Reanalysis of the electrostatic component of the persistence length of poly(styrene sulphonate)

Abstract

The electrostatic component of the persistence length ( L el ) for three molecular-weight preparations of poly(styrene sulphonate) was determined from magnetic birefringence measurements (m.b.m.) by Weill and Maret in 1982. The ratio L el / L a el plotted as a function of the Debye-Huckel screening length (λ DH ) exhibited significant deviations from unity over the range24 ⩽ λ DH (a˚) ⩽ 108, where L a el is the analytical expression for the electrostatic persistence length for finite chain lengths given by Odijk. Weill, Maret and Odijk attempted to explain the discrepancy between theory and experiment by invoking ‘semidilute’ solution conditions for the long-chain results concomitant with a different degree of counterion condensation than used for the short-chain results. It is proposed in the present paper that a ‘three-region’ model for the counterion distribution about the polyion accurately represents the m.b.m. data without having to invoke differential counterion condensation properties of short and long chains, or having to postulate semidilute solution conditions. The three regions are: (1) Manning-type condensation on the surface of the polyion; (2) a counterion sheath in the vicinity of the polyion; and (3) the remaining solution.