Birefringence and Shrinkage during Thermal Reversion of Oriented Polymers

Thermally-induced changes in shrinkage and birefringence in oriented polycarbonate have been studied using video recordings. It was found that the decrease in birefringence was not in phase with the development of shrinkage except in a narrow temperature range near to T _g . It is evident that these two phenomena are controlled by different aspects of molecular orientation. In another set of experiments the development of birefringence in thick polystyrene mouldings could not be explained either by residual stresses nor by frozen-in flow orientation. It is believed that these two observations are related and that thermal stresses due to the temperature gradients present during cooling are responsible for the introduction of localized orientation of molecule segments that is largely independent of main chain orientation. Related ideas are found in the literature on the changes in birefringence caused by stresses applied to glassy polymers for extended periods and these are compared with the observations presented here and discussed.