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Measurement and evaluation of polyaniline degradation
Polymer Degradation and Stability 41 (1993) 69-76
Measurement and evaluation of polyaniline degradation C. Q. Cui, X. H. Su & Jim Y. Lee Department of Chemical Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 0511 (Received 17 September 1992; accepted 31 September 1992)
The hydrolytic degradation of polyaniline (PANi) by electrochemical oxidation 1 M HCIO4 is examined semi-quantitatively. It is suggested that the extent of degradation can be measured in terms of the incorporation of the hydrolysis products in the polymer matrix as evaluated by the ratio of the second anodic peak current to the first, iB/iA, in the voltammogram of polyaniline (PANi) redox. The incorporation, in conjunction with partial polymer dissolution and subsequent thinning of the polymer deposit, fragmentation of PANi fibrils and extensive crosslinking, result in the densification of the polymer surface, loss of electrical conductivity and finally lead to the deactivation of the electrochemical activity. Two regions can be identified in the degradation process. In the active dissolution region, PANi is partially dissolved and a good fraction of the hydrolysis products is incorporated into the remaining film, changing both the morphology and the structure of PANi in due course. In the passivation region, the polymer is totally deactivated electrochemically as a result of the morphological modification. It is believed that degradation proceeds mainly in the surface layers of the polymer, resulting in low utilization of the polymer redox change in thick films and films deposited at high anodic deposition potentials.
INTRODUCTION
degradation that occurs concurrently with the electrodeposition of PANi also disrupts the orderly growth of the polymer chains, 12-~4 impairing the performance of PANi in practical applications. Therefore, the degradation of PANi is a problem that must be reckoned with, understood and resolved before the polymer can deliver its full potential in the many application areas suggested for it. The mechanism and kinetics of PANi degradation have been reported previously. 3,4,14-17 pbenzoquinone (BQ) was concluded to be the final and the main soluble degradation product in the over-oxidation of PANi, ~3 and its presence has been verified by UV/visible absorption spectroscopy. ~2"~6 The hydrolytic degradation of PANi is believed to proceed formally as follows: -
Application of electroactive polyaniline (PANi) in areas such as rechargeable batteries, 1,2 electrochromic displays,3 microelectronic devices, 4 corrosion inhibitors, 5 catalytic electrode materials, 6'7 ion exchange 8 and selective gas permeation membranes 9, and generation of light images 1° demand high stability from the polymer. The polymer is atmospherically more stable than polyacetylenes, but it nonetheless undergoes rapid hydrolytic degradation through reactions with water when it is polarized more positively than 0.7 V (v. SCE) in 1 M HCI. 3 Although the degradation hardly occurs in non-aqueous electrolytes, ~ the reduced rates of redox reactions in such environments may not be acceptable to some applications. Hydrolytic
PA~+ + H20 - e---->PA~_I + (BQ and/or PAP)
(1) where PAP is p-aminophenol or its oligomers. Stilwell & Park 17 studied the hydrolytic degrada-
Polymer Degradation and Stability 0141-3910/93/$06.00 (~) 1993 Elsevier Science Publishers Ltd. 69
70
C. Q. Cui, X. H. Su, Jim Y. Lee
tion in H2SO4 using the rotating ring-disk electrode technique. The rate of hydrolysis for oxidized PANi in the quinoid form was found to depend on film thickness and follow consecutive first order kinetics. The observed dependencies of the hydrolysis reaction on the acidity of the degradation medium and the sulphate concentration was consistent with the Schiff base hydrolysis mechanism. In the voltammogram of cyclic potential sweep (CPS) deposition of polyaniline, the first anodic peak at -0.17 V (A) and the second anodic peak at -0.52 V(B) are often assigned to the oxidation of PANi to the polaronic state and the oxidation of the products from hydrolytic degradation of PANi respectively. The ratio of the peak currents, ia/iA, was suggested as a measure of the extent of incorporation of degradation products in the polymer film electrodeposited. 18"~9 This method of quantitatively estimating the extent of incorporation of hydrolysis products in PANi has been used in the understanding of the fundamental differences of the deposition techniques, and the effects of deposition conditions on the electrochemical properties of PANi deposit. ~9 Moreover, the ratio serves also as an indicator for the quality of the polymer deposit. A small ratio is indicative of a smaller extent of incorporation and subsequently a polymer film of better structural integrity and morphology. TM In this paper, the ratio will be used to study the characteristics of PANi films polarized to the degradation potential both voltammetrically and potentiostatically in 1 M HCIO4. The degradation tendency of PANi films of different growth structure and thickness will also be discussed.
