Comparison of shear stress and wall slip measurement techniques on a linear low density polyethylene

Small gap, torsional, parallel plate experiments allow the measurement of shear stress and slip velocity at shear rates comparable to those measured in slit and capillary rheometers. Using this technique we compare data from parallel plate measurements, where effects of pressure and temperature are negligible, to those from pressure driven flows where they are not. The shear stress for all techniques agrees within experimental error at low shear rates. At higher shear rates the slit and capillary measurements diverge from the parallel plate measurements highlighting the effect of pressure and temperature on viscosity. Also, breakdown of the Cox–Merz rule at slightly higher shear rates is apparent when comparing dynamic and steady shear data from the parallel plate experiments. Since the complex viscosity diverges in the same manner as the viscosity derived from pressure driven flow it seems as if agreement of the complex viscosity with the viscosity derived from pressure driven flow is fortuitous. With the capillary geometry, there is no sign of slip effects at low stress levels probably due to experimental limitations while the torsional rheometer shows slip at all shear stresses. There is no critical stress for the onset of slip but there is a critical stress where slip increases dramatically.