A simple method to estimate the oxidation state of polyanilines

The electrical conductivity and other properties of polyaniline (PANI) and its derivatives are highly dependent on its oxidation states. In order to obtain a simple method to determine the oxidation state, a systematic study was performed to investigate the interconversion between pernigraniline (PB) PANI form to emeraldine (EB) one, and from EB to leucoemeraldine (LEB), by ultraviolet/visible (UV/Vis) spectroscopy. UV/Vis spectra were carried out for different oxidation degrees of PANI/N-methyl-2-pyrrolidinone (NMP) solutions. The redox states interconversion was made in NMP solutions in ambient air and/or inert atmosphere. LEB base presents only one peak in the absorbance spectra, associated with the π–π* transition, while the other two forms, EB and PB, exhibit another peak of lower energy. This second peak, related to a molecular exciton in EB, has its maximum intensity diminished as the EB is reduced or oxidized. By plotting the relative intensity of these peaks vs. the oxidation degree, it was possible to identify, with a reasonable accuracy, the oxidation state of PANIs. This method was also applied to poly(o-methoxyaniline) (POMA).