Polymorphism in polymers; its implications for polymer crystallisation

The aim of this article is to extend the earlier reported observations on t he role of transient metastable phases in polymer crystallisation in relati on 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 press ure using polyethylene as a model substance. During transformation from the hexagonal to the orthorhombic phase it is shown that in the process of gro wth, 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 thex help of in-situ optical microscopy and transmission electron microscopy. The observations are that the newly transformed cryst al acts as a nucleation centre for many new crystals starting in the hexago nal phase. It is also noticed that with increasing supercooling multilayeri ng dominates. Subsequently, the distinction between primary and secondary t hickening has been made and its morphological consequences will be discusse d. In its wider generality, the experimental findings indicate that in poly ethylene at atmospheric pressure crystallisation occurs via the hexagonal p hase. When extended to atmospheric pressure, the morphological features giv e further insight into spherulite formation. The observations have been ext ended to other polymers such as nylon, paraffins, poly-di-alkyl siloxanes, trans-1,4 polybutadiene etc. The proposed viewpoint on the crystal size inf luence in phase transition has been extended to polymer processing as will be illustrated briefly for the case of processing of the intractable polyme r ultra-high-molecular-weight polyethylene (UHMW-PE). (C) 2000 Kluwer Acade mic Publishers.