Fiber structure development of poly(ethylene terephthalate-co-isophthalate) copolymer

Isophthalic acid copolymerization is generally used to control the properties of polyethylene terephthalate fiber. To investigate the controlling mechanism, the effects of copolymerization on orientation-induced crystallization behavior and smectic phase formation after necking were analyzed by acquiring X-ray diffraction images during the continuous drawing process. Isophthalic acid copolymerization suppressed orientation-induced crystallization during the spinning and drawing processes. It also decreased the d-spacing and apparent thickness of the smectic phase, while the integrated intensity and length of the smectic phase were not affected. Furthermore, the melting point and crystallinity decreased, and the thermal shrinkage of drawn fibers increased, while the birefringence and tensile strength were unchanged. Almost no change in the molecular orientation and microfibril length presumably resulted in the fiber strength being retained.