Stress-Optical Properties of PVC above the Glass Transition

Abstract

Studies of the photoelastic behavior of a PVC homopolymer film have been carried out in the rubbery state above the glass transition (up to 160°C) to complement studies already carried out in the glassy state. Some measurements were also made on the same samples in the glassy state for completeness. Step-loading experiments were used, as in previous experiments in the glassy state. The results obtained are somewhat complex, indicating that this polymer is considerably different in nature from a simple amorphous polymer such as polystyrene. There is a progressive shift in the zero load values of birefringence (when the sample is unloaded from increasing stress levels), indicating that a progressive structure change is taking place as load level is increased. This could be a result of progressive plastic yield or breaking and reforming of stress-sensitive crosslinks. Below the glass transition this shift takes place in two stages, which may be related to the secondary beta transition and major glass transition of the polymer. The stress-optical coefficient changes from small negative values to large positive values as temperature is increased through the glass transition; a zero value is observed at 80°C. In addition to the abrupt change of SOC at the glass transition, a second abrupt change is observed at 145°C which is the temperature of annealing of the film; annealing conditions may therefore be playing a very important role in determining the solid-state structure of the polymer.