Solutions of polyelectrolytes and mechanochemical systems

Abstract

Abstract The behavior of polyelectrolyte molecules in aqueous solution has been discussed. It has been shown that the pronounced increased in viscosity of polymeric acids upon ionization is due to the change of shape of the molecule due to electrostatic repulsive forces. Nonionized polymeric acids assume the form of highly hypercoiled rigid spheres obeying Einstein's viscosity law, the hypercoiled form being presumably due to strong intermolecular hydrogen bonding and van der Waals' interactions. The polymeric molecules open up with increasing ionization, and assume the form of freely linked threads, obeying Staudinger's law, at about 10% ionization. At high degrees of ionization, they stretch to rod‐like filaments, with the viscosity increasing as the square of the molecular weight. The shape of the molecules is also dependent on the concentration of the solution and on the amount of foreign ions present. An excess of the latter annihilates the electrostatic field and reduces the viscosity to that of normal random coils. Polyampholytes of the type of copolymers of methacrylic acid and vinylpyridine show the expected behavior: the viscosity is at a minimum at the isoelectric point and increases in both the acidic and alkaline regions with the increase in free charge. The field energy of polyelectrolyte molecules has been determined thermodynamically by means of the potentionmetric titration. It has been shown to be dependent upon the molecular weight, the concentration of the polymer, and the ionic strength. The charge of the field energy per ionized group has been found to be independent of the molecular weight. The electrostatic field energy has been calculated theoretically from statistical and electrostatic considerations and an approximated formula has been arrived at by using a simplified model. The calculated field energies have been compared with those derived from potentiometric titration and found to be in good agreement with each other. The deviations from the theory, the particular shape of the viscosity versus degree of ionization, and field energy versus ionization curves, have been attributed mainly to the existence of contractile van der Waals' forces, operative between the segments of the polymeric molecules. These forces act in addition to the usual rubberlike configurational elasticity. An attempt has been made to explain these effects on a quantitative basis. Mechanochemical systems capable of transforming chemical energy of ionization into mechanical energy of stretch have also been described. These systems, typified by cross‐linked gels and fiber gels of polymeric electrolytes, can be made to work in isothermal cycles in a manner reminiscent of the physicochemical mechanism of the muscle. A cycle has been analyzed in which a system is made to work between two pH values (corresponding to chemical potentials), and two degrees of ionization, under the influence of an exterior force.