Experimental observation and numerical simulation of transient "stress fangs" within flowing molten polyethylene

We report experimental observations and matching numerical simulation for t he time-dependent start-up flow of two molten polyethylenes (PEs) within a slit entry and exit geometry. For the case of a low density polyethylene (L DPE), an unexpected transient, birefringence "stress fang" was observed dow nstream of the slit exit. The stress fang consisted of a localized region o f stress concentration. The stress fang, however, was not observed for a li near low density polyethylene (LLDPE) sample subjected to the same processi ng condition. A matching time-dependent numerical simulation of the flow is also presented. Using a split Lagrangian-Eulerian method for simulating tr ansient viscoelastic flow with the multimode pom-pom constitutive equation, the general features of the stress fangs were predicted for the LDPE. In a ddition, the simulation did not predict stress fangs for the LLDPE. The pap er demonstrates that for this particular case the pom-pom model can success fully discriminate the complex flow behavior of different PEs, and shows th at the presence (or otherwise) of a stress fang is sensitive to the particu lar theology of the polymer that arises from long chain branching. (C) 2001 The Society of Rheology.