Determination of the viscoelastic properties of a homologous series of the fermentation polymer xanthan gum

A homologous series of xanthan gum was investigated in terms of its viscoelastic properties. Xanthan gum exhibits an unusually high stability to temperature and salts. In an 0.