were referenced to the C1s neutral carbon peak at 284.6 eV to compensate for surface charging effects. In peak synthesis, the line widths (full width at halfmaximum, fwhm) of the Gaussian peaks were maintained constant for all components in a particular spectrum. The surface morphology of the films was investigated using a Nanoscope IIIa scanning atomic force microscope. All atomic force microscope (AFM) images were collected in air under a constant force mode (scan size, 5.0 µm; scan rate of 3.0 Hz).

Results and Discussion UV-Induced Graft Copolymerization of VBzCl on PANI Surface. The reaction of VBzCl with freestanding EB film in the presence of UV light results in a color change of the film from brown to blue, and the color remains during the washing and drying process. This color change gives the first indication that the film has been changed from the base state to the electrically conductive salt form. The XPS N1s and Cl2p core-level spectra of the EB films before and after reaction with VBzCl for different periods of time under UV irradiation are shown in Figure 1. The N1s core-level spectrum of the EB film in Figure 1a can be curve-fitted with two major components with binding energy (BE) at about 398.2 and 399.4 eV, attributable to the imine (-Nd) and amine (-NH-) nitrogen, respectively.16,17 Upon UV-induced reaction with VBzCl, the imine signal decreases in intensity and a new high-BE tail attributable to the N+ components appears (1.451.50 and 2.9-3.0 eV from amine peak). The corresponding Cl2p core-level spectrum at the initial stages of the reaction (Figure 1d) can be resolved into three spin-orbit split doublets (Cl(2p3/2) and Cl(2p1/2)), with the BE for the Cl(2p3/2) peaks at about 197.1, 198.6, and 200.2 eV.18,19 The first and the last BE components suggest the presence of ionic (Cl-) and covalent (-Cl) chlorine species, respectively. The intermediate chloride species, Cl* (dashed component in the Cl2p spectra in Figure 1), which has been widely observed,20-23 is more appropriately associated (15) Jaeger, R. C. Introduction to Microelectronic Fabrication; Neudeck, G. W., Pierret, R. F., Eds.; Addison-Wesley Publishing Company: Reading, MA, 1993; p 66. (16) Tan, K. L.; Tan, B. T. G.; Kang, E. T.; Neoh, K. G. Phys. Rev. B 1989, 39, 8070. (17) Kang, E. T.; Neoh, K. G.; Tan, T. C.; Khor, S. H.; Tan, K. L. Macromolecules 1990, 23, 2918. (18) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Chastain, J., Ed.; PerkinElmer Corporation: Eden Prairie, MN, 1992; p 63. (19) Kang, E. T.; Neoh, K. G.; Woo, Y. L.; Tan, K. L.; Huan, H. A.; Wee A. T. S. Synth. Met. 1993, 53, 333. (20) Mirrezaei, S. R.; Munro, H. S.; Parker, D. Synth. Met. 1988, 26, 169. (21) Tan, K. L.; Tan, B. T. G.; Kang, E. T.; Neoh, K. G. J. Chem. Phys. 1991, 94, 5382. (22) Dannetun, P.; Lazzaroni, R.; Salaneck, W. R.; Scherr, E.; Sun, Y.; MacDiarmid, A. G. Synth. Met. 1991, 41-43, 645.

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Table 1. Surface Compositions of VBzCl-Grafted EB Freestanding Films sample

N/C

Cl/C

Cl/N

-Nd/N

EB EB-UV5 EB-UV10 EB-UV20 EB-UV30 EB-UV40 EB-UV60 EB-H4 EB-H8

0.17 0.16 0.13 0.096 0.056 0.024 0.008 0.13 0.10

0.032 0.044 0.076 0.078 0.082 0.089 0.053 0.049

0.20 0.34 0.80 1.17 3.41 12.0 0.41 0.49

0.46 0.32 0.28 0.23 a a a 0.13 0.07

a

surface compos ition -NH/N N+/N (Cl- + Cl*)/Cl 0.54 0.58 0.58 0.58 a a a 0.66 0.67

0 0.10 0.14 0.19 a a a 0.21 0.26

0.60 0.33 0.26 0.14 0.05 0.95 0.38 0.43

0.84 1.2 0.91 a a a 0.82 0.93

1012 2 × 108 2 × 106 3 × 106 1 × 107 1 × 108 4 × 109 2 × 105 1 × 105

The N1s signal is too low to allow for proper deconvolution.

Scheme 2

with anionic chloride species resulting from the chargetransfer interactions between the halogen and the metallike conducting state of the polymer chain. As the reaction time increases, the Cl- and Cl* components decrease and the -Cl component becomes the dominant feature in the Cl2p spectrum. When the reaction time increases to beyond 40 min, the intensity of the N1s signal is too low to allow proper deconvolution of the peak (Figure 1g), and by 60 min, the N1s signal is hardly discernible (Figure 1i). By that time, the Cl2p spectrum also indicates that the Cl exists solely as the -Cl species. The surface compositions of the EB films as calculated from the XPS spectra are given in Table 1. For those spectra of the VBzCl-modified films that can be deconvoluted, the N+/(Cl- + Cl*) ratio is close to unity, indicating that the Cl- and Cl* serve as counterions to the positively charged nitrogen of polyaniline and charge neutrality is

