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

Abstract

The linear and nonlinear shear rheology of entangled polystyrene (PS) solutions diluted by styrene oligomers with various lengths was compared with the shear rheology of a pure melt having the same number of entanglements (Z) during startup shear and step-shear strain experiments using a cone partitioned-plate geometry. By fixing the same Z, the shear rheology of the PS solutions and the melt shows some universal features in the linear and nonlinear regimes. Undershoot of the shear stress growth coefficient is observed during the startup flow of the PS solutions and depends strongly on the length of the oligomers. The Rotation Zero Stretch model captures the stress overshoot and the steady shear viscosity quantitatively, except at the high shear rates when undershoot is observed. Stress relaxation after step-shear strain experiments reveals that the PS solutions show a transition from type A damping (close to the Doi–Edwards prediction) to type B (weaker than the Doi–Edwards prediction), while the pure melt having the same Z shows a type A response, which suggests that the length of the oligomers influences the nonlinear damping response. The nonuniversality of the nonlinear damping response of the solutions and the melt is possibly due to the changes in flow-induced friction reduction during step-shear strain deformation.