A Unifying Scheme for Polymer Crystallization Based on Recent Experiments with Wider Implications for Phase Transformations

This lecture is aimed to link the “main stream” subject of chain folded polymer crystallization to the “speciality stream” of extended chain crystallization, the latter as typified by the crystallization of polyethylene under pressure. This is achieved through a scheme based on some new experimental material comprising the recognition of thickening growth as a primary growth process of lamellae and of the prominence of metastable phases, specifically of the mobile hexagonal phase in polyethylene. The scheme relies on the consideration of crystal size as a stability determining factor, namely on melting point depression, which in general is different for different polymorphs. It is shown that under specificable conditions phase stabilities can invert with size, i.e. a phase which is metastable for infinite size can become the stable one when the phase is sufficiently small. When applying this condition to crystal growth it follows that a crystal in such a situation will appear and grow in a phase that is different from that in its state of ultimate stability, maintaining this state as a metastable one or transforming into the ultimate stable state during growth according to circumstances. The consequences of such deliberations, of potential significance to all phase transformations also beyond polymer crystallization, are being developed throughout the paper.