Polymorphism in polymers : its implications for polymer crystallisation

The aim of this article is to extend the earlier reported observations on the role of transient metastable phases in polymer crystallisation in relation with the initial crystal size. In this article experimental evidence is provided to bridge the gap between single crystal formation in the melt at elevated pressure and temperatures vs. crystallisation at atmospheric pressure using polyethylene as a model substance. During transformation from the hexagonal to the orthorhombic phase it is shown that in the process of growth, a crystal goes through thermodynamically stable and metastable states before transformation to the orthorhombic phase occurs. The crystal growth, on transformation to the thermodynamically stable orthorhombic phase, has been followed with the help of in-situ optical microscopy and transmission electron microscopy. The observations are that the newly transformed crystal acts as a nucleation centre for many new crystals starting in the hexagonal phase. It is also noticed that with increasing supercooling multilayering dominates. Subsequently, the distinction between primary and secondary thickening has been made and its morphological consequences will be discussed. In its wider generality, the experimental findings indicate that in polyethylene at atmospheric pressure crystallisation occurs via the hexagonal phase. When extended to atmospheric pressure, the morphological features give further insight into spherulite formation. The observations have been extended to other polymers such as nylon, paraffins, poly-di-alkyl siloxanes, trans-1,4 polybutadiene etc. The proposed viewpoint on the crystal size influence in phase transition has been extended to polymer processing as will be illustrated briefly for the case of processing of the intractable polymer ultra-high-molecular-weight polyethylene (UHMW-PE)