Wall Slip and Boundary Effects in Polymer Shear Flows

Polymer – surface interactions strongly influence many important industrial and rheological flows. In particular, polymer melts and solutions slip against the surface; this has long been associated with sharkskin and spurt in extrusion, and recent experimental observations suggest that slip also plays a role in the formation of enhanced concentration fluctuations in entangled polymer solutions. Analyses of melt flow in model geometries, incorporating simple slip models which include the effects of chain orientation and stretching, and employing common linear and nonlinear viscoelastic models for the polymer stress, demonstrate that slip can lead to short-wave hydrodynamic instabilities. The results predict values of the critical recoverable shear, the critical shear rate, and the frequency of distortion that are consistent with experimental observations for sharkskin during extrusion of linear polyethylenes. Further analysis, employing slip models generalized to include the effects of normal load (i.e. pressure) at the surface, confirms and unifies previous theoretical work: shear stress dependent slip models cannot predict instabilities consistent with sharkskin; and pressure dependent slip leads to instability due to illposedness at the boundary. Computational studies performed for polymer solutions highlight the importance of boundaries on the flow behavior. Computations were