Rheo-optical observations of sharkskin formation in slit-die extrusion

In this paper we present new experimental evidence indicating that the pres ence of adsorbed polymer chains in the die exit region of a slit die is a n ecessary feature in the production of sharkskin on the extrudate. Specifica lly, we use a flow birefringence technique to visualize the level of molecu lar deformation in the die exit region. By varying the applied shear stress level, we are able to find the critical molecular deformation in the die e xit to produce sharkskin when the no-slip boundary condition prevails. We t hen perform the same experiment on a slit die with the slip boundary condit ion in the die exit. It is observed that no sharkskin roughening of the ext rudate occurs even at a much higher level of retardance. (C) 2001 The Socie ty of Rheology.