Relationship between the microstructure and rheology of micellar solutions formed by a triblock copolymer surfactant

We study the relationship between microstructure and rheology of spherical micelles formed by a triblock copolymer consisting of polyethylene oxide and polypropylene oxide in aqueous solutions. Small angle neutron scattering (SANS) is used to determine the self-association and hydration of the micelles at various polymer concentrations and temperatures. The intermicellar interaction can be described as a hard core repulsion with surface attraction. At elevated temperatures, the polymeric micelles exhibit a higher degree of association, dehydration, and surface adhesion. The low shear viscosity of the micellar solution is evaluated as a sum of the hydrodynamic contribution and a contribution from the interparticle interaction. The latter part is calculated based on the formula proposed by de Schepper, Smorenburg, and Cohen [Phys. Rev. Lett. 5, 2178 (1993)]. We adopt Baxter's model of a hard sphere with an adhesive surface to evaluate the interparticle structure factor and find that the surface attraction effectively increases the viscosity at high volume fractions. To calculate the relative viscosity at low shear rate of the polymeric micellar solutions, we use the volume fractions and intermicellar interaction potentials extracted from SANS data analysis. We obtain excellent agreement between the calculated viscosity values and the experimental measurements.