Rheology of Concentrated Isotropic and Anisotropic Xanthan Solutions. 2. A Semiflexible Wormlike Intermediate Molecular Weight Sample

The rheology of a semiflexible wormlike xanthan sample (Fraction X2F2, M w = 4.80 × 10 5 g/mol) has been studied in oscillatory and steady shear modes in aqueous NaCl (0.01, 0.1, 1.0 M) at polymer concentrations spanning the isotropic, biphasic, and fully anisotropic regimes. The rheological observables change with polymer weight fraction W and shear rate/frequency in much the same way as described in the companion paper for lower molecular weight sample X13F3 ( M w = 1.54 × 10 5 g/mol). This semiflexible, intermediate molecular weight sample is more readily aligned under the steady shear field than the rodlike, low molecular weight sample X13F3. For the anisotropic solutions, the steady shear viscosity η shows shear thinning behavior at high and low shear rate γ̇ and shear thickening at intermediate γ̇, unusual behavior that is presumably linked to the negative first normal stress difference sometimes seen in lyotropic liquid crystalline systems. A plot against W of the crossover frequency ω c at which the loss tangent is unity is “V” shaped with the minimum corresponding to the phase boundary concentration between the isotropic and biphasic regions. The Cox−Merz rule is generally not obeyed. The dynamic viscosity η * > η at most frequencies for systems that are isotropic or predominantly isotropic; the situation is reversed in anisotropic or predominantly anisotropic systems. In contrast to observations for X13F3, η and η * are sensitive to salt concentration in the isotropic as well as the anisotropic phase. Estimates of the mean separation of xanthan molecules in the equilibrium anisotropic phase show that separations exceed those at which hydration forces between biological macromolecules are postulated to be dominant. Electrostatic interactions play a prominent role in dictating the concentrations of the coexisting isotropic and anisotropic phases.