The protonation deprotonation hysteresis in polyaniline

The changes in intrinsic oxidation state ([=N-] and [-NH-] content) and protonation level ([N+]/[N] ratio) of emeraldine (EM) base powders as a function of protonic acid loadings resulting from the protonation-deprotonation cycle have been assessed quantitatively by X-ray photoelectron spectroscopy. A hysteresis effect was observed for the protonation-deprotonation cycle involving HCl, but not for H2SO4. The hysteresis has been attributed in part to covalent HCl addition to the polymer and the associated reduction in intrinsic oxidation state. A hysteresis effect involving a significant increase in intrinsic oxidation state was observed in the once acid-base cycled EM films cast from N-methylpyrrolidinone, but not for powders. The phenomenon has been attributed to the more efficient hydrolysis of powder samples in aqueous acid media due to the large effective surface area.