Non-Newtonian behaviour of hydrolysed polyacrylamide in strong elongational flows: a transient network approach

Abstract

In this paper we report a new technique that links molecular behaviour and macrorheology in idealized elongational flow systems. The effective elongational viscosity of aqueous solutions of hydrolysed polyacrylamide (widely used and studied in applied hydrodynamics) is determined and correlated with the various stages of chain stretching and transient network formation. Beyond a critical strain rate, strong non-Newtonian dilatant effects are observed. These are unmistakably due to the existence of transient networks, which arise as a consequence of entanglements becoming mechanically effective at timescales shorter than the disentanglement time. We report strong parallels between observations in our idealized experiments and dilatant effects commonly observed in other flow systems that contain appreciable elongational components. These effects had been previously generally attributed to the viscosity enhancement due to the stretching of isolated molecules. On the basis of our observations, which include the coil-stretch transition, we are forced to reinterpret such effects as also due to the development of transient entanglement networks.