Structural change in an unoriented

amorphous PET film annealed at temperatures below T

and the effect of ezcess enthalpy relaxation on oermeation rates of CO2 gas and toluene liquid in treated samples have been studied. The results suggest that the amount of excess enthalpy relaxation as determined from the endothermic peak in T

interval

the Tg and density all increase with annealing time

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

the permeation rte of CO2 gas in treated samples reduced. While the apparent permation rate of toluene liquid increased with annealing time because ofacreation 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.