EXPERIMENTAL A conventional three electrode system was used. The working electrode is a 5 mm rotating glassy carbon (GC) disk electrode in a Teflon sheath. A large platinum gauze and a saturated calomel reference electrode (SCE) were used as the counter and the reference electrodes respectively. Prior to each experiment, the GC electrode was carefully washed with N-methyl pyrrolidone (NMP), polished with diamond paste, rinsed thoroughly with deionized water and dried on a clean tissue. The reproducibility of the experimental results was significantly
better with this pretreatment of the working electrode. All chemicals used were of the Analytical Reagent grade. Aniline (BDH) was further purified by distilling over zinc dust to remove oxidized impurities. The clear aniline distillate was stored in the dark under nitrogen. PANi was deposited from solutions containing 0-2 M of the clear aniline distillate in 1MHCIO4. The electrolyte was deaerated by flowing nitrogen for 10 min before each electrochemical run. All runs were carried out at room temperature. Electrodeposition and electrochemical characterization of PANI films were carried out on an EG & G model 273 potentiostat/galvanostat under compiter control. Subsequent to potentiostatic deposition at 0-8V for 4 min for a deposition charge around 170 mC/cm 2, PANi was degraded in 1 M HCIO4 by potentiostatic polarization at 1.0 V, and by voltammetric polarization from - 0 . 2 V to 1.0 V at 50 mV/s. The electrochemical activity of degraded PANi was assessed by its voltammetric response in the same electrolyte, using the potential range of - 0 . 2 1.0 V at 50 mV/s. UV/visible spectroscopy and scanning electron microscopy (SEM) were used to examine the structural and morphological differences between the pristine and the degraded forms of PANi. In both cases PANi was deposited potentiostatically on platinum foils at 0.8 V for 43 mC/cm 2 (thin film, for UV spectroscopy) and 200mC/cm 2 (thick film, for SEM) respectively. Degradation in this case was limited to potentiostatic polarization at 1.0V in 1 M HCIO4. N-methyl pyrrolidone (NMP) was chosen as the solvent for PANi in UV/visible spectroscopic measurements. SEM samples were copiously washed, vacuum dried and stored under nitrogen atmosphere prior to examination by a JSM-T330A scanning electron microscope. RESULTS A N D DISCUSSION Surface morphology and structure of degraded PANi films Figure 1 compares the surface morphology of degraded and fresh PANi films. The degradation in this case consisted of potentiostatic polarization at 1-0 V in 1 M HCIO4 for 30 min. Although the morphology in both cases is still predomin-
71
Measurement and evaluation of polyaniline degradation
incorporated into the polymer, resulting in diminished performance of PANi in practical applications. The changes in PANi structure due to hydrolytic degradation were also examined by UV/visible spectroscopy (Fig. 2). The spectra are dominated by two absorption peaks located at 325 nm and 620nm respectively. The 325 nm peak is due to localized exciton formation and can be assigned to a ~r-~r* transition of the benzenoid structure, red shifted by the auxochromic effect of lone pairs on the nitrogen atom. ]3c'2°'2~ The absorption at 620 nm results in a blue colour and is characteristic of the benzenoid-quinoid transition in the emeraldine form of PANi base. 13 (NMP) can easily deprotonate PANi in the emeraldine oxidation state, particularly when the amount of polymer dissolved is small). The decrease in the intensity of both peaks as polarization time increases is indicative of polymer degradation and the reduction in film thickness) 3b More important, both peaks display a blue shift and the decrease in their wavelengths also increases with polariza-
Polarisation t i m e (min)
Fig. 1. Surface morphology of pristine (A) and degraded PANi films (B). The film was degraded by potentiostatic polarization in 1 MHCIO4 solution at 1-0 V for 30 min.
ated by the fibrous structure that is characteristic of PANi deposition from HCIO4, the polymer fibrils in degraded PANi are shorter and are crosslinked more extensively. In addition, there are also irregularities along the fibrils in the form of large and coarse aggregates. The orderliness of the polymer matrix is reduced as a result of the disruption in directionality. The aggregates are probably induced by the hydrolytic degradation of PANi such as that indicated by reaction (1). It is generally believed that both polymer growth and hydrolytic degradation proceed through a common intermediate generated by the overoxidation of PANi to the bipolaronic state (diradical dications). In the absence of aniline in the electrolyte (the present experimental condition), the polymer is consumed by hydrolysis without replenishment from polymerization. The products from hydrolytic degradation could be
P < 0"7~
I
250
575 Wavelength (rim)
900
Fig. 2. UV/visible absorbance spectra of degraded PANi films. The polarization times are shown in the figure.
72
C. Q. Cui, X. H. Su, Jim Y. Lee
tion time. As reported previously for the oligomeric leuco bases of the PANi type, 22 the wavelength is a function of the number of monomer units in the oligomer and it increases with the degree of polymerization. The blue shift of the absorption peaks is therefore indicative of the shortening of the polymer chains. The decrease in the polymer chain length after degradation is also evident from the results of SEM examination. A shoulder at 290nm (as indicated by the up-arrow in Fig. 2) was noted after 10 min of polarization. The peak may be assigned to oligomers with short chains and hydrolysis products arising from the degradation. 22 The development of the shoulder into a peak was complete in 80 min. In about the same time the peak at 320-325 mm that is characteristic of polymers of considerable chain length was reduced to a shoulder (the down-arrow in Fig. 2), and disappeared altogether after 120 min of polarization. This suggests that degraded PANi is a mixture of polymers of different lengths and possibly some hydrolysis products. The increase in quinoid moieties in degraded polymer film brings about the localization of polarons in PANi. According to the model proposed by Sum 2.000 j
I
I
et al., 23 with greater localization, more excitation
strength will be transferred to the valence band (VB) to the upper polaron level transition (UPLT) and the polaron level in PANi is shifted closer to the VB. The physical consequence of polaron localization is the increase in film resistance which is often found in PANi degradation. Therefore, with hydrolytic degradation of PANi, the thickness of the polymer deposit necessarily decreases because of the consumptive nature of the hydrolysis reactions. The length of PANi fibrils is shortened and some of the hydrolysis products are assimilated into the degraded film. These consequences would produce significant changes in the electrochemical behaviour of the polymer that can be identified by cyclic voltammetry. Voltammetric characteristics of PANi in degradation Figure 3 shows the voltammograms of PANi during degradation with different polarization times. In the voltammograms, peaks A and C are generally identified as the oxidation of the base or protonated benzenoid units in PANi to the J
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I 0.600
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-0.200
73
Measurement and evaluation of polyaniline degradation polaronic state and their subsequent oxidation to the bipolaronic state, respectively. TM Peak A, in particular, is often used to assess the electroactivity of the polymer film. Peak B is assigned to the oxidation of hydrolysis products such as BQ, PAP or partially soluble PANi containing quinone moieties. '2 An increase in the polarization time reduces the cathodic charge or the net charge in the voltammograms and increases the separation between each redox pair. This is demonstrative of the deleterious nature of the degradation process which consumes PANi that can undergo reversible redox reactions, and the increase in film resistance as a result of the shortening of the length of PANi oligomers and the incorporation of hydrolysis products in the remaining film. These general voltammetric characteristics of PANi degradation have also been reported by o t h e r s . 3'4'14c'16'24 Therefore, the voltammetric characteristics of PANi degradation is parallel to the changes in the physical properties as noted by SEM and UV/visible spectroscopy. For a more quantitative understanding of the degradation reactions, the ratio of the anodic peak currents, iB/iA, and the ratio (1-- iA/i°A) where i~, is the anodic peak current at the start of polarization, are plotted against the polarization time in Fig. 4. The ratios are proportional to the extent of incorporation of hydrolysis products in the film and the extent of PANi consumption, respectively. The use of iB/iA as a measure of the quality of PANi films has been advocated in our previous work. ~8 In the early stages of polarization, both ratios are found to increase linearly with the increase in polarization time. As polarization continues to 11 min and beyond, the rate of increase of the ratio (1-- iA/i°A) is suppressed, and the ratio iB/iA approaches a steady state value as indicated by the plateau in Fig. 4. In the linear region, the increase in the extent of PANi consumption and the incorporation of hydrolysis products with time as a result of the active dissolution of PANi due to hydrolysis reactions. The kinetics of hydrolysis relative to that of PANi redox is difficult to extract from the plots but the variation of iB/iA with time is best explained by reactions of different orders. As the degradation of PANi increases the fragmentation of the polymer fibrils and the incorporation of hydrolysis products in PANi, a decrease in the electronic conductivity of the film with polarization time is expected.
