‘Polyaniline’: Protonic acid doping of the emeraldine form to the metallic regime

The emeraldine base form of polyaniline, which consists of equal numbers of reduced [(C6H4)N(H)(C6H4)N(H)] and oxidized [(C6H4)N(C6H4)N] repeat units, is doped to the metallic conducting regime by aqueous 1 M HCl. The product is believed to consist of equal numbers of the reduced repeat units and the diprotonated oxidized repeat units, The resulting material appears to have significant π delocalization in the polymer backbone. The doping is of a type not hitherto observed in a conducting polymer. It results in the formation of an environmentally stable nitrogen base salt rather than of a carbonium ion as in conventional oxidative p-doping of a polymer. Unlike all other conducting polymers, its conductivity depends on two variables, namely the degree of oxidation and the degree of protonation of the material. Various relationships between conductivity, pH of the acid dopant solutions, percentage doping and pKa of the protonated emeraldine salt are described and discussed.