Crystallisation of single-site polyethylene blends investigated by thermalfractionation techniques

Branched polyethylenes, low density polyethylenes (LDPE) or long-chain bran ched very low density polyethylenes (VLDPE), were blended with VLDPEs conta ining short branches. The melting behaviour of pure copolymers and their bl ends were investigated using differential scanning calorimetry (DSC) after applying stepwise isothermal crystallisation ('thermal fractionation'). The rmal fractionation separates polymers according to their branching densitie s and fractionated curves used to determine the short-chain branching distr ibution (SCB), crystallisation and miscibility of blends. When both polymer s have similar unbranched segments, they may co-crystallise if they are mis cible in the melt. The co-crystallisation is observed to occur in all sets of blends, however, the extent of co-crystallisation varies from blend to b lend. The blends of metallocene-catalysed VLDPE1 and LDPEs show significant deviation from the additivity rule indicating the greater co-crystallisati on and hence melt miscibility at all compositions. The extent of co-crystal lisation decreases for the VLDPE1 blends containing long-chain branched VLD PE2 and increases for Ziegler-Natta-catalysed VLDPE3-VLDPE2 blends, as the VLDPE2 content increases. DSC fractionated curves allow detailed examinatio n of co-crystallisation and miscibility of blends that is also comparable t o the results gained by temperature rising elution fractionation. (C) 2001 Elsevier Science Ltd. All rights reserved.