ESR properties of polyaniline in a wide pH range

• ESR signal of polyaniline is still very strong at pH 10 but disappearance at pH > 11.
• ESR signal disappearance of polyaniline accompanied by deprotonation at pH > 11.
• Deprotonation of polyaniline depends on pKb of pH functional group in polyaniline.
• Visible spectra of polyaniline show no occurrence of deprotonation at pH ⩽ 7.
• Deprotonation of polyaniline is accompanied by the reduction step.

In this paper, polyaniline was synthesized using cyclic voltammetry in a potential range between −0.20 and 1.10 V (vs. SCE), in a solution consisting of 0.20 M aniline and 0.60 M H2SO4. The electron spin resonance (ESR) measurements of the emeraldine salt of polyaniline were done in a wide pH range of 0.7 (0.20 M H2SO4 solution) to 12.0 of 0.30 M Na2SO4 solution. It was found that the ESR signal of polyaniline was still very strong at pH 10.0 and then its ESR signal disappeared at pH > 10.0. This result is unexpected because polyaniline was electrochemically inactive and its conductivity vanished at pH > 5.0 based on the previous reports. The visible spectra of polyaniline showed no change in the absorption peak position in a pH range of 0.7–7.0, indicating no occurrence of deprotonation; while its absorption peak shifted markedly towards the short wavelength direction from pH 8.0 to 12.0, as a result polyaniline film become purple, indicating occurrence of deprotonation. The ESR signal disappearance and deprotonation of polyaniline are strongly related to pKb values of pH functional group in polyaniline chains. The entire deprotonation process of polyaniline is suggested to include the conventional deprotonation step, i.e.; without charge transfer and the reduction step of polyaniline that was confirmed with electrochemical measurements.