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 p olarization, and dielectric constants at finite temperatures are deter mined through minimization of the Gibbs free energy calculated using c onsistent 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 d ebye. Increasing temperature results in a decrease in polarization due to: (i) an increase in the unit cell volume, (ii) a decrease in the l ocal electric field, and (iii) an increase in the magnitude of dipole oscillations. It is found that the dipole oscillation is consistent wi th the excitation of a single rotational lattice mode. (C) 1995 Americ an Institute of Physics.