Stress Relaxation in Monodisperse Entangled Polymer Melts: Correlation Between Viscoelastic Response and Single-Chain Relaxation via Molecular Dynamics Simulations

Abstract

We study stress relaxation in several types of entangled monodisperse linear polymer melts by comparing the shear stress relaxation modulus, $G(t)$, with the end-to-end vector autocorrelation function, $P(t)$. The study includes three Kremer-Grest bead-spring models with varying chain stiffness, as well as a chemistry-specific coarse-grained model of \emph{cis}-1,4-polybutadiene. For each model, multiple chain lengths were simulated, spanning a range of $N/N_e = 5$-$50$ entanglements per chain. We observe that in all cases the behavior of $G(t)$, beyond the short-time Rouse regime, is accurately described by $G^0_{\mathrm{N}}[P(t)]^2$, where the chain-length-independent prefactor $G^0_{\mathrm{N}}$ denotes the plateau modulus. This correlation is consistent with both double reptation and dynamic tube dilation models of polymer relaxation, although the two models are based on different physical pictures. The double reptation model represents the melt as a transient network in which stress relaxation is governed by the survival probability of pairwise entanglements. The dynamic tube dilation model, however, assumes that the tube of constraints surrounding a polymer chain progressively enlarges as relaxation proceeds. The relation $G(t) = G^0_\mathrm{N}[P(t)]^2$ can serve as a basis for determining the plateau modulus and the corresponding entanglement length. It also simplifies the modeling of $G(t)$, since an accurate analytical expression for $P(t)$ is sufficient to describe the long-time behavior of $G(t)$. We further compare the simulation data for $P(t)$ and $G(t)$ with theoretical predictions.