Metallic transport in polyaniline

Despite nearly three decades of materials development, the transport properties in the 'metallic state' of the so-called conducting polymers are still not typical of conventional metals(1-7). The hallmark of metallic resistivity-a monotonic decrease in resistivity with temperature-has not been obtained at temperatures over the full range below room temperature; and a frequency dependent conductivity, sigma(omega), typical of metals has also not been observed. In contrast, the low-temperature behaviour of 'metallic' polymers has, in all previous cases, exhibited an increase in resistivity as temperature is further decreased, as a result of disorder-induced localization of the charge carriers(1-4). This disorder-induced localization also changes the infrared response such that sigma(omega) deviates from the prediction of Drude theory(5-7). Here we report classic metallic transport data obtained from truly metallic polymers. With polyaniline samples prepared using self-stabilized dispersion polymerization(8), we find that for samples having room-temperature conductivities in excess of 1,000 S cm(-1), the resistivity decreases monotonically as the temperature is lowered down to 5 K, and that the infrared spectra are characteristic of the conventional Drude model even at the lowest frequencies measured.