Structure of drawn polyethylene terephthalate

Measurements of birefringence, infrared dichrosim, and x‐ray diffraction have been made on crystallized and uncrystallized polyethylene terephthalate films stretched up to 500%. The results show that, at high elongations, stretching induces high linear orientation of the chain molecules parallel to the direction of stretch. In the uncrystallized samples, a biaxial orientation in which the plane of the molecules tends to be parallel to the plane of the film can be detected in the x‐ray diffraction patterns and from the birefringence measurements. This higher orientation, as well as the usual linear orientation, is enhanced by crystallization, owing to directional crystal growth on oriented crystal nuclei which develop during the stretching operation. At low extension, pronounced tipping of the crystalline regions occurs during crystallization. As a result, the preferred direction for the fiber axis of the crystallites varies from a direction approximately perpendicular to the direction of stretch at low extension, to a direction almost parallel to the direction of stretch at the highest extensions.