Molecular mobility in the glass transition region in cold-drawn films of vinylidene fluoride-tetrafluoroethylene copolymers with different morphology

For two films of vinylidene fluoride-tetrafluoroethylene copolymer having identical chemical compositions but differing in initial morphology, the structural rearrangements taking place upon their cold drawing were studied. As was found, the cold drawing of both films is accompanied by a decrease in the degree of crystallinity. This trend leads to a decrease in the activation parameters in the alpha(a)-relaxation region in the vicinity of the glass transition. Orientation of the sample with a nonspherulite morphology gives rise to a far more dramatic amorphization. In this case, the processes of stress-induced softening are accompanied by an increase in the fractional content of polymer chains in a disordered state. As a result, both cooperative and local dynamics are intensified. For the sample with a spherulite morphology, its cold drawing is accompanied by the development of microcracks. According to the data of IR spectroscopy, the principle reason for this behavior is related to a reduction in the packing density in the interlamellar regions of the initial (isotropic) sample. Therefore, the activation barriers of thermal motion are decreased, thereby promoting concentration of the fluctuation stresses on the chain backbone. The development of microcracks favors a more complete orientation of lamellas along the direction of tensile drawing, but the ordering of interlamellar chains becomes less perfect. The oriented films containing microcracks with a higher free volume appear to be characterized by a quite unexpected increase in the relaxation times of the microbrownian motion in the region of the glass transition. This observation is related to the development of numerous stressed tie-chains with a reduced conformational entropy in an amorphous phase. A threefold increase in the residual and maximum polarization as seen in the curves of electric hysteresis for the cold-drawn films is associated with two factors. An ordered arrangement of ferroelectric crystals along the direction of tensile drawing assists an increase in the projection of their corresponding dipole moments on the direction normal to the film. Upon a decrease in the activation parameters of molecular motion in the interphase regions, a certain fraction of chains which may exist either in a paraelectric or anti ferroelectric state may experience an irreversible transition to a ferroelectric phase.