FLUORESCENCE STUDIES OF AMPHIPHILIC POLY(METHACRYLIC ACID)-BLOCK-POLYSTYRENE-BLOCK-POLY(METHACRYLIC ACID) MICELLES

A-B-A block copolymers (A = poly(methacrylic acid), B = polystyrene) have been prepared by anionic polymerization. These amphiphilic copolymers can form stable micelles in water/1,4-dioxane mixtures as well as in water or an aqueous buffer. These micelles are presumed to have a polystyrene core and poly(methacrylic acid) shell. The ability of the micelles to solubilize and release hydrophobic species was studied by fluorescence methods, primarily using pyrene as a fluorescence probe. The following processes were studied: (1) the effect of pyrene loading on monomer/excimer fluorescence ratio and quenching by Cu2+; (2) the rate of exchange between micelles containing pyrene and other aromatic species by the time dependence of either their monomer/excimer ratio or sensitized fluorescence after mixing micelles; (3) the time dependence of the fluorescence quenching of pyrene following the addition of small molecules (N,N-dimethylaniline, CCl4). The following conclusions were obtained: (1) a significant fraction (ca. 20-30%) of the pyrene molecules were on or near the polystyrene-water interface (this depends on loading); (2) diffusion of the probe out of the micelle is the rate-determining step in the release and exchange of large hydrophobes. This process is very slow in its later phases and probably represents slow diffusion from the core of the polystyrene region of the micelle.