Dilute solutions of diblock copolymers in a selective solvent: I. Evidence for spherical star like micelles

Diblock copolymer polystyrene-polyisoprene PS-PI is studied in dilute solution at room temperature in methylcyclohexane, a good solvent for polyisoprene and a theta solvent for polystyrene at T_θ = 70°C. The molecular mass of the copolymer is 1.1 × 10⁶ g/mol and the polystyrene weight fraction is 36%. The diblock copolymer aggregates into star like micelles which are studied by static and quasielastic light scattering. Both techniques lead to a full characterization of the micelles. Well below the overlap concentration, the data are strongly dependent on micelle interactions. The scattered intensity (the diffusion coefficient) presents a maximum (a minimum) at a scattering vector which is concentration dependent. From this dependence we infer that the micelles are in a liquid phase, the mean distance between first neighbor being 400 nm at a concentration C = 10^-3 g/cm³. From these measurements we can deduce the mobility per monomer and demonstrate that the micelles move in an effective medium having the macroscopic viscosity of the solution.