Langmuir, Vol.16, No.12, 5324-5332, 2000
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(CH2CH2O)(25)C12H25 (C12E25) (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 C12E25 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] approximate to 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 (lambda) remaining practically the same (ca. 10 ms). The value of G(0) increases with increasing [NaCl], passing through a maximum at [NaCl] approximate to 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 lambda . 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.
Keywords:WATER-SOLUBLE COPOLYMERS;NONRADIATIVE ENERGY-TRANSFER;VINYLETHER COPOLYMERS;ASSOCIATIVE POLYMERS;AQUEOUS-SOLUTION;AMPHIPHILIC TERPOLYMERS;RESPONSIVE ASSOCIATIONS;POLY(ETHYLENEOXIDE);FLUORESCENCE PROBE;CHAIN-LENGTH