Dynamic light scattering from asymmetric block copolymers in neutral good solvents: Evidence of association in the disordered state

Dynamic light scattering (DLS) has been used to explore the properties of asymmetric styrene-isoprene (SI) block copolymers in concentrated solutions. Concentrations were always well below those necessary to access the order-disorder transition in neutral good solvents. The samples include SI(10-50), SI(36-9), and SIS(10-100-10), where the numerical suffixes denote the block molecular weights in kilodaltons; experimental emphasis was placed on SI(10-50). The DLS intensity correlation functions in the neutral good solvents, THF and toluene, were dominated by a slow mode that first appeared at a concentration c⁺ ≈ 4c*, where c* is the coil overlap concentration. The decay rate of this mode scaled approximately as the third power of the scattering wavevector, and the excess scattered intensity decreased with increased scattering angle. These results were tentatively ascribed to the onset of substantial concentration fluctuations, that exhibited cylindrical, or wormlike structures. Measurements in solvents of known selectivity, dioxane and cyclohexane, and on a copolymer of the opposite composition, SI (36-9), indicated that the intermolecular association was driven by the effectively repulsive interactions between styrene and isoprene segments, rather than by solvent selectivity.