AbstractPoly(ethylene terephthalate) (PET) fibers having a range of structure and morphology were prepared by heat‐setting commercial PET yarn at temperatures of 100–250°C for 5 min under two conditions, while the yarn was free to relax and when it was held taut at constant length. The crystallinity, crystallinity orientation, and crystallite size were determined by X‐ray diffraction while birefringence was measured with the help of optical microscopy. The amorphous orientation factor was computed from the structural parameters. The coupling between the crystalline and amorphous regions was determined using the Takayanagi model. While crystallinity and crystallite orientation values for the corresponding samples of the free‐annealed and taut‐annealed series did not show very large differences, the free‐annealed samples had much lower amorphous orientation, especially when heat‐set at higher temperatures. Also while the free‐annealed samples showed a predominantly series type of coupling between the crystalline and amorphous regions, the taut‐annealed samples showed a significant degree of parallel coupling. It is shown that samples in which there is distinct phase separation between the crystalline and amorphous regions have a predominantly se
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