Abstract: Effects of physical aging on the tensile properties, stress yielding mechanism and submicroscopic structure of amorphous PES copolymer film were studied by DSC, stressstrain curve and fluorescence spectrum measurements. It was shown that an endothermic peak appeared on the DSC curve and the T_g increased, its initial modulus and yield stress increased while the elongation decreased. The viewpoint of cohensional entanglement (CE) was employed to interpret the experimental results, which indicated that the stress yielding of aged PES copolymer was the destruction of the CE formed during aging. The study of fluorescence spectrum proved that the excited phenylenerings of amorphous PES copolymer with high density phenylenerings in its main chain could form an excimer with the surrounding unexcited phenylenerings, and the emission peak intensity of excimer was strengthened with the increase of aging time. The result of electronmicroscopy investigation indicates that PES copolymer has formed a local order structure during the physical aging.

Key words: PES copolymer, stress yielding, fluoresence spectrum, local order structure 