Low-temperature relaxation in isotropic vinylidene fluoride-tetrafluoroethylene copolymer films prepared by various methods

Ferroelectric vinylidene fluoride-tetrafluoroethylene (VDF-TFE) copolymers exhibit two low-temperature relaxation processes with parameters depending on the degree of crystallinity. The beta process is observed at a lower temperature and is related to relaxation involving conformational defects of a ferroelectric phase. The activation parameters of this process depend on the sample morphology, The passage from crystals with folded chains to those with extended chains is accompanied by a significant increase in the enthalpy and entropy for the beta process of relaxation. The alpha (a) process taking place at a higher temperature is due to the microbrownian motion of a disordered phase in the vicinity of the glass transition. An analysis of the whole body of data shows that the amorphous phase occurs predominantly in the form of an interphase layer at the crystal boundaries, where a certain long-range order is retained. A special feature of the VDF-TFE copolymers, distinguishing this system from the other crystallizable polymers, is that the mesomorphous state of the interphase layer is characterized by a lattice with zero spontaneous polarization. This fact implies that a paraelectric phase (coexisting with the ferroelectric phase below the Curie point) is localized at the boundaries of the polar beta -modification. This circumstance explains some correlations between the parameters of ferroelectric phenomena in the system studied. In particular, it is demonstrated that the activation parameters of the alpha (a)-beta process may significantly affect characteristics such as the residual and spontaneous polarizations. It was found that the characteristics of chain dynamics in the disordered phase may control the plastic flow processes involved in the uniaxial drawing of the VDF-TFE copolymers.