Further Studies of Intercrystalline Links in Polyethylene

Abstract

A previous study of intercrystalline links in polyethylene, carried out using fractionated polymer only, has now been extended to mixtures of fractions and to whole polymer. There is little difference between links grown from concentrated (50%) solutions of polyethylene in n-C32H66, regardless of the molecular weight distribution of the polymer. These links exhibit surfaces that are more or less featureless and the evidence indicates that they are composed of molecules crystallized, for the most part, with extended conformations. However, when these links are used as substrates upon which polyethylene is deposited epitaxially from dilute (1% or less) solution in later, quite separate, crystallization experiments, lamellar overgrowths are formed which demonstrate unambiguously the crystallization of molecules in folded conformations. The marked difference between these cases is attributed to entanglements which, as crystallizing molecules are ``reeled in'' to growth fronts, cause neighboring molecules to become extended and partly aligned in directions normal to these fronts. Links grown from concentrated solutions are built up from such molecules whose conformations are grossly disturbed by this crystallization-induced orientation. However, epitaxial overgrowths deposited from dilute solution are built up from molecules whose conformations in solution are unaffected, and these then crystallize conventionally with folded conformations.