Enhancement of Electrical Stability of Polyaniline Films in Aqueous

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Enhancement of Electrical Stability of Polyaniline Films in Aqueous Media by Surface Graft Copolymerization with Hydrophobic Monomers Baozong Zhao, K. G. Neoh,* F. T. Liu, and E. T. Kang Department of Chemical and Environmental Engineering, National University of Singapore, Kent Ridge, Singapore, 119260

K. L. Tan Department of Physics, National University of Singapore, Kent Ridge, Singapore, 119260 Received March 19, 1999. In Final Form: July 14, 1999 Surface modification of free-standing polyaniline (PANi) films and PANi coating on low density polyethylene (LDPE) substrates via UV-induced graft copolymerization with hydrophobic monomers was carried out. Pentafluorostyrene (PFS) and styrene were successfully graft copolymerized on the PANi surfaces, rendering them hydrophobic. The effects of UV graft copolymerization time, graft copolymerization temperature, and monomer concentration on the graft concentration were investigated. The pristine and graft-modified films were characterized using both surface and bulk analytical techniques. For the pristine PANi films, the loss of counterions from the surface region of the film occurs very rapidly in deionized water. This loss is very effectively retarded by surface graft copolymerization with PFS, hence preserving the PANi’s conductivity even upon prolonged immersion in deionized water. This enhancement in the electrical stability of the PANi film was also achieved in moderately basic aqueous medium.

Introduction As one of the oldest conductive polymers, polyaniline (PANi) has been extensively investigated for its special characteristics and potential applications.1 However, commercial use of PANi is largely limited due to the poor processability, electrical stability, and environmental compatibility.2 PANi in the emeraldine (EM) state can be easily obtained by the polymerization of aniline under fairly mild conditions,3 and its electronic properties can be changed drastically by oxidation/reduction and deprotonation/reprotonation. The conductivity of PANi is very dependent on the protonation level with the conductivity changing by more than a factor of 1010 between the undoped and doped states. PANi doped by mobile inorganic anions such as Cl- or ClO4- readily undergoes partial undoping when immersed in water due to the migration of the anions out of the polymer matrix.4 In basic solutions, the undoping process becomes even more rapid. Various methods have been employed to immobilize the counterions in the polymer matrix in order to enhance PANi’s stability in aqueous solutions. Self-doped PANi with sulfonic acid (-SO3-) groups substituted into the aniline ring has been synthesized by treating PANi with fuming sulfuric acid.5,6 The copolymerization of aniline and metanilic acid also results in about 20% of the aniline * To whom correspondence should be addressed. Tel.: +65-8742186. Fax: +65-779-1936. E-mail: [email protected]. (1) Kang E. T.; Neoh, K. G. and 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) Nechtschein, M.; Genoud, F.; Menardo, C.; Mizoquchi, K.; Travers, J. P.; Villeret, B. Synth. Met. 1989, 29, E211. (4) Neoh, K. G.; Kang E. T.; Tan, K. L. Polym. Degrad. Stab. 1994, 43, 141. (5) Yue J.; Wang, Z. H.; Cromack, K. R.; Epstein, A. J.; MacDiarmid, A. G. J. Am. Chem. Soc. 1991, 113, 2665. (6) Kang E. T.; Neoh, K. G.; Woo, Y. L.; Tan, K. L. Polym. Commun. 1990, 32, 412.

