Evolution of the conducting state of polyaniline from localized to mesoscopic metallic to intrinsic metallic regimes

The results of microwave dielectric constant (ϵmw) and dc conductivity (σdc) of doped polyaniline (PAN) samples in different charge delocation regimes are reported. With camphor culfonic acid doped PAN prepared in m-cresol solvent (PAN-CSA (m-cresol)) respresenting the most delocalized material, a negative ϵmw is measured indicating an intrinsic metallic state. The Drude model at low frequency is used to estimate a relatively small plasma frequency ωp ⋍ 0.015 eV and an anomalously long scattering time τ ⋍ 1.2 × 10−11 s. This τ implies the importance of phonon back scattering for the most delocalized electrons as expected for the open Fermi surface of a highly anisotropic metal. Depending on the crystallinity, there are two different charge localization regimes in PAN samples: uncoupled metallic islands and coupled “mesoscopic” metallic state. The three-dimensional nature of metallic state is demonstrated by the correlation of low temperature dielectric response with the parallel and perpendicular crystalline domain lengths obtained from x-ray data. The charge localization to delocalization transition dependence on the polymer processing will be discussed. We present the temperature dependent nonmetal-metal transition for some preparations of PAN-CSA (m-cresol). The intrinsic conductivity of metallic PAN is estimated to be ∼ 107 S/cm.