SAXS from four-arm polyelectrolyte stars in semi-dilute solutions

We have performed small-angle X-ray scattering experiments on semi-dilute solutions of highly charged star polyelectrolytes. Poly(sodium acrylate) (PANa) and poly(cesium acrylate) (PACs) stars with four arms were successfully synthesized by a combination of atom transfer radical polymerization and chemical modifications. Over a wide range of polyelectrolyte concentration C_p, these two systems as well as their equivalent linear polyelectrolytes were investigated at different ionic strengths. Scattering experiments show the existence of a scattering peak denoted as q_max, which disappears with the addition of a simple electrolyte, evidencing the electrostatic character of the interactions. We have also studied the effect of the charge parameter and the nature of the counterion (Na and Cs) on the scattering properties of these star polyelectrolytes. In the case of PACs, q_max scales with the polyelectrolyte concentration as C_p^1/2 over the whole range of studied concentrations, whereas it scales as C_p^1/2 (for C_p < 45 mg·ml^-1) and C_p^1/4 (for C_p > 45 mg·ml^-1) in the case of PANa.