Structure of amphiphilic block copolymer micelles in aqueous dispersions

Abstract

Two molecular weight series of Poly(butadiene)-Poly(ethylene oxide) diblock copolymers have been synthesised using anionic polymerisation techniques. The amphiphilic nature of the copolymers results in micelles being formed on dispersion in water. Dynamic light scattering was employed to ascertain the critical micelle concentration and micelle dimensions. Small-angle X-ray and neutron scattering were used to investigate high concentration dispersions providing micelle dimensions and an insight into the nature of the interactions between micelles from the structure factor, which develops at higher concentrations. The detailed model used polymer brush theory to fit the small-angle scattering data at low concentrations in the absence of interparticle interactions. Micelle dimensions determined by model fitting matched well with those predicted from theory. At higher concentrations when these interactions are dominant, a Yukawa potential between micelles was used to model the observed structure factor. The unsaturation of the poly(butadiene) chains comprising the core of the micelle facilitated post-polymerisation cross-linking of the core using a redox- initiated free-radical polymerisation at room temperature. Dynamic light scattering was employed to determine the micelle dimensions, with small angle X-ray and neutron scattering used to investigate higher concentration dispersions. The micelle cores were seen to contract by circa 10-40% upon cross-linking in relation to the virgin micelles, resulting in the junction points of the coronal chains on the surface of the micelle core coming closer together. Interestingly the thickness of the corona decreased in relation to the virgin micelles, a phenomenon due to the presence of inorganic ions from the cross-linking reaction reducing the thermodynamic quality of the solvent for the poly(ethylene oxide) brush, causing it to partially collapse.