Kinetics of Structural Relaxation in Physical Aging of Amorphous Poly (Ethylene Terephthalate)

Structural relaxation in amorphous poly (ethylene terephthalate) (PET) quenched from various temperatures above and below the glass transition temperature Tg (ca 68°C) has been studied in terms of time dependence of the specific volume. Experimental results were analyzed using the phenomenological multiparameter model of Moynihan and co-workers to give numerical values of kinetic parameters characterizing the structural relaxation. The kinetics of PET struc-tural relaxation could be divided into two different processes. One was in a temperature range below Tg-20°C, where the kinetic parameters remained constant while the time (ta) and tempera-ture (Ta) of aging varied. The relaxation time of this process was strongly dependent on the instan-taneous state of the PET glass and showed a broad distribution. The other process was in a temperature range from Tg-20°C to Tg, where the kinetic parameters changed with Ta, and the re-laxation time showed an Arrhenius-like temperature dependence and a less broad distribution. Moreover, it was found that thermal pretreatment in the vicinity of Tg was likely to stabilize the glassy PET structure and lead to some extreme retardation in long-term segmental rearrangements and cooperative motions.