Deformation mechanism of amorphous poly(ethylene terephthalate) as a function of molecular weight and entanglements

Abstract

The deformation mechanism of amorphous poly(ethylene terephthalate)(PET) has been studied as a function of molecular weight and entanglement density in predrawn films. PET chips (intrinsic viscosity = 0.6–4.9 dl g –1) were dissolved in mixed solvent of 1,1,1,3,3,3-hexafluoro-2-propanol and dichloromethane. The polymer solutions (polymer conc. = 2–40 wt%) were frozen at −30°C and then most of the solvent was removed at −30°C under vacuum, resulting in amorphous films with various entanglement densities. The films were drawn by solid-state coextrusion at 70°C. It was found that the initial polymer concentration used for film preparation had a marked effect on the maximum achievable extrusion draw ratio ( EDR max) of the films, especially for higher molecular weights. The optimum concentration decreased and the highest EDR max increased with increasing molecular weight. The deformability was also affected by the stress-induced crystals, which might act as net points. The tensile modulus and strength of drawn films were related to draw ratio and molecular weight. The higher the draw ratio and molecular weight, the higher were the tensile properties of drawn samples. The improved efficiency of draw with higher molecular weight was explained by the suppression of disentanglement and relaxation of oriented amorphous molecules during deformation; both are important in the development of structural anisotropy and continuity along the draw direction.