Time-dependent birefringence in glassy polymers during stress-relaxation and recovery

Abstract

The birefringence of polystyrene and polycarbonate have been measured during uniaxial stress relaxation tests. The birefringence of polystyrene shows an instantaneous positive increment on applying a tensile deformation; during stress relaxation the value showed a time-dependent change in the negative direction and the total birefringence eventually became more negative than the value measured prior to the deformation. On unloading, the overall trend was for the birefringence to change towards the level measured in the as-prepared sample prior to deformation but an initial departure in the opposite direction was observed. With polycarbonate, the birefringence changed instaneously on applying the deformation and remained steady during stress relaxation. On unloading the birefringence fell instaneously by an amount less than the increment observed on applying the deformation. Thereafter the birefringence continued to fall, approaching the original value with a time constant similar to that governing stress relaxation. These results are interpreted with reference to the molecular structures, in particular the location of the highly polarizable aromatic rings.