AbstractStress–relaxation studies between room temperature and 140°C were made on oriented multifilament poly(ethylene terephthalate) (PET) yarn and on four other samples which had been prepared by heat‐setting this yarn at 180 and 230°C while free to relax and when held taut at constant length. Using the time–temperature superposition principle, the data were superimposed, and the master curves of relaxation modulus were constructed for each sample; the relaxation–time spectra for these samples were computed using the Alfrey approximation. The results revealed that (i) the free‐annealed samples show higher visco‐elasticity than the corresponding taut‐annealed samples and (ii) the relaxation–time spectra show two prominent relaxation processes on logarithmic time scale in the case of all the samples; the two relaxations being more intense in the free‐annealed samples. These observations are interpreted in terms of fiber structure and morphology, which had been studied earlier; the amorphous orientation in the free‐annealed samples is low, and they have a more distinct separation of the crystalline and amorphous phases. These factors facilitate molecular relaxation processes in the non‐crystalline phase; the relaxation are therefore relatively more intense in these samples, and they exhib
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