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Tube theory predicts the same behaviour for both, and why they differ is disputed.","posed_since":"2003","precise":"In uniaxial extension of monodisperse entangled polystyrene at strain rates above the inverse Rouse time, melts show steady extensional viscosity $\\eta_E \\sim \\mathrm{eps}_{dot}^{-1/2}$ while semidilute and concentrated solutions with matched entanglement number show extensional hardening, in conflict with tube models including chain stretch. Identify the mechanism (proposed: flow-induced reduction of monomeric friction by segment orientation) and predict $\\eta_E(\\mathrm{eps}_{dot})$ for melts and solutions from one theory.","problem_ref":"5561b3e75091c150147b66731e8e6b93e34a957b19e63321aebc2238198dfe26","references":"","settled_by":"A constitutive model with independently measured parameters that reproduces $\\eta_{E}(eps_{dot})$ for melts and solutions, supported by rheo-optical or simulation measurements of the friction coefficient under flow.","status_note":"Steady thinning of PS melts in fast extension was reported in 2003 (Bach, Almdal, Rasmussen and Hassager, Macromolecules, https://doi.org/10.1021/ma034279q); the friction-reduction explanation is not universally accepted.","title":"Why entangled melts and solutions differ in fast extensional 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