Aging effects on the transport properties in conducting polymer polypyrrole

We present electronic transport studies of the aging process in naphtalene-sulfonate-doped polypyrrole. They include in situ conductivity measurements as a function of the aging time up to 1 month, and conductivity and thermoelectric power measurements in the temperature range 300-15 K for different aging times at 120 degrees C in room atmosphere. We show that while the short-aging-time decay of the conductivity may be accounted for by a law of the type sigma(0)-sigma(t(a))proportional to root t(a), the long-aging-time evolution is well described by a stretched exponential, sigma=sigma(0)exp[-(T-a/tau)(1/2)]. Moreover, two distinct temperature dependences have been identified: (i) sigma=sigma exp[-(T-0/T)(1/2)] for aged samples and (ii) sigma=sigma(0)exp[-T-1/T+T-0] for as-synthesized or lightly aged samples. The thermal variation of the thermoelectric power can be described by the following law: S(T) =AT+B+C/T, where the relative weight of the linear term, A, appears to be a decreasing function of the aging time. All the results are comprehensively explained in terms of conducting grains separated by insulating barriers in which the conduction is controlled by a hopping process of the charge carriers between the grains. The aging phenomenon is found to consist of a decrease of the grain size, in parallel with a broadening of the barriers, as in a corrosion process. As the aging time increases, the size of the conducting grains decreases and then goes below a critical value that is responsible for a crossover in the transport mechanism and therefore in the time dependence of the conductivity as experimentally observed. In the aged samples, this model leads to the existence of a single expression that accounts for both the temperature and the aging-time dependences of the conductivity, i.e., 1n sigma(t(a),T)proportional to-(t(a)/T)(1/2).