Cross-Nucleation between Concomitantly Crystallizing α- And γ-Phases in Polypivalolactone: Secondary Nucleation of One Polymorph on Another

Cross-nucleation between polymorphs occurs when a new crystal structure nucleates on the surface of a pre-existing crystal of a different modification. The understanding of the phenomenon is still mostly phenomenological and qualitative. Here, we report quantitative measurements of cross-nucleation rate of the α phase of polypivalolactone on its γ phase, during melt crystallization over a large temperature range. The cross-nucleation kinetics is well modeled as heterogeneous secondary nucleation of one polymorph on another, in which the formation of a viable α-phase nucleus on the surface of γ-crystals is the rate-limiting step. According to this analysis, cross-nucleation can occur because the interfacial energy between the two structures is small, and the nucleation free energy barrier is similar to that for secondary nucleation within the same polymorph in crystal growth. Some peculiar aspects of cross-nucleation with respect to conventional heterogeneous nucleation, arising from the continuous growth of the nucleating substrate, are also highlighted.