Transport and EPR studies of polyaniline: A quasi-one-dimensional conductor with three-dimensional ‘‘metallic’’ states

Abstract

It is an open question if ‘‘metallic’’ polymers have one-dimensional or three-dimensional conduction states. We investigate this issue by studying a model polymer, the HCl-doped emeraldine salt form of oriented polyaniline (PAN-ES) through the temperature dependence of the dc conductivity, thermoelectric power, complex microwave dielectric constant, electron paramagnetic resonance (EPR), and electric-field dependence of conductivity. The thermopower, microwave dielectric constant, and EPR data suggest that the electrons are three dimensionally delocalized. We propose that oriented PAN-ES consists of coupled parallel chains that form ‘‘metallic’’ bundles. These bundles are the ‘‘crystalline’’ regions of the polymer in which the electron wave functions are three dimensionally extended. This is in contrast to conventional quasi-one-dimensional conductors (many ‘‘metallic’’ charge-transfer salts) in which conducting chains are essentially isolated. However, between bundles are the amorphous (less-ordered) regions in which charge hopping dominates the macroscopic conductivity. The formation of the ‘‘metallic’’ bundles is proposed to be the result of a significant charge-interchain-transfer rate inside the crystalline regions. The implications of the results for the improvement of conductivity are addressed.