Estimation of the relative stiffness of the molecular chain in polyelectrolytes from measurements of viscosity at different ionic strengths

A method was developed that allowed comparison of the stiffness of the chain in different polyelectrolyte from measurements of the intrinsic viscosity at different concentrations of added monovalent (sodium) salt. The response to salt was quantitatively expressed as the slope of straight lines relating the intrinsic viscosity to the reciprocal of the square‐root of the ionic strength. This slope increased considerably with increasing molecular weight of the polyelectrolyte, and could serve to characterize the response to salt of different substances only when comparison was made at a constant molecular weight. An empirical parameter, B, which is the slope corresponding to an intrinsic viscosity of 1.0 at an ionic strength of 0.1 M could be correlated to the unperturbed dimensions of the molecules. A method of extrapolation, enabling the determination of B from measurements of viscosity on only one sample of unknown molecular weight, was evaluated. The empirically found correlation between Band some well established parameters of stiffness did not contrast predictions from the “fuzzy‐sphere model” of Fixman, provided that reasonable assumptions regarding ion‐binding and the interaction between polymer and solvent were made.