Watching the Polymorphic Transition from a Field-Induced to a Stable Crystal by Dielectric Techniques

We prepared a metastable polymorph of vinyl ethylene carbonate by inducing crystallization of the supercooled liquid via an electric field of about 80 kV cm-1. Electric fields with sufficiently low frequencies are required to observe polymorphism for this material. Depending on the temperature protocol prior to crystallization, the field-induced crystal structure can be obtained either with high polymorphic purity or mixed with seed crystals of the ordinary stable polymorph. At the crystallization temperature of Tc = 198 K, the pure field-induced polymorph shows no sign of a polymorphic transition, while at the same temperature, this crystal structure converts to the stable state in a matter of several hours if seeded by the stable polymorph. Interestingly, we find that the permittivity changes linearly with the volume fraction of the field-induced polymorph during the conversion, so that the polymorph transition can be followed in real time by dielectric techniques.