Strain Hardening of Various Polymer Melts

Various strain-hardening features of polymer melts in uniaxial extension are described. Of special interest is the dependence of strain hardening on elongational rate. Long-chain branched polyethylene, commercial polystyrenes and their blends with high molar mass components exhibit strain hardening becoming more pronounced with increasing strain rate. For linear polypropylenes, strain hardening is not found within the experimental window. Adding a small amount of a linear polyethylene with ultrahigh molar mass (UHMWPE) results in strain hardening increasing with decreasing rate. Rouse times are used to qualitatively interpret the experimental results on polystyrene. The non strain hardening behavior of linear PP and HDPE can be made plausible by the Rouse model. But for a qualitative understanding of the rate dependence of strain hardening of the PP/UHMWPE blend, the existence of a separate phase of the minor component and special deformation processes at the interface with the matrix have to be postulated.