Rheo-optical properties of polymers: Relaxation and recovery during absorption and desorption of organic liquids

As part of a study in elucidating the mechanisms by which crazing is initiated in polymers, we have measured the force and birefringence decay which occurs in several polymers when placed in contact with methanol. In all cases, the force decay can be used to determine the rate of bulk diffusion of the liquid into the polymer. In contrast, the birefringence decay appears to be related to shear relaxation mechanisms which are highly dependent on the type of material. For example, in the case of simple rubbers such as polyisoprene, the rates of decay of force and birefringence are identical within experimental error. In contrast, in amorphous thermoplastics the force decay is typically an order of magnitude faster than the birefringence decay. These results are discussed in terms of distortional and orientational birefringence and with respect to the morphology of the glassy state.