Study of phase transition of ferroelectric polymers by use of molecular dynamics and x-ray diffraction

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Molecular dynamics simulations of the ferroelectric phase (low temperature phase) and a hypothetical paraelectric phase (high temperature phase) of poly(vinylidene fluoride) (PVDF) have clarified the following results.(1) Control of macroscopic electric fields is necessary for the simulation of the ferroelectric phase of PVDF. The effects of the electric fields suggest that the electrostatic interaction stabilizes the ferroelectric phase of poly(vinylidene fluoride).(2) The ferroelectric phase is transformed into nonpolar structures in the medium without polarization. The transition between the nonpolar phase and the ferroelectric phase is observed at low temperatures and suggests a possibility of antiferroelectric phase.(3) The simulation of a nonpolar phase at high temperature shows a characteristic rotation and inversion motion of dipoles and the sliding diffusion of the molecular chain, It shows a possibility of the paraelectric phase.(4) A nonlinear lattice size dependence of the anisotropic stress is observed at a high temperature phase of PVDF. This phenomenon gives an explanation of the structural phase transition accompanied by the stretching of the chain length induced by the pressure.