SAXS from polyelectrolyte solutions under shear: Xanthan and Na-hyaluronate examples

Xanthan and Na-hyaluronate (Na-Hy) polysaccharides belong to a class of pol yelectrolytes that show a maximum in the reduced viscosity as a function of polyelectrolyte concentration. It is also well documented that xanthan sol utions present a polyelectrolyte scattering maximum as a function of the wa vevector q. However, despite considerable expanded experimental efforts, no scattering peak was observed in Na-Ky systems when using light or neutron scattering techniques. In this work and far the first time, we report that only the use of high brilliance of synchrotron radiation at rest and under shear enables to highlight the expected small-angle scattering peak in the Na-Hy polyelectrolyte system. At zero shear rate, the scattering profile re veals a very diffuse and hardly detectable maximum at ideal experimental co nditions (i.e., semidilute "salt-free" polyelectrolyte concentration-or low ionic strength- and momentum transfer a). As the shear rate is increased, this very small maximum is magnified and reveals clearly the expected polye lectrolyte nature of Na-Hy not yet observed using other radiation (light or neutrons). The system undergoes a typical and progressive change from isot ropic to anisotropic phase, when increasing the shear, confirming the origi n of the peak and the role of the electrostatic interactions on the structu ral order in polyelectrolyte systems. These results are compared to those o btained under the same conditions on the xanthan polyelectrolyte system whe re, in addition to the so-called polyelectrolyte scattering peak, a second- order peak in the scattering profile is observed as a consequence of the sh ear.