Protonation and deprotonation of polyaniline films and powders revisited

The changes in the intrinsic oxidation states of thin films of emeraldine (EM) base as a function of treatment time in aqueous acid during the protonation-deprotonation cycle were quantitatively assessed by X-ray photoelectron spectroscopy (XPS). For protonation carried out in 0.05 M HCl, 1 M HCl, 1 M H2SO4 and 2 M HClO4, an acid treatment time of 1 h followed by rapid deprotonation in NaOH increased the intrinsic oxidation state of the films to that of nigraniline (NA). However, lower intrinsic oxidation states than that of NA were obtained for films with increasing acid exposure time. The increase in intrinsic oxidation state after one cycle of acid-base treatment was not readily observed in EM powders. The dense morphology of the EM films cast from N-methylpyrrolidinone (NMP) solutions must have retarded the diffusion of water molecules into the films to cause the hydrolysis reaction. The hydrolysis reaction is known to cause a decrease in the intrinsic oxidation of the polymer.