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.