The effect of orientation of nuclei on the stress-induced crystallization of polyethylene is discussed. Changes of stress and birefringence during elongation and subsequent relaxation process have been measured at molten state and slightly crystallized state for low density polyethylene. The behavior of stress and birefringence in both process for molten polymer were shown to be consistent with Kuhn-Grun's theoretical equation. The film, being quenched after elongation, had preferred a-axis orientation. The behavior of stress and birefringence during elongation for partially crystallized polymer have not coincided with rubber elastic theory because of the contribution of orientation of spherulitic nuclei in the melt, but these were shown to accord with rubber-like behavior in the relaxation process. These film had preferred b-axis orientation to the stress direction.
Dependence of crystal orientation on elongation temperature and cooling condition have been investigated for low density polyethylene and high density polyethylene. The film, elongated from partially crystallized state, had the mixture of preferred a- and b- axis orientation to the stress direction. It is considered that characteristic crystal orientation is achieved with stress-induced crystallization of molten polymer and with ordered crystallization starting from the oriented spherulitic nuclei.