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

A multiscale modeling\nframework is adapted for the <i>in silico</i> prediction\nof nonlinear shear rheology of entangled polymer melts.\nThis approach begins with an all-atom model and progresses to two\ncoarser-grained models, the coarse-grained and slip-spring models,\nthrough a bottom-up parametrization process informed by the structure\nand dynamics of finer-grained models. The steady shear viscosity of\nentangled polymer melts is calculated by performing simple shear simulations\nwith both the coarse-grained and slip-spring models. Using atactic\npolystyrene melts as a target, the steady shear viscosities are calculated\nunder a wide range of shear rates and are found to be in quantitative\nagreement with experimental measurements, serving to provide an example\nof the prediction of nonlinear rheology of an entangled material starting\nfrom a purely atomistic model without freely adjustable parameters.\nAt high shear rates, when the Weissenberg number is above unity, the\nsteady shear viscosity exhibits a power law behavior with ∼\nγ̇<sup>–0.7</sup>, as captured by the coarse-grained\nmodel of slightly entangled melts. As the shear rate decreases, the\ntransition from the power-law regime to the shear-rate-independent\nplateau regime depends on the molecular weight and is reproduced by\nthe slip-spring model. Integrating the coarse-grained and slip-spring\nmodels permits the prediction of the steady shear viscosity of entangled\npolystyrene melts over 6 decades of time.