NONLINEAR CREEP-BEHAVIOR OF PET

The non-linear creep behaviour has been studied on PET films at room t emperature. A particular value of the stress, sigma(c), was used to ch aracterize the change between the linear to the non-linear domain. The variations of the elastic modulus, the relaxed modulus and sigma(c) r evealed great sensitivity to the morphology of the crystallization. A molecular model of non elastic deformation, assuming (i) hierarchical correlated molecular motion, and (II) nucleation and expansion of shea red-microdomains, was used to analyse the role of stress on anelastici ty. To take into account the two-phase structure of semicrystalline fi lms, a phenomenological series/parallel model was applied to express t he mechanical coupling between amorphous and crystalline phases. Quant itative agreement was found between theoretical predictions and experi mental data for low and high stresses. However, there was a discrepanc y in the rate of recovery because the model predicts a strain recovery slower than the experimental behaviour. Consequently, it is proposed to develop further the molecular model mentioned above by specifying t he energy profile of a sheared-microdomain and its stress dependence. Then, the difference between creep and recovery strain rate could be e xplained.