0.8
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polarization time in degradation.
Indeed, at polarization times of 11 min and beyond where at least - 5 3 % of the initial amount of PANi is consumed, the ohmic polarization in the electrode due to increased film resistivity is large enough to retard the hydrolysis reaction, and the rate of change of (1-iA/i°a) decreases. In the meanwhile a time independent iB/iA reflects that the structure and composition of the film remain virtually unchanged with time. The passivation of PANi is therefore realized in this region where the polymer loses almost all of its electroactivity. For ease of discussion that follows, this region will be addressed as the 'passivation region' whereas the region where both ratios vary linearly with polarization time will be addressed as the 'active dissolution region'. It has been identified by the rotating ring-disk electrode technique and UV/visible spectroscopy that hydrolysis products are partially soluble and the major soluble species in them is B Q . 14'16"17 Therefore, according to the change in i~/iA and (1-- iA/i°A) with polarization time, it can be concluded that the rate of incorporation of hydrolysis products in the film is higher in the active dissolution region than in the passivation region. The hydrolysis products formed in the passivation region are mostly soluble in the solution so that the extent of incorporation of hydrolysis products is invariant with polarization time. The observation that incorporation of hydrolysis products hardly occurs in the passivation region is in good agreement with the results of Stilwell & Park, '6 which showed that the ratio of UV absorbance of soluble species in the
C. Q. Cui, X. H. Su, Jim Y. Lee
74
solution to the change in potentiostatic oxidation of PANi also levels off at long polarization times. 16 In addition, the rapid increase in the absorbance to charge ratio with time in the initial stages of polarization can be perceived as a consequence of the incorporation of partially soluble hydrolysis products in the film. The dependence of the extent of incorporation on film thickness can also be accounted for likewise. Since the diffusion of soluble hydrolysis products to the solution is not a rate limiting step, 17 the PANi matrix of the thicker film in the initial stage of degradation is capable of retaining more hydrolysis products than that of the thinner film in the later stage of degradation after the polymer has been substantially consumed by the hydrolysis reactions. Therefore, the ratio in/iA could more effectively reflect the extent of PANi degradation in practical applications. In the voltammetric polarization of PANi, the change in the ratio of in/iA with the number of cycles was also examined (Fig. 5). The ratio follows the same trend as that in potentiostatic polarization, namely the increase in the ratio with the number of cycles eventually approaches a steady state value. In the active dissolution region, the ratio in/iA increases with the number of cycles. The smaller steady state value and the poorer linearity of the increase could be due to the non-equivalence of polarization times under potentiostatic and voltammetric conditions.19 PANi was totally deactivated after cycling in 1MHCIO4 solution from - 0 . 2 to 1.0V at 50 mV/s for more than 200 cycles. Therefore, the
results from both the potentiostatic and voltammetric degradation of PANi are comparable, and attest to the usefulness of the ratio of the anodic peaks currents, iB/iA, in following the extent of PANi degradation by measuring the extent of incorporation of hydrolysis products in the polymer film. Effect of properties of P A N i film on its degradation
In as much as the properties of PANi deposits are affected by the growth c o n d i t i o n s , 12,j3a'13b'25-28 the degradation, i.e. instability of PANi is likewise dependent on the properties of the deposit. According to Stilwell & Park, ~7the loose fibrous structure in thick films could increase the accessibility of the solution to the polymer interior, thereby increasing the rate of PANi dissolution. Figure 6 shows the time dependency of iB/iA for three films of different deposition charge. As deposition charge is a good measure of film thickness, the physical measurement of thickness was not attempted in the experiments. Regardless of the thickness, there are still active dissolution and passivation regions in the degradation process. If the 'life expectancy' of PANi in degradation is defined as the time to reach passivation, then the lives are 9 min, 11 min and 14 min for PANi films deposited at 0.8 V for 2 min, 4 min and 8 min, respectively. Therefore, the life of PANi is not increased linearly with film thickness. More specifically, the steady state value of in/iA in the passivation
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Measurement and evaluation of polyaniline degradation region actually decreases with thickness. The results seem to suggest that hydrolysis reactions in degradation are limited to the surface region of the polymer film only. The occurrence of hydrolysis reactions results in extensive fragmentation and crosslinking of the polymer chains. This, together with the incorporation of partially soluble hydrolysis products may have produced morphological changes in the PANi structure that hinders the diffusion of the electrolyte into the polymer interior. The degradation of PANi is suppressed as a consequence. The limited access of electrolyte to the polymer interior suppresses the degradation. This should not be construed as advantageous because the electricity is reduced simultaneously. The utilization of the redox charge in the polymer is actually poorer and the polymer indeed has a shorter 'useful life'. In PANi deposition, the morphology of the film can easily be changed from a loose, well-defined fibrous structure to a dense, poorly defined structure by increasing the anodic limit of the deposition potential. This fact was explored in further experiments to substantiate the argument of evolving morphology in degradation. Figure 7 shows the dependence of ia/iA on polarization time for films deposited at different potentials for the same deposition charge of 171 mC/cm 2. The ratio of iB/iA before polarization is indicative of the impurity incorporated during deposition, and is higher for films deposited at higher anodic potentials, indicating the imperfection inherent in high potential depositions. '8.'9 The films deposited at 1
-
0.9
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Fig. 7. Dependence of iB/iA on polarization time for PANi films deposited at different anodic potentials for a charge of 171 mC/cm 2. The deposition minerals are indicated in the legend.
