Polyethylene — Copolymer Model Blends: Morphology and Mechanical Properties

Abstract

Random copolymers of ethylene and α-olefins made by low pressure polymerization are known to be mixtures of chains which vary considerably in comonomer content.¹ Mirabella, et al.² have shown that such composition heterogeneity in linear low density polyethylene (LLDPE, in that case ethylene-butene copolymers) leads to the formation of dispersed, submicron size domains of highly branched polymer, presumably by phase separation in the melt. A similar conclusion was reached by Deblieck and Mathot³ regarding the morphology of ethylene-octene copolymers. It was further suggested by Mirabella, et al.² that the superior plane strain fracture toughness of LLDPE (J_C = 40 kJ/m² at −20°C, versus ~ 2 kJ/m² for unbranched polyethylene homopolymer) results from the presence of the dispersed rubber-like regions. Their conclusion was underscored by the observation that conventional low density polyethylene (LDPE) with similar density, branching and molecular weight, but characterized by a narrower composition distribution, has no melt phase separation and a low toughness, about the same as that of unbranched homopolymer (PE).