The size factor in phase transitions: its role in polymer crystal formation and wider implications

Abstract

It is shown that under specifiable circumstances stabilities of competing phases can invert with size: specifically, that a phase which is metastable when of infinite size can become the stable one when of sufficiently small dimensions. It follows that phase development, crystal growth in particular, may start in a phase variant which becomes metastable when the new phase is fully developed. If it stays in this form Ostwald’s rule of stages will seem to be obeyed, if it transforms into the phase of ultimate stability the past history of phase development becomes obliterated. In the special instance of flexible polymers, polyethylene in particular, there can be thickening growth while in the metastable ‘mobile’ phase (hexagonal phase in polyethylene), hence residence within this phase will determine the lamellar thickness, consequently also the final texture of the crystallizing material. Based on these considerations the two so far essentially disconnected areas of chain folded and extended chain type crystallization can be visualised within a unified frame work with new, broadened perspectives for the whole subject of polymer crystallization. In addition, the scheme creates a junction between thermodynamic (stability) and kinetic (rates) aspects of phase transitions in wider generality.