EFFECTS OF THE CRYSTALLINITY ON THE BETA-RELAXATION OF POLY(ETHYLENE-TEREPHTHALATE)

The beta relaxation of PET is studied as a function of its crystallini ty. The variation of the magnitude of tan Phi is explained taking into account the mechanical coupling of crystalline and amorphous phases b y means of a series-parallel model which is intermediate between the p urely parallel and series ones. The adjustable parameter which is cons idered in such a model is related to the amorphous volume fraction whi ch ensures the mechanical coupling and which appears to be propertiona l to the reciprocal long period. The morphology analysis shows that th e variation of the crystalline fraction (between 0.32 and 0.53) during treatments carried out for one hour between 120 degrees C and 230 deg rees C affects mainly the crystalline lamellae thickness while the lon g period evolves much less. Therefore, the coupling fraction remains a lmost unchanged. Moreover, an important feature of this work is that t he beta relaxation in semicrystalline polymers does not linearly evolv es with the crystallinity while beta relaxation does not exist in the crystalline part.