  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.
