Mechanical response of amorphous and semicrystalline poly(ethylene terephthalate) and modelling in frame of quasi point defect theory

The thermomechanical activation of deformation in amorphous and (38%crystalline) semicrystalline poly(ethylene terephthalate) (PET) has been investigated using dynamic mechanical tests and large strain experiments. Activation parameters for both materials are determined in the neighbourhood of the glass transition temperature using stress relaxation experiments. The complete mechanical behaviour of amorphous PET is then analysed in the frame of a molecular model or ‘quasi point defect’ theory. With this aim, a new method is proposed, based on only three isochronal measurements, and leading to the determination of the set parameters. This method makes it possible to reproduce the stress–strain curve over a range of temperatures, as well as the relevant activation parameters. Finally some qualitative explanations are given for the mechanical behaviour of semicrystalline PET.