75
potentials more positive than 0.8 V indeed have shorter fibrils and more crosslinking resulting in a relatively dense structure.J9 This kind of structure would hinder the diffusion of electrolyte to the polymer interior to a greater extent, resulting in a shorter useful life of the polymer. This is confirmed by the earlier onset of the passivation region, which defines the useful life of the polymer, for films deposited at higher potentials (Fig. 7). Indeed the onset of passivation decreases monotonically with the deposition potential. Therefore, the value of iB/iA in evaluating and understanding the extent and effects of PANi degradation is again demonstrated.
CONCLUSIONS The ratio of the second to the first anodic peak current in the voltammogram of PANi redox, iB/iA, which was previously suggested as a measure of the extent of impurity incorporation during deposition, is used again here to indicate semi-quantitatively the hydrolytic degradation of PANi in 1 MHCIO4. With the degradation of PANi, the film thickness is reduced in association with the shortening of the length of PANi chains and the incorporation of the hydrolysis products in the film. Based on the dependency of the ratio on polarization time, two regions can be identified. In the active dissolution region that commences early in either potentiostatic or voltammetric polarizaiton, the ratio increases rapidly with polarization time. There is dissolution of PANi at this stage but a good fraction of the hydrolysis products is retained by the film. The incorporation of hydrolysis products in the polymer changes the morphology of the polymer matrix that makes the passage of electrolyte to its interior increasingly difficult. When degradation finally evolves into the passivation region, dissolution of PANi is inhibited and little hydrolysis products are incorporated into the film and a steady state value for iB/iA is obtained. The time to the onset of passivation can therefore be used as an indicator of the polymer's useful lifespan in practical applications as the polymer is totally deactivated electrochemically beyond that. It is found that the lifespan of the polymer does not increase linearly with film thickness. As degradation proceeds mainly in the surface region of the polymer, the resulting densification
76
C. Q. Cui, X. H. Su, Jim Y. Lee
of the p o l y m e r surface renders a large fraction of the polymer interior inaccessible for electrochemical activity, which also explains the lower utilization of redox charge in thick p o l y m e r films. A similar situation also arises with P A N i f o r m e d at high anodic deposition potentials. The incorporation of products from the hydrolytic degradation of P A N i is therefore precursory to the demise of the polymer's functionality in practical applications.
ACKNOWLEDGEMENT The authors would like to express their gratitude to Mr S. K. Tung in the D e p a r t m e n t of Mechanic Engineering for the S E M examination of P A N i electrodeposits.
REFERENCES 1. Kitani, A., Kaya, M. & Sasaki, K., J. Electrochem. Soc., 133 (1986) 1069. 2. MacDiarmid, A. G., Yang, L. S., Huang, W. S. & Humphrey, B. D., Synth. Met., 18 (1987) 393. 3. Kobayashi, T., Yoneyama, H. & Tamura, H., J. Electroanal. Chem., 161 (1984) 419. 4. Paul, E. W., Ricco, A, J. & Wrighton, W. S., J. Phys. Chem., 89 (1985) 1441. 5. Noufi, R., Nozik, A. J., White, J. & Warren, L. F., J. Electrochem. Soc., 129 (1982) 2261. 6. Kost, K. M. & Bartak, D. E., Anal. Chem., 60 (1988) 2379. 7. Mu, S., Xue, H. & Qian, B., J. Electroanal. Chem., 304 (1991) 7.
8. Oyama, N., Ohsaka, T. & Shimizu, T., Anal. Chem., 57 (1985) 1526. 9. Fabrizio, M., Furlanetto, F., Mengoli, G., Musiani, M. M. & Paolucci, F., J. Electroanal. Chem., 323 (1992) 197. 10. Yoneyama, H., Takahashi, N. & Kuwabata, S., J. Chem. Soc., Commun. (1992) 173. 11. Fiordiponti, P. & Pistoia, G., Electrochim. Acta, 34 (1989) 215. 12. Shim, Y.-B., Won, M.-S. & Park, S.-M., J. Electrochem. Soc., 137 (1990) 538. 13. (a) Stilwell, D. E. & Park, S.-M.; J. Electrochem. Soc., 135 (1988) 2254; (b) 135 (1988) 2491; (c) 136 (1989) 427. 14. Kobayashi, T., Yoneyama, H. & Tamura, H., J. Electrochem. Soc., 177 (1984) 293. 15. Glarum, S. H. & Marshall, J. H., J. Electrochem. Soc., 134 (1987) 2160. 16. Stilwell, D. E. & Park, S.-M., J. Electrochem. Soc., 135 (1988) 2497. 17. Stilwell, D. & Park, S. M., J. Electrochem. Soc., 136 (1989) 688. 18. Cui, C. Q., Ong, L. H., Tan, T. C. & Lee J. Y., J. Synth. Mat., 58, (1993) 147. 19. Cui, C. Q., Ong, L. H., Tan, T. C. & Lee, J. Y., J. Electroanal. Soc. (1992) 346, (1993) 477. 20. Monkman, A. P., Bloor, D., Stevens, G. C., Stevens, J. C. H. & Wilson, P., Synth. Met., 29 (1989) E277. 21. McCall, R. P., Ginder, J. M., Leng, J. M., Ye, H. J., Manohar, S. K., Masters, J. G., Asturias, G. E. & MacDiarmid, A. G., Phys. Rev. B, 41 (1990) 5202. 22. Zach, P., J. Polym. Sci., 10 (1972) 2379. 23. Sum, U., Fesser, K. & Butter, H., Solid State Comm., 61 (1987) 607. 24. Jiang, R. & Dong, S., Synth. Met., 28 (1988) 255. 25. Zotti, G., Cattarin, S. & Comisso, N., J. Electroanal. Chem., 239 (1988) 387. 26. Nunziante, P. & Pistoia, G., Electrochim. Acta, 34 (1989) 223. 27. LaCroix, J.-C. & Diaz, A. F., J. Electrochem. Soc., 135 (1988) 1457. 28. Inzelt, G., J. Electroanal. Chem., 279 (1990) 169.