rings having covalently bonded -SO3- groups.7 Another method is to prepare doped PANi films by blending PANi and polymeric or organic acids in NMP.8,9 These methods result in greater stability of PANi in water but the conductivity of such PANi is significantly lower than that of PANi doped by Cl- or ClO4- anions. Furthermore, upon treatment with a base, the PANi synthesized by the abovementioned methods rapidly revert to the insulating state. Thus, so far we are not aware of any method of preparing PANi films that can retain high conductivity under basic conditions. In this paper, we report on the surface graft copolymerization of PANi films with hydrophobic monomers. The effects of monomer concentration, the nature of the monomers, and the UV-irradiation time on the surface graft copolymerization were investigated in detail. The pristine and graft-modified films were characterized by surface as well as bulk analytical techniques. The enhancement of the electrical stability of the graftmodified PANi free-standing films and PANi coatings on low-density polyethylene (LDPE) substrates in water and basic solutions is reported. Experimental Section Films Preparation. Polyaniline was synthesized via the oxidative polymerization of aniline with ammonium persulfate in 1 M H2SO4 according to the method described in the literature for 1 M HCl.2 It was converted to EM base 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 for 24 h. Free-standing emeraldine base (EM base) film of about 1020 µm in thickness was prepared by casting from N-methyl-2pyrrolidinone (NMP) solution containing 5 wt % of EM base. The (7) Neoh, K. G.; Kang E. T.; Tan, K. L. Synth. Met. 1993, 60, 13. (8) Neoh, K. G.; Pun, M..Y.; Kang E. T.; Tan, K. L. Synth. Met. 1995, 73, 209. (9) Chen, S. A.; Lee, H. T. Macromolecules 1995, 28, 2858.

10.1021/la990328+ CCC: $18.00 © 1999 American Chemical Society Published on Web 09/10/1999

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Table 1. Contact Angle of PFS-Grafted PANi Films sample

pristine

UV-5a

UV-8

UV-10

UV-20

UV-30

UV-55

UV-70

UV irradiation time (min) contact angle (deg)

0 45

5 66

8 84

10 88

20 91

30 92

55 98

70 99

a Number denotes the period of UV irradiation in minutes during preparation of sample, e.g., for UV-5, the graft copolymerization was carried out for 5 min under UV-irradiation.

EM base film was reprotonated by immersion in 1 M HClO4 solution for 24 h, and then dried under reduced pressure for another 24 h. These films will be denoted as PANi films in the subsequent discussion. In the preparation of PANi coating on low-density polyethylene (LDPE) film, the LDPE film (obtained from Goodfellow Inc. of Cambridge, UK) of dimensions 15 × 15 × 0.125 mm was first pretreated by O2 plasma for 60 s, and then immersed in the above-mentioned polymerizing solution of aniline for 4 h. The film was then dried under reduced pressure for 24 h. Surface Graft Copolymerization. The monomers used for surface grafting copolymerization with the PANi are pentafluorostyrene (PFS) (Aldrich Chem. Co.) and styrene (Fluka Chem. Co.). The monomers were either used in pure form, or in xylene (mixture of isomers) or tetrahydrofuran (THF) solution. A drop of monomer was placed on the surface of a PANi film or a PANi coated LDPE film. The film was then sandwiched between two pieces of quartz plates. The assembly was then exposed to UVirradiation for a period of time at 50 °C, with rotation at a constant speed of 90 rpm. In some experiments, graft copolymerization was carried out on one side of the film only. In this case, the monomer was added only to the side facing the UV source. After a fixed period of time, the grafted film was removed from the quartz plates and soaked in 100 mL of chloroform with constant stirring for 24 h to remove the homopolymer and unreacted monomer. It was then dried under reduced pressure before being characterized and subjected to the electrical stability tests. Electrical Stability Tests. The PANi and PANi coated LDPE films were immersed in 100 mL of deionized water for a period of time under ambient conditions. The films were dried under reduced pressure before being subjected to surface and bulk characterization. The same immersion procedure was also used with aqueous solutions of different pH values instead of the deionized water. The pH value was adjusted by adding HCl or NaOH to deionized water. The films used were small enough relative to the volume of the solution used so that the pH of the solution can be considered as essentially constant during the course of the experiment. Sample Characterization. The electrical conductivity of the film was measured by the two-probe method and reported as the sheet resistance (Rs) in Ω/0 (the resistivity is equal to Rs × thickness of the sample).10 The surface contact angle was measured by the sessile drop method using the Rame-Hart contact angle goniometer (model 100-00). The bulk chemical composition was analyzed using a Perkin-Elmer model 2400 CHN elemental analyzer. The bulk Cl content was measured by the wet chemical titration method. The changes in the UV-visible absorption spectra of PANi coated LDPE films were monitored on an UV-vis-NIR scanning spectrophotometer (Shimadzu UV3101 PC), using the pristine LDPE film as the reference. The surface compositions were measured using X-ray photoelectron spectroscopy (XPS) on a VG ESCALAB MK II spectrometer with a Mg KR X-ray source (1253.6 eV photons). The X-ray source was run at a reduced powder of 120 W (12 kV and 10 mA). The core level spectra were obtained at a photoelectron take off angle of 75°. The pressure in the analysis chamber was maintained at 10-8 mbar or lower during the measurements. To compensate for surface charging effects, all binding energies were referenced to the C1s neutral carbon peak at 284.6 eV. In spectral deconvolution, the full width at half-maximum of the peak components in a spectrum was kept constant. Surface chemical compositions were determined from peak area ratios corrected (10) Jaeger, R. C. In Introduction to Microelectronic Fabrication; Neudeck, G. W., Pierret, R. F., Eds.; Addison-Wesley Publishing Company: Reading, MA, 1993; p 66. (11) Neoh, K. G.; Kang, E. T.; Tan, K. L. J. Polym. Chem. B, 1997, 101, 726.

