Study of the molecular structure of PET films obtained by an inverse stretching process Part 2: crystalline reorganization during longitudinal drawing

Abstract

The transverse stretching, under a constant drawing force, of a one-way drawn poly(ethylene terephtalate) film has been studied. This type of stretching leads to an equilibrium deformation under load which increases mainly with the applied stress and to a lesser degree with temperature. The crystalline phase of such samples has been characterized using wide-angle X-ray diffraction. The orientation of the chain axis and the normal to the phenylene ring have been analysed as well as the crystalline size, as a function of the draw ratio and the stretching temperature. The crystallinity is constant during stretching. The orientation distribution of the chain axes as well as the length of the crystals shows that the crystals which were preferentially oriented with their c-axes along the first stretching direction rotate rapidly towards the actual draw direction and grow up in length as the stretching proceeds. At the same time, the length of the crystals aligned along the other direction of the film plane decreases rapidly, indicating a break-down of some of these crystals. The orientation in the crystalline phase is uniquely controlled by the draw ratio. No relaxation with temperature is observed. This stretching method is compared with the constant speed drawing of one-way drawn films, where no rotation of large crystallites can be detected.