Stability studies of polyaniline

Polymer Degradation and Stability 27 (1990) 107-117

Stability Studies of Polyaniline K. G. N e o h , a E. T. K a n g , a S. H. K h o r " & K. L. T a n b Department of Chemical Engineering, b Department of Physics, National University of Singapore, Kent Ridge, Singapore 0511 (Received 13 January 1989; accepted 28 January 1989)

ABSTRACT The stability of conductive polyaniline has been assessed at elevated temperatures, under high current density and in organic solvents. A decrease in electrical conductivity (cr) occurs when the protonated polyaniline is subjected to temperatures above 60°C. However, at temperatures below IO0°C, the decrease in a is reversible. The polymer can sustain a current density of 2A/cm z over a 24 h period with no adverse effect but breaks down rapidly with a current density over lOA/cm z. Although the protonated polyaniline shows very limited solubility in organic solvents, its electrical conductivity can be significantly affected by the solvent. X-ray photoelectron spectroscopy results support the postulate that the degradation of protonated polyaniline in solvents is due to the loss of chlorine anions and/or the conversion of some of the anions to covalently bonded chlorine. Polyanilinetetracyanoethylene complex appears to have better electrical stability than the conventional protonated polyaniline.

INTRODUCTION The polyaniline family of conducting polymers has been the subject of extensive investigations. This class of material can be transformed from an insulating state to a conducting one by acid treatment and/or by electrochemical means. 1 -a The reversibility of these reactions and the greater stability in air compared with other electroactive polymers, such as polyacetylene, have made the potential uses of polyaniline in batteries or 107 Polymer Degradation and Stability 0141-3910/89/$03'50 © 1989 Elsevier Science Publishers Ltd, England. Printed in Great Britain

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other electrical/electronic devices attractive. A number of recent studies on the electronic structure of polyanilines 2'4- 7 have contributed to a better understanding of the oxidation/reduction processes and the conduction mechanism. In any potential practical application, a knowledge of the stability and degradation mechanism of this class of conducting polymer is also of primary importance. The ageing of polyaniline salts in air at room temperature s and at 150°C, (Ref. 9), and the effect of water vapor 1° on conductivity have been reported. In this paper, we report on stability studies which have investigated the effects of temperature, current density and organic solvents on polyaniline.

EXPERIMENTAL Protonated polyaniline (PAn-HC1) was synthesized via the oxidative polymerization of aniline by ( N H 4 ) 2 S 2 0 s in aqueous HC1. l Four different stability experiments were performed: (i) thermal stability with no passage of current, (ii) electrical conductivity (a) measurements at various temperatures, (iii) electrical stability under high current density and (iv) effects of exposure to organic solvents. The weight changes of the polymer with temperature were monitored with a Netzsch STA 409 simultaneous thermogravimetric-differential thermal analyzer at a heating rate of 10°C/min in either N 2 o r air. For each run, about 8 mg of sample was used with 100ml/min of gas at approximately atmospheric pressure. For electrical conductivity measurements, the powdery polymer was compressed into pellets of 1.2 cm diameter and about 0.1 cm thickness and the standard collinear four-probe method was used. The four-probe assembly consists of a Teflon frame with platinum contacts. In determining the variation of tz with temperature, the ambient (air) temperature was slowly and continuously increased to 150°C and a current of 5 mA was used to determine t~ at regular intervals. The electrical stability tests were performed by subjecting each pellet to a continuous flow of current while it was exposed to laboratory air at room temperature. The conductivity of the pellet was continuously monitored while a constant DC power supply (Hewlett Packard Model 6212B) was used to maintain a steady current even if the conductivity changed during the course of the experiment. To assess the effects of organic solvents on the polymer, about 0.5 g of powder was dispersed in 100 ml of solvent and left for about 18-20h. The organic solvents tested were chloroform, ethanol, toluene, THF, acetonitrile and DMSO. After the treatment period, the sample was washed with additional solvent (except in the case of DMSO, where ethanol was used in the final rinsing to remove DMSO) and dried