Measurement and evaluation of polyaniline degradation C. Q. Cui, X. H. Su & Jim Y. Lee Department of Chemical Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 0511 (Received 17 September 1992; accepted 31 September 1992)
The hydrolytic degradation of polyaniline (PANi) by electrochemical oxidation 1 M HCIO4 is examined semi-quantitatively. It is suggested that the extent of degradation can be measured in terms of the incorporation of the hydrolysis products in the polymer matrix as evaluated by the ratio of the second anodic peak current to the first, iB/iA, in the voltammogram of polyaniline (PANi) redox. The incorporation, in conjunction with partial polymer dissolution and subsequent thinning of the polymer deposit, fragmentation of PANi fibrils and extensive crosslinking, result in the densification of the polymer surface, loss of electrical conductivity and finally lead to the deactivation of the electrochemical activity. Two regions can be identified in the degradation process. In the active dissolution region, PANi is partially dissolved and a good fraction of the hydrolysis products is incorporated into the remaining film, changing both the morphology and the structure of PANi in due course. In the passivation region, the polymer is totally deactivated electrochemically as a result of the morphological modification. It is believed that degradation proceeds mainly in the surface layers of the polymer, resulting in low utilization of the polymer redox change in thick films and films deposited at high anodic deposition potentials.
INTRODUCTION
degradation that occurs concurrently with the electrodeposition of PANi also disrupts the orderly growth of the polymer chains, 12-~4 impairing the performance of PANi in practical applications. Therefore, the degradation of PANi is a problem that must be reckoned with, understood and resolved before the polymer can deliver its full potential in the many application areas suggested for it. The mechanism and kinetics of PANi degradation have been reported previously. 3,4,14-17 pbenzoquinone (BQ) was concluded to be the final and the main soluble degradation product in the over-oxidation of PANi, ~3 and its presence has been verified by UV/visible absorption spectroscopy. ~2"~6 The hydrolytic degradation of PANi is believed to proceed formally as follows: -
Application of electroactive polyaniline (PANi) in areas such as rechargeable batteries, 1,2 electrochromic displays,3 microelectronic devices, 4 corrosion inhibitors, 5 catalytic electrode materials, 6'7 ion exchange 8 and selective gas permeation membranes 9, and generation of light images 1° demand high stability from the polymer. The polymer is atmospherically more stable than polyacetylenes, but it nonetheless undergoes rapid hydrolytic degradation through reactions with water when it is polarized more positively than 0.7 V (v. SCE) in 1 M HCI. 3 Although the degradation hardly occurs in non-aqueous electrolytes, ~ the reduced rates of redox reactions in such environments may not be acceptable to some applications. Hydrolytic
PA~+ + H20 - e---->PA~_I + (BQ and/or PAP)
(1) where PAP is p-aminophenol or its oligomers. Stilwell & Park 17 studied the hydrolytic degrada-
Polymer Degradation and Stability 0141-3910/93/$06.00 (~) 1993 Elsevier Science Publishers Ltd. 69
70
C. Q. Cui, X. H. Su, Jim Y. Lee
tion in H2SO4 using the rotating ring-disk electrode technique. The rate of hydrolysis for oxidized PANi in the quinoid form was found to depend on film thickness and follow consecutive first order kinetics. The observed dependencies of the hydrolysis reaction on the acidity of the degradation medium and the sulphate concentration was consistent with the Schiff base hydrolysis mechanism. In the voltammogram of cyclic potential sweep (CPS) deposition of polyaniline, the first anodic peak at -0.17 V (A) and the second anodic peak at -0.52 V(B) are often assigned to the oxidation of PANi to the polaronic state and the oxidation of the products from hydrolytic degradation of PANi respectively. The ratio of the peak currents, ia/iA, was suggested as a measure of the extent of incorporation of degradation products in the polymer film electrodeposited. 18"~9 This method of quantitatively estimating the extent of incorporation of hydrolysis products in PANi has been used in the understanding of the fundamental differences of the deposition techniques, and the effects of deposition conditions on the electrochemical properties of PANi deposit. ~9 Moreover, the ratio serves also as an indicator for the quality of the polymer deposit. A small ratio is indicative of a smaller extent of incorporation and subsequently a polymer film of better structural integrity and morphology. TM In this paper, the ratio will be used to study the characteristics of PANi films polarized to the degradation potential both voltammetrically and potentiostatically in 1 M HCIO4. The degradation tendency of PANi films of different growth structure and thickness will also be discussed.
