Anomalous Temperature Dependence Of Isotactic Polypropylene Alpha-On-Beta Cross-Nucleation Kinetics

A particular kind of heterogeneous nucleation, i.e., cross nucleation, is sometimes observed in polymorphic substances, when a new crystal structure nucleates on the surface of a crystal of a different modification. Here we show a unique and apparently incongruous nucleation behavior in polymorphic isotactic polypropylene (i-PP). The rate of cross-nucleation of the monoclinic a-phase on the trigonal beta-phase crystals increases with increasing temperature, in the vicinity of the a crystals melting point. This behavior is contrary to that of the heterogeneous nucleation kinetics of the same crystal on various solid substrates, and also to the previously reported cases of cross-nucleation rate of other polymorphic systems, both exhibiting the expected decrease with temperature in the same range of undercoolings. i-PP alpha-on-beta cross nucleation apparently eludes the nucleation theory. The results are explained as a manifestation of a kinetic competition between alpha-on-beta cross-nucleation and growth of beta-crystalline seeds, and finally reconciled with the current understanding of nucleation. These new findings indicate that further theoretical efforts are needed to include the cross-nucleation phenomenon in the framework of a comprehensive understanding of polymorphic crystallization. Incidentally, this study highlights the intrinsic limits of the, industrially desirable, promotion of beta-phase formation in polypropylene.