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Abstract
A combination of in situ conductivity, ESR and UV-vis measurements as a function of the electrode potential is applied to study in detail the pH-influence of aqueous solutions containing sulfate anions and the effect of nonaqueous solution with LiClO4 on polyaniline (PANI) and polypyrrole (PPy). Upon electrochemically oxidizing both polymer films in acidic solution, the conductivity increases by a factor of 104-105. Changing pH from 0.8 to 7 lowers the conductivity values and the anodic potential limit, to which the polymers can be taken without irreversible degradation. Even at pH = 7 considerable conductivity changes are detected.
According to ESR measurements, maximum spin density is found during the transition of the polymers between the insulating and the fully conducting state. With UV-vis spectroscopy only in the case of PANI in acidic solution an optical absorption located at 430 nm can clearly be ascribed to the spin state, but at pH = 7 this characteristic absorption band disappears.
On the basis of the polaron-bipolaron model, the results strongly favor a bipolaronic conduction model for PANI and PPy. The observed pH-effect can be explained with a diminished chemical stability of bipolarons in aqueous solution of higher pH.