Thermoelectric power and conductivity of different types of polypyrrole

We have measured the thermoelectric power and conductivity as a function of temperature of a wide range of polypyrrole samples, including a film of so luble polypyrrole synthesized chemically, and wrinkled films synthesized us ing indium-tin oxide electrodes; other samples investigated include high-co nductivity polypyrrole films synthesized at different temperatures and curr ent densities, films grown on nonconducting substrates, and polypyrrole gas sensors. The thermoelectric powers are remarkably similar and metal-like f or the medium and high conductivity samples but show nonzero extrapolations to zero temperature for wrinkled samples. The temperature dependence of co nductivity tends to be greater for samples of lower conductivity. In contra st to polyaniline and polyacetylene, a crossover to metallic sign for the t emperature dependence of conductivity at higher temperatures is not observe d in any of our samples; the fluctuation-induced tunnelling and variable-ra nge hopping expressions account for nearly all our conductivity data except for low-temperature anomalies. (C) 1999 John Wiley & Sons, Inc.