Universality and nonuniversality in nonlinear shear rheology of entangled polystyrene solutions and melts with the same number of entanglements.

This article investigates the influence of oligomeric styrene solvents of varying chain lengths on the linear and nonlinear shear rheology of entangled polystyrene (PS) solutions with a fixed number of entanglements per chain (Z), comparing them to pure PS melts having the same Z. The study finds universality in linear viscoelastic behavior and steady-state shear thinning across solutions and melts, while nonlinear features such as stress undershoot during startup shear and nonlinear damping after step-shear strain differ depending on oligomer size. The Rotation Zero Stretch (RZS) model quantitatively captures stress overshoot and steady shear viscosity but does not predict the undershoot observed at high shear rates, which is stronger in solutions with shorter oligomers and may relate to flow-induced chain tumbling. Additionally, stress relaxation experiments reveal a transition from type A damping (close to Doi–Edwards theory) in melts to weaker type B damping in solutions, attributed to differences in alignment-induced friction reduction influenced by oligomer length and possibly flow-induced heterogeneous morphology.