An analysis of DC conductivity in terms of degradation mechanisms induced by thermal aging in polypyrrole/polyaniline blends

Measurements of the d.c. electrical conductivity on thermally treated polypyrrole/polyaniline (PPy/PANI) samples, in which the PPy content increased by 10% w.w. starting from pure PANI to pure PPy, followed a σ(t, T) = σ0(t)exp[−(T0/T)1/2] law. This is consistent with a heterogeneous structure of the granular metal type, in which aging is accompanied by the shrinking of the conductive grains causing the decrease of the sample conductivity, a process which is described by the increase of the parameter T0. The preexponential factor σ0(t) depends on the intrinsic conductivity of the grains and geometrical factors affecting the carrier paths through the energy barriers, as are the grain size distribution and the mean volume occupied by the conducting grains in the material. It was found that for the samples as a whole the thermal aging law, which predicts ln σ(t, T)∝t1/2 is followed for a given temperature T, where t is the time of the thermal treatment, in accordance with a granular metal type structure. On the other hand, the preexponential factor σ0(t) decreases with the aging, following a different law [σ(t = 0, T)−σ(t, T)]/σ(t = 0, T)∝t1/2, where σ(t = 0, T) is the initial value of σ0(t), that of the fresh sample. This law reveals an aging caused by a degradation proceeding into the interior of the grains in a diffusion-like manner. So, the two different laws of aging, one from T0 and the other from σ0, reveal that the aging does not simply reduce the size of the grains, but affects their interior, this degradation decreases with depth.