CHARACTERIZATION OF THE CORE OF POLYSTYRENE BLOCK POLY(METHYL METHACRYLATE) POLYMER MICELLES BY ENERGY-TRANSFER

Nonradiative direct energy transfer (DET) experiments have been employed to characterize the core of block copolymer micelles. Polystyrene-block-poly(methyl methacrylate) polymers (PS-PMMA) with a chromophore at the junction have been prepared by multistep living anionic polymerization. The chromophore at the junction is either a donor, phenanthrene (Phe) for polymer 1 (M(n)(PS) = 11K, M(n)(PMMA) = 25K), or an acceptor, anthracene (An) for polymer 2 (M(n)(PS) = 11K, M(n)(PMMA) = 26K). In 30/70 (w/w) dioxane-methanol mixtures, these polymers spontaneously form monodisperse block copolymer micelles. By quasi-elastic light scattering (QELS) and viscosity measurements we find that the hydrodynamic radius R(H) = 19 nm and that these micelles are made of 140 polymer units. In these micelles, donors and acceptors are embedded in the core/shell interface. Two approaches are considered to study the interface where DET occurs. In the first, the interface is taken as the flat surface of a perfect sphere. In the second, the interface is assumed to be a fractal medium. These two analyses of the fluorescence decays (combined with computer simulations of the DET kinetics) give evidence that the core is swollen by the solvent and that the core/shell interface is diffuse. An apparent fractal dimension of 2.3 is recovered.