Relative Thermodynamic Stability of α and β PVDF Crystal Phases: A Molecular Simulation Methodology

Poly(vinylidene fluoride) (PVDF) is a piezoelectric polymer, with the crystalline β-phase having the highest polarity among all its phases. A multistage transformation process is developed, using molecular dynamics simulations, to compute the free energy difference between α- and β-phases of PVDF. Methods of free energy perturbation and Jarzynski's equality were used to determine Helmholtz free energy change, ΔF, for the individual stages, from which the Gibbs free energy difference, ΔG, between the α- and β-phases was calculated. Infinitely large crystals modeled using periodic boundaries with 36 chains and 12 monomers in each chain were used for the study. All-atom simulations were performed with the force fields previously developed for PVDF. In concurrence with experimental observations, the α-phase was found to be thermodynamically more stable at normal temperature and pressure conditions. The β-phase was found to be more stable at high and low temperatures and high pressure.