The classical rheological theories of entangled polymeric liquids are built upon two pillars: Gaussian statistics of entanglement strands and the assumption that the stress arises exclusively from the change of intramolecular configuration entropy. We show that these two hypotheses are not supported by molecular dynamics simulations of polymer melts. Specifically, the segment distribution functions at the entanglement length scale and below deviate considerably from the theoretical predictions, in both the equilibrium and deformed states. Further conformational analysis reveals that the intrachain entropic stress at the entanglement length scale is substantially smaller than the total stress, indicative of a considerable contribution from interchain entropy. Lastly, the relation between entanglement strand entropic stress and macroscopic stress exhibits a bifurcation behavior during deformation and stress relaxation, which cannot be accounted for by the classical theories.
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