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under reduced pressure. The weight and conductivity of the sample were recorded. Analytical techniques used for sample characterization include ultra-violet (UV)-visible absorption spectroscopy (using a Shimadzu UV260 spectrophotometer), infra-red (IR) absorption spectroscopy (using a Perkin Elmer Model 682 spectrophotometer) with the polymer samples dispersed in KBr pellets, and X-ray photoelectron spectroscopy (XPS). The XPS measurements were made on a VG Scientific ESCALAB MkII spectrometer with a MgK0t X-ray source (1253"6 eV photons). The powdery polymer samples were mounted on standard sample studs with double-sided adhesive tapes. All core-level spectra were referenced to the Cls neutral carbon peak at 284-6eV and the peak areas for various elements were corrected by experimentally determined instrumental sensitivity factors.

RESULTS A N D DISCUSSION Comparison of the thermogravimetric (TG) scans of PAn-HC1 in N 2 and in air (Fig. l) shows that for temperatures less than 250°C, there is no significant difference in the weight loss behaviour. The PAn-HC1 sample in both air and N 2 shows an initial small weight loss from 30°C to 100°C. This weight loss may be due to residual water and/or HCI from the synthesis process. After 100°C there is essentially no weight change until about 225°C. 1.0

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et al.

From 250°C to 350°C the sample heated in air showed a slower rate of weight loss. It has been postulated that when PAn-HCI is heated in air at 150°C, its total chlorine content changes little but it is transformed to a deprotonated structure accompanied by chlorine substitution at the aromatic ring. 9 The TG data from the air and N 2 runs would be consistent with the postulate that the formation of covalent chlorine makes it more stable to heat above 250°C than the mainly ionically bonded chlorine present in the protonated structure. The rapid weight loss observed in air for temperatures greater than 350°C is due to the formation of volatile products resulting from the oxidation of the polymer. After PAn-HC1 is treated with NH4OH, the deprotonated product shows a smaller initial weight loss (between 30°C and 100°C) than the PAn-HC1 and there is minimal weight loss in N 2 below 500°C. Even at 700°C, the deprotonated product retains 75% of its original weight. The conversion of chloride ions to covalently bonded chlorine can be expected to decrease a.9 The electrical conductivity at various temperatures normalized by that at room temperature, tr/aRr, is plotted in Fig. 2. The reported value of o- at any temperature is the value measured after l0 min at a constant ambient temperature. The electrical conductivity of the PAnHC1 sample is temperature dependent, being weakly thermally activated before showing a sharp decrease after about 60°C. The decrease in ~ appears to be reversible if the sample is heated to temperatures below 100°C and then

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Stability studies o[polyaniline

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left to cool in ambient air (Fig. 3). With higher heat treatment temperatures, a will increase upon cooling but not to the level prior to heating. (The values of normalized conductivity in Figs 2 and 3 are not exactly equal due to different heat treatment conditions.) The recovery in conductivity is a slow process and the water vapor in the air appears to be an essential factor in this process since the sample cooled in the dessicator shows only minimal recovery. Water vapor has also been shown to increase the r o o m temperature conductivity o f both the base and the hydrochloride by a factor of ---2. t° However, the decrease in a upon heating to temperatures above 100°C is probably not due to loss of adsorbed water vapor alone. When the sample is cooled after such treatment, it achieves a room temperature value of o- which decreases with the heat treatment period. XPS experiments on a P A n - H C I sample heated to 150°C show a total chlorine to nitrogen ratio

