Birefringence of Rubber during Creep and Recovery

Measurements of creep and recovery of lightly cross-linked polybutadiene, carried out in both shear and compression geometries, reveal two deviations from the stress optical law: the birefringence increases during the constant stress creep, and the birefringence is not zero during the stress-free recovery. Most interestingly, the sign of the birefringence during recovery was found to be opposite to that during creep. This implies that the chain segments have a net orientation opposite to the macroscopic deformation.