Dynamic birefringence of polyolefins

The use of the dynamic birefringence method was extended to the study of the relationship between structural changes and mechanical properties for polyethylene, atactic and isotactic polypropylene, and polybutene‐1. The phase angle between birefringence and strain was measured which permitted the resolution of the strain‐optical coefficient into real and imaginary parts. The real and imaginary parts of the dynamic relaxation modulus were also measured and compared with the optical data. Results for the crystalline polymers were interpreted in terms of the two‐stage mechanism of deformation, involving spherulite deformation followed by internal crystalline reorientation. For the atactic polymer, the viscoelastic flow of amorphous regions contributes to the birefringence. The temperature dependence of birefringence and stress is similar for this mechanism but not for the crystalline contribution.