Effect of macromolecular orientation on the structural relaxation mechanisms of poly(ethylene terephthalate)

In this work we used Dielectric Spectroscopy (DS) and Thermally Stimulated Depolarisation Currents (TSDC) analysis to obtain values of fragility indexes as defined in the ‘Strong-fragile’ glass former liquid concept. DS and TSDC measurements have been performed on a series of polyethylene terephthalate (PET) samples with various degrees of macromolecular orientations. These orientations have been obtained by means of uniaxial drawing performed above the glass transition temperature. Samples with draw ratios λ=1, λ=2 and λ=5 have been obtained. From TSDC measurements, the fragility index of the glassy state (mg) can be obtained, while DS measurements allow to reach the fragility index value of the liquid-like state (m). Furthermore, for low draw ratio, the material is practically wholly amorphous and its liquid state can be classified as ‘fragile’ glass forming liquid while for semi-crystalline PET (draw ratio λ=5), it is shown the remaining amorphous phase becomes stronger (m varies from 170 to 60 and mg varies from 81 to 21).