Molecular simulation of the melting of PVDF crystal phases

Polyvinylidene fluoride (PVDF), a semicrystalline polymer, exhibits polymorphism of its crystal phase, with some crystal phases having piezoelectric properties. The melting of PVDF crystal phases a, /1, y, S and e was studied using molecular dynamics simulations. Effects of the PVDF chain length on crystal phase melting were investigated for crystals with 12, 24, and 36 monomers in each chain as well as infinitely large crystals modelled with chains having no ends (loop) and connected through periodic boundaries. In line with the experimental observations, a-, y- and /1-phases exhibited melting temperature (T,,,) in the increasing order. The /1-phase exhibited the highest melting enthalpy (dH,,,) amongst all the phases. T,,, was found to be dependent on the molecular conformation and not on the arrangement of chains. Similar to the experimental observations, T,,, and dH,,, increased with the increase in chain length of crystal. The melting temperatures for the loop system agree well with melting temperatures for finite size crystals extrapolated to infinite length, however the enthalpies are much lower for the loop crystals, because the topological constraints do not allow the melt to fully relax.