Intrinsic viscosity of small spherical polyelectrolytes: Proof for the intermolecular origin of the polyelectrolyte effect

Abstract

Spherical polystyrenesulfonate particles in the size range between 7 nm<R<50 nm are synthesized via crosslinking copolymerization in microemulsion and subsequent sulfonation via polymer reactions. These model polyelectrolytes, when carefully purified, show the qualitative aspects of the polyelectrolyte effect, i.e., large excess viscosities with a strong increase of the intrinsic viscosity with decreasing concentration. A quantitative evaluation of these data on the basis of a modified Hess–Klein relation reveals that the complete dependence on polymer as well as on salt concentration can be fitted with one parameter only, the effective charge number per particle, Zeff. The specific viscosity increases with decreasing particle size and inverse particle density, but no simple explanations for the found relations can be given. Since conformational changes play only a minor role for spherical systems, the comparison of the concentration dependence of the reduced viscosities of linear chains with those of the spherical polyelectrolytes allows for a differentiation between intra- and intermolecular effects. It is qualitatively shown that a major contribution to the polyelectrolyte effect is caused by intermolecular interactions, i.e., the increase of the electrostatic screening length and interparticle-coupling with decreasing concentration. The quantitative description of the concentration and molecular weight dependence of the reduced viscosity of linear polyelectrolytes in salt-free solution reveals that Zeff does apparently not depend on molecular weight, the found molecular weight dependence of the reduced viscosity is due to the increase of the hydrodynamic radius, only. In addition, our modified Hess–Klein model also describes some quantitative features of the viscosity curves, such as the molecular weight dependent shape of the maxima. Deviations between theoretical description and experimental data which become significant for smaller linear polyelectrolytes are attributed to a concentration dependent coil expansion.