We describe detailed investigations on the self-assembly of polybutadiene-block-poly(2-vinylpyridine) diblock copolymers into so-called crew-cut micellar aggregates in aqueous media. Three analogous diblock copolymers with different short segments of poly (2-vinylpyridine) were synthesized via anionic polymerization. The self-assembled aggregates are studied in dioxane/water mixtures by means of dynamic light scattering! and transmission electron microscopy. Depending on the added volume fraction of water, spherical micelles. branched cylindrical micelles, and vesicles can be found. These aggregates can be cross-linked in a facile fashion using a UV photoinitiator. The structures of the crew-cut aggregates remain intact during cross-linking, yielding stabilized polymeric nanoparticles. A transfer of these nanoparticles into common, only slightly selective solvents, in which no aggregates or different type of aggregates would exist without cross-linking, is possible. The cross-linked nanoparticle structures remain unchanged during this process. Moreover, a new pathway for the phase transition of cylindrical micelles to vesicles was found and is elucidated.