Consistent modeling of nonlinear shear and elongational start-up data of entangled polystyrene solutions

Abstract Nonlinear shear and elongational start-up data of three entangled PS solutions consisting of the same weight fraction of a linear long-chain polystyrene PS-600 k and three different styrene oligomeric solvents were recently reported by Cui et al. (Rheol Acta 64:97-105, 2025). The solvents are two linear styrene oligomers of different molecular weights as well as a star styrene oligomer. We show that start-up of shear viscosity and apparent normal stress difference as well as start-up of elongational viscosity can consistently be described by the Rotation Zero Stretch (RZS) model (Rheol Acta 63:573, 2024; Phys Fluids 36:093124, 2024), which is based on the tube model and a flow-strength sensitive evolution equation of stretch. In extensional flows, the RZS model reduces to the Enhanced Relaxation of Stretch (ERS) model (J Rheol. 65:1413, 2021). The modeling is based exclusively on the linear-viscoelastic characterization of the solutions and a consistent set of Rouse stretch relaxation times for PS-600 k. Graphical Abstract