SIMULTANEOUS EPR AND ELECTROCHEMICAL MEASUREMENTS ON POLYANILINE IN AMBIENT-TEMPERATURE MOLTEN-SALTS

Simultaneous EPR and electrochemical measurements have been carried out on polyaniline (PAn) prepared by monomer oxidation in an acidic aqueous solution and investigated in an ambient temperature ionic liquid, which consists of a mixture of aluminum chloride and 1-methyl-3-ethylimidazolium chloride. The maximum EPR response was found at the point where half the total observed charge had been passed in both cyclic voltammetry and potential step experiments. A one-to-one relationship between the number of spins observed and the number of electrons removed was found to about 25% of full oxidation. Experiments are explained in terms of two unresolved one-electron steps, with a thermodynamic comproportionation equilibrium among the neutral, polaron, and bipolaron states. The equilibrium constant K(com) of the reduced form, alpha, and of the oxidized form, beta, changes with the conductivity and ionic environment of the film. The bipolaron is favored in the initial doping process, and the polaron is dominant in the final doping stage. The EPR response of the polaron decays with a half-life between 8 and 17 s.