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Block copolymer micelles formed by diblock and triblock copolymers of styrene and methacrylic acid were characterized in solution in a mixed solvent with 80 vol % of dioxane and 20 vol % of water. Methods of static light scattering, quasielastic light scattering, differential refractometry, viscometry, sedimentation velocity, and densitometry were used. No unattached unimer molecules were observed. Three independent methods were employed for obtaining micellar weights. They agreed well with each other. No anomalous behavior was observed by any method. The micellar solutions were shown to contain almost exclusively single micelles; only a few samples (those producing the largest micelles) contained micellar clusters. The micelles behaved hydrodynamically and thermodynamically as impermeable spheres. The structure of the micellar shell was characterized in some detail. Relations between the aggregation number and the hydrodynamic radius of the micelles on the one hand and the sizes of the styrene and methacrylic acid blocks on the other were presented as scaling type phenomenological equations.Physicochemical properties of self-assembling molecules are of ever increasing experimental and theoretical interest. This is not only due to the intriguing phenomenon of selfassembling but also due to important applications of such materials in coatings, adhesives, thin films, microfabrication of electronic devices, pharmaceutical and photographic technologies, oil recovery, etc. Among the most important self-assembling systems are polymeric micelles.

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Micelles formed from amphiphilic block copolymers are known to effect aqueous solubilization of hydrophobic molecules. The kinetics of uptake or release can be monitored by fluorescence if the solubilizate is a fluorophore. The primary objective of this paper is the characterization of the release kinetics in aqueous solution of two hydrophobic fluorescent probes (pyrene and phenanthrene) loaded into polymer micelles composed of the following diblock polymers:  polystyrene-block-poly(methacrylic acid), poly(tert-butyl acrylate)-block-poly(2-vinylpyridine), poly(2-vinylpyridine)-block-poly(ethylene oxide). Polystyrene latex particles were also studied for comparison. The release process was analyzed by a model of diffusion out of a sphere and the diffusion constants we measure are very small (10-18−10-16 cm2/s), depending on the core and probe. An exception is poly(2-vinylpyridine) for which the release was too fast for our measurement technique. Independent measurements of the partition coefficient of the probes between the micelle and water demonstrated that the micelles are very effective at solubilization (partition coefficients from 3 × 104 to 3 × 105 were obtained, depending on the micelle−probe combination). Consideration of the partition coefficient, fluorescence quenching of the solubilized probe by Tl+, and the release kinetics has suggested a “three-region” model for solubilization of hydrophobic molecules in this class of polymer micelles:  (1) The first is the core, which for several of our systems is glassy. Diffusion from a glassy core is very slow. (2) The second is an “inner corona”, composed of the hydrophilic block polymer which may be swollen by water but its ionization, by gain or loss of protons, is suppressed. The important role of the inner corona in solubilization was not appreciated by us in our earlier study of phenanthrene released from two different PS−PMA micelles. In some cases the majority of the solubilized probe appears to be located in this region. (3) Finally, there is an “outer corona” for which the chains are not crowded and which may sustain a significant charge density. Probe molecules solubilized in this region are accessible to the Tl+ quencher.

Polystyrene-block-poly(2-vinylpyridine) (PS-b-PVP) was synthesized by anionic polymerization. Micellization of PS-b-PVP was accomplished by dissolving the polymer in a 45:50:5 volume ratio methanol:dioxane:water mixture followed by a stepwise dialysis into a 0.1 M HCl solution. The hydrophobic PS blocks form the core of the micelles, and the protonated PVPH+ forms the outer shell. Addition of poly(2-vinylpyridine)-block-poly(ethylene oxide) (PVP-b-PEO) and titration to a pH above 10 causes a coprecipitation of the PVP blocks of both copolymers, forming three-layered micelles that we refer to as “onion” micelles. The micellar structures were investigated by static and dynamic light scattering and transmission electron microscopy. The weight ratio of PVP-b-PEO to PS-b-PVP was varied during the onion micelle preparation to study the fraction of PVP-b-PEO that is adsorbed on the PS-b-PVP “core micelle”. The stability with respect to dilution of the micelles formed from PS-b-PVP and of the onion micelles was studied.

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