Rheology of concentrated isotropic and anisotropic rigid polysaccharide solutions: Xanthan and scleroglucan

The rheology of concentrated isotropic and anisotropic solutions of rigid polysaccharides xanthan and scleroglucan has been studied with a Rheometrics RFS II Fluids Spectrometer in both steady shear and oscillatory modes. Two xanthan samples, one of low molecular weight and is rigid rodlike, the other of intermediate molecular weight and is serniflexible wormlike, have been prepared and their rheological behaviors are found to be quite different from each other. The rigid rodlike sample behaves mostly like Newtonian fluid, and obeys the Cox-Merz rule for all the solutions studied. No cross over of storage and loss modulus is observed because the rigid xanthan sample is too short and rigid to form network. But the semiflexible xanthan sample possesses gel-like properties when it is in the concentrated isotropic regime and the strength of network formed increases with salt concentration. It does not obey the Cox-Merz rule in any salt concentration, and crossover of storage and loss moduli is observed. Although the two xanthan samples behave differently, they do have common points: both have viscosity peaks corresponding to the phase boundary concentration for isotropic and biphasic regimes, and the rheological parameters do not increase further with concentration in fully anisotropic regime, unlike flexible high molecular weight xanthan samples. The scleroglucan sample studied has very interesting properties: In isotropic regime, it is like Newtonian fluid and obeys the Cox-Merz rule like rigid rodlike xanthan, but it follows Matheson's theory in terms of the viscosity increase with concentration from isotropic to fully anisotropic regimes, similar to flexible xanthan. And yet the scleroglucan sample is not flexible enough to form networks as observed for intermediate molecular weight xanthan. So, the scleroglucan sample studied has something in common with every xanthan conformation, either rigid rodlike, or semiflexible, or flexible, but they are also different.