(237h) Slip-Link Simulations of Entangled Polymers in Planar Extensional and Planar Mixed Flows with Comparison to Experiments

The non-equilibrium dynamics of entangled polymers still remains a challenging problem. It is now well understood that the nonequilibrium dynamics of topological constraints or entanglements in an entangled system is the key element in understanding the rheology and flow dynamics of these complex fluids. The modeling of entangled polymers for ?fast flows?, in particular, is complicated due to the interplay between many dynamic chain mechanisms including reptation, tube-length fluctuations, constraint renewal/release, and chain stretch. Recent experimental data of the extensional viscosity of monodisperse polystyrene melts (Bach et al. Macromolecules, 2003, Luap et al. Rheo. Acta 2005) has revealed a failure in the available models for entangled polymers. The experimental data shows an exponent of -0.5 in the extensional thinning region, in contrast to the predictions of DE/DEMG models which predict a thinning exponent of -1. In the present work, we have extended a slip-link based model which was originally proposed by Masubuchi et. al. (J. Chem. Phys. 2001) to simulate planar extensional flow using Kraynik-Reinelt boundary conditions. The original model incorporates the ?individual? mechanisms of reptation and tube length fluctuation as well as the collective contributions arising from the 3D network structure of the entangled chains including constraint release. Based on our extended model, we present a physical explanation for the failure of DE/DEMG models in predicting the correct thinning exponent. We explain the dynamics behind extensional thinning based on the orientation and stretch distributions of chains at steady state and propose that chain orientation plays an important role in thinning dynamics. We also present a detailed comparison of our planar extensional viscosity simulations to the experimental data of Sridhar et. al. Finally, we extend the model to model planar mixed flows using modified Kraynik-Reinelt boundary conditions. We compute the birefringence of entangled system in mixed flows to understand the effect of flow type on the chain orientation characteristics as well as to compare with available experiments. Finally, we discuss certain properties of the mesoscopic slip-link model and their direct effect on our rheological predictions including the presumed constant constraint renewal/release frequency and the decrease in number of entanglements for fast flows.