Molecular distribution analysis of melt-crystallised ethylene copolymers

A simple technique employing differential scanning calorimetry (DSC) to investigate the molecular structure of ethylene copolymers is presented in this paper. Three commercial Ziegler-Natta catalysed linear low density polyethylenes (LLDPE) and a commercial single-site catalysed very low density polyethylene (VLDPE) were subjected to continuous cooling at a slow rate of 0.08degreesC min(-1). Like other thermal fractionation techniques, the slow continuous cooling (SCC) technique segregates polymers according to their branching density, allowing the short chain branching (SCB), lamellar thickness (L) and methylene sequence length (MSL) distribution to be determined using the DSC melting curves. It was found that the single-site catalysed VLDPE exhibits narrow SCB, L and MSL distributions, with shorter methylene sequences. In contrast, the Ziegler-Natta catalysed LLDPEs have much broader bimodal SCB, L and MSL distributions, with less SCB, and are composed of both short and long methylene sequences. LLDPEs have thicker lamellae compared with VLDPE (12.9, 12.5, 9.8 nm versus 6.7 nm) and the lamellar thickness values are consistent with the results measured by transmission electron microscopy (TEM). The slow continuous cooling and stepwise cooling techniques are complementary: the former provides a continuous distribution profile and the latter a well-defined histogram for the lamellar thickness. The results obtained are qualitatively comparable to those gained by temperature rising elution fractionation (TREF), because the cooling rate used here is of the order of the rates used in TREF analysis