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 0.08 • C 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.
Temperature modulated differential scanning calorimetry (TMDSC) was employed to study the melting and crystallization behavior of various polyethylenes (PEs). Samples of high density PE (HDPE), low density PE (LDPE), linear low density PE (LLDPE), and very low density PE (VLDPE) with different crystal structures and morphologies were prepared by various thermal treatments (isothermal crystallization and slow, fast, and dynamic cooling). The reversing and nonreversing contributions, measured on the experimental time scale, were varied, depending on the crystal stability. A relatively large reversing melt contribution occurs for unstable crystals formed by fast cooling compared to those from slow cooling treatments. All samples of highly branched LDPE, LLDPE, and VLDPE showed a broad exotherm before the main melting peak in the nonreversing curve, suggesting crystallization and annealing of crystals to more stable forms. Other samples of HDPE, except when cooled quickly, did not show any significant crystallization and annealing before melting. The crystallinity indicated that dynamically cooled polymers were much more crystalline, which can be attributed to crystal perfection at the lamellar surface. A reversible melting component was also detected during the quasiisothermal TMDSC measurements. Melting is often accompanied by large irreversible effects, such as crystallization and annealing, where the crystals are not at equilibrium. Such phenomena during a TMDSC scan provide information on the polymer thermal history.
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