Structure and Rheology of Solutions and Gels of Stiff Polyelectrolyte at High Salt Concentration

The rheology and the structural evolution of xanthan solutions and gels upon increasing polymer concentration were monitored in the absence and in the presence of cross-linker (chromium chloride). Direct visualization of the unperturbed structure of these systems by freeze-fracture transmission electron microscopy revealed the microphase-separated morphology with percolated polymer skeleton composed of aggregated chains. The thickness of the skeleton increases with increasing polymer concentration and at the addition of cross-linker, indicating enhancement of microphase separation. Upon cross-linking, the networks acquire higher plateau modulus G', but it becomes less dependent on polymer concentration C than in the un-cross-linked state (G' similar to C-1.35 and G' similar to C-2.1, respectively). The molecular imaging also unraveled the presence of backfolded xanthan segments and many sharp kinks at distances smaller than the persistence length, suggesting the presence of flexible hinges between double-helical fragments of the polysaccharide, which is in agreement with small-angle X-ray scattering data.