Frequency of the AC Electric Field Determines How a Molecular Liquid Crystallizes

The ability to control crystallization is of central importance to many technologies and pharmaceutical materials. Electric fields have been shown to impact crystallization, but little is known about the mechanism of such effects. Here we report on our observations of how the frequency of an external electric (ac) field changes the crystallization rate and the partitioning into distinct polymorphs of vinylethylene carbonate. We find that the field effects are pronounced only for frequencies below a certain threshold, which is orders of magnitude below that characterizing molecular orientation but consistent with the reorientation of polar crystal nuclei of radius r < 3 nm. We conclude that the electric field opens an additional nucleation pathway by lowering the free-energy barrier to form a polymorph that melts at a temperature similar to 20 K below that of the ordinary crystal. This lower melting polymorph is not obtained at zero electrical field.