POLYMER CRYSTALLIZATION - SURVEY AND NEW TRENDS WITH WIDER IMPLICATIONS FOR PHASE-TRANSFORMATIONS

After reviewing the present status of research on the crystallization of flexible polymers some new perspectives in the field, as arising from ongoing research, will be presented. First, a brief resume is given of the experimental material on chain-folded crystallization (here to be termed 'main stream') on polyethylene as the model substance. This is followed by a brief account on extended chain type crystallization as realizable, in the case of polyethylene, under hydrostatic pressure through the intervention of a mobile phase (here termed 'speciality stream'). This is followed by a review of theoretical attempts to account for the experimental facts with special emphasis on open-ended issues and on connections with the main body of theories on crystal growth outside the polymer field. The new perspectives are aimed at linking the two streams, chain-folded and chain-extended, through a scheme based on new experimental recognitions on polyethylene but with potential relevance to all crystal growth, in fact phase transitions. It relies on crystal size as a variable, leading to the realistic possibility that phase stabilities can invert with size, i.e. a phase which is metastable for infinite size can become stable when the phase is sufficiently small. When this condition is applied 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. These general thermodynamic considerations can be linked to kinetic criteria accounting, amongst others, for the frequently observed dominance of metastable phases, thus providing a combined thermodynamics- and kinetics-based justification to Ostwald's rule of stages. In the case of polymers such transient phases can have greatly enhanced influence owing to the associated high chain mobility. The latter enables thickening growth through the agency of sliding diffusion, which can then create altogether new possibilities in the field of polymer crystallization.