Nonlinear Rheology of Telechelic Associative Polymer Networks: Shear Thickening and Thinning Behavior of Hydrophobically Modified Ethoxylated Urethane (HEUR) in Aqueous Solution

Abstract

Flow behavior was examined for a 1.0 wt % aqueous solution of hydrophobically modified ethoxylated urethane (HEUR; Mw = 4.6 × 104). In the linear viscoelastic regime, the solution exhibited single-Maxwellian behavior attributable to thermal reorganization of the transient network composed of strings of HEUR flower micelles. Under shear flow at intermediate shear rates γ just above the equilibrium relaxation frequency 1/τ, the solution exhibited thickening characterized by monotonic increase of the viscosity growth function η+(t;γ) with time t above the linear η+(t) and by the steady-state viscosity η(γ) larger than the zero-shear viscosity η0. However, at those γ, the first normal stress coefficient growth function Ψ1+(t;γ) and its steady-state value Ψ1(γ) remained very close to the linear Ψ1+(t) and Ψ1,0 and exhibited no nonlinearity. In addition, the relaxation times of the viscosity and normal stress coefficient decay functions η–(t;γ) and Ψ1–(t;γ) measured after cessation of steady flow agreed...