The viscous behaviour of dilute solutions of a strong polyelectrolyte (polystyrene sulphonate)

Few investigations of the behaviour of polyelectrolyte solutions have been carried to sufficiently low concentrations to ascertain the limiting value of the reduced specific viscosity at infinite dilution. Recent investigations with polymers of weak acids (e. g. polymethacrylates) have indicated that owing to the combined effects of change of shape and interaction, the reduced viscosity exhibits a maximum value at a finite concentration below which it decreases rapidly. Since other factors such as hydrolysis may play a part in the case of such polyelectrolytes, an investigation has been made into the viscous behaviour of polymers of the strong acid styrene sulphonic acid and its sodium salt in dilute solutions. These materials show a maximum reduced specific viscosity at a concentration of ca. 0∙002%, which appears to be real, although its mag­nitude is greatly influenced by the purity of the water and small concentrations of ions have a very marked effect. It thus cannot be concluded with certainty that the observed increase of the reduced specific viscosity with concentration at low concentrations is due to interaction effects. The molecular weights of the fractions used were determined from their mean sedimentation constants and intrinsic viscosities using the equation of Flory & Mandelkern (1952). From these molecular weights, mean end-to-end lengths of the particles under different conditions were derived by the relation of Flory & Fox (1951). It was found that at the maximum of the viscosity curve, the extension of the particle in water is about half of the total contour length and is sufficient to permit of particle interaction at these concentrations. The length is greatly diminished by increase of the ionic strength, and even if interaction occurs at low concentrations, it ceases to be the dominant factor when the concentration is greater than 0∙003% in most cases. Measurements were also made of the electrical conductance of the polymer in the salt and acid forms; in the latter it was found that although the degree of dissociation is low, the equivalent conductance is nearly independent of the concentration. This would appear to mean that the dissociation of the hydrogen ions is determined by the micro-environment of the acid groups within the particles and is scarcely influenced by the concentration of polymer particles in the solution.