Nonlinear optical investigation of the bulk ferroelectric polarization in a vinylidene fluoride/trifluoroethylene copolymer

The ferroelectric polarization of vinylidene fluoride-trifluoroethylene copolymers (70/30 mol %) is investigated using complementary measurements of the surface charge and the second harmonic intensity (at 530 nm) generated in the polymer from a Nd:YAG laser beam (at 1060 nm). Due to the nonzero electrical conductivity of the polymer (especially above room temperature), the nonlineear optical technique provides better measurement of the bulk polarization, its changes with time, the applied electric field, and temperature. First, we present optical results obtained on polarized films after removal of the electrodes: they confirm the proportionality between the second harmonic intensity and the square of the ferroelectric polarization, and the centrosymmetric character of the paraelectric phase; they also show the temperature dependence of the remanent polarization in the crystal phase. Second, we present simultaneous measurements of the surface charge and of the second harmonic intensity under very low-frequency applied voltage through transparent electrodes. The field dependence of the measured polarization is discussed in terms of a microstructural analysis of the dielectric properties in the semicrystalline material. It is also shown that unipolar voltages applied during long periods of time are able to produce charge injection and space charge in the vicinity of the electrodes which can result in screening of the applied electric field.