Optical birefringence of polyisobutylene during creep and recovery

Creep and recovery rheooptical measurements were conducted on linear polyis obutylenes, with viscosity-average molecular weights in the range from 4.2 x 10(5) to 4.7 x 10(6) g/mol. To isolate the contribution of chain ends to the birefringence, the temperature of each experiment was adjusted to yield equivalent creep strains for each sample. Higher molecular weight PIE exhi bited less change in the birefringence during creep, but the birefringence during recovery was independent of molecular weight. The sign of the birefr ingence during the stress-free recovery was opposite (i.e., corresponding t o tension) to that during creep, even though the macroscopic deformation wa s compression.