New explanation for chain folding in polymers

Abstract

Polymer crystals are characteristically thin in the molecular direction¹ so that materials such as polyethylene and nylon have an extremely finely divided texture consisting of crystal lamellae interspaced with non-crystalline regions (melting points are depressed by 20 ^°C or more). Many distinctive properties of these polymers follow as a consequence of this phenomenon, often known as chain folding because molecules generally fold back and forth across the thin dimension of the crystals. The currently accepted explanation of the origin of chain folding in terms of 'nucleation' ('LH' theories²) appears to be inconsistent with the observed shapes of crystals³ (see also Wunderlich⁴). In this paper I describe how an approximate treatment of an alternative model^5,6 involving rough growth surfaces and molecular 'pinning' can give analytical results in line with experiment. The aim is to derive the result in such a way as to concentrate on the physical mechanisms involved and to discuss the kinetic barriers caused by molecular continuity. If there turns out to be any connection between chain folding of synthetic molecules and folding in proteins, it will be clear that the models discussed here may be more readily extended to the latter than LH models², because LH models² are specific to crystals and not applicable, for example, to the ribbon structures which are formed from β sheets^7,8.