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

The beta relaxation of PET is studied as a function of its crystallinity. The variation of the magnitude of tan Phi is explained taking into account the mechanical coupling of crystalline and amorphous phases by means of a series-parallel model which is intermediate between the purely parallel and series ones. The adjustable parameter which is considered in such a model is related to the amorphous volume fraction which ensures the mechanical coupling and which appears to be propertional to the reciprocal long period. The morphology analysis shows that the variation of the crystalline fraction (between 0.32 and 0.53) during treatments carried out for one hour between 120 degrees C and 230 degrees C affects mainly the crystalline lamellae thickness while the long period evolves much less. Therefore, the coupling fraction remains almost unchanged. Moreover, an important feature of this work is that the beta relaxation in semicrystalline polymers does not linearly evolves with the crystallinity while beta relaxation does not exist in the crystalline part.