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 s olutions. Concentrations were always well below those necessary to acc ess the order-disorder transition in neutral good solvents. The sample s include SI(10-50), SI(36-9), and SIS(10-100-10), where the numerical suffixes denote the block molecular weights in kilodaltons; experimen tal emphasis was placed on SI(10-50). The DLS intensity correlation fu nctions in the neutral good solvents, THF and toluene, were dominated by a slow mode that first appeared at a concentration c(+) approximate to 4c, where c* is the coil overlap concentration. The decay rate of this mode scaled approximately as the third power of the scattering w avevector, and the excess scattered intensity decreased with increased scattering angle. These results were tentatively ascribed to the onse t of substantial concentration fluctuations, that exhibited cylindrica l, or wormlike structures. Measurements in solvents of known selectivi ty, dioxane and cyclohexane, and on a copolymer of the opposite compos ition, SI(36-9), indicated that the intermolecular association was dri ven by the effectively repulsive interactions between styrene and isop rene segments, rather than by solvent selectivity. (C) 1998 John Wiley & Sons, Inc.