STUDY OF THE MOLECULAR-STRUCTURE OF PET FILMS OBTAINED BY AN INVERSE STRETCHING PROCESS - PART 2 - CRYSTALLINE REORGANIZATION DURING LONGITUDINAL DRAWING

The transverse stretching, under a constant drawing force, of a one-wa y drawn poly(ethylene terephtalate) film has been studied. This type o f stretching leads to an equilibrium deformation under load which incr eases mainly with the applied stress and to a lesser degree with tempe rature. The crystalline phase of such samples has been characterized u sing wide-angle X-ray diffraction. The orientation of the chain axis a nd 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 t emperature. The crystallinity is constant during stretching. The orien tation distribution of the chain axes as well as the length of the cry stals shows that the crystals which were preferentially oriented with their c-axes along the first stretching direction rotate rapidly towar ds the actual draw direction and grow up in length as the stretching p roceeds. At the same time, the length of the crystals aligned along th e other direction of the film plane decreases rapidly, indicating a br eak-down of some of these crystals. The orientation in the crystalline phase is uniquely controlled by the draw ratio. No relaxation with te mperature is observed. This stretching method is compared with the con stant speed drawing of one-way drawn films, where no rotation of large crystallites can be detected. (C) 1998 Elsevier Science Ltd. All righ ts reserved.