Coil overlap in moderately concentrated polyelectrolyte solutions: effects of self-shielding as compared with salt-shielding as a function of chain length

Abstract

The generalized intrinsic viscosity {η} (hydrodynamic volume of the solute at arbitrary polymer concentration c) – introduced by analogy to the intrinsic viscosity [η] – provides access to the degree of coil overlap Ω for polyelectrolyte solutions in pure water or in saline water. The experimental basis of this investigation consists in viscosity measurements as a function of c for a large number of sodium polystyrene sulfonate (Na-PSS) samples covering the molecular weight range from 0.91 to 1000 kg mol⁻¹. The accurate modeling of these dependencies with a maximum of three parameters yields detailed information on Ω as a function of (c[η]) in the absence and in the presence of extra salt. It demonstrates that the polymer character of PSS-Na is lost as the number of monomeric units falls below approximately 15. In the case of pure water the extent of coil overlap resulting at given grows markedly with rising M. The minimum share of self-shielding in the total shielding – resulting for a large surplus of extra salt – depends on Ω. It starts from zero at infinite dilution and approaches limiting values on the order of 90% in all cases. The transition between these two values spreads out over increasing ranges of Ω as M becomes larger. Also discussed are generalized Kuhn–Mark–Houwink relations for constant c and unexpected concentration dependencies of Ω.