Elucidating the Molecular Rheology of Entangled Polymeric Fluids via Comparison of Atomistic Simulations and Model Predictions

The startup and steady shear flow properties of an entangled, monodisperse linear polyethylene liquid (C1000H2002) were investigated via virtual experimentation using nonequilibrium molecular dynamics. The simulation results for transient and steady-state rheological functions were directly compared with several variations of the tube model. These comparisons demonstrated that in the linear viscoelastic regime where the shear rate was lower than the reciprocal disengagement time of the liquid (i.e., (gamma) over dot < tau(-1)(d)) all the models examined were reasonably capable of predicting shear flow behavior of both shear and normal stresses under both steady-state and startup conditions. Within the shear rate range tau(-1)(d)< (gamma) over dot < tau(-1)(e) (where tau(e) is the entanglement time of the liquid), all models performed poorly under both steady-state and startup conditions due to inaccurate evolution equations for the tube orientation tensor and the tube stretch. The neglect of entanglement dynamics leading to individual molecular tumbling dynamics in the models appeared to be the primary source of error. At high shear rates ((gamma) over dot> tau(-1)(e)), the entanglement dynamics influenced the stress significantly, and all model predictions for shear and normal stresses diverged from the simulated values. A simple modification to the stress expression was proposed to incorporate the entanglement dynamics, which resulted in good quantitative agreement of predicted steady-state and startup shear and normal stresses of NEMD-guided model predictions with the simulated data. Additional results demonstrated that the overshoot and undershoot in the shear viscosity under startup of shear flow were correlated to tube orientation and chain tumbling dynamics, respectively. The dynamics of the tube stretch variable and normal stress differences were directly correlated to the behavior of the energetic potential between atomic interaction sites, which exhibited clear signs of tumbling dynamics of individual molecules for (gamma) over dot well below tau(-1)(R).