Effect of Electrode Material on Contact High-Voltage Polarization in a Vinylidene Fluoride-Hexafluoropropylene Copolymer

High-voltage polarization processes occurring in alternating fields of various frequencies in extruded films of vinylidene fluoride-hexafluoropropylene copolymers are studied. Remanent polarization typical for ferroelectric polymers with a polar cell appears during crystallization in the nonpolar alpha phase. This effect is associated with the presence of a highly imperfect ferroelectric (or antiferroelectric) phase in the copolymer, where chains assume planar zigzag conformations. The magnitude of remanent polarization depends on the frequency of the applied bipolar sawtooth voltage. This observation may be explained by the fact that the resulting field is created not only by the external source but also by the space charge that is contributed by carriers injected from electrodes. With the use of deposited Au and Al, the role of electrode material in polarization and conduction processes is ascertained. In the case of Al, Al2O3 and new Al-C chemical bonds are formed on the polymer surface. Owing to formation of these additional dielectric layers at the copolymer/metal interface, electrodes may be blocked partially. The above new chemical bonds facilitate formation of deeper surface traps for carriers; as a result, the remanent surface potential appears in polarized samples even after storage in the short-circuit mode.