Figure 1. XPS spectra of PANi films, (a) to (d) the wide-scan spectra and (e) to (h) the C1s core-level spectra of pristine PANi film and PFS-grafted PANi films UV-5, UV-30 and UV-70, respectively. with the appropriate experimentally determined sensitivity factors, and are accurate to (10%.11 The surface morphology of the films was investigated by atomic force microscopy (AFM). The AFM studies were carried out using a Nanoscope III scanning atomic force microscope. All images were collected in air under constant force mode (scan size 20 µm, set point 3.34 V, scan rate 0.7 Hz).

Results and Discussion The success of surface graft copolymerization with the hydrophobic monomers is ascertained by the increase in the water contact angle of the free-standing PANi film as shown in Table 1. The water contact angle increases from 45° to as high as 99° after the UV-induced graft copolymerization with the hydrophobic monomer pentafluorostyrene (PFS). The data suggest that the extent of the graft, and thus the thickness of the hydrophobic layer, increase with the graft copolymerization time. The success of surface graft copolymerization was further ascertained by comparing the wide-scan and C1s XPS spectra of PANi films before and after graft copolymerization with PFS (denoted as PFS-grafted films in short), as shown in Figure 1. The graft-modified films show a F1s peak at 687.7 eV in the wide-scan spectra12 (Figure 1b to 1d), and the intensity of this peak increases with the UV graft (12) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. In Handbook of X-ray Photoelectron Spectroscopy; Chastain, J., Ed.; PerkinElmer Corporation: Minneapolis, MN, 1992; pp 46-47.

Enhancement of Electrical Stability of Polyaniline Films

Figure 2. Graft concentration and sheet resistance of PFSgrafted PANi films as a function of UV-irradiation time.

copolymerization time. The C1s core-level spectra of the graft-modified films also show a C-F peak component at 287.8 eV, in addition to the C-C, C-H (at 284.6 eV), C-N (at 285.5 eV), and C-O (at 286.0 eV) peaks13 of the pristine film (compare Figures 1f, 1g, and 1h to Figure 1e). Similarly, the intensity of C-F peak increases with UV graft copolymerization time, suggesting that an increase in the thickness of the graft layer has been achieved. The curve-fitted core-level spectra are used in the calculation the graft concentration, which is defined as the number of repeat units of the graft chain per aniline unit. graft concentration ) (area of F1s peak × sensitivity factor)/5 (area of C1s peak - area of F1s peak × sensitivity factor × 8/5)/6

