SHARKSKIN TEXTURING INSTABILITIES IN THE FLOW OF POLYMER MELTS

The capillary Bow of molten polymers has received much attention becau se at high Bow rates the extrusion of the polymer melt is commonly acc ompanied by instabilities. These manifest themselves as surface distor tions, called melt fracture, in the final plastic product. We have mod eled this Row, incorporating a stick-slip boundary condition at the ca pillary wall. The boundary condition is enforced by a model for the lo cal state of the polymer, which undergoes a phase transition controlle d by the local shear. We numerically solve the model. coupled to the h ydrodynamic Row and to a Maxwell model for viscoelasticity. In various regimes, the model exhibits steady flow, periodic oscillations, and m ore complicated spatiotemporal structures, which explain the sharkskin texturing observed in melt fracture.