Polymorphism and cross-nucleation in poly(1,3-dioxolan)

The isothermal crystallization from the melt of a poly(1,3-dioxolan) having a number-average molecular weight of 5100 was investigated by optical microscopy, differential scanning calorimetric and X-ray diffraction. A single spherulitic morphology (Phase I) of negative sign is observed at crystallization temperatures below 19 °C while multiple morphologies are observed at crystallization temperatures above 19 °C (Phases IIa, IIb and III). Phase IIa crystallizes first and is characterized by a low birefringence until its sporadic transformation into Phase IIb, characterized by a sudden increase in birefringence. Phase IIb acts as a nucleating agent for Phase III (cross-nucleation process). Phases I and III exhibit distinct growth rates that are two orders of magnitude lower than the transformation rate of Phase IIa into Phase IIb. Relationships between the crystalline structures of the different morphologies and their melting temperatures are also discussed. The transformation IIa/IIb is easily detected optically by a huge increase in birefringence but there is a more subtle Ia/Ib transformation which is hardly seen optically, although it gives rise to a large increase in melting temperature of that phase. [Display omitted]