Studies on piezoelectric organic polymer by molecular dynamics simulation.

Abstract

We investigate the physical and electrical properties of ferroelectric organic polymer, poly-vinylidene fluoride (PVDF). Piezoelectricity is interesting property usually belong to ceramic materials, where PVDF is special that it possesses the strongest piezoelectricity among organic polymers. The prominent electrical property of PVDF is based on its structure. PVDF is known to exist in at least five phases, among them the most important structures are $\alpha$-phase and $\beta$-phase.

We perform density functional theory calculations and molecular dynamics simulation to investigate physical, structural and dynamical properties of crystalline PVDF. The phase transition is essential background of ferroelectric properties of PVDF, where study on the detailed mechanism is still in progress. We approach to the mechanism of phase transition in atomistic scale and examined closely. We examine the change of the conformational energy and the corresponding structure of each phase depending on the lattice parameters of the orthorhombic crystalline structure. From this information, we construct the path that connects the point where the $\alpha$ phase is most stable to the point where the $\beta$ phase is most stable, and identify the subregion in the lattice parameter space where $\alpha$ and $\beta$ phases have the same energy. In this sub-region, we locate the point which gives the lowest conformation energy for both $\alpha$ and $\beta$ phases, and examine the behavior of the lowest energy profile and corresponding change of intermediate structures as the conformation of the PVDF chain transforms from $\alpha$ phase to $\beta$ phase.

Molecular dynamics simulations were also performed to explore large scale model of PVDF as polymer film is made up of large amount of monomers. The force field parameters are originally designed from the elaborated \emph{ab initio} calculation. The original force field also regards the electrical propertie...