Solution Properties of Micelle Networks Formed by Nonionic Surfactant Moieties Covalently Bound to a Polyelectrolyte: Salt Effects on Rheological Behavior

Rheological properties of aqueous solutions of micelle networks formed by a copolymer of sodium 2-(acrylamido)-2-methylpropanesulfonate (AMPS) and an associative macromonomer, methacrylate substituted with HO(CH 2 CH 2 O) 25 C 12 H 25 (C 12 E 25 ) (20 mol % content in the copolymer), were investigated as a function of the polymer concentration ( C p ), added salt concentration ([NaCl]), and shear stress. The polymer-bound C 12 E 25 surfactant moieties form micelles via intra- and interpolymer association. These polymer-bound micelles are bridged by polymer chains and hence form a network structure. The solution viscosity of this micelle network increases markedly with increasing C p near 10 g/L (at [NaCl] = 0.10 M) and beyond this C p the number of micelle bridges greatly increases with increasing C p . The extent of the micelle bridging depends on [NaCl]. When C p is sufficiently high, where interpolymer associations are favorable, steady-state viscosity measured at a low-shear rate increases with increasing [NaCl], exhibiting a maximum value at [NaCl] ≈ 0.13 M (at C p = 25.0 g/L), and then decreases as [NaCl] is further increased. The micelle network solutions exhibit shear-dependent viscosity behavior, i.e., Newtonian behavior at low shear rates (<30 s - 1 ), shear thickening at intermediate shear rates (30−200 s - 1 ), and shear thinning at higher shear rates (>200 s - 1 ). Furthermore, the solutions behave as a viscoelastic fluid at low polymer concentrations ( C p ≤ 25.0 g/L) but they show significant elastic properties with increasing C p, both storage and loss moduli increasing greatly with increasing C p and the two moduli becoming close to each other. When C p is increased to ca. 100 g/L or higher, the micelle network system exhibits gel-like behavior with a plateau modulus ( G 0 ) increasing markedly with increasing C p whereas a terminal relaxation time ( λ ) remaining practically the same (ca. 10 ms). The value of G 0 increases with increasing [NaCl], passing through a maximum at [NaCl] ≈ 0.13 M (at C p = 25 g/L), and then decreases with further increasing [NaCl]. Thus, the viscosity is virtually governed by G 0 but not by λ . The micelle bridge is dynamic in nature, which makes the disruption and re-formation of the micelle bridge occur reversibly in response to external stimuli.