Concomitant crystallization and cross-nucleation in polymorphic polymers

Crystallization of polymorphic polymers can lead to different structures, starting from the same melt or solution. Comprehensive understanding of crystallization modalities in such systems is of primary technological and scientific relevance, because it can enable prediction - and possible control - of the polymorphic composition of a material and, in turn, of its properties. Several structuring pathways are possible. A given structure can develop either directly or through successive metastable states. The latter case obeys the so-called Ostwald’s rule of stages. Under particular thermodynamic and kinetic conditions, two or more polymorphs can nucleate and grow concomitantly. Moreover, cross-nucleation is observed when a faster growing polymorph nucleates on a previously existing polymorph. This chapter is focused on concomitant crystallization and cross-nucleation between polymer polymorphs, two topics that have seldom been considered in the past and lack critical review. First, the scattered pieces of information in the polymer literature are collected and discussed. Within this framework, we present two relevant examples taken from our own work, concerning concomitant crystallization of poly(pivalolactone) polymorphs and cross-nucleation in seeded crystallization of isotactic poly(1-butene).