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Title: Why entangled melts and solutions differ in fast extensional flow
Plain: 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: 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.
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.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: contested
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.
Posed since: 2003