STRUCTURE-CONDUCTIVITY RELATION FOR POLYPYRROLE WITH A 2-DIMENSIONAL MICROSCOPIC STRUCTURE

Polypyrrole (PPy) films with dominating two-dimensional microscopic structures show higher electrical conductivities and a weaker temperature dependence when compared to the corresponding one-dimensional material. For the temperature-dependent conductivity data, sigma(T), a reasonable fit to an analytic expression is obtained for the fluctuation-induced tunneling mechanism. We use improved expressions for the Sheng formula to give a realistic connection between the fit parameters and the barrier characteristics which include the height of the barrier and the effective mass of the tunneling carriers. From these expressions, we determine the nominal length of the tunnel junctions to be 8.5 Angstrom and the cross section to be 6 Angstrom(2). Based on these values we suggest a structure model to explain the parameters derived from the electrical conductivity. In this model, the polymer consists of islands with a two-dimensional (macrocyclic) structure. These islands of finite size are crosslinked by segments of one-dimensional Pqr chains, the latter acting as tunnel junctions. Accordingly, the parameters obtained from our analysis of the temperature dependence correspond roughly to a chain length of two monomeric units of the crosslinking one-dimensional chain segments and to the cross section of the pi-electron system, respectively.