Strain hardening of various polyolefins in uniaxial elongational flow

In this contribution correlations between the type of strain hardening of long-chain branched polyethylenes and the level of their zero shear-rate viscosities in comparison to linear polyethylenes are described. For polyethylenes of various branching structures four different types of strain hardening can be observed. Type I: strain hardening is approximately independent of elongational rate. Type II: strain hardening decreases with increasing elongational rate. Type III: strain hardening increases with increasing elongational rate. In addition to that, materials are found which do not show strain hardening within the experimental window (type IV). Qualitative correlations with the dependence of the zero shear-rate viscosity on the weight average molecular mass can heuristically be deduced. Polyethylenes of type IV fulfill the well-established relationship eta(0) similar to M-w(3.4), samples of types I and II give higher zero shear-rate viscosities than the linear products. Type III samples generally exhibit zero shear-rate viscosities lower than the linear relationship. As the dependence of the zero shear-rate viscosities on the molecular mass offers some insight into the branching structure of a polyethylene the differences in strain hardening can heuristically be related to the molecular structure. These correlations can be transferred to different types of long-chain branched polypropylenes giving some hint to their molecular structure. (C) 2003 The Society of Rheology.