The detailed analysis of the time resolved luminescence of a specifically designed fluorescent molecular rotor enables the direct monitoring of the self-assembly of a poly(N,N-dimethylacrylamide)-block-polystyrene (PDMA-b-PS) copolymer into core-corona nanoparticles. Comparison with bulk PS indicates hard confinement of the micelle core, due to the solvation of the hydrophilic PDMA corona.