Light scattering and viscoelasticity in aqueous mixtures of oppositely charged and hydrophobically modified polyelectrolytes

Rheological and intensity (ILS) and dynamic Light scattering (DLS) experime nts were performed on semidilute aqueous mixtures of various compositions o f oppositely charged and hydrophobically modified polyelectrolytes. The ass ociative phase separation usually observed when mixing oppositely charged p olyelectrolytes is restricted to a fairly narrow mixing region when the pol ymers are hydrophobically modified. Measurements were carried out at mixing ratios both before and after this two-phase area. The rheological properti es in the vicinity of the two-phase region show that the elastic response d ominates even at fairly low frequencies, indicating the existence of strong intermolecular interactions. The time correlation data obtained from the D LS experiments revealed the existence of two relaxation modes, one single e xponential at short times followed by a stretched exponential at longer tim es. The fast mode is always diffusive, and the extracted hydrodynamic corre lation length as well as the static one from ILS increases toward phase sep aration of the mixture. The slow relaxation time reveals that the dynamics slowed down in the vicinity of phase separation, and the angular dependence of the slow mode is stronger than that of the fast mode and increases grad ually as the tao-phase region is approached These features that are attribu ted to enhanced hydrophobic associations can be rationalized in the framewo rk of the coupling model of Ngai. The fractal dimension, determined from IL S, drops toward phase separation, and this trend suggests a more "open" net work structure at this stage.