Stress-induced crystallization of poly(ethylene terephthalate)

Quenched amorphous films of poly(ethylene terephthalate) (PET) are stretched at temperatures less than T_g; changes in density, wide-angle x-ray diffraction, and small-angle light scattering are observed. The density increase upon stretching is attributed to an increase in crystallinity accompanied by an increase in the intensity of somewhat diffuse wide-angle x-ray diffraction and of both V_V and H_V small-angle light scattering patterns. The formation of oriented rodlike superstructure may be discerned from small-angle light scattering. Annealing of these samples increases the crystallinity as measured from density and leads to an increase in the perfection of crystalline and supercrystalline structure as measured by wide-angle x-ray diffraction and small-angle light scattering. The rodlike morphology changes to form spherulitelike aggregates as observed by small-angle light scattering and light micrographs. A model is proposed to explain the observations. Studies are extended to stretching films of PET above their T_g and observing changes in birefringence, density, wide-angle x-ray diffraction and small-angle light scattering as a function of elongation and stretching temperature. The formation of defomed spherulitelike superstructure may be discèrned from light micrographs. Results are compared with those obtained upon stretching films below T_g.