Release Kinetics Studies of Aromatic Molecules into Water from Block Polymer Micelles

Abstract

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.