The tube model has long served as a cornerstone of entangled polymer dynamics and rheology. We test some of the key concepts of the tube model by investigating the equilibrium dynamics of a free chain in a fixed-topology polymer network using molecular dynamics simulation. Our results reveal significant position-dependent dynamical heterogeneity manifested as a rugged free energy landscape for the longitudinal motion, resulting in additional friction due to the topological interactions, and a “tube dilation” mechanism arising from the relaxation of entanglements formed by loop strands in the network in a “flip-flop” motion. On timescales less than the Rouse time, the mean-square displacement of the center of mass of the free chain displays a subdiffusive behavior that is slower than the expected $t^{1/2}$ scaling. On longer timescales, the effects of topological friction amount to a renormalized monomer friction that is several times the intrinsic value. These results challenge some of the fundamental assumptions of the tube model, even under equilibrium conditions.

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• Polymer conformation & topology
• Polymer entanglement
• Rheology
• Polymer theory
• Tube theories