Viscosity/molecular weight relation in bulk polymers—II. Onset of non-Newtonian Flow

Abstract

The strong dependence of polymer melt viscosity on molecular weight can be attributed to the ability of polymer molecules to participate in entanglements. When unidirectional shear stress is applied, the melt viscosity reduces because of molecular motion which results in a reduced statistical requirement for segmental cooperation. Qualitative examination of polymer motion suggests that the shear stress required to reduce melt viscosity from its Newtonian flow varies inversely with molecular weight and depends on the degree of polymer polydispersity. Experiments confirm the proposed dependence of the initial shear stress for non-Newtonian flow on molecular weight, and indicate the potential use of this shear stress as a characteristic of the high molecular weight portion of the distribution in a given polymer.