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We report results of small angle neutron scattering measurements made onsodium polystyrene sulfonate in aqueous salt solutions. The correlation lengthand osmotic compressibility are measured as a function of polymer (c) and addedsalt ($c_S$) concentrations, and the results are compared with scalingpredictions and the random-phase approximation (RPA). In Dobrynin et al'sscaling model the osmotic pressure consists of a counter-ion contribution and apolymer contribution. The polymer contribution is found to be two orders ofmagnitude smaller than expected from the scaling model, in agreement withearlier observations made on neutral polymers in good solvent condition. RPAallows the determination of single-chain dimensions in semidilute solutions athigh polymer and added salt concentrations, but fails for $c_S < 2$ M. The \chiparameter can be modelled as the sum of an intrinsic contribution and anelectrostatic term: $\chi \simeq \chi0+K/c_S^{1/2}$, where $\chi_0 > 0.5$ isconsistent with the hydrophobic nature of the backbone of NaPSS. The dependenceof $\chi_{elec} \simeq 1/c_S^{1/2}$ disagrees with the random-phaseapproximation ($\chi_{elec} \simeq 1/c_S$), but agrees with the lightscattering results in dilute solution and Dobrynin et al's scaling treatment ofelectrostatic excluded volume.

Screening lengths and osmotic compressibility of flexible polyelectrolytes in excess salt solutions