Structure and Viscosity of Spherical Polyelectrolyte Microgels: A Model for the Polyelectrolyte Effect?

Abstract

Spherical microgels made of polystyrene sulfonic acid can be regarded as slightly swollen polyelectrolyte “balls” that cannot change their conformation due to the inner cross-linking. Because of their theory-compatible shape (spherical), the practical absence of intramolecular effects and van der Waals interactions, they are regarded as ideal models to examine and benchmark polyelectrolyte theories. In salt-free aqueous solution, both the static structure factor and the solution viscosity are analyzed in terms of their dependencies on the polyelectrolyte concentration, and typical polyelectrolyte behavior is observed, i.e., a steep increase of the reduced specific viscosity with decreasing concentration as well as formation of ordered solution structures. A more complete picture is provided by the systematic examination of macroions over one decade of radii (5nm < R < 70 nm). This mesoscopic range of molecular mass covers the gap between linear polyelectrolytes and classical latex spheres . Solution structuring, phase separation behavior and theconcentration dependence of interparticle distances show the importance of electrostatic attractive forces. This is confirmed by salt concentration and cross-linking-density dependent experiments. Scattering data are also analyzed in a refmed version of the Coulomb potential/MSA within the primitive model that show the existence of effective repulsive and attractive forces. The concentration dependent solution viscosity curves are well described by a modemode coupling approximation introduced by Hess and Klein. This formalism also allows the recalculation of the viscosity behavior from the light scattering curves, resulting in a quantitative agreement between measured and calculated values. Finally, the resulting picture is transferred to high precision measurements on linear polyelectrolytes of the same chemical composition, where direct comparison is expected to reveal the effects specific to linear structures. It is seen that the predominant effect is still intermolecular in nature, i.e. due to the interaction potential, but an effect due to chain stretching can also be identified. This effect is more pronounced for small polymers than for large polymers.