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, Δ<i>F</i>, for the individual stages, from which the Gibbs free energy difference, Δ<i>G</i>, 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.