Sustained release properties of arborescent polystyrene-graft-poly(2-vinylpyridine) copolymers

The release kinetics of small molecules from dendritic graft copolymer micelles incorporating an arborescent polystyrene (PS) core and a poly(2-vinylpyridine) (P2VP) shell were investigated in dilute HCl solutions by fluorescence and UV spectroscopies. The redistribution of pyrene and perylene among arborescent micelles was studied by the fluorescence resonance energy transfer (FRET) technique, and was characterized by an initial burst in exchange followed by gradual equilibration of the probes. Fluorescence quenching experiments demonstrated that the diffusion coefficient of pyrene increased for copolymer micelles of higher generations, suggesting a more porous shell structure for the higher generation arborescent PS-g-P2VP copolymers. In vitro release tests for indomethacin and lidocaine monitored by UV spectroscopy showed that sustained release characteristics were achieved, the release rate being higher for lidocaine due to its higher water solubility at low pH. The release rate of indomethacin increased for lower generation micelles and for higher micelle loadings, in agreement with a diffusion-controlled release mechanism. An increasing fraction of the indomethacin molecules loaded in the micelles remained trapped for higher generation copolymers. The diffusion coefficient and the release rate of indomethacin were calculated by fitting the solution of Fick's second law of diffusion to the experimental data. While the initial release rate decreased for higher generations, the trends observed for the diffusion coefficients were similar to those determined for pyrene in the fluorescence quenching experiments. This result is again consistent with a more diffuse shell structure for higher generation micelles, possibly due to the enhanced electrostatic repulsions between the charged P2VP chains. (C) 2008 Elsevier Ltd. All rights reserved.