(422j) Molecualr Origin of Shearbanding in Entangled Polymeric Solutions

In recent years, a number of studies have suggested that the occurrence of shear banding in entangled polymeric solutions is a consequence of the interplay between flow induced concentration fluctuation, and concentration dependence of shear and normal stresses. To that end, theoretical analysis based on the two-fluid approach has used to examine the role of shear-induced macromolecular migration on the formation of banded velocity profiles. Specifically, to delineate the role of shear-induced migration on shear banding, constitutive equations with non-monotonic and monotonic flow curves with the two-fluid approach were employed. These analyses provide evidence that a flow-induced concentration redistribution leads to a shear-banded flow structure with a lower concentration in the high shear rate band. Recent experiments performed using in-situ Rheo-PTV measurements to obtain the velocity and concentration profiles for a PBD-DOP solution with an average entanglement density of thirty-eight, in a Taylor-Couette flow geometry show a lower polymer concentration in the fast band and a higher concentration in the slow band. Therefore, existence of nonuniform polymer concentration is postulated to be a prerequisite for formation banded flow structures in entangled polymeric solutions.

Although these studies provide evidence that shear-induced migration can cause shear banding, few issues require clarification before one can unequivocally claim that shear banding in entangled polymeric solutions is a consequence of shear induced migration of macromolecules. For instance, the time scale for development of concentration fluctuations is extremely long, i.e., O(10⁶) of the disengagement time. Also, the results of aforementioned studies are at odds with high-resolution planar large amplitude oscillatory flow confocal microscopy/rheometry of a polymeric solution composed of -lambda-phage DNA with an average entanglement number of three-hundred In fact, these experiments observe shear-banded velocity profiles at high flow rates at significantly shorter time scales without any evidence of a concentration difference between the high and low shear bands. Hence, to elucidate the molecular origin of shear banding, high-fidelity coarse-grained dissipative particle dynamics (DPD) simulations of entangled polymeric solutions over a wide range of shear rates has been carried out. Our results show a novel coupling between inhomogeneous entanglement density and shear induced concentration to be the molecular origin of banded flow structures in entangled polymeric solutions.