Why entangled melts and solutions differ in fast extensional flow
In plain words
When an entangled polymer melt is stretched fast, its resistance to stretching drops, while a solution of the same polymer with the same number of entanglements becomes stiffer. Tube theory predicts the same behaviour for both, and why they differ is disputed.
Precise statement
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.
What would settle it
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 in the literature
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.