Melt strain hardening of various high density polyethylenes

Strain hardening of polymer melts is a rheological property interesting from an application and a fundamental point of view. While strain hardening of long-chain branched polyethylenes and polypropylenes and their relation to the molecular structure have widely been discussed in the literature, there are only few publications on the behavior of polyethylenes of high density (HDPE). Moreover, the results available from the literature are different with respect to the dependence of strain hardening on the elongational rate. Some samples show a negligible strain hardening, but most of them reveal hardening increasing with the decreasing rate. The lack of comprehensive molecular characterizations restricts a deeper understanding. Thus, in this work, the elongational behavior of various HDPEs was studied and their molecular structure characterized using high-temperature gel-permeation chromatography coupled with laser-light scattering at small elution volumes. In addition, the zero-shear viscosity as a function of molar mass was considered. It was found that molecules of very high molar masses or few long branches effecting long relaxation times may be the reason for strain hardening increasing with the decreasing rate. The underlying chemical reaction is not understood in detail, but it seems very probable that the catalysts and the processes used for the polymerization of HDPE may be able to generate molecular structures with a strong effect on strain hardening.