ИСТИНА

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 Gʹ, but it becomes less dependent on polymer concentration C than in the uncross-linked state (Gʹ ~ C1.35 and Gʹ ~ C2.1, respectively). The molecular imaging also revealed the presence of backfolded xanthan segments and many sharp kinks at distances smaller than the persistence length, which allowed us to suggest the presence of flexible hinges between double-helical fragments of this polysaccharide, so that the macromolecular structure of xanthan can be regarded as a "broken rod" rather than a wormlike one, which is in agreement with SAXS data. Thus, the present study provided many new insights into the morphology of xanthan networks as well as into the molecular origins of xanthan rheology, which is quite important taking into account the development of new injection fluids for enhanced oil recovery and control of their rheological properties.