where the stoichiometric factors of 5 and 8 are introduced to account for the 5 F atoms and 8 C atoms in each PFS unit, respectively. The factor of 6 accounts for the 6 C atoms per aniline unit. This graft concentration value is only used for comparing the grafting results under different conditions. From Figure 2, the increase in graft concentration with UV irradiation time is obvious, with the most rapid increase occurring during the first 10 min. Grafting cannot be achieved on the film in the absence of UV irradiation. The induction time for the UV-induced graft copolymerization is less than 5 min. The sheet resistance (Rs) increases with UV irradiation time, consistent with the increase in the graft concentration. An increase in UV irradiation time from 10 to 70 min results in the doubling of the graft concentration and an order of magnitude increases in Rs. The three-dimensional AFM images of pristine and PFSgrafted PANi films are shown in Figure 3. The surface of the pristine film is relatively smooth with a root-meansquare (RMS) roughness value of 12.3 nm, consistent with the value reported ealier.14 However, the surface roughness increases substantially with the graft concentration. From the AFM images of the grafted films (especially Figure 3c) it is discernible that the film surface consists of a series of stria with superimposed “peaks”, where the grafted polymer units amass along the vertical (Z) direction when graft concentration increases. (13) Neoh, K. G.; Kang, E. T.; Tan, K. L. Polymer 1993, 34, 1630. (14) Silk, T.; Hong, Q.; Tanm, J.; Compton, R. G. Synth. Met. 1998, 93, 59.

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The effect of monomer concentration on grafting efficiency can be seen in Figure 4. Since only a small fraction of the monomer used becomes grafted on the film surface, the graft concentration obtained using different monomer concentrations does not exhibit much difference for monomer concentration greater than 50 vol %. However, at lower monomer concentration (107Ω/0 within 5 h of immersion in water. The sharp increase in Rs suggests that the deprotonation in the surface region of the PANi film is rather rapid in water. For the PFS-grafted sample UV-10, its Rs increases only by 1 order of magnitude in the same period. The behavior of the graft-modified sample of UV-5 is similar to that of the pristine film, since its graft concentration is rather low (only 0.1). On the other hand, although the resistance of the UV-70 sample is nearly 2 orders of magnitude higher than the resistance of the pristine PANi film, the former film still retains its conductivity even after immersion in water for over one week. Electrical stability tests were also carried out on PANi film spin coated with a 0.5 wt % of poly(PFS) (average molecular weight of 47 000) in chloroform. The Rs of the poly(PFS) coated PANi film before water treatment is close to that of UV-70 sample. However, after 5 h in water, the Rs of the former film increases to >107Ω/0, similar to that observed for the pristine PANi film. Thus, the surface graft copolymerization of PFS on PANi is much more effective than the spin coating of poly(PFS) on PANi in retarding the deprotonation of PANi in water. The higher degree of effectiveness of surface graft copolymerization with PFS in stabilizing the electrical conductivity of the PANi film in water as compared to styrene is shown in Figure 6a. In both cases, the UVirradiation period was 30 min and pure monomer was used. The Rs of the PFS-grafted PANi film after water treatment is about an order of magnitude lower than that of the styrene grafted film, though the two samples have almost the same Rs before immersion in water. It can also be seen that the PFS-grafted UV-30 sample is almost as effective as the UV-70 sample in maintaining PANi’s conductivity after prolonged immersion in water (comparing Figure 6a with Figure 5) even though the graft concentration of UV-30 is less than half of that of UV-70 (Figure 2). The XPS N1s core level spectra of the pristine and PFSgrafted PANi film (UV-30) before and after the water immersion test are shown in Figure 7. These spectra were deconvoluted into the following peak components: imine (-Nd) peak at 398.2 eV, amine (-NH-) peak at 399.4 eV

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Figure 3. AFM images of pristine and PFS-grafted PANi films with different graft concentrations.

