Influence of external factors on the micellization process and aggregate structure of poly(oxy)styrene-poly(oxy)ethylene block copolymers

Solutions of the polyoxystyrene-polyoxyethylene block copolymer SO17EO65, where SO denotes polystyrene oxide block as the hydrophobic block and EO the polyethylene oxide block as the hydrophilic block, in mixtures of water (a selective solvent for the EO blocks) and 1,4-dioxane (a good solvent for both blocks) were studied by surface tension and light scattering measurements. Surface and micellar structural parameters have been analyzed as a function of solvent composition. The critical micelle concentration increases and the micellar aggregation number decreases, respectively, as the amount of 1,4-dioxane in the binary solvent increases as a consequence of the enhanced solubility of the SO blocks in the solvent mixture, causing the lowering of the interfacial tension between the hydrophobic blocks in the micellar core and the solvent; therefore, to achieve thermodynamic equilibrium, the micelle size decreases. In addition, static light scattering (SLS) has been proved to be a useful technique to detect the lower boundary of the transition between a dilute micellar solution (sol) to a local-ordered micellar solution (soft gel) resulting from a percolation mechanism. Comparison of the sol-soft gel boundaries obtained from SLS for copolymers SO17EO65 and EO67SO15EO67 with those previously derived by rheology is made. Finally, changes in the autocorrelation function of the solutions at the boundary obtained from dynamic light scattering are also analyzed.