Conductivity degradation due to thermal aging in conducting polyaniline and polypyrrole

The decrease of the electrical conductivity of polyaniline and polypyrrole due to thermal aging is reported. The d.c. conductivity is measured in the temperature range 300-80 K for aging times from 0 to 11 h at 120 degrees C in room atmosphere. The conductivity of polyaniline decreases with aging time according to the law sigma = sigma(o)exp[-(t/tau)(1/2)]. Polypyrrole diverges either from the above or sigma(o)-sigma alpha t(1/2). Moreover, polyaniline follows sigma = sigma(o)exp[-(T-o/T)(1/2)] law with T-o increasing with aging time. This can be explained by a conduction mechanism consisting of electron tunnelling between conducting grains embedded into an insulating matrix. Polypyrrole follows a sigma = sigma(o)exp[-T-1/(T + T-o)] law until about 5 h of aging time. For longer heating its behaviour diverges from the predictions of known models of conduction in polymers. The above can be attributed to differences of the aging process in the two compounds. In polyaniline, aging is accompanied simply by a decrease of the grain size, the separation of which increases from 27 to 54 Angstrom after 10 h of heating at 120 degrees C. In polypyrrole, for short aging times, the grain size remains constant, their separation being about 60 Angstrom. Longer aging leads to a thermally activated conductivity whose mechanism is obscure. (C) 1998 Elsevier Science S.A.