Figure 4. Graft concentration of PFS-grafted PANi film as a function of monomer concentration in xylene and THF (UV irradiation time of 30 min).

and positively charged nitrogen (N+) peak at g400.8 eV.1,15,16 A comparison of the spectra of the pristine and PFS-grafted PANi films before water treatment (Figure 7a and Figure 7d respectively) clearly shows that the extent of protonation (as indicated by the [N+]/[N] ratio) decreases during the grafting process. Nevertheless, all the -Nd are still protonated. Upon treatment with water, the pristine film undergoes more rapid deprotonation, as (15) Tan, K. L.; Tan, B. T. G.; Kang, E. T.; Neoh, K. G. Phys. Rev. B 1989, 39, 8070. (16) Kang E. T.; Neoh, K. G.; Tan, T. C.; Khor, S. H.; Tan, K. L. Macromolecules 1990, 23, 2918.

Figure 5. Resistance of pristine and PFS-grafted PANi films with different graft concentrations after immersion in water.

indicated by the decrease of N+ units (Figure 7 and Table 2). The deprotonation process is also monitored by the decrease in the amount of the counterion, ClO4-. The anions give rise to a peak at 207.4 eV in the Cl2p core level spectrum17 and the effect of water treatment on the (17) Neoh, K. G.; Kang E. T.; Tan, K. L. J. Phys. Chem. 1991, 95, 10151.

Enhancement of Electrical Stability of Polyaniline Films

Langmuir, Vol. 15, No. 23, 1999 8263 Table 2. Surface and Bulk Compositions of PANi Films time of surface bulk immersion in water (h) -Nd/N -NH-/N N+/N F/C Cl/N Cl/N Pristine PANi

PFS-grafted (UV-30)

Figure 6. Comparison of resistance change of PANi films after immersion in water, (a) grafted with PFS versus grafted with styrene (with the same UV irradiation time of 30 min), and (b) the grafted side versus the ungrafted side.

Figure 7. XPS N1s core-level spectra, (a) to (c) pristine PANi film before and after 1h and 24 h immersion in water, respectively, and (d) to (f) PFS-grafted PANi film (UV-30) before and after 1h and 24 h immersion in water, respectively.

[Cl]/[N] ratio is shown in Table 2. The XPS results show that, for the pristine PANi film, [Cl]/[N] decreases by about 70% in the first hour of water treatment, and few ClO4anions remain after 24 h. On the other hand, for the PFSgrafted film, the decrease is 20% in the first hour, and about 30% in 24 h. The decrease in the [Cl]/[N] ratio (from

0 1 24

0 0.30 0.40

0.47 0.54 0.52

0.53 0.16 0.08

s s s

0.56 0.45 0.17 0.35 0.04 0.31

0 1 24

0 0.08 0.10

0.56 0.63 0.67

0.44 0.29 0.23

0.27 0.40 0.42 0.25 0.32 0.41 0.23 0.27 0.39

XPS) of the films also closely matches the decrease in [N+]/[N] ratio. The result from the XPS analysis is indicative of the composition in the surface region of the films only. Hence, the [Cl]/[N] ratio is also determined from bulk elemental analysis to obtain a more representative value of the entire film. The surface and bulk [Cl]/ [N] ratios are compared in Table 2. Thus, although the loss of anions from the surface region of the pristine film is extensive, the bulk [Cl]/[N] ratio is still 70% of the original value after 24 h of water treatment. However, the loss of anion from the film’s surface greatly decreases the conductivity as shown in Figure 5. In the case of the PFS-grafted film, the bulk [Cl]/[N] ratio before water treatment is similar to that of the pristine film although the surface [Cl]/[N] of the former is significantly lower. After 24 h of water treatment, the PFS-grafted film still retains more than 90% of the ClO4- anions. The slow migration of ClO4- anions out of the bulk of the PANi film is further illustrated by the behavior of the PANi film with one side graft copolymerized with PFS and the other side left ungrafted. Comparing the resistance changes of the ungrafted side and the grafted side (Figure 6b) when the film is treated with water, one can see that the behavior of the grafted side is similar to that of the PANi graft film copolymerized with PFS on both sides, while the ungrafted side exhibits the same behavior as the pristine PANi film (Figure 5). This result suggests that despite the presence of a concentration gradient of the ClO4- anions along the film’s thickness, the migration of the anions from the bulk of the film out through the ungrafted surface is not sufficient to affect the conductivity of the grafted surface even after one week of immersion in water. The results presented so far were obtained with free standing films. A comparison of the deprotonation behavior of the pristine and PFS-grafted thin PANi coating on LDPE was also carried out using UV-visible absorption spectroscopy. The spectrum of the pristine PANi coating in Figure 8a shows a long absorption tail, which has been assigned to intraband free carrier excitations, together with the polaron band at 430 nm, and the π-π* transition at 320 nm which is not well defined. These are characteristic features of protonated EM.18 No significant difference is found between the pristine and the PFS-grafted PANi film (Figure 8b), except for the slightly higher intensity of the 320 nm band together with a lower intensity of the 430 nm band in the grafted film, which indicates that some deprotonation has occurred during the grafting operation (due to the effects of the monomer and the chloroform washing procedure). This surface deprotonation effect contributes to an increase in the film resistance after grafting. After the pristine film is immersed in water, the intensity of the bands at 830 and 430 nm decreases, and the intensity of 320 nm band (18) Cao, Y.; Smith, P.; Heeger, A. J. Synth. Met. 1989, 32, 263.