maintained. We postulate that the Cl- and Cl* species are produced via a mechanism similar to the concurrent doping and N-alkylation of EB by alkyl halides that we reported earlier.24 In the N-alkylation of EB, we have earlier proposed that the imine nitrogen is preferentially attacked by alkyl halides. The XPS data in Table 1 shows that the -NH-/N ratio remains constant in the initial stages of the reaction (UV irradiation time < 30 min) while the -Nd/N ratio decreases and the N+/N ratio increases. This is consistent with the proposed mechanism where the N+ results from the N-alkylation of the -Nd units. As can be seen from the XPS data, with increasing reaction time, covalent chlorine (-Cl) becomes the main Cl species. Since this species is associated with the VBzCl groups, these groups must have graft copolymerized on the film surface and could not be removed upon washing with chloroform. The presence of a surface-grafted poly(VBzCl) layer is further ascertained by an increase in surface hydrophobicity as indicated by an increase in the water contact angle from about 55° for the pristine EB film to about 75° for EB-UV20. The very low N1s signal at long reaction time (>40 min) also suggests that a layer of poly(VBzCl) has been formed over the EB film and as the thickness of this layer increases, the contribution of the EB film to the XPS signal is attenuated. The absence of Cl- and Cl* peak components in the Cl2p spectra at long reaction time is also consistent with the presence of the poly(VBzCl) layer, which comprises only -Cl associated with the VBzCl unit. From the XPS data, we can thus propose a representation of the VBzCl graft-modified PANI as in Scheme 2 where the different possible structures of the graft units (not in stoichiometric proportions) are shown. The XPS results are representative only of the composition at the outermost surface of the film, as determined by the probing depth of the X-ray (107 Ω/sq within 1 h of treatment in water and to >109 Ω/sq after 24 h treatment. The N+/N and ClO4-/N ratios of EB-HClO4 after treatment in water for 24 h drop substantially from 0.53 to 0.04 and from 0.67 to 0.06, respectively. However, the N+/N and (Cl- + Cl*)/N ratios of the graft-modified film (EB-UV20) only drop slightly from 0.19 to 0.14 and from 0.21 to 0.15, respectively, and its Rs increases by less than 1 order of magnitude when exposed to water for 24 h. At pH ) 12, the undoping of the EB-HClO4 film is even more rapid while the Rs of the EB-UV20 film changes by less than 1 order of magnitude after 10 h treatment (Figure 8b). UV-visible absorption spectroscopy was also used to monitor the undoping behavior of the VBzClgrafted EB films. Figure 9 shows the absorption spectra

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EB-UV20. After 10 h of immersion in water, the former increases by more than 3 orders of magnitude to 108 Ω/sq, whereas the latter does not change significantly from 5 × 106 Ω/sq (Figure 8a). As mentioned earlier, these films obtained using the heat-induced method have a high degree of alkylation (as given by the (Cl- + Cl*)/Cl ratio) with a low degree of polymerization of the VBzCl monomer (as indicated by the -Cl/Cl ratio and water contact angle). It was found that the (Cl- + Cl*)/N ratio of EB-H8 decreased from 0.28 to 0.02 after treatment in water for 24 h, suggesting that significant undoping occurred owing to the migration of the counterions out of the polymer matrix into water. These results show that the stability of the graft-copolymerized samples by heat-induced methods is not much improved over that of the EB-HClO4 film. Conclusion

Figure 9. UV-visible absorption spectra of PE-UV20 coating after treatment in media of (a) pH ) 6 and (b) pH ) 12 for various periods of time.

of PE-UV20 before and after treatment in water and in a basic solution of pH of 12. It is clear from Figure 9a that although there are some changes in the absorption spectrum with time of treatment in water, the spectrum after 24 h is still that of a PANI salt. When the pH is increased to 12, the emergence of a new band at 630 nm is obvious after 24 h treatment. The appearance of the 630 nm band indicates that substantial undoping of the coating has occurred. These UV-visible absorption spectroscopy results are consistent with the observed increasing trend of Rs reported in Figure 8. The importance of the hydrophobic VBzCl graft copolymer layer on the PANI film surface in stabilizing the electrical conductivity in aqueous media is illustrated by the relative ineffectiveness of the EB-H4 and EB-H8 samples in water. The Rs of EB-H4 and EB-H8 increases much more rapidly after treatment in water than that of

The reaction of polyaniline base films in the emeraldine state with vinyl benzyl chloride under UV irradiation and heat was investigated in this work. In both methods, alkylation of the imine nitrogen of the polyaniline units results in the formation of positively charged nitrogen, as well as Cl- anions that serve as the counterions to the polyaniline. As a result, the polyaniline is converted to the conducting salt state. Concurrent with the alkylation process, the copolymerization of the vinyl benzyl chloride results in the formation of a hydrophobic graft copolymer layer on the polyaniline surface. The UV-induced method is more effective than the heat-induced method in enhancing the graft copolymerization, and a longer UV irradiation time gives rise to a higher graft concentration. The hydrophobic graft copolymer layer reduces the surface conductivity but imparts to the polyaniline a high degree of electrical stability in aqueous media by retarding the migration of the counterions out of the polymer matrix. The protective effect of the graft layer is decreased only at high pH (>10). In contrast, the samples obtained via the heat-induced method have a higher surface conductivity owing to the low graft copolymer concentration, but the electrical stability in aqueous media is not significantly enhanced over that of a pristine polyaniline salt doped by mobile anions. LA000967G