SECONDARY FLOW OF ENTANGLED POLYMER FLUIDS IN PLANE COUETTE SHEAR

We report new evidence of secondary flow during plane Couette shearing of entangled polystyrene solutions and an entangled polyisobutylene m elt. The secondary how is shown to be driven by normal stress imbalanc es at the free edges of the materials, and is accompanied by spatial v ariations in birefringence. In the polymer solutions, the secondary fl ow, though unmistakable, is weak and is only apparent from tracer part icle visualization experiments using a video microscopy technique. In the polymer melt, however, secondary flow is much stronger and causes gross shape changes in sheared samples that are easily visualized with the naked eye. We also show that several details of the plane Couette secondary flow are correctly predicted by a simple differential const itutive equation in the narrow gap approximation. Our findings suggest that the secondary flow's effect on sliding plate rheological measure ments could be minimized by (a) using large plates with narrow gaps, i .e., small sample aspect ratios; (b) performing measurements at low We issenberg numbers (Wi = tau(L) gamma); and (c) employing measuring tec hniques such as laser birefringence that permit stresses to be determi ned close to the center of the shearing surfaces of the plane Couette cell. In addition to secondary flow, we find significant levels of sli p during steady shearing of entangled polystyrene solutions. The slip, though qualitatively similar to the entangled slip predictions of Bro chard and deGennes [Langmuir 8, 3033-3037(1992)], is unusual because a t low slip velocities V-s the slip lengths b = V-s/gamma are of the or der of 150 mu m, which is much larger than expected for entangled slip . Furthermore, slip in the solutions is shown to be nonisotropic with the slip velocity manifesting a similar shear rate dependence to the b irefringence. These findings suggest that the slip law for entangled p olymers may be more complex than previously thought. (C) 1996 Society of Rheology.