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

Flow behavior was examined for a 1.0 wt % aqueous solution of hydrophobically modified ethoxylated urethane (HEUR; M w = 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 γ̇ just above the equilibrium relaxation frequency 1/τ, the solution exhibited thickening characterized by monotonic increase of the viscosity growth function η+(t;γ̇) with time t above the linear η+(t) and by the steady-state viscosity η­(γ̇) larger than the zero-shear viscosity η0. However, at those γ̇, the first normal stress coefficient growth function Ψ1 +(t;γ̇) and its steady-state value Ψ1(γ̇) 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;γ̇) and Ψ1 –(t;γ̇) measured after cessation of steady flow agreed with those in the linear regime. All these results suggested that the network strands were just moderately stretched to show no significant finite extensible nonlinear elasticity (FENE) effect and that the number density ν of the network strands was negligibly affected by the shear at γ̇ just above 1/τ. A simple transient Gaussian network model incorporating neither the FENE effect nor the increase of ν suggested that the thickening of η+(t;γ̇) and η­(γ̇) and the lack of nonlinearity for Ψ1 +(t;γ̇) and Ψ1(γ̇) could result from reassociation of the HEUR strands being in balance with the dissociation but anisotropically enhanced in the shear gradient direction. In contrast, at γ̇ ≫ 1/τ, η+(t;γ̇) exhibited overshoot above the linear η+(t) and then approached η­(γ̇) < η0, whereas Ψ1 +(t;γ̇) stayed below the linear Ψ1 +(t) and approached Ψ1(γ̇) ≪ Ψ1,0 after exhibiting a peak. The relaxation of η–(t;γ̇) and Ψ1 –(t;γ̇) after cessation of flow was considerably faster than that in the linear regime. These nonlinear thinning features at γ̇ ≫ 1/τ were attributable to the flow-induced disruption of the HEUR network (and decrease of ν).