Bottom-up Multiscale Simulation of Nonlinear Rheology for Entangled Polymer Melts: Using Atactic Polystyrene as An Example

A multiscale modeling framework is adapted for the <i>in silico</i> prediction of nonlinear shear rheology of entangled polymer melts. This approach begins with an all-atom model and progresses to two coarser-grained models, the coarse-grained and slip-spring models, through a bottom-up parametrization process informed by the structure and dynamics of finer-grained models. The steady shear viscosity of entangled polymer melts is calculated by performing simple shear simulations with both the coarse-grained and slip-spring models. Using atactic polystyrene melts as a target, the steady shear viscosities are calculated under a wide range of shear rates and are found to be in quantitative agreement with experimental measurements, serving to provide an example of the prediction of nonlinear rheology of an entangled material starting from a purely atomistic model without freely adjustable parameters. At high shear rates, when the Weissenberg number is above unity, the steady shear viscosity exhibits a power law behavior with ∼ γ̇^-0.7, as captured by the coarse-grained model of slightly entangled melts. As the shear rate decreases, the transition from the power-law regime to the shear-rate-independent plateau regime depends on the molecular weight and is reproduced by the slip-spring model. Integrating the coarse-grained and slip-spring models permits the prediction of the steady shear viscosity of entangled polystyrene melts over 6 decades of time.