Birefringence and stress relaxation in polyisobutylene

Relaxation of birefringence and stress in simple extension has been studied for polyisobutylene at ‐26.0°C and 25.0°C for extension ratios ranging from about 1.2 to 2.0. The dependence of both the stress and birefringence on the extension ratio, for a given time, is well described by equations of the Mooney–Rivlin form. The Mooney–Rivlin treatment of the birefringence experiments was found to reinforce but not add to information available from the stress‐relaxation experiments alone. At 25.0°C, the stress‐optical coefficient is found to be stress dependent. Possible explanations of this behavior are discussed. An experiment is also reported in which the birefringence is observed under zero stress immediately after cutting a strained sample. The zero stress birefringence is believed to be due to regions of stress‐induced crystallization taking a finite time to melt after the stress is removed.