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; M w = 4.6 × 10 4 ). 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 γ̇just 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 η 0 . However, at those γ, the first normal stress coefficient growth function Ψ 1 + (t;γ) and its steady-state value Ψ 1 (γ) remained very close to the linear Ψ 1 + (t) and Ψ 1,0 and exhibited no nonlinearity. In addition, the relaxation times of the viscosity and normal stress coefficient decay functions η − (t;γ) and Ψ 1 − (t;γ) 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 γ̇just 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;γ) and η(γ) and the lack of nonlinearity for Ψ 1 + (t;γ) and Ψ 1 (γ) 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;γ) exhibited overshoot above the linear η + (t) and then approached η(γ) < η 0 , whereas Ψ 1 + (t;γ) stayed below the linear Ψ 1 + (t) and approached Ψ 1 (γ) ≪ Ψ 1,0 after exhibiting a peak. The relaxation of η − (t;γ) and Ψ 1 − (t;γ) 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).

Concentration dependence of rheological properties of telechelic associative polymer solutions

“…In this appendix, we briefly show the anisotropic bridge formation model proposed in Ref [18]. We also show that the sparse network model formulated in this work reduces to the anisotropic bridge formation model under some conditions.…”

“…(The details are shown in Appendix A.) If we compare dynamic equations (31), (32) and (15) with the anisotropic bridge formation model [18], we find that Φ(r, t)P (0, t) works as the source function for a newly constructed bridge. The source function in the anisotropic bridge formation model was introduced rather phenomenologically and its molecular meaning was left rather arbitrary.…”

“…We expand the function Φ [m] ss (r) into a Hermite polynomial series, following the previous work [18]. …”

Shear-Induced Heterogeneity in Associating Polymer Gels: Role of Network Structure and Dilatancy

“…Near the overlap concentration, sticker clusters can be bridged by polymer strands and form an interconnected volume spanning networka physical gel [1][2][3]. Such gels are found in both natural and synthetic systems, and display a striking array of rheological behavior, including strain stiffening [4], negative normal stresses [5], shear thickening [6,7], shear thinning [8], and shear banding [9][10][11][12][13][14][15].Despite the ubiquity and versatility of physical gels, a fundamental understanding of the interplay between their microstructure, dynamics, and rheological properties remains a challenging and open problem. For instance, while experiments and simulations of associative networks (including both AP [13][14][15] and colloidal [16] gels) under simple shear have observed spatial inhomogeneities in both shear rate and density, suggesting some form of sheargradient concentration coupling (SCC) [17][18][19][20], the microscopic mechanism for the instability is unclear.…”

“…Near the overlap concentration, sticker clusters can be bridged by polymer strands and form an interconnected volume spanning networka physical gel [1][2][3]. Such gels are found in both natural and synthetic systems, and display a striking array of rheological behavior, including strain stiffening [4], negative normal stresses [5], shear thickening [6,7], shear thinning [8], and shear banding [9][10][11][12][13][14][15].…”