Model for aging in HCl-protonated polyaniline: Structure, conductivity, and composition studies

We present a comprehensive study of the aging process in HCl-doped polyaniline exposed to air. A complete set of measurements was carried out on three series of samples stemmed from the same preparation batch, and aged at 100, 120, and 140 ^∘{}C for times up to one month. All the samples were studied by x-ray-diffraction and conductivity measurements in the 4.2--300-K temperature range, and by thermogravimetric and elemental analyses. The analysis of the x-ray data points out the heterogeneous character of the aging process. An amorphous phase (F phase) is shown which grows at the expense, first, of the amorphous emeraldine salt (ES-I) phase and, then, of the crystalline ES-I phase. We show that whatever the aging time, the conductivity can be described in terms of hopping between conducting grains separated by insulating barriers leading to σ(T)=σ₀·exp[-(T₀/T)1/2]. The thermogravimetric and elemental analyses data give evidence of several chemical transformations: (i) a slight dedoping due to HCl evolution, (ii) an oxidation of the polymer backbone, and (iii) a chlorination of the rings. We propose a picture for the aging mechanism that accounts for the whole set of data with a particular emphasis on the quantitative evolution of both the F phase volume content and the hopping conductivity parameter T₀. The pristine conducting grains would consist of a crystalline core of emeraldine salt (ES-I) surrounded by ``paracrystalline'' and amorphous ES-I shells. The aging phenomena would then appear as a nibbling mechanism of conducting grains due to a progressive transformation of the doped polyaniline into the F phase, starting from the grain periphery, i.e., the most disordered parts of the material, toward the crystalline cores. Moreover, this model allows one to reproduce quantitatively the kinetics of the conductivity decrease at a given temperature.