Development of orientation with drawing in polystyrene films: effect of time and temperature

A series of experiments has been undertaken on uniaxially drawn polystyrenes with a range of melt-flow indices. The samples were drawn at constant crosshead speed at temperatures above the glass transition temperature range. The molecular orientation as measured by optical birefringence was measured in samples that had been quenched to room temperature at different times after the deformation stopped. Some samples were held at constant length while immersed in an oil bath at 115 8C and the resulting shrinkage stress was monitored. The initial peak in shrinkage stress was found to be proportional to the room temperature birefringence via a stress-optical coefficient. Furthermore this shrinkage stress, for samples quenched immediately after drawing, was shown to be equal to the true stress during the drawing experiment at the moment of quenching. The Doi-Edwards characteristic relaxation times calculated using the weight average molecular weight were shown to provide a good semi-quantitative explanation of the data.