Relationships Between Ac Dielectric Nonlinearities and Molecular Conformations in Ferroelectric Langmuir-Blodgett Polymer Films

The capacitance of poly(vinylidenefluoride-trifluoroethylene) copolymer ultrathin films was investigated as a function of frequency, ac drive, and temperature. In the ferroelectric (FE) phase, the capacitance of the copolymer films increases with ac drive and shows an ac dielectric nonlinearity. On the contrary, the capacitance decreases with ac drive in the paraelectric (PE) phase. Since the maximum ac drive is much smaller than the coercive field, the contribution of the polarization switching to the increase of the capacitance can be ruled out. The ac drive dependence of the capacitance is believed to reflect the information of the molecular configuration, the intra- and interchain interactions.