Characterization of polyisoprene-b-poly(methyl methacrylate) diblock copolymer micelles in acetonitrile

In acetonitrile, polyisoprene-b-poly(methyl methacrylate) (PI-PMMA) diblock copolymers form starlike micelles with a dense core of the insoluble PI bl ocks and a soft solvent-swollen corona of the soluble PMMA blocks. Static a nd dynamic light scattering experiments in combination with viscosity measu rements show that these micelles behave hydrodynamically as hard spheres. T he block copolymers are labeled at the block junction, with a single fluore scent dye, either a donor chromophore (phenanthrene) or an acceptor chromop hore (anthracene), These dyes are confined to the interface during self-ass embly. Fluorescence energy-transfer experiments on molecularly mixed micell es of donor- and acceptor-labeled copolymers provide a core radius of 7.6 /- 0.8 nm and a number-average aggregation number (N-n(agg)) Of 98 +/- 22 u nder the assumption that the energy transfer takes place on a surface of a sphere. Simulations in terms of a Helfand-Tagami junction distribution prof ile confirm that the core-corona interface of the PI-PMMA micelles is thin (ca. 0.9 nm) and that almost all of the energy transfer occurs within a nar row interfacial region. From the static light scattering measurements of th e mixed micelles a weight-average aggregation number (N-w(agg)) Of 127 +/- 6 is obtained. The ratio N-w(agg)/N-n(agg) = 1.3 agrees with size polydispe rsity of the micelles obtained from the analysis of dynamic light scatterin g data. The experimental corona thicknesses are in good agreement with thos e calculated from expressions describing starlike block copolymer micelles.