Birefringence Effects in Acrylonitrile Polymers. II. The Nature of the 140°C Transition

Abstract

Birefringence measurements on solvent-cast films of polyacrylonitrile homopolymer under dead load are reported as a function of temperature and as a function of time at constant temperature. It is shown that a large positive birefringence effect is associated with the 240°C aromaticization reaction in PAN. The high negative birefringence state produced by temperature cycling and first reported in Part I of the present series is found to require a small dead load on the sample during cooling for its formation, and is observed to form under conditions of almost constant length. From the interaction between the large negative effect and the positive effects of the nitrile conjugation reaction, it is deduced that the magnitude of the large negative effect is directly proportional to the concentration of nitrile groups present. It is proposed that the 140°C transition in PAN is a result of the association-dissociation equilibrium between nitrile dipoles on adjacent chains. The temperature dependence of the equilibrium constant for this association is calculated from the birefringence ``cooling curve,'' giving an enthalpy change of 48 kcal/mole, which suggests that association of dipole multiples is involved. Creep, dynamic mechanical, and dielectric loss data from the literature are correlated on the basis of this proposal concerning the nature of the transition.