Phase behavior and viscoelasticity of AOT microemulsions containing triblock copolymers

Mixtures of AOT (sodium bis(2-ethylhexyl) sulfosuccinate)/water/decane microemulsions with polyethylene oxide/polyisoprene/polyethylene oxide (PEO/PI/PEO) triblock copolymers form highly associated solutions with unusual phase behavior and concentration dependence of the viscoelastic moduli. A gas-liquid phase transition, reported here for the first time in these mixtures, is attributed to the entropic gain accompanying the conversion of loops to bridges, in accord with theories for polymer brushes. The homogeneous condensed phase is highly viscous and elastic. The volume fraction dependence of the high-frequency modulus conforms to expectations from theories for either reversible networks or solutions of flowerlike micelles. The apparent low shear, or plateau, viscosity, on the other hand, exhibits a maximum at a volume fraction roughly twice that at the phase boundary. This unusual behavior is predicted qualitatively for solutions of flowerlike micelles, but the quantitative aspects of the response are more difficult to rationalize.