Control of Crystallization Pathways by Electric Fields

Abstract

Polar molecular materials subject to high electric fields of magnitude E lead to situations which are usually characterized by dipole energies (μE) that remain small compared with the thermal energy, i.e., μE << k_BT. As a result, typical nonlinear dielectric effects are very small and electric fields are expected to have little impact on the net molecular orientation and on thermodynamic potentials. Nevertheless, static electric fields in the range from 40 to 200 kV cm⁻¹ were observed to impact the crystallization dynamics and pathway of a polar molecular glass-former: vinyl ethylene carbonate (VEC), a derivative of propylene carbonate. Various temperature/field protocols have been employed to reveal the effect of a static electric field on the crystallization behavior of VEC. The volume fraction of the liquid state is measured via the dielectric permittivity. The rate of crystallization could be accelerated by more than a factor of 10, either by applying a field near the glass transition temperature, T_g, and then taking the sample to a higher crystallization temperature T_c without field, or by taking the sample directly from T > T_m to T_c, where T_m is the melting temperature, and then applying an electric field. Interestingly, crystallization promoted by electric fields gave rise to a new polymorph that could not be obtained in the absence of an electric field. The signature of this new structure is a melting temperature that was observed to be 20 K below that of the ordinary crystal of VEC. Because VEC is a simple polar molecule, these field-induced features are expected to occur in many other materials having sufficient permanent dipole moments. Our results highlight the important role of an external electric field as additional control variable to influence the crystallization tendency of molecular glass-formers, and provide new opportunities in pharmaceutical science or organic electronics.