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

The results of microwave dielectric constant (ε{lunate}_mw) and dc conductivity (σ_dc) 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 ε{lunate}_mw is measured indicating an intrinsic metallic state. The Drude model at low frequency is used to estimate a relatively small plasma frequency ω_p {reversed tilde equals} 0.015 eV and an anomalously long scattering time τ {reversed tilde equals} 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.