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

Nonradiative direct energy transfer (DET) experiments have been employ ed to characterize the core of block copolymer micelles. Polystyrene-b lock-poly(methyl methacrylate) polymers (PS-PMMA) with a chromophore a t the junction have been prepared by multistep living anionic polymeri zation. The chromophore at the junction is either a donor, phenanthren e (Phe) for polymer 1 (M(n)(PS) = 11K, M(n)(PMMA) = 25K), or an accept or, 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 sc attering (QELS) and viscosity measurements we find that the hydrodynam ic 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 co re/shell interface. Two approaches are considered to study the interfa ce 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 t o 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 interf ace is diffuse. An apparent fractal dimension of 2.3 is recovered.