Abstract
The strain‐optical coefficient and relaxation modulus were simultaneously measured for high‐density polyethylene at various temperatures ranging from 12 to 100°C. Not only horizontal but also vertical shifts were necessary to obtain smooth master curves by the application of time–temperature superposition. However, the relaxation modulus decreases with rising temperature while the strain‐optical coefficient increases. This behavior indicates that the variation of the relaxation modulus and the strain‐optical coefficient with time can not be explained by a decrease in crystallinity with rising temperature since a decrease in crystallinity usually causes a decrease in the strain‐optical coefficient with time can not be explained by a decrease in crystallinity with rising temperature since a decrease in crystallinity usually causes a decrease in the strain‐optical coefficient. It was emphasized that another explanation should be sought for the vertical shift in the time–temperature superposition of the time‐dependence curves of the relaxation modulus and the strain‐optical coefficient at various temperatures. The master curve of the strain‐optical coefficient at various temperatures. The master curve of the strain‐optical coefficient or the optical distribution function of relaxation times determined from it serve to distinguish the type and thermal history of the polyethylene.