Structure of polystyrene-block-poly(ethylene oxide) diblock copolymer micelles in water

Micellization in water of two homologous series of AB-type diblock copolymers, copolymers of polystyrene (PS) as the A block and poly(ethylene oxide) (PEG) as the B block, were investigated by small-angle X-ray scattering (SAXS) and dynamic light scattering (DLS). The copolymers have molecular weights <(M)over bar (n)> in the range 2000--34800, and have in a given series, the same number of repeating units of the PS block, (N-PS = 10 and 38), and a variable number of repeating units of the PEO block (N-PEO values in the range 23-704). In order to avoid secondary association of micelles, a dialysis technique was used to prepare the micellar systems, in the case of copolymers having high <(M)over bar (n)> values of the PS block. The experimental micelle properties such as the core radius R(c) and the aggregation number N of non-equilibrium structures, so called ''frozen micelles'', obtained by dialysis, were found to be independent of the copolymer characteristics. However, for equilibrium structures, obtained by direct solubilization of the copolymers (N-PS = 10) in water, R(c) and N were found to decrease with increasing N-PEO for the homologous series.