Exploring molecular origins of sharkskin, partial slip, and slope change in flow curves of linear low density polyethylene

This paper explores the molecular mechanism for sharkskin formation on extrudate of linear low density polyethylenes (LLDPE) and investigates the rheological origin of a characteristic curvature (i.e., a slope change) in the flow curve of LLDPE. Rheological measurements, performed at various temperatures from 160 to 240 degrees C with a controlled-pressure capillary rheometer and a variety of dies, suggest that the slope change in the flow curve, interpreted by many as demonstrating wall slip in the die land, arises from a combination of interfacial slip and cohesive failure due to chain disentanglement, first initiated on the die wall in the exit region. Since the disentanglement state is unstable for the adsorbed chains within a certain stress range below the critical stress for the global stick-slip transition, a partial slip flow cannot sustain itself and occurs only periodically. This time-dependent molecular entanglement-disentanglement fluctuation produces the sharkskin like extrudate in the regime where the slope change takes place. Sharkskin dynamics are found to precisely correlate with chain relaxation processes. Specifically, the characteristic time scale tau(i.e., the sharkskin periodicity) is found to be of the same magnitude and have the same WLF (Williams-Landel-Ferry) temperature dependence as that of the characteristic molecular relaxation time tau* as determined by oscillatory shear measurements in a parallel-plate flow cell. The LLDPE resins are also observed to undergo interfacial stick-slip transitions as well as a rarely seen cohesive slip-slip transitions at various temperatures. (C) 1996 Society of Rheology.