In situ FTIR–attenuated total reflection spectroscopic investigations on the base–acid transitions of polyaniline. Base–acid transition in the emeraldine form of polyaniline

Abstract

Emeraldine refers to the semi-oxidized state of polyaniline and it can exist in the base form or the protonated form (salt) generated by treatment of the base form with an acid. Using a newly applied doping system NaReO₄/HReO₄, the investigation of the base–acid transition process of emeraldine has been carried out by ex situ and in situ FTIR spectroscopic methods. In situ FTIR–ATR (attenuated total reflection) measurements performed during the base–acid transition process clearly show the change in the structure of the polymer film observable. The physical properties of emeraldine can be changed considerably by varying the pH of the media. The base form of emeraldine is an insulator and treatment with acidic media results in the highly conducting salt form (proton doping). The protonation of emeraldine starts at pH 6. The intensity of a broad absorption band at wavenumbers higher than 2000 cm^–1(0.25 eV) depends strongly on the pH of the electrolyte. This absorption band bleaches completely with increasing pH, indicating a strong correlation between this band and the metallic character of the polymer. On the basis of in situ FTIR–ATR measurements, a reaction path of acid–base transitions has been proposed.