Stretching-induced crossover to the metallic state in iodine-doped polyacetylene samples

We report the coexistence of metallic and non-metallic states within different portions of samples of heavily iodine-doped stretched polyacetylene based upon the measurements of a huge negative dielectric constant at microwave frequency (εmw) for the center portions and a relatively small and positive εmw for the neck and end portions. A Drude model in the low frequency limit is applied to estimate a transport time and a plasma frequency for the metallic portions of the samples. A modified Drude model including one-dimensional localization effects is used to account for the temperature dependence of the microwave dielectric constant and conductivity. For the non-metallic portions, an interrupted metallic strand model, which represents the material as a bundle of metallic wires or polymer chains interrupted by insulating defects, is applied to analyze the localization behavior probed by the positive dielectric constant. The variation of dielectric response of samples stretched before doping and with aging after doping points to the key role of inhomogeneous disorder in charge transport for these fibrillar materials with incomplete crystallinity.