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Figure 9. Sheet resistance of pristine and PFS-grafted PANi films after immersion in media of different pH for 1 h.

Figure 8. UV-visible absorption spectra of (a) PANi coating on LDPE before and after immersion in water, (b) PFS-grafted PANi coating on LDPE before and after immersion in water. The time on the spectra indicates the period of immersion in water.

increases, together with a new band at 635 nm. The presence of the 635 nm band attributable to the excitonlike transition involving the benzoid and quinoid rings19 is indicative of the process whereby protonated EM is changed to the EM base state. A comparison between the spectra of the pristine and grafted films after the same period in water (Figure 8a and 8b) indicates that the deprotonation rate is much lower in the grafted film than in the pristine film. The deprotonation level of the grafted film after 2 h in water is comparable to that of the ungrafted film after 0.25 h in water. It should be noted that the deprotonation process in the PANi coated LDPE film proceeds at a much faster rate than in the free standing PANi film. This phenomenon is consistent with (19) McCall, R. P.; Ginder, J. M.; Leng, J. M.; Ye, H. J.; Manohar, S. K.; Masters, J. G.; MacDiarmid, A. G.; Epstein, A. J. Phys. Rev. B 1990, 41, 5202.

the fact that the PANi coating is much thinner and deprotonation occurs readily in the surface region of the film, as seen earlier from the XPS results in Table 2. The PFS-grafted films show resistance to deprotonation even in aqueous media with pH >7, as shown in Figure 9. Only a slight increase in Rs was observed for the UV-70 sample in media with pH from 2 to 10, whereas there is a 4 order of magnitude increase in the resistance for the pristine film. The fact that the conductivity of the UV-70 (graft concentration of 1.2) film is preserved for a wider range of pH compared to that of the UV-20 (graft concentration 0.6) film suggests that a higher graft concentration is particularly useful in strong basic medium. However, it can be seen from Figure 9 that beyond the pH value of 11, deprotonation is accelerated and the protective effect of the graft layer is diminished under high OH- concentration. Conclusion Hydrophobic monomers such as pentafluorostyrene (PFS) and styrene can be successfully graft copolymerized on the surface of PANi film. A longer UV irradiation time and higher temperature would give rise to a higher graft concentration. The graft copolymerization can be successfully accomplished with monomer concentrations higher than 50 vol %, either in xylene or THF. PFS is more effective than styrene in forming a hydrophobic graft layer. The grafted film exhibits significantly higher stability in maintaining its conductive state than the pristine film, either in deionized water or in aqueous media with pH from 2 to 10. The protective effect is diminished when the pH exceeds 10. The deprotonation of PANi film in water occurs rapidly on the surface but the migration of the anions out of the bulk of the film is relatively slow. Thus, the surface graft copolymerization of PANi with a hydrophobic monomer is a practical way to retard deprotonation. LA990328+