Decoding the steady elongational viscosity of monodisperse linear polymers using tube-based modeling

Abstract

The current coarse-grained picture to represent polymer chain dynamics under uniaxial extensional flow (based on the Doi–Edwards model) fails to predict some scaling dependencies of material properties on deformation rate observed experimentally, specifically the monotonic thinning behavior of polymer melts. Recently, new mechanisms based on the concept of monomeric friction reduction have been proposed to explain this peculiar behavior; however, it is difficult to include them in the framework of the standard tube model. Therefore, in this work, we propose an alternative treatment which does not rule out friction reduction but uses a different approach. It considers that the chain can stretch up to a certain level that we determine based on the Pincus blob picture, in place of determining to which extend the chain stretch is reduced compared to its finite extensibility. To this end, we revisit the extensional rheological data of polystyrene melts and see how the specificities of chains under strong elongational flow can be integrated into a tube model. This requires accounting for possible flow-induced chain orientation, stretching, and disentanglement. In particular, we extend the picture of Pincus blobs and define different levels of stretch that a chain can reach as a function of the extensional rate by invoking a rate-dependent blob picture. While this approach requires introducing an additional parameter to describe the stretch relaxation time, the results are in good agreement with the experimental observations. This alternative but sound approach should contribute to the on-going discussion on the elongation of entangled polymers.