Resonance characteristics and polarization profile of partially poled P(VDF-TrFE) copolymer

Thick films of polyvinylidene fluoride/trifluoroethylene (P(VDF-TrFE)) copolymer with the composition 80/20 mol-% were poled at elevated temperature. The dielectric spectrum of this film shows two resonance peaks which correspond to the fundamental and second harmonic of the thickness vibration mode. Even harmonics of the thickness mode are usually not observed as they are not coupled to the electrical excitation. Piezoelectric and pyroelectric profiling experiments using both the pressure wave propagation (PWP) method and the laser intensity modulation method (LIMM) show that the copolymer has an inhomogeneous polarization profile which leads to the observed second order harmonic. In particular, these methods indicate that an unpoled region exists near the cathode side of the sample. By fitting the observed dielectric spectrum to a model of two Lorentz oscillators, the oscillation strengths have been estimated and found to he consistent with those obtained from the appropriate Fourier coefficients of the measured pyroelectric profile.