First principle calculations of molecular polarization switching in P(VDF-TrFE) ferroelectric thin Langmuir-Blodgett films

This paper reports first principle calculations and analysis of the molecular mechanism of the polarization switching in polyvinylidene fluoride and its copolymer with trifluoroethylene (P(VDF-TrFE)) using semi-empirical and ab initio quantum chemical methods based on the HyperChem 7.5 and Gaussian98 programs. The simulations were performed for different copolymer contents in P(VDF-TrFE) - (70:30), (60:40) and pure PVDF. The calculated values of the dipole moment and average polarization of the molecular chains show a clear hysteresis under varying electric field with polarization saturated at ∼0.1-0.14 C m^-2. The calculated coercive fields (corresponding to the rotation of molecular chains to opposite orientation) are consistent (within an order of magnitude) with experimental data obtained for thin films (E_c = 5-18 MV cm^-1). In the absence of external electric fields, the interactions between several molecular chains lead to the orientation of all dipole moments along one direction parallel to the chain plane. This model corresponds to the PVDF layer on the dielectric surface. For the electric field in the perpendicular direction, all chains are rotated along this direction corresponding to the model of conductive substrate.