Ultrafast crystallization of polar polymer melts

Abstract

Primary nucleation of poly(vinylidene fluoride) (pVDF) from bulk entangled amorphous polymer melts has been observed from molecular dynamics (MD) simulations. This is the first instance of spontaneous primary nucleation of an entangled bulk polymer in a MD simulation. The melt-to-crystal phase transition occurs via a three-stage process. The ordered phase is found to consist of mainly chain-folded lamella with predominantly adjacent re-entry. The observed crystal polymorphs correspond to either the well known β phase or form I polymorph or a crystal structure that does not coincide with any of the known experimental polymorphs for pVDF, but is found to be strikingly similar to the β structure but with antiparallel alignment of the dipole moments normal to the polymer chain axis. The time required for the onset of nucleation is found to decrease with the number of monomers in the polymer, while the opposite is found for the growth rate of the crystal. We find that the crystallization of the polymer is mediated by electrostatics; in fact the crystal structure spontaneously melts when the electrostatic interactions are removed.