The conditions of fibre formation by melt spinning of a thermoplastic elastomeric poly(ether ester) based on poly(butylene terephthalate) and poly(ethylene oxide) glycol with molecular weight of 1000 are studied. Some properties of both the starting polymer and the final fibres are discussed. A highly regular structure of the fibre is observed by scanning electron microscopy.

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A sample of high-elastic-textured poly (ethylene terephthalate) (PET) yarns were subjected to different conditions of heat-setting, i.e., different temperatures and overfeeds. The structural changes were characterized by different physical methods. The orientation factors of crystalline and amorphous regions were calculated. The yarn tensions acting during the SET process were measured and corrected to the structural data. The results obtained show that two processes take place during the SET process: relaxation of the molecule segments in amorphous regions and change of crystallite orientation in microfibrils. A simple model for explanation of the structural changes in simultaneously draw-textured PET yarns is proposed.