EXPERIMENTAL A conventional three electrode system was used. The working electrode is a 5 mm rotating glassy carbon (GC) disk electrode in a Teflon sheath. A large platinum gauze and a saturated calomel reference electrode (SCE) were used as the counter and the reference electrodes respectively. Prior to each experiment, the GC electrode was carefully washed with N-methyl pyrrolidone (NMP), polished with diamond paste, rinsed thoroughly with deionized water and dried on a clean tissue. The reproducibility of the experimental results was significantly
better with this pretreatment of the working electrode. All chemicals used were of the Analytical Reagent grade. Aniline (BDH) was further purified by distilling over zinc dust to remove oxidized impurities. The clear aniline distillate was stored in the dark under nitrogen. PANi was deposited from solutions containing 0-2 M of the clear aniline distillate in 1MHCIO4. The electrolyte was deaerated by flowing nitrogen for 10 min before each electrochemical run. All runs were carried out at room temperature. Electrodeposition and electrochemical characterization of PANI films were carried out on an EG & G model 273 potentiostat/galvanostat under compiter control. Subsequent to potentiostatic deposition at 0-8V for 4 min for a deposition charge around 170 mC/cm 2, PANi was degraded in 1 M HCIO4 by potentiostatic polarization at 1.0 V, and by voltammetric polarization from - 0 . 2 V to 1.0 V at 50 mV/s. The electrochemical activity of degraded PANi was assessed by its voltammetric response in the same electrolyte, using the potential range of - 0 . 2 1.0 V at 50 mV/s. UV/visible spectroscopy and scanning electron microscopy (SEM) were used to examine the structural and morphological differences between the pristine and the degraded forms of PANi. In both cases PANi was deposited potentiostatically on platinum foils at 0.8 V for 43 mC/cm 2 (thin film, for UV spectroscopy) and 200mC/cm 2 (thick film, for SEM) respectively. Degradation in this case was limited to potentiostatic polarization at 1.0V in 1 M HCIO4. N-methyl pyrrolidone (NMP) was chosen as the solvent for PANi in UV/visible spectroscopic measurements. SEM samples were copiously washed, vacuum dried and stored under nitrogen atmosphere prior to examination by a JSM-T330A scanning electron microscope. RESULTS A N D DISCUSSION Surface morphology and structure of degraded PANi films Figure 1 compares the surface morphology of degraded and fresh PANi films. The degradation in this case consisted of potentiostatic polarization at 1-0 V in 1 M HCIO4 for 30 min. Although the morphology in both cases is still predomin-
71
Measurement and evaluation of polyaniline degradation
incorporated into the polymer, resulting in diminished performance of PANi in practical applications. The changes in PANi structure due to hydrolytic degradation were also examined by UV/visible spectroscopy (Fig. 2). The spectra are dominated by two absorption peaks located at 325 nm and 620nm respectively. The 325 nm peak is due to localized exciton formation and can be assigned to a ~r-~r* transition of the benzenoid structure, red shifted by the auxochromic effect of lone pairs on the nitrogen atom. ]3c'2°'2~ The absorption at 620 nm results in a blue colour and is characteristic of the benzenoid-quinoid transition in the emeraldine form of PANi base. 13 (NMP) can easily deprotonate PANi in the emeraldine oxidation state, particularly when the amount of polymer dissolved is small). The decrease in the intensity of both peaks as polarization time increases is indicative of polymer degradation and the reduction in film thickness) 3b More important, both peaks display a blue shift and the decrease in their wavelengths also increases with polariza-
Polarisation t i m e (min)
Fig. 1. Surface morphology of pristine (A) and degraded PANi films (B). The film was degraded by potentiostatic polarization in 1 MHCIO4 solution at 1-0 V for 30 min.
ated by the fibrous structure that is characteristic of PANi deposition from HCIO4, the polymer fibrils in degraded PANi are shorter and are crosslinked more extensively. In addition, there are also irregularities along the fibrils in the form of large and coarse aggregates. The orderliness of the polymer matrix is reduced as a result of the disruption in directionality. The aggregates are probably induced by the hydrolytic degradation of PANi such as that indicated by reaction (1). It is generally believed that both polymer growth and hydrolytic degradation proceed through a common intermediate generated by the overoxidation of PANi to the bipolaronic state (diradical dications). In the absence of aniline in the electrolyte (the present experimental condition), the polymer is consumed by hydrolysis without replenishment from polymerization. The products from hydrolytic degradation could be
P < 0"7~
I
250
575 Wavelength (rim)
900
Fig. 2. UV/visible absorbance spectra of degraded PANi films. The polarization times are shown in the figure.
72
C. Q. Cui, X. H. Su, Jim Y. Lee
tion time. As reported previously for the oligomeric leuco bases of the PANi type, 22 the wavelength is a function of the number of monomer units in the oligomer and it increases with the degree of polymerization. The blue shift of the absorption peaks is therefore indicative of the shortening of the polymer chains. The decrease in the polymer chain length after degradation is also evident from the results of SEM examination. A shoulder at 290nm (as indicated by the up-arrow in Fig. 2) was noted after 10 min of polarization. The peak may be assigned to oligomers with short chains and hydrolysis products arising from the degradation. 22 The development of the shoulder into a peak was complete in 80 min. In about the same time the peak at 320-325 mm that is characteristic of polymers of considerable chain length was reduced to a shoulder (the down-arrow in Fig. 2), and disappeared altogether after 120 min of polarization. This suggests that degraded PANi is a mixture of polymers of different lengths and possibly some hydrolysis products. The increase in quinoid moieties in degraded polymer film brings about the localization of polarons in PANi. According to the model proposed by Sum 2.000 j
I
I
et al., 23 with greater localization, more excitation
strength will be transferred to the valence band (VB) to the upper polaron level transition (UPLT) and the polaron level in PANi is shifted closer to the VB. The physical consequence of polaron localization is the increase in film resistance which is often found in PANi degradation. Therefore, with hydrolytic degradation of PANi, the thickness of the polymer deposit necessarily decreases because of the consumptive nature of the hydrolysis reactions. The length of PANi fibrils is shortened and some of the hydrolysis products are assimilated into the degraded film. These consequences would produce significant changes in the electrochemical behaviour of the polymer that can be identified by cyclic voltammetry. Voltammetric characteristics of PANi in degradation Figure 3 shows the voltammograms of PANi during degradation with different polarization times. In the voltammograms, peaks A and C are generally identified as the oxidation of the base or protonated benzenoid units in PANi to the J
I
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0.500 -0.000 H
_
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I 0.600
I 0.400
I 0.200
I 0.000
e (vl Fig. 3. Cyclic voltammograms of degraded PANi films with different polarization times.
