MOLECULAR DESCRIPTION OF CONSTANT-LOAD STRETCHING OF AMORPHOUS POLY(ETHYLENE-TEREPHTHALATE) FILMS

This paper is devoted to a molecular description of the deformation of amorphous isotropic poly(ethylene terephthalate) (PET) films at tempe ratures slightly above the glass transition temperature under constant load. The deformation is qualitatively described by chain relaxation phenomena occurring before stress-induced crystallization, which are f ollowed by the equilibration of a rubber-like network. The junction po ints include both trapped entanglements and crystalline units. The str ucture of this network is characterized by the number of segments betw een crosslinks. This parameter is calculated by comparing the predicti ons of the rubber elasticity theory (without Gaussian approximation) w ith the experimentally observed draw ratios under given conditions of temperatures and loads. It is shown that light loads induce soft netwo rks leading to high draw ratios. The predictions of the molecular orie ntation derived from this treatment are in good agreement with birefri ngence data on a large variety of samples.