Comonomer distribution in polyethylenes analysed by DSC after thermal fractionation

Ethylene copolymers exhibit a broad range of comonomer distributions. Therm al fractionation was performed on different grades of copolymers in a diffe rential scanning calorimeter (DSC). Subsequent melting scans of fractionate d polyethylenes provided a series of endothermic peaks each corresponding t o a particular branch density. The DSC melting peak temperature and the are a under each fraction were used to determine the branch density for each me lting peak in the thermal fractionated polyethylenes. High-density polyethy lene (HDPE) showed no branches whereas linear low-density polyethylenes (LL DPE) 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 form ed in the liquid phase, with those formed by single-site catalysts. The met allocene 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 ro om temperature thus fractionation of ULDPE should continue to sub-ambient t emperatures. The thermal fractionation is shown to be useful for determinin g the crystallisation behaviour of polyethylene blends.