Preparation of ultra-high modulus linear polyethylenes; effect of molecular weight and molecular weight distribution on drawing behaviour and mechanical properties

Abstract

A systematic investigation of the effect of molecular weight and molecular weight distribution on the cold drawing behaviour of linear polyethylene has been undertaken. In the molecular weight range studied, the natural draw ratio was very sensitive to the morphology of the initial material; spectacular effects on the natural draw ratio were observed provided that an optimum initial morphology was achieved. These effects can be related to both molecular weight and molecular weight distribution. The extensional modulus and melting behaviour of the drawn material was also examined. To a first approximation the extensional modulus related to the natural draw ratio, and at very high draw ratios (∼30) extremely high extensional moduli (∼700kbar) were obtained. The structure and properties of the drawn material did, however, also depend on the molecular weight and molecular weight distribution. In particular, when certain molecular weight requirements were satisfied, the oriented samples showed the presence of extended chain material. It does, however, appear that although differences in molecular weight and molecular weight distribution give rise to differences in extensional moduli, the presence of extended chain crystallization per se is not a necessary requirement for the production of high modulus material.