Thermal characterization of ethylene polymers prepared with metallocene catalysts

A new fractionation technique, segregation fractionation by differential scanning calorimetry (DSC), was applied to both lab-scale and commercial metallocene copolymers produced with a variety of comonomers. Comparisons were made to other fractionation methods, such as temperature rising elution fractionation (TREF) and crystalline analysis fractionation (CRYSTAF). The new method represents an alternative tool for the relative qualitative analysis of chemical composition distribution (CCD) and the technique was found useful for the characterization of comonomer unit distribution. Differences in the catalyst system and in the thermal treatment were reflected in the melting pattern of the copolymers, obtained after fractionation. The new method was used to study CCD broadening in lab scale slurry polymerizations, and it was found that the effect of polymerization time and comonomer feed system on the CCD can be followed by segregation fractionation measurements: Dynamic mechanical analysis (DMA) was also used to evaluate the heterogeneity of the polyethylene structure. Studies of β-transitions showed differences in the tan δ-density correlations, which were influenced by the comonomer type. In most cases the intensity of the β-transition increased with branching (decreasing density), but for ethylene-1-octadecene copolymers the situation was opposite at lower densities. This may be attributed to the crystallization of C₁₆H₃₃ branches.