Flow Instabilities as Stress Limiters for Molten Polymers

Abstract

Molten polymers and concentrated solutions in which the molecular weight is well above the critical value for entanglements can exhibit quite high shear and tensile stresses in flows that can be easily generated in the laboratory and in processing machinery. Various phenomena occur at these high stresses that have the effect of inhibiting the increase in stress as the rate of deformation is further increased. In the laboratory, these phenomena interfere with efforts to evaluate constitutive equations under conditions of large strain and strain rate, and in plastics forming operations they can produce unacceptable defects in extrudates. The occurrence of such phenomena is often attributed to some inadequacy of the apparatus generating the deformation.