We use molecular simulations to probe the local viscoelasticity of an entangled polymer melt by tracking the motion of embedded nonsticky nanoparticles (NPs). As in conventional microrheology, the generalized Stokes-Einstein relation is employed to extract an effective stress relaxation function $G_{\mathrm{GSE}}(t)$ from the mean square displacement of NPs. $G_{\mathrm{GSE}}(t)$ for different NP diameters $d$ are compared with the stress relaxation function $G(t)$ of a pure polymer melt. The deviation of $G_{\mathrm{GSE}}(t)$ from $G(t)$ reflects the incomplete coupling between NPs and the dynamic modes of the melt. For linear polymers, a plateau in $G_{\mathrm{GSE}}(t)$ emerges as $d$ exceeds the entanglement mesh size $a$ and approaches the entanglement plateau in $G(t)$ for a pure melt with increasing $d$. For ring polymers, as $d$ increases towards the spanning size $R$ of ring polymers, $G_{\mathrm{GSE}}(t)$ approaches $G(t)$ of the ring melt with no entanglement plateau.

• Received 27 June 2017
• Revised 17 October 2017

DOI:https://doi.org/10.1103/PhysRevLett.120.057801