PREDICTING POLYMER MELT BEHAVIOR NEAR THE INCEPTION OF WALL SLIP IN OSCILLATORY SHEAR

Large amplitude oscillatory shear (LAOS) experiments on molten high-de nsity polyethylene (HDPE) conducted by Hatzikiriakos and Dealy have sh own that the shear stress response is highly unstable above a critical shear stress amplitude of sigma(c) = 0.09 MPa. The contour of this cr itical stress amplitude on a Pipkin diagram is termed the slip isoplet h. For stresses well below the slip isopleth, transient network theory has been shown to describe the LAOS behavior of this material accurat ely. However, the slip isopleth predicted by this theory is well below the measured isopleth. This discrepancy suggests that there could be a region on a Pipkin plot, where wall slip can occur, without giving r ise to unstable stresses.