Polymer crystals

Publisher Website
Google Scholar
Abstract
The major developments in our understanding of polymer crystals and the kinetics of their formation have been many. It is now established that adjacent re-entry folding of the molecule occurs on crystallisation from solution, but when crystallised from the melt the molecular trajectory approximates a random coil when viewed from a distance. From a closer viewpoint, it appears that adjacent re-entry folding is only one contributor and that any given molecule exits and re-enters several crystals in a manner that is determined by the crystallisation temperature. In addition, it is now accepted that the interface between the crystal and the random melt may be macroscopic in nature. Variously referred to as rigid amorphous phase or interzonal material, the extent of formation of such an intermediate state is strongly dependent on the rigidity of the given polymer molecule. Our understanding of the kinetics of crystallisation has taken a major step forward through the realisation that three regimes exist, dependent on the relative rates of secondary nucleation and surface spreading. At the time of writing, the third regime is believed to be unique to long chain molecules, whereas the first two regimes have been observed in atomic solids. Although we are now beginning to understand the influence of molecular length and microstructural impurity levels on regime behaviour, our definitive knowledge is still in its formative stages. A further important development is the recognition that the skeletal structure of common superstructural crystalline aggregates, such as spherulites, is determined by a small number of leading or dominant lamellae and that the bulk of the crystallinity develops behind the growth front. Such behaviour had been recognised for many years in low-crystallinity polymers, but is now a recognised feature of growth in common high-crystallinity polymers also.