The structure of concentrated polyelectrolyte (PE) solutions is studied. There is a lack of experimental data on these systems. Specifically, no orientational order has ever been observed in solutions of flexible or semi-flexible PE, even though the existence of a nematic phase has been theoretically predicted at high concentration. We have measured the position q in the reciprocal space of the peak that is present in the correlation function of aqueous solutions of highly charged PE, obtained by scattering measurements (SAXS and SANS). We combined high spatial resolution and a very wide concentration range to compare the variation of q versus the concentration c, with the predicted scaling laws q*~ca. The intrinsic stiffness of PE is the key parameter of our study. Indeed, PSS, a flexible PE, exhibits 3 regimes already described: a=1/2 -semi-dilute regime- a=1/4 -concentrated regime- an “ionomeric” regime at extreme concentrations, for which a=0. PDADMAC is semi-flexible and exhibits a unique series of exponents involving 4 regimes: semi-dilute and concentrated regimes with a=1/2 and 1/4, followed by 2 regimes associated with a=1 and 1/2 respectively, which are new and unpredicted for PE solutions. PaMSS has an intermediate intrinsic stiffness, and exhibits hybrid behaviour. The semi-dilute and concentrated regimes are followed by an a=1 regime, and then an “ionomeric” regime for extreme concentrations. We introduce a simple model which explains the succession of the a=1 and 1/2 regimes as the appearance of an orientational order. We also discuss the differences between SAXS and SANS measurements, and the role played by the ionic strength, when salt is added.
