Electrostatic self-assembly of oppositely charged copolymers and surfactants:: A light, neutron, and X-ray scattering study

We report on the formation of colloidal complexes resulting from the electrostatic self-assembly of polyelectrolyte-neutral diblock copolymers and oppositely charged surfactant. The copolymers investigated are asymmetric and characterized by a large neutral block. Using light, neutron, and X-ray scattering experiments, we have shown that the colloidal complexes exhibit a core-shell microstructure. The core is described as a dense microphase of micelles connected by the polyelectrolyte blocks, whereas the shell is a diffuse brush made from the neutral chains. For all copolymer/surfactant systems, we show the existence of a critical charge ratio Z(C) (similar to1) above which the formation of hierarchical structures takes place. Copolymers of different molecular weight and polyelectrolyte blocks have been studied in order to assess the analogy with another type of core-shell aggregates, the polymeric micelles made from amphiphilic copolymers. The present results indicate that the radius of the core depends essentially on the degree of polymerization of the polyelectrolyte block and not on that of the neutral chain. On the other hand, the size of the overall colloid increases with increasing molecular weights of the copolymers. Taking advantage of the resolution of X-ray scattering, we have also shown that the micelles in the core of the aggregates are structurally disordered.