-0.200
73
Measurement and evaluation of polyaniline degradation polaronic state and their subsequent oxidation to the bipolaronic state, respectively. TM Peak A, in particular, is often used to assess the electroactivity of the polymer film. Peak B is assigned to the oxidation of hydrolysis products such as BQ, PAP or partially soluble PANi containing quinone moieties. '2 An increase in the polarization time reduces the cathodic charge or the net charge in the voltammograms and increases the separation between each redox pair. This is demonstrative of the deleterious nature of the degradation process which consumes PANi that can undergo reversible redox reactions, and the increase in film resistance as a result of the shortening of the length of PANi oligomers and the incorporation of hydrolysis products in the remaining film. These general voltammetric characteristics of PANi degradation have also been reported by o t h e r s . 3'4'14c'16'24 Therefore, the voltammetric characteristics of PANi degradation is parallel to the changes in the physical properties as noted by SEM and UV/visible spectroscopy. For a more quantitative understanding of the degradation reactions, the ratio of the anodic peak currents, iB/iA, and the ratio (1-- iA/i°A) where i~, is the anodic peak current at the start of polarization, are plotted against the polarization time in Fig. 4. The ratios are proportional to the extent of incorporation of hydrolysis products in the film and the extent of PANi consumption, respectively. The use of iB/iA as a measure of the quality of PANi films has been advocated in our previous work. ~8 In the early stages of polarization, both ratios are found to increase linearly with the increase in polarization time. As polarization continues to 11 min and beyond, the rate of increase of the ratio (1-- iA/i°A) is suppressed, and the ratio iB/iA approaches a steady state value as indicated by the plateau in Fig. 4. In the linear region, the increase in the extent of PANi consumption and the incorporation of hydrolysis products with time as a result of the active dissolution of PANi due to hydrolysis reactions. The kinetics of hydrolysis relative to that of PANi redox is difficult to extract from the plots but the variation of iB/iA with time is best explained by reactions of different orders. As the degradation of PANi increases the fragmentation of the polymer fibrils and the incorporation of hydrolysis products in PANi, a decrease in the electronic conductivity of the film with polarization time is expected.
0.8
am
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0 II1
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m
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~
;
~
~
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20
Polarisation Time, min
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polarization time in degradation.
Indeed, at polarization times of 11 min and beyond where at least - 5 3 % of the initial amount of PANi is consumed, the ohmic polarization in the electrode due to increased film resistivity is large enough to retard the hydrolysis reaction, and the rate of change of (1-iA/i°a) decreases. In the meanwhile a time independent iB/iA reflects that the structure and composition of the film remain virtually unchanged with time. The passivation of PANi is therefore realized in this region where the polymer loses almost all of its electroactivity. For ease of discussion that follows, this region will be addressed as the 'passivation region' whereas the region where both ratios vary linearly with polarization time will be addressed as the 'active dissolution region'. It has been identified by the rotating ring-disk electrode technique and UV/visible spectroscopy that hydrolysis products are partially soluble and the major soluble species in them is B Q . 14'16"17 Therefore, according to the change in i~/iA and (1-- iA/i°A) with polarization time, it can be concluded that the rate of incorporation of hydrolysis products in the film is higher in the active dissolution region than in the passivation region. The hydrolysis products formed in the passivation region are mostly soluble in the solution so that the extent of incorporation of hydrolysis products is invariant with polarization time. The observation that incorporation of hydrolysis products hardly occurs in the passivation region is in good agreement with the results of Stilwell & Park, '6 which showed that the ratio of UV absorbance of soluble species in the
C. Q. Cui, X. H. Su, Jim Y. Lee
74
solution to the change in potentiostatic oxidation of PANi also levels off at long polarization times. 16 In addition, the rapid increase in the absorbance to charge ratio with time in the initial stages of polarization can be perceived as a consequence of the incorporation of partially soluble hydrolysis products in the film. The dependence of the extent of incorporation on film thickness can also be accounted for likewise. Since the diffusion of soluble hydrolysis products to the solution is not a rate limiting step, 17 the PANi matrix of the thicker film in the initial stage of degradation is capable of retaining more hydrolysis products than that of the thinner film in the later stage of degradation after the polymer has been substantially consumed by the hydrolysis reactions. Therefore, the ratio in/iA could more effectively reflect the extent of PANi degradation in practical applications. In the voltammetric polarization of PANi, the change in the ratio of in/iA with the number of cycles was also examined (Fig. 5). The ratio follows the same trend as that in potentiostatic polarization, namely the increase in the ratio with the number of cycles eventually approaches a steady state value. In the active dissolution region, the ratio in/iA increases with the number of cycles. The smaller steady state value and the poorer linearity of the increase could be due to the non-equivalence of polarization times under potentiostatic and voltammetric conditions.19 PANi was totally deactivated after cycling in 1MHCIO4 solution from - 0 . 2 to 1.0V at 50 mV/s for more than 200 cycles. Therefore, the
results from both the potentiostatic and voltammetric degradation of PANi are comparable, and attest to the usefulness of the ratio of the anodic peaks currents, iB/iA, in following the extent of PANi degradation by measuring the extent of incorporation of hydrolysis products in the polymer film. Effect of properties of P A N i film on its degradation
In as much as the properties of PANi deposits are affected by the growth c o n d i t i o n s , 12,j3a'13b'25-28 the degradation, i.e. instability of PANi is likewise dependent on the properties of the deposit. According to Stilwell & Park, ~7the loose fibrous structure in thick films could increase the accessibility of the solution to the polymer interior, thereby increasing the rate of PANi dissolution. Figure 6 shows the time dependency of iB/iA for three films of different deposition charge. As deposition charge is a good measure of film thickness, the physical measurement of thickness was not attempted in the experiments. Regardless of the thickness, there are still active dissolution and passivation regions in the degradation process. If the 'life expectancy' of PANi in degradation is defined as the time to reach passivation, then the lives are 9 min, 11 min and 14 min for PANi films deposited at 0.8 V for 2 min, 4 min and 8 min, respectively. Therefore, the life of PANi is not increased linearly with film thickness. More specifically, the steady state value of in/iA in the passivation
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Measurement and evaluation of polyaniline degradation region actually decreases with thickness. The results seem to suggest that hydrolysis reactions in degradation are limited to the surface region of the polymer film only. The occurrence of hydrolysis reactions results in extensive fragmentation and crosslinking of the polymer chains. This, together with the incorporation of partially soluble hydrolysis products may have produced morphological changes in the PANi structure that hinders the diffusion of the electrolyte into the polymer interior. The degradation of PANi is suppressed as a consequence. The limited access of electrolyte to the polymer interior suppresses the degradation. This should not be construed as advantageous because the electricity is reduced simultaneously. The utilization of the redox charge in the polymer is actually poorer and the polymer indeed has a shorter 'useful life'. In PANi deposition, the morphology of the film can easily be changed from a loose, well-defined fibrous structure to a dense, poorly defined structure by increasing the anodic limit of the deposition potential. This fact was explored in further experiments to substantiate the argument of evolving morphology in degradation. Figure 7 shows the dependence of ia/iA on polarization time for films deposited at different potentials for the same deposition charge of 171 mC/cm 2. The ratio of iB/iA before polarization is indicative of the impurity incorporated during deposition, and is higher for films deposited at higher anodic potentials, indicating the imperfection inherent in high potential depositions. '8.'9 The films deposited at 1
-
0.9
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~ 0.7
~
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75
potentials more positive than 0.8 V indeed have shorter fibrils and more crosslinking resulting in a relatively dense structure.J9 This kind of structure would hinder the diffusion of electrolyte to the polymer interior to a greater extent, resulting in a shorter useful life of the polymer. This is confirmed by the earlier onset of the passivation region, which defines the useful life of the polymer, for films deposited at higher potentials (Fig. 7). Indeed the onset of passivation decreases monotonically with the deposition potential. Therefore, the value of iB/iA in evaluating and understanding the extent and effects of PANi degradation is again demonstrated.
