MODEL FOR AGING IN HCL-PROTONATED POLYANILINE - STRUCTURE, CONDUCTIVITY, AND COMPOSITION STUDIES

We present a comprehensive study of the aging process in HCl-doped pol yaniline exposed to air. A complete set of measurements was carried ou t on three series of samples stemmed from the same preparation batch, and aged at 100, 120, and 140 degrees C for times up to one month. All the samples were studied by x-ray-diffraction and conductivity measur ements in the 4.2-300-K temperature range, and by thermogravimetric an d elemental analyses. The analysis of the x-ray data points out the he terogeneous character of the aging process. An amorphous phase (F phas e) is shown which grows at the expense, first, of the amorphous emeral dine salt (ES-I) phase and, then, of the crystalline ES-I phase; We sh ow that whatever the aging time, the conductivity can be described in terms of hopping between conducting grains separated by insulating bar riers leading to sigma(T)= sigma(0).exp[-(T-0/T)(1/2)] The thermogravi metric and elemental analyses data give evidence of several chemical t ransformations: (i) a slight dedoping due to HCl evolution, (ii) an ox idation of the polymer backbone, and (iii) a chlorination of the rings . We propose a picture for the aging mechanism that accounts for the w hole set of data with a particular emphasis on the quantitative evolut ion of both the F phase volume content and the hopping conductivity pa rameter T-0. The pristine conducting,:rains would consist of a crystal line 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 transform ation of the doped polyaniline into the F phase, starting from the gra in periphery, i.e., the most disordered parts of the material, toward the crystalline cores. Moreover, this model allows one to reproduce qu antitatively the kinetics of the conductivity decrease at a given temp erature. [S0163-1829(98)05936-0].