Studies of the Comonomer Distributions in Low Density Polyethylenes Using Temperature Rising Elution Fractionation and Stepwise Crystallization by DSC

Abstract

The comonomer distributions of commercial linear low density polyethylenes (LLDPE) and linear very low density polyethylenes (VLDPE) produced with traditional high activity Ziegler–Natta (Z–N) catalysts were characterized by temperature rising elution fractionation (TREF). In order to develop faster characterization methods the polymers were also characterized using a segregation fractionation technique (SFT) based on a stepwise crystallization by differential scanning calorimetry (DSC). Comparative studies of SFT and TREF demonstrated that SFT provides an alternative tool for the relative qualitative analysis of the chemical composition distribution (CCD) and the technique is useful to characterize the heterogeneity in comonomer unit distribution. Lamellar thickness distributions can be calculated from the DSC endo-therms by applying the Thomson–Gibbs equation. The SFT technique was also applied to commercial single-site (metaliocene) LLDPE and VLDPE polymers. In spite of their more homogeneous structure compared with the Z–N copolymers, which contain many active sites, these single site copolymers also gave thermograms resolved into several peaks.