Structure and properties of polyaniline films prepared via electrochemical polymerization. I: Effect of pH in electrochemical polymerization media on the primary structure and acid dissociation constant of product polyaniline films

Electrochemical polymerization of aniline was conducted in constant current mode at different pH ranging from 0.2 to 3.7 in an ITO glass electrode-quartz cell assembly for in situ monitoring of the changes in potential, amount of charge passed through, and u.v.-visible absorption (u.v.-Vis) spectra during polymerization. u.v.-Vis spectra of product polyaniline (PAn) films were determined in different pH solutions to evaluate the apparent acid dissociation constants (pKa)(app) from plots of log{(Abs(HAX) - Abs(obs))/(Abs(obs) - Abs(A)) versus pH, where Abs(obs), Abs(HAX), and Abs(A) are the observed absorption strength and those of the fully protonated and deprotonated species, respectively, for a given wavelength light. In PAn films prepared at low pH ( < 1.0), emeraldine form PAn chains with relatively long average conjugation length but broad distribution were produced, while in those prepared at high pH ( > 1.5), PAn chains with short conjugation length were produced and later deteriorated as the polymerization proceeded, because the polymerization potential eventually exceeded a critical value ( + 0.8 V) beyond which degradation of produced PAn chains took place. (C) 1998 Elsevier Science Ltd. All rights reserved.