Effect of heat treatment on the dielectric relaxation of polyacrylonitrile. III. Role of duration of heat treatment

Dielectric relaxation of polyacrylonitrile (PAN) heat-treated at 120°C in vacuum for durations varying from 4 to 24 h is studied in order to investigate the effect of the duration of heat treatment. Decrease in dielectric constant increment and loss peak height and an increase in relaxation temperature on heat treatment are observed at all durations of heat treatment. At the studied temperature of heat treatment the observed changes in dielectric relaxation were found to increase with duration of heat treatment showing maxima at a 16 h duration and then a decrease at a higher duration of heat treatment. The results are discussed in terms of the variation of degree of intermolecularly bound nitrile groups with the duration of heat treatment. Glass transition temperature calculated from the dielectric data, through WLF theory, showed a variation, similar to that of the degree of bound nitrile groups, with duration of heat treatment. The observed variation of T_g is attributed to the interlinking effect produced by the bound nitrile group pairs on the segmental mobility.