MICELLIZATION IN BLOCK POLYELECTROLYTE SOLUTIONS .3. STATIC LIGHT-SCATTERING CHARACTERIZATION

Block polyelectrolyte micelles formed by poly(styrene-b-sodium acrylate) in aqueous solutions were characterized by static light scattering(SLS). Initially, the solutions contained gel-like particles; the kinetics of disentanglement of these particles were measured from the intensity of the scattered light at different angles as a function of heating time at 100 degrees C. It was found that after ca. 50 h of heating no further changes occurred in the scattered intensity. The effect of different sodium chloride concentrations on the aggregation numbers (N), radii of gyration (R(g)), and second virial coefficients (A(2)) of the resulting micellar solutions was determined for two block copolymers, PS(6)-b-PANa(180) and PS(23)-b-PANa-(300). It was found that N increased as a function of salt concentration at low salt contents, but the values remained constant above ca. 0.10 M NaCl. A range of samples with PS block lengths ranging from 6 to 71 units and PANa block lengths ranging from 44 to 780 units was measured in 2.5 M NaCl. As expected, the length of the insoluble block had a much greater effect on the aggregation numbers than that of the soluble block. The data were examined according to the scaling predictions of the star model and several mean-field models. Comparison with several of the models showed good agreement with experimental values of N, calculated core radius (R(c)), and R(g) as a function of block lengths. The core radii values of the micelles agreed very well with those determined independently for similar samples measured in the solid state by small-angle X-ray scattering(SAXS). From this result, it was concluded that the micelles in 2.5 M NaCl exist singly, i.e., that no supermicellar aggregates are present and that the core is solvent free.