Structural change in an unoriented

amorphous PET film annealed at temperatures below Tgand the effect of excess enthalpy relaxation on permeation rates of CO2 gas and toluene liquid intreated samples have been studied. The results suggest that the amount of excess enthalpy relaxa-tion as determined from the endothermic peak in Tg interval

the Tg and density all increase withannealing time

but the trans-conformation component of samples decreases. No change of struc-ture in the amorphous phase was found other than the normal densification of the molecular chain packing or a reduction in free volume had occurred during the annealing regimes.Therefore

the permeation rate of CO2 gas in treated samples reduced.While the apparent permeation rate of tokuene liquid increased with annealing time because of a creation of extensive cracks at film surface arising from both the increase in embrittlement of polymer and the swelling action of toluene liquid on treated samples.