Polyacrylonitrile fiber behavior. II. Dependence on structure and environment

Abstract

The effect of comonomer, draw ratio, and annealing temperature on the glass transition temperature T_g and on a number of properties of polyacrylonitrile fibers has been examined. The coefficients of axial thermal expansion above T_g were found to be positive and unusually high. In water, they increased with increasing draw ratio and annealing temperature, but these effects could not (or hardly) be observed in decalin. The axial thermal expansion is compared with that of polyamide fibers, where the coefficient was positive on unstretched fibers but negative on drawn ones throughout the temperature range (25–150°C.) examined. Implications for the structure of polyacrylonitrile fibers and a number of peculiarities in their glass transition are discussed. Most of the results were in line with a structure based on stiff molecules twisted out of a straight conformation by intramolecular repulsive interaction between adjacent dipoles. An unexpectedly large proportion of compounds examined were found to exert a plasticizing action which could be related most readily to the dipole moments. The effect is attributed to a reduction in molecular stiffness and is compared with the solution behavior of the polymer. An increase in “yield point” with increasing molecular size of tensile testing medium provided evidence for an increase in volume during initial extension.