Dynamics of PS-PMMA diblock copolymers in toluene

Abstract

The dynamic behaviour of two polystyrene-polymethylmethacrylate (PS-PMMA) diblock copolymers has been studied in toluene by quasielastic light scattering in the vicinity and above the overlap concentration C* of the copolymer chains. At the macromolecular scale two relaxation modes have been found near C*. The slow mode has been ascribed to the classical cooperative motion of the transient physical network formed by the copolymer molecules. It is identical to the motion that is observed in the equivalent binary PS/toluene solution where PS has the same molecular weight as the molecular weight of the whole copolymer and where the PS and the PS-PMMA concentrations are equal. The fast mode is a structural mode that reflects the fluctuations of concentration of one species with respect to the other. Above C* a third motion appears that is not anticipated by the theories. The frequency of this mode is very low and decreases when the copolymer concentration increases. This mode is attributed to the formation of aggregates that can occur even at relatively low concentrations (4C*) for the systems under investigation.