Rheological behavior and molecular relaxation of polycations with macrocounterions in aqueous solutions

Counterions play significant roles in the characteristics of polyelectrolytes, whereas many problems still remain unsolved. Among most previous research studies, counterions were too small to be “seen” by light scattering. In this article, we prepare macrocounterions which are single/double carboxylate ion-terminated polyethylene glycols (Mn ≈ 5000 g mol−1). The solution properties of polycations with macrocounterions are investigated through rheological, conductivity and dynamic light scattering measurement. On the basis of the scaling theory, the static and dynamic parameters of polyelectrolytes are analyzed over wide concentration ranges. In rheological measurement, a critical concentration () has been observed. When the concentration is below , the role of macrocounterions can be ignored; while above , macrocounterions determine the longest relaxation and prevent entanglement of polycations. Moreover, the same turning point exists in the dependence of conductivity and viscosity on the molar fraction of macrocounterions, which indicates that macrocounterions can't serve as brushes condensed on the polycation until their molar fraction is above the effective charge density of polyelectrolytes. In dynamic light scattering, we investigate the relaxation of polycations with macrocounterions on the length scale of electrostatic blobs, where the size of macrocounterions becomes significant. In comparison with ordinary polyelectrolytes, it is observed that both fast and slow diffusive relaxations of polycations with macrocounterions are significantly slowed down, and the fast mode relaxation becomes dominant. Since no independent relaxation of macrocounterions has been observed, it is supposed that the fast mode relaxation is the coupled relaxation of polycations and macrocounterions.