Micelle Formation of Random Copolymers of Sodium 2-(Acrylamido)-2-methylpropanesulfonate and a Nonionic Surfactant Macromonomer in Water As Studied by Fluorescence and Dynamic Light Scattering

Abstract

A polyelectrolyte-bound nonionic surfactant (HO(CH2CH2O)25C12H25) (C12E25) system was prepared by copolymerization of sodium 2-(acrylamido)-2-methylpropanesulfonate (AMPS) and a methacrylate substituted with HO(CH2CH2O)25C12H25 in N,N-dimethylformamide initiated by 2,2‘-azobis(isobutyronitrile). The contents of the surfactant macromonomer unit (fDE25) in the copolymers are in the range 10−30 mol %. Dry samples of these copolymers are soluble in water, polymer-bound surfactants undergoing micellization. The micelle formation was studied by fluorescence and quasielastic light scattering (QELS) techniques in 0.1 M NaCl aqueous solutions. Steady-state fluorescence data for pyrene probes solubilized in water in the presence of the copolymers suggest that the polymer-bound surfactants associate in both intra- and interpolymer fashions to form micelles. The interpolymer hydrophobic associations were found to commence to occur at a relatively well-defined polymer concentration (Cp), which can be regarded as an apparent “cmc” for interpolymer micelle formation. Such cmc values are 10−100 times smaller than that for discrete C12E25 molecules, the values increasing with increasing fDE25. Mean aggregation numbers (Nagg) of the terminal dodecyl groups at an end of the (CH2CH2O)25 spacer group were estimated by a time-dependent fluorescence method. Nagg values were found to be of the same order of that for the discrete C12E25 micelles although the values for the polymer-bound surfactant micelles are somewhat larger than that for the free micelle. The Nagg values were relatively constant over a significant range of Cp. QELS data indicated bimodal distributions of relaxation times with a fast and a slow relaxation mode. The slow mode component is due to polymer aggregates that include a number of micelle units formed by polymer-bound C12E25 groups. On the other hand, the fast mode component may be attributed to either a “unimeric” micelle (a micelle formed by a single polymer chain) or an “oligomeric” micelle (a micelle formed by a small number of polymer chains). Hydrodynamic radii (Rh) for the slow mode component are on the order of 60−170 nm exhibiting a tendency to increase with increasing Cp whereas those for the fast mode are 6−8 nm independent of Cp. Rh for discrete C12E25 micelle was found to be ca. 5 nm under the same conditions. On the basis of the characterization, a hypothetical micelle model was proposed where micelle units formed from polymer-bound C12E25 moieties are bridged by polymer chains yielding a micelle network structure.