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

In this paper we present new experimental evidence indicating that the presence of adsorbed polymer chains in the die exit region of a slit die is a necessary feature in the production of sharkskin on the extrudate. Specifically, we use a flow birefringence technique to visualize the level of molecular 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 exit to produce sharkskin when the no-slip boundary condition prevails. We then perform the same experiment on a slit die with the slip boundary condition in the die exit. It is observed that no sharkskin roughening of the extrudate occurs even at a much higher level of retardance. (C) 2001 The Society of Rheology.