Crystallisation of blends of LLDPE with branched VLDPE

Abstract

Blends of linear low-density polyethylene (LLDPE) and very low-density polyethylene (VLDPE) with long chain branching have been prepared by extrusion mixing. All copolymers have similar branch lengths and are commercial ethylene–octene copolymers. The copolymers and blends were subjected to crystallisation (“thermal fractionation”) by stepwise cooling interspersed with isothermal periods and the fractionated samples were examined by differential scanning calorimetry (DSC). Thermal fractionation by DSC separates copolymers and blends according to their branching densities. Thermal fractionation data were used to calculate branching distribution in polyethylenes using calibration curves obtained from the literature. It is found that LLDPE contains a broad distribution of branching densities whereas the VLDPEs contain a narrow distribution, though with shorter average lengths between branches. Where both polymers have common melting endotherms in the thermally fractionated blends they may co-crystallise if they are mutually miscible in the melt. In blends containing low amounts of VLDPEs, the branching observed are combined effects of each individual polymer indicating that polymers retain some of their individual features after blending. The blends with high VLDPE amounts show some miscibility in the melt suggesting co-crystallisation between the copolymers may be occurring.