DYNAMICS OF COMPOSITION FLUCTUATIONS IN DIBLOCK COPOLYMER SOLUTIONS FAR FROM AND NEAR TO THE ORDERING TRANSITION

Dynamic light scattering in the polarized geometry has been used to in vestigate the dynamics of composition fluctuations in solutions of two poly(styrene-block-1,4-isoprene) diblock copolymers in a nonselective , good solvent both far from and near to the ordering transition. Four different relaxation mechanisms were identified. One relaxation relat es to the cooperative diffusion of copolymer chains, and it behaves si milarly to that in semidilute homopolymer solutions. As concentration increases, two additional relaxations gain in amplitude: the wavevecto r-independent internal or breathing mode, predicted by theory, with am plitude increasing with scattering angle, and the recently established for copolymer melts diffusive relaxation; the relaxation times of bot h processes increase with copolymer concentration. In addition, anothe r very slow process is evident at low scattering angles and is related to the long-range density fluctuations. A theoretical analysis is pre sented that predicts the existence and the behavior of the first three relaxations. The new diffusive relaxation is attributed to the compos ition polydispersity of the diblocks; its intensity is proportional to the degree of polydispersity; its rate is governed by the self-diffus ion of the copolymer chains; both its intensity and its relaxation tim e increase with the copolymer concentration, in contrast to the behavi or of the cooperative diffusion.