Temperature and molecular weight dependence of the zero shear‐rate viscosity of an entangled polymer melt from simulation and theory

Abstract

In a previous article, we described how the frequencydependent complex shear modulus and the time-dependent shear stress relaxation modulus for a highly entangled polybutadiene (PBD) melt can be obtained from molecular dynamics (MD) simulations of an unentangled PBD melt. In that work, we obtained from simulations of an unentangled melt all properties required for the prediction of the dynamic shear modulus with three reptation theories for the dynamics of entangled melts of linear, monodisperse polymers. More recently, we showed how the high-frequency (glassy) behavior of PBD can be obtained directly from MD simulations. The calculated complex and stress relaxation shear moduli for a PBD melt with a molecular weight of 1.3 z 10 Da at 298 K were found to be in excellent agreement with experimental data. In this work, we investigate the ability of MD simulations of the unentangled melt, in conjunction with reptation theory, to reproduce the molecular weight and temperature dependence of the viscoelastic properties of PBD. Here we concentrate on the low-frequency/long-time dynamics that determine the zero shear-rate viscosity, a property that has been extensively studied for PBD as a function of molecular weight and temperature.