A Unifying Scheme for Polymer Crystallization with Wider Implications for Phase Transformations

Abstract

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 thermodynamic variable, 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 for 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. These general thermodynamic considerations can be connected with previously established kinetic criteria, by which a given phase transformation starts through the particular polymorph which is stable down to the smallest size, a form in which also growth is fastest. The above scheme is applicable, in its widest generality to all phase transformations, and as such also provides a previously unsuspected thermodynamic justification to Ostwald’s Rule of Stages. In the specific field of polymer crystallization it can have certain explicit consequences and it is to these that the paper is being addressed. Amongst others it should provide a widened base from which the manifold effects observed in the cyrstallization of flexible polymers can be approached from a newly unified viewpoint.