‘‘Ordered’’ structure in dilute solutions of sodium polystyrenesulfonates as studied by small-angle x-ray scattering

Abstract

The small angle x-ray scattering measurements were performed for aqueous solutions of sodium polystyrenesulfonates having relatively narrow molecular weight distributions. As was observed for other synthetic macroions, polynucleotide and proteins, a single, broad peak was observed. The scattering vector at the peak position (Sm) was shifted toward larger values with increasing polymer concentration and toward lower values with increasing salt concentration, which confirmed earlier observations with polyacrylate and poly-L-lysine. The molecular weight dependence of the scattering behavior, which was earlier observed, was confirmed to be true for samples with Mw of 74 000, 18 000, and 4600. The mixture of two fractions with different Mw’s gave a scattering curve which was again different from the composite curve obtained with the parent curves before mixing. A similar situation was observed for the mixture of polystyrenesulfonate and polyacrylate. Thus, it was concluded that the observed single peak indicates the presence of an intermolecular ordering, not an intramolecular ordering. The intermacroion distance (2Dexp) was thus calculated by using the Bragg equation. 2Dexp decreased with increasing polymer concentration and increased (not decreased) with increasing concentration of added salt and Mw. The 2Dexp values thus obtained were smaller beyond the experimental error than 2D0, a theoretical distance calculated from the concentration by assuming the uniform distribution of the macroions throughout the solution and the 2D0/2Dexp value amounted to 3.5 for high molecular weight samples. This fact indicates the presence of an intermacroion attractive interaction. When two fractions with different Mw’s were compared at a given number concentration of macroions, the 2Dexp value for the sample of a larger Mw was smaller than that for the fraction of a smaller Mw. This implies that the attraction must be intensified, though unexpectedly, with increasing valency of the macroions, confirming our earlier interpretation that the attraction could be brought into existence through the intermediary of counterions present between macroions. The larger the number of the counterions, the stronger the intermacroion attraction. With increasing temperature, the peak was slightly lowered and the 2Dexp became smaller, though very slightly. Rough agreements between x-ray and neutron scattering data were noted. On the basis of the direct microscopic observation of a similar ordering of polymer latex particles, it was suggested that the macroions in the ordered regions were subject to violent thermal motion, and that paracrystalline distortion of the ordered structures allowed us to observe only a single broad peak.