Dynamics and branch-point motion in entangled symmetric star polymers examined via slip-spring and molecular dynamics simulations of rheology, diffusion, and scattering functions

This study examines the dynamics of entangled star polymers using the slip-spring (SS) model, comparing its predictions quantitatively with molecular dynamics simulations of the Kremer-Grest (KG) model and qualitatively with experimental data. We demonstrate that branch-point (BP) diffusion in the SS model is only modestly inhibited by increasing the functionality (f) or by suppressing constraint release. This cannot be explained by the naïve picture in which hopping motion is induced by the loss of entanglements on f - 2 arms, suggesting diffusive mechanisms that require escape from entanglements only on fewer arms. After confirming that the SS model qualitatively reproduces experimental results for linear rheology, BP-to-arm-end vector relaxation, and diffusion, we examine the mean-squared displacements (MSDs) and intermediate scattering functions (ISFs) computed by both models. While linear viscoelastic behavior is known to be almost independent of f, the KG model shows clear f-dependence of MSD and ISF. With an appropriately determined number of entanglements, the SS model reproduces the KG results well at long times, the regime that a coarse-grained model is expected to describe. Deviations appear at short times, possibly because, unlike the KG model, the SS model assumes Gaussian statistics at all length scales. To discuss the differences between the SS and KG models, we examined the radial distribution functions between BPs in the KG model, which show that close approach between two BPs becomes increasingly unlikely as f increases, and this can be interpreted as an effective repulsion between BPs. We discussed how this observation may inform possible future extensions of the SS model.