Rheology of Entangled Polymeric Liquids through Simulations of the Primitive Chain Network Model with Finite Extensibility

摘要

We here report on some modifications of the Primitive Chain Network (PCN) model, originally proposed in [Y. Masubuchi et al., J. Chem. Phys. 115, 4387 (2001)], which both refine the model and make it suitable for predicting nonlinear rheological response in fast flows. Simulation results are compared with some existing viscoelastic data on monodisperse polystyrene melts [Schweizer et al., J. Rheol. 48, 1345 (2004); Bach et al., Macromolecules 36, 5174 (2003)], by using as single adjustable parameter (to be assigned once and for all) a basic relaxation time which relates to the coarse graining of the PCN model. Essentially quantitative prediction of linear viscoelasticity is achieved. Without further parameters, the nonlinear behavior of the polystyrene melts in start-up of shear and uniaxial elongational flow is also reproduced by the simulations. Specifically, the viscosity growth functions are quantitatively predicted, whereas transient elongational viscosities are reasonably reproduced only up to extensional rates of the order of the reciprocal Rouse time. At higher elongation rates, although the strain hardening effect is correctly captured by the simulations, steady state values are not. Notwithstanding this latter limitation, the simulation model appears adequate to portray the rheological behavior of entangled melts, both in the linear and the nonlinear range.