Crystallisation of single-site polyethylene blends investigated by thermal fractionation techniques

Branched polyethylenes, low density polyethylenes (LDPE) or long-chain branched very low density polyethylenes (VLDPE), were blended with VLDPEs containing short branches. The melting behaviour of pure copolymers and their blends were investigated using differential scanning calorimetry (DSC) after applying stepwise isothermal crystallisation (‘thermal fractionation’). Thermal fractionation separates polymers according to their branching densities and fractionated curves used to determine the short-chain branching distribution (SCB), crystallisation and miscibility of blends. When both polymers have similar unbranched segments, they may co-crystallise if they are miscible 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 blend. The blends of metallocene-catalysed VLDPE1 and LDPEs show significant deviation from the additivity rule indicating the greater co-crystallisation and hence melt miscibility at all compositions. The extent of co-crystallisation decreases for the VLDPE1 blends containing long-chain branched VLDPE2 and increases for Ziegler–Natta-catalysed VLDPE3–VLDPE2 blends, as the VLDPE2 content increases. DSC fractionated curves allow detailed examination of co-crystallisation and miscibility of blends that is also comparable to the results gained by temperature rising elution fractionation.