THE NATURE OF COUPLING BETWEEN THE CRYSTALLINE AND AMORPHOUS PHASES AND ITS EFFECT ON THE PROPERTIES OF HEAT-SET POLY(ETHYLENE-TEREPHTHALATE) FIBERS

Poly(ethylene terephthalate) (PET) fibres having a wide range of struc tures and morphologies were prepared by heat-setting commercial PET ya rn at fixed temperatures between 100 and 255 degrees C under two condi tions, i.e. while the yarn was free to relax and when it was held taut at constant length. The structural and morphological characteristics of the crystalline and amorphous phases of the fibre were obtained by using standard methods. The sonic modulus, boiling-water shrinkage, re covery during an axial load-elongation test, and uptake of disperse dy e by the fibre were determined experimentally. The dependence of these physical properties on structure is critically examined in this paper . It is shown that, in addition to the well-known structural character istics such as the content and orientation of the amorphous phase, siz e and size distribution of crystallites, birefringence, etc., the natu re of coupling between the crystalline and amorphous phases in the fib re, which differs significantly in the two sets of fibres, can play an important role in determining fibre properties.