Heat setting of oriented poly(ethylene terephthalate): Effects of deformation mode, strain level, and heating medium



We continue a previous study of the α and α′ mechanical relaxation processes in oriented poly(ethylene terephthalate) by means of heat-setting experiments on drawn filaments between room temperature and the melting region. Both processes were exhibited in both torsion and bending deformations, but heat setting was found consistently to be greater in bending than in torsion: this was accounted for by the known anisotropy of the α relaxation. In both modes of deformation, increasing strain level gave reduced recovery, and results suggested this arose from high stresses activating at room temperature the stress relaxation mechanisms, which at low stresses occurred only at elevated temperatures (α if as-drawn, α′ if preset). Results were found to be independent of whether blown air or stirred oil was used as heat transfer medium, in contrast to some previous reports in the literature; the reason lay in differences in the precise sequences of deformation and temperature employed.