Electrical conductivity of pyrolyzed polyacrylonitrile

Using ultrapure samples of polyacrylonitrile (PAN) of 485,000 or 150,000 average molecular weight solution cast in dimethylformamide, the dc conductivity ([sgrave]) of pyrolyzed PAN (PANP) films has been studied for pyrolysis temperatures (Tp) of 280-435°C. Conductivity measurements made during pyrolysis indicate the onset of a dramatic increase in [sgrave] for Tp of 390-435°C. Conductivities as high as 5 (ohm-cm)−1 have been observed for Tp < 435°C. This situation contrasts sharply with previous literature which had indicated that a increased uniformly and monotonically with Tp for 200°C < Tp < 900°C and that values of [sgrave] > 1 (ohm-cm)−1 were observed only for Tp > 600°C. Our results indicate that the maximum value of [sgrave] obtained is not a strong function of Tp (390°C < Tp < 435°C) or of molecular weight. However, the rate of increase of [sgrave](t) is strongly dependent on Tp in this range. After pyrolysis, repeated heating and cooling below Tp do not alter [sgrave](T). IR spectra show that the sudden increase in [sgrave] is correlated to the formation of conjugated C=C and C=N bonds. The shape of [sgrave](T) suggests that conduction is probably due to hopping.