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

Polypyrrole (PPy) films with dominating two-dimensional microscopic st ructures show higher electrical conductivities and a weaker temperatur e dependence when compared to the corresponding one-dimensional materi al. For the temperature-dependent conductivity data, sigma(T), a reaso nable fit to an analytic expression is obtained for the fluctuation-in duced tunneling mechanism. We use improved expressions for the Sheng f ormula to give a realistic connection between the fit parameters and t he 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 Ang strom and the cross section to be 6 Angstrom(2). Based on these values we suggest a structure model to explain the parameters derived from t he electrical conductivity. In this model, the polymer consists of isl ands 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 ob tained from our analysis of the temperature dependence correspond roug hly to a chain length of two monomeric units of the crosslinking one-d imensional chain segments and to the cross section of the pi-electron system, respectively.