Liquid-crystalline solutions of electrically conducting polyaniline

Abstract

Polyaniline (PANI), fully protonated with camphor sulfonic acid (CSA), was found to form liquid-crystalline solutions in m-cresol. The existence of this interesting state of matter was inferred from observations of opalescence, depolarization of plane-polarized light and the existence of a critical concentration for the onset of the formation of the liquid-crystalline phase. This critical concentration was found, in accord with existing theory, to decrease monotonically as the viscosity or molecular weight of the polyaniline increased. By shearing the liquid-crystalline solutions onto glass substrates and subsequent precipitation in acetone, oriented polyaniline films were obtained. The existence of a liquid-crystalline phase implies a rather rigid conformation of the fully protonated polyaniline in solution. Importantly, the emeraldine base form of PANI remained isotropic in solutions in m-cresol up to very high concentrations. Thus it is suggested that protonation induces a chain stiffening and extension of the polyaniline macromolecules (relative to the insulating emeraldine base) resulting from a combination of electronic delocalization and counter-ion ‘crowding’.