Non-equilibrium microstructural and transport characteristics of glassy poly (ethylene terephthalate)

Abstract

In this work the effects of penetrant-induced microstructural changes in amorphous poly (ethylene terephthalate) (PET)) are examined via CO 2 transport, Fourier transform infrared spectroscopy and differential scanning calorimetry employed in coordination. The effects of sub- T g thermal annealing are examined as well, in both as-received and pre-conditioned samples. In the CO 2 transport study, a new parameter estimation procedure is introduced which makes use of differential permeation data to obtain the relevant sorption/transport parameters embedded in the dual-mode sorption/transport model. Exposure of amorphous PET to CO 2 is found to increase the overall free volume of the sample as reflected by increases in the penetrant saturation constant, C' H, with CO 2 pressure and time of exposure. At sufficiently high penetrant activities and/or long exposure times, the sample becomes fully plasticized; this effect is also evident in the accelerated consolidation process apparent by differential permeation. From the FTIR data, changes in local conformational isomer ratios of the ethylene glycol moieties of ET chains near and below the glass transition region due to thermal effects and that resulting from CO 2 exposure are isolated and linked to changes in free volume. The elevated gauche levels resulting from conditioning treatments are paralleled by variations in C' H until full plasticization occurs. The parallel reduction in the gauche fraction and C' H resulting from thermal annealing, can, on the other hand, be ascribed to conversion of the isomer to the higher energy trans conformation. In DSC studies, while C' H continues to increase with CO 2 exposure time, the endothermic peak also increases, after passing through a minimum. This behavior is attributed to the redistribution of free volume during the heating cycle. Similar to the case of as-received films, annealing of pre-conditioned films subsequently reduces the intensity of the endothermic peak after a period of free volume readjustment.