Effects of thermal contraction on structure and properties of PET fibers

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Abstract
A series of poly(ethylene terephthalate) fibers of various molecular weights was first drawn to a practical maximum draw ratio and then allowed to contract thermally under tension for 10, 20, and 38%. These contracted fibers exhibit a high degree of plasticity even when tested at—100°C and extension rates of 1300%/sec. An attempt is made to explain this behavior by means of a systematic study of morphological changes which occur during thermal contraction. The interpretation of the results of small-angle and wide-angle x-ray diffractions, infrared spectroscopy, and birefringence suggest the existence of two types of amorphous domains; those separating the adjacent crystallites in the microfibril and those separating the microfibrils. It is speculated that the molecules in these two domains respond differently to thermal effects and stress, and that the interfibrillar amorphous domain consists of highly extended molecules. It is shown that the thermal contraction, which does not involve major changes in the degree of crystallinity, proceeds by several mechanisms. At low degrees of contraction, the most important mechanism is the contraction of the microfibrils. At high levels of contraction, the shrinkage proceeds to a large degree via relative displacement of the microfibrils and the contraction of extended interfibrillar tie molecules. The conclusions regarding the structure of these fibers are corroborated by means of transmission electron microscopy of thin cross-sections.