A MODEL OF CRYSTAL POLARIZATION IN BETA-POLY(VINYLIDENE FLUORIDE)

A model of the crystal polarization of beta-poly(vinylidene fluoride) utilizing an atomic potential energy function based on the shell model of electronic polarization is developed. Lattice constants, crystal polarization, and dielectric constants at finite temperatures are determined through minimization of the Gibbs free energy calculated using consistent quasi-harmonic lattice dynamics. Molecular dynamics is used to include the effects of thermal oscillations of the dipoles. We find that in going from a single chain in vacuum to a chain packed in the crystal the repeat unit dipole increases by approximately 50% or 0.9 debye. Increasing temperature results in a decrease in polarization due to: (i) an increase in the unit cell volume, (ii) a decrease in the local electric field, and (iii) an increase in the magnitude of dipole oscillations. It is found that the dipole oscillation is consistent with the excitation of a single rotational lattice mode. (C) 1995 American Institute of Physics.