Charged Diblock Copolymers at Interfaces: Micelle Dissociation Upon Compression

We use grazing incidence X-ray scattering to study the surface micellization of charged amphiphilic diblock copolymers poly(styrene-block-acrylic acid) at the air-water interface. Scattering interference peaks are consistent with the formation of hexagonally packed micelles. The remarkable increase of inter-micelle distance upon compression is explained by a dissociation of micelles into a brush. Hence, surface micelles reorganize, whereas micelles of the same copolymers in solutions are 'frozen'. We show indeed that the energetic cost of unimer extraction from micelles is much lower for surface than for solution. Finally, a model combining electrostatic interactions and micelle/brush equilibrium explains surface pressure vs. area without free parameters. keywords - soft matter, liquids and polymers, biological physics, chemical physics and physical chemistry.