Quasi-elastic light scattering by polystyrene-poly(methyl methacrylate) diblock copolymer solutions in a non-selective solvent in the vicinity of the overlap concentration

The dynamic properties of a styrene (S)-methyl methacrylate (MMA) diblock copolymer in semidilute solutions of toluene have been investigated by quasi-elastic light scattering. One relaxation mode has been found in the low-concentration regime near the overlap concentration C^*. This mode is identified as the cooperative mode which is observed in semidilute solutions of the equivalent polystyrene (PS) homopolymer. A second mode appears simultaneously at higher concentrations. This slower diffusive mode is ascribed to the diffusion of the center of mass of the clusters formed by the copolymer molecules. The fast structural mode which is predicted by the theory^1–3 is not observed. It is suggested that this mode, which must have a small amplitude, is hidden by the other two relaxation modes.