Stick−Slip Transition in Capillary Flow of Polyethylene. 2. Molecular Weight Dependence and Low-Temperature Anomaly

The present study focuses on molecular characteristics and temperature dependence of a superfluid-like stick−slip transition in a series of highly entangled high-density polyethylene (PE) resins and their rheological behavior in pressure-driven capillary flow. It is found that at sufficiently high stresses the capillary flow characteristics are largely dictated by the stick−slip transition occurring at the PE/die wall interfaces. The transition magnitude is found to have an explicit and strong molecular weight M w dependence reminiscent of bulk chain entanglement, i.e., the extrapolation length b c ∝ M w 3.4 . In addition to its normal temperature dependence for T ≥ 200 °C as described in paper 1, the critical stress for the stick−slip transition ceases to scale linearly with T from 200 down to 160 °C for all four molecular weights studied. The extrapolation length b c at the transition also becomes much larger at a lower T . The anomaly is clearly interfacial in nature and may involve a flow-induced ordered mesophase near the PE/wall interfaces.