Assessment of the intrinsic birefringence of partially oriented poly(ethylene terephthalate) yarns

Partially oriented poly(ethylene terephthalate) yarns (PET POY) have been studied with respect to their birefringence, sonic modulus, and stress-optical properties in an effort to extract the values for the intrinsic birefringences of the crystalline (Δn_c°) and amorphous (Δn_a°) regions. The data have been analyzed within the framework of both the sonic modulus-birefringence treatment of Samuels and Dumbleton, and the Gaussian rubber elasticity theory using the usual shrinkage stress-birefringence relations. The following values have been determined: Δn_c° = 0.22 and Δn_a° = 0.19. The work was undertaken to resolve the discrepancy in the published literature values of these two parameters.