Gandara Amarasinghe

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

Abstract 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.

Comonomer Distribution in Polyethylenes Analysed by DSC After Thermal Fractionation

Ethylene copolymers exhibit a broad range of comonomer distributions. Thermal fractionation was performed on different grades of copolymers in a differential scanning calorimeter (DSC). Subsequent melting scans of fractionated polyethylenes provided a series of endothermic peaks each corresponding to a particular branch density. The DSC melting peak temperature and the area under each fraction were used to determine the branch density for each melting peak in the thermal fractionated polyethylenes. High-density polyethylene (HDPE) showed no branches whereas linear low-density polyethylenes (LLDPE) exhibited a broad range of comonomer distributions. The distributions depended on the catalyst and comonomer type and whether the polymerisation was performed in the liquid or gas phase. The DSC curves contrast the very broad range of branching in Ziegler—Natta polymers, particularly those formed in the liquid phase, with those formed by single-site catalysts. The metallocene or single-site catalysed polymers showed, as expected, a narrower distribution of branching, but broader than sometimes described. The ultra low-density polyethylenes (ULDPE) can be regarded as partially melted at room temperature thus fractionation of ULDPE should continue to sub-ambient temperatures. The thermal fractionation is shown to be useful for determining the crystallisation behaviour of polyethylene blends.

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.