Thermal degradation of the electrical conductivity in polyaniline and polypyrrole composites

The thermal aging in various conducting polyaniline and polypyrrole composites protonated with hydrochloric acid, containing polymers with sulphonic or phosphoryl groups is investigated. The DC conductivity was measured in the temperature range 300-80 K for aging times from 0 to 300 h at 70 degrees C in room atmosphere. The conductivity decreases with aging time according to the law sigma= sigma(0)exp[-(t/tau)(1/2)], indicating an inhomogeneous structure of the granular metal type. Time tau, which specifies the aging rate, varies with the porosity and the composition of the composites from 40 to 3000 h. The porosity and the presence of sulphonic or phosphoryl groups retards the aging process. The conductivity of the composites with temperature T follows the sigma= sigma(0)exp[-T-1/(T + T-0)] law predicted by Sheng's model of fluctuation-induced tunneling (FIT), which assumes conductive grains separated by insulating barriers in accordance with the thermal aging law mentioned above. Thermal degradation is attributed to the release of HCl from the samples, which reduces the protonated-conducting phase. (C) 2000 Elsevier Science S.A. All rights reserved.