The effects of orientation and crystallinity on the solvent-induced crystallization of poly(ethylene terephthalate). II. Physical structure and morphology

This series of papers summarizes the results of an experimental research program designed to establish the effects of preexisting orientation and crystallinity on the solvent‐induced crystallization of poly(ethylene terephthalate) films. Dimethylformamide was used as a model for a strongly interacting solvent. This paper deals with the morphological and structural modifications induced by the solvent crystallization process. The effects on the structure and morphology of the solvent crystallization were dominated by surface cavitation, creation of voids, and by changes in the degree of crystallinity. The surface structure varied from a spherulitic cavitated structure for films of low orientations (low draw ratios) to a smooth surface at the higher draw ratios. In addition to the change in the surface morphology, an internal void structure was formed due to crystallization taking place in a swollen state. The formation of voids is dependent on the treatment temperature, draw ratio, and the method of solvent removal. The voids collapse upon annealing at high temperatures. Increases in the degree of crystallinity and changes in orientation were dependent on the treatment temperature. The crystallite size goes through a maximum at an intermediate degree of orientation. The crystalline orientation decreases with increasing treatment temperature. The structures formed during solvent treatments did not show any characteristic premelting endotherms at the low draw ratios. In the case of higher draw ratios, a small premelting endotherm was noticeable at a temperature 70°C greater than that of the solvent treatment.