Micellization of amphiphilic diblock copolymers: Corona shape and mean-field to scaling crossover

The aggregation behaviour of six amphiphilic: poly(ethylene-propylene)-bloc k-polyethylene oxide (PEP-PEO) copolymers in water was investigated with th e aim to study systematically the influence of a growing soluble block on t he structural properties of the micelle. The polymers were prepared such th at they differ in the PEO content over a large range of molecular weights, while the PEP size was kept constant. The micelles were characterized by sm all-angle neutron scattering (SANS) involving contrast variation experiment s. Data analysis nas performed by a spherical core/shell model with either constant or radial decreasing density profile in the corona and constant de nsity in the core for both cases. The main new result is that we have obser ved a coronal shape crossover from a micelle with homogeneous density distr ibution for polymers of low PEO content to a starlike micelle with a hyperb olic density profile for polymers of high PEO content. With respect to the aggregation number this corresponds to a crossover from the mean-field to t he scaling behavior of a starlike micelle.