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 (Δ n c °) and amorphous (Δ n 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: Δ n c ° = 0.22 and Δ n a ° = 0.19. The work was undertaken to resolve the discrepancy in the published literature values of these two parameters.