Crystallisation of blends of LLDPE with branched VLDPE

Blends of linear low-density polyethylene (LLDPE) and very low-density poly ethylene (VLDPE) with long chain branching have been prepared by extrusion mixing. All copolymers have similar branch lengths and are commercial ethyl ene-octene copolymers. The copolymers and blends were subjected to crystall isation ("thermal fractionation") by stepwise cooling interspersed with iso thermal periods and the fractionated samples were examined by differential scanning calorimetry (DSC). Thermal fractionation by DSC separates copolyme rs 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 con tains a broad distribution of branching densities whereas the VLDPEs contai n a narrow distribution, though with shorter average lengths between branch es. Where both polymers have common melting endotherms in the thermally fra ctionated blends they may co-crystallise if they are mutually miscible in t he melt. In blends containing low amounts of VLDPEs, the branching observed are combined effects of each individual polymer indicating that polymers r etain some of their individual features after blending. The blends with hig h VLDPE amounts show some miscibility in the melt suggesting co-crystallisa tion between the copolymers may be occurring. (C) 2000 Elsevier Science Ltd . AU rights reserved.