CONCLUSIONS The ratio of the second to the first anodic peak current in the voltammogram of PANi redox, iB/iA, which was previously suggested as a measure of the extent of impurity incorporation during deposition, is used again here to indicate semi-quantitatively the hydrolytic degradation of PANi in 1 MHCIO4. With the degradation of PANi, the film thickness is reduced in association with the shortening of the length of PANi chains and the incorporation of the hydrolysis products in the film. Based on the dependency of the ratio on polarization time, two regions can be identified. In the active dissolution region that commences early in either potentiostatic or voltammetric polarizaiton, the ratio increases rapidly with polarization time. There is dissolution of PANi at this stage but a good fraction of the hydrolysis products is retained by the film. The incorporation of hydrolysis products in the polymer changes the morphology of the polymer matrix that makes the passage of electrolyte to its interior increasingly difficult. When degradation finally evolves into the passivation region, dissolution of PANi is inhibited and little hydrolysis products are incorporated into the film and a steady state value for iB/iA is obtained. The time to the onset of passivation can therefore be used as an indicator of the polymer's useful lifespan in practical applications as the polymer is totally deactivated electrochemically beyond that. It is found that the lifespan of the polymer does not increase linearly with film thickness. As degradation proceeds mainly in the surface region of the polymer, the resulting densification
76
C. Q. Cui, X. H. Su, Jim Y. Lee
of the p o l y m e r surface renders a large fraction of the polymer interior inaccessible for electrochemical activity, which also explains the lower utilization of redox charge in thick p o l y m e r films. A similar situation also arises with P A N i f o r m e d at high anodic deposition potentials. The incorporation of products from the hydrolytic degradation of P A N i is therefore precursory to the demise of the polymer's functionality in practical applications.
ACKNOWLEDGEMENT The authors would like to express their gratitude to Mr S. K. Tung in the D e p a r t m e n t of Mechanic Engineering for the S E M examination of P A N i electrodeposits.
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8. Oyama, N., Ohsaka, T. & Shimizu, T., Anal. Chem., 57 (1985) 1526. 9. Fabrizio, M., Furlanetto, F., Mengoli, G., Musiani, M. M. & Paolucci, F., J. Electroanal. Chem., 323 (1992) 197. 10. Yoneyama, H., Takahashi, N. & Kuwabata, S., J. Chem. Soc., Commun. (1992) 173. 11. Fiordiponti, P. & Pistoia, G., Electrochim. Acta, 34 (1989) 215. 12. Shim, Y.-B., Won, M.-S. & Park, S.-M., J. Electrochem. Soc., 137 (1990) 538. 13. (a) Stilwell, D. E. & Park, S.-M.; J. Electrochem. Soc., 135 (1988) 2254; (b) 135 (1988) 2491; (c) 136 (1989) 427. 14. Kobayashi, T., Yoneyama, H. & Tamura, H., J. Electrochem. Soc., 177 (1984) 293. 15. Glarum, S. H. & Marshall, J. H., J. Electrochem. Soc., 134 (1987) 2160. 16. Stilwell, D. E. & Park, S.-M., J. Electrochem. Soc., 135 (1988) 2497. 17. Stilwell, D. & Park, S. M., J. Electrochem. Soc., 136 (1989) 688. 18. Cui, C. Q., Ong, L. H., Tan, T. C. & Lee J. Y., J. Synth. Mat., 58, (1993) 147. 19. Cui, C. Q., Ong, L. H., Tan, T. C. & Lee, J. Y., J. Electroanal. Soc. (1992) 346, (1993) 477. 20. Monkman, A. P., Bloor, D., Stevens, G. C., Stevens, J. C. H. & Wilson, P., Synth. Met., 29 (1989) E277. 21. McCall, R. P., Ginder, J. M., Leng, J. M., Ye, H. J., Manohar, S. K., Masters, J. G., Asturias, G. E. & MacDiarmid, A. G., Phys. Rev. B, 41 (1990) 5202. 22. Zach, P., J. Polym. Sci., 10 (1972) 2379. 23. Sum, U., Fesser, K. & Butter, H., Solid State Comm., 61 (1987) 607. 24. Jiang, R. & Dong, S., Synth. Met., 28 (1988) 255. 25. Zotti, G., Cattarin, S. & Comisso, N., J. Electroanal. Chem., 239 (1988) 387. 26. Nunziante, P. & Pistoia, G., Electrochim. Acta, 34 (1989) 223. 27. LaCroix, J.-C. & Diaz, A. F., J. Electrochem. Soc., 135 (1988) 1457. 28. Inzelt, G., J. Electroanal. Chem., 279 (1990) 169.