K. G. Neoh et al.

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(C1/N) of 0"54 and a covalent chlorine to chloride anion ratio (-C1/C1-) of 0.62. The corresponding values prior to heat treatment are 0.51 and 0.42. This indicates that the irreversible conversion of ionic chlorine to covalent chlorine becomes significant at temperatures greater than 100°C. Polyaniline also forms a conductive complex with tetracyanoethylene (TCNE) when the deprotonated base is treated with TCNE solution. The resulting complex (for example, with a T C N E : P A n mole ratio of ~0"6:1) has a room temperature conductivity (aRT) of 0"24 S/cm in contrast with about 2 S/cm for PAn-HC1. However, the conductivity of the P A n - T C N E complex shows greater electrical and thermal stability than PAn-HC1. As can be seen in Fig. 2, a for the P A n - T C N E complex increases with temperature such that its value at 150°C is more than 50% higher than the room temperature value. At 150°C, the electrical conductivities of the P A n TCNE complex and PAn-HC1 become comparable because a/aRT for the two samples are about 1"5 and 0.2 at 150°C and the corresponding values of the conductivities before heating (aRT) are 0"24 and 2 S/cm. Hence, at 150°C, a for the P A n - T C N E and PAn-HC1 samples are 0.36 and 0"4S/cm, respectively. However, after exposing the P A n - T C N E complex for 18 h at 150°C, its conductivity at this temperature decreases to almost 25% of its room temperature value. The differences in stability and degradation mechanisms of PAn-HC1 and P A n - T C N E complexes reflect the different chemical and electronic structures of these two conducting polymer complexes. The detailed synthesis and characterization of the P A n - T C N E complex will be presented in a separate paper. The ability of PAn-HC1 to sustain a continuous current flow is illustrated in Fig. 4. All experiments were conducted in air at 23°C and a relative humidity of 70%. The polymer shows no discernible change in a after 24 h at 2AICM2

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Effect of current density on electrical conductivity, ao is the initial conductivity.

Stability studies o/'poO,aniline

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a current density of 2A/cm 2. As the current density is increased beyond 8A/cm 2, the polymer breaks down rapidly. Comparison of PAn-HCI with chemically synthesized polypyrrole (PPY)-halogen complexes shows that the former has a higher current carrying capacity than PPY-C12 complex but is inferior to PPY-I 2 and PPY-Br 2 complexes. 11 The P A n - T C N E complex also has a higher current carrying capacity than PAn-HC1. Of the six organic solvents tested, chloroform has the least effect on P A n HC1 while DMSO affects the polymer most adversely. There is essentially no weight change after chloroform treatment and a correspondingly shows only a slight decrease (Fig. 5). In contrast, when DMSO is used, a 14% weight loss is observed and a is decreased by a factor of 20. The organic solvents apparently alter the chlorine content and nature in PAn-HC1. The C12p XPS core-level spectra for the chloroform and DMSO treated samples are presented in Fig. 6. Each spectrum can be deconvoluted into two major components. The peak component at the lower binding energy of around 197 eV is attributable to chloride anion while the higher binding energy component at around 200"3eV corresponds to the covalently bonded chlorine. Comparison of these two spectra indicates clearly that the ratio of chloride anion to covalently bonded chlorine is lower for the DMSO treated sample. To obtain a better idea of how the change in chlorine content and nature affects o, the ratios CI/N and C1-/N were calculated from the XPS 1.0

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BIHDING ENERGY (eV) Fig. 6. Cl2p XPS core-level spectra of (a) DMSO treated PAn-HCI and (b) chloroform treated PAn-HCI.

spectra and plotted in Fig. 5 together with a for the six PAn-HCI samples treated with solvents. The PAn-HC1 sample before solvent treatment has C1/N and C1-/N ratios of 0.51 and 0"37, respectively. With solvent treatment, the C1/N ratio becomes less than the initial value, but the trend in decreasing C1/N does not correspond to that of tr. On the other hand, the decrease in C1-/N follows the decrease in a closely. Hence the data indicate that the decline in a upon solvent treatment is due to the loss of chloride anions either into solution or conversion into covalently bonded species. Thus, the C1-/N ratio, rather than the C1/N ratio, should be used for correlating the conductivity data in protonated polyaniline. The UV-visible absorption spectrum of the DMSO-soluble fraction of PAn-HC1 is shown in Fig. 7. The absorption spectrum of the DMSO soluble fraction shows a band at about 310 nm followed by a shoulder at 440 nm and a long absorption tail extending to the near IR region. This spectrum shows similarities with the in-situ spectrum of thin polyaniline film deposited under galvanostatic conditions in HC1.12 The fully reduced form of polyaniline shows a weak broad band at 900 nm and a strong sharp band at 335 nm. As the potential increases and the film is oxidized, the long wavelength band increases in intensity and shifts to shorter wavelength, a weak band at 440 nm appears and then disappears, and the band at 335 nm initially shifts to 320nm and then back to 340nm. 12 The insoluble fraction after DMSO treatment also shows similarities to the original PAn-HC1. The IR absorption spectrum of the former is shown

Stabilio, studies ~f polyaniline

115

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in Fig. 8. The intensity of the band at 1580cm -t relative to that at 1490cm-~ remains almost unchanged by DMSO treatment. These bands have been assigned to quinone and benzoid ring stretching, respectively.13 The long absorption tail from 4000cm-~ to about 1700cm-1, and the intense and broad band at 1140cm- ~ assignable to the vibration due to the protonated structure also remain prominent. In view of these similarities between the DMSO treated sample and PAn-HC1, it is not surprising that the thermal and electrical behaviour of these two samples follow the same MICROMETRES 25

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116

K.G. Neoh et al.

trend (Figs 1 and 2). Although a of the D M S O sample is only about 5% of that of PAn-HC1 due to the loss of a portion of the chloride anions, the decrease in a with temperature (Fig. 2) is probably due to the same process as occurs in PAn-HC1, i.e. the loss of adsorbed water vapor and the conversion of the remaining chloride anions to covalently bonded chlorine.

CONCLUSION Comparison of the stability studies of PAn-HC1 with earlier results obtained with chemically synthesized PPY-halogen complexes ~ indicates that the conductivity of PAn-HC1 degrades more readily on heating and under current than the P P Y - B r 2 complex. For the latter, the decline in a on heating corresponds to the onset of major weight loss at 140°C whereas decrease in a of the former occurs at about 60°C. The decrease in a of P A n HC1 is probably due to the initial loss of adsorbed water vapor followed by the conversion of chloride anions to covalent chlorine. The differences in stability between these two conducting polymers are related to the different doping mechanisms. This is also evident when the stability of PAn-HC1 is compared with that of P A n - T C N E complex under identical conditions. The degradation of a of PAn-HC1 when treated with organic solvents is highly dependent on the nature of the solvent, with D M S O having the most adverse effect among the six solvents tested. It appears that degradation in a is related to either the loss of chloride anions and/or the conversion of the anions into covalently bonded species.

REFERENCES 1. Chiang, J. C. & MacDiarmid, A. G., Synth. Met., 13 (1986) 193. 2. MacDiarmid, A. G., Chiang, J. C., Richter, A. F. & Epstein, A. J., Synth. Met., 18 (1987) 285. 3. Epstein, A. J., Ginder, J. M., Zuo, F., Bigelow, R. W., Woo, H. S., Tanner, D. B., Richter, A. F., Huang, W. S. & MacDiarmid, A. G., Synth. Met., 18 (1987) 303. 4. Salaneck, W. R., Lundstrom, I., Hjertberg, T., Duke, C. B., Conwell, E., Paton, A., MacDiarmid, A. G., Somasiri, N. L. D., Huang, W. S. & Richter, A. F., Synth. Met., 18 (1987) 291. 5. Snauwaert, Ph., Lazzaroni, R., Riga, J. & Verbist, J. J., Synth. Met., 16 (1986) 245. 6. Hjertberg, T., Sandberg, M., Wennerstrom, O. & Lagerstedt, I., Synth. Met., 21 (1987) 31. 7. Epstein, A. J., Ginder, J. M., Zuo, F., Woo, H. S., Tanner, D. B., Richter, A. F., Angelopoulos, M., Huang, W. S. & MacDiarmid, A. G., Synth. Met., 21 (1987) 63.

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8. Munstedt, H., Polymer, 29 (1988) 296. 9. Hagiwara, T., Yamaura, M. & lwata, K., Synth. Met., 25 (1988) 243. 10. Angelopoulos, M., Ray, A., MacDiarmid, A. G. & Epstein, A. J., Synth. Met., 21 (1987) 21. 11. Neoh, K. G., Kang, E. T. & Tan, T. C., Poly. Deg. and Stab., 21 (1988), 93. 12. Bloor, D. & Monkman, A., Synth. Met., 21 (1987) 175. 13. Tang, J., Jing, X., Wang, B. & Wang, F., Svnth. Met., 21 (19881, 231.