Granular-rod model for electronic conduction in polyaniline

We present here a granular-rod model for the metallic state of the conducting polymer, polyaniline. In this model the metallic islands correspond to single strands of the polymer. The macroscopic conductivity results from anisotropic three-dimensional variable-range hopping in the network of metallic rods. We incorporate the experimentally observed temperature dependence of the charge carrier density and show that this model is capable of explaining (1) the temperature dependence of the conductivity σ=σ0exp[-(T0/T)1/2], (2) the doping dependence of T0, (3) the anomalous 1/T dependence of the thermoelectric power, as well as (4) the linear increase of the Pauli susceptibility with dopant concentration. We show that the quantitative agreement between our predictions for the doping dependence of T0, the temperature dependence of the thermoelectric power, and the experimental data is excellent. We also illustrate that the temperature range where variable-range hopping is valid is decreased below the experimentally observed temperature range over which σ=σ0exp[-(T0/T)1/2] if the metallic islands correspond to three-dimensional bundles of the polymer strands. It would appear then that a single strand model of the metallic state of polyaniline is more consistent with a variable-range hopping picture.