FLUOROMETRIC AND QUASI-ELASTIC LIGHT-SCATTERING STUDY OF THE SOLUBILIZATION OF NONPOLAR LOW-MOLAR MASS COMPOUNDS INTO WATER-SOLUBLE BLOCK-COPOLYMER MICELLES

The diblock copolymer polystyrene-block-poly(tert-butylmethacrylate) was prepared by anionic polymerization, and a portion was hydrolyzed to prepare polystyrene-block-poly(methacrylic acid). Some of the copolymers were end-tagged with either less than one on average or a chain of four 2-vinylnaphthalene moieties at the end of the polystyrene block, while others were tagged with exactly one dansyl group via the SO2 linkage at the end of the poly(methacrylic acid) block. Micelles were initially prepared in mixtures of H2O/1,4-dioxane (80 vol %) and then dialyzed into water to remove the dioxane. Steady-state and time-dependent fluorescence spectroscopy along with static and quasi-elastic light scattering (QELS) were used to study the solubilization of organic molecules into the micelles for different pHs, ionic strengths, and copolymer concentrations. It was found that a weight ratio of up to 5:1 solubilized benzene to polystyrene can be taken up by these micelles. Light scattering and fluorimetric measurements indicate that solubilization occurs simultaneously into micellar cores and into hydrophobic domains in the shells. Moreover, it is likely that the swelling of the micellar core and the increase in the core volume are partially compensated for by the decrease in shell thickness possibly due to an increased amount of hypercoiled-hydrophobic structure. Probe mobility was monitored by following excimer formation and the time-dependent fluorescence anisotropy of naphthalene in the cores and dansyl in the shells. The fluorescence properties proved to be sensitive indicators of core swelling during solubilization. It was also found that dansyl labels experience a much more complex and less polar microenvironment than expected for an aqueous medium. QELS and viscosity measurements indicate that, at low ionic strength and at a relatively low degree of neutralization of the carboxylic acid groups, the electrostatic repulsion of highly charged spherical micelles results in the formation of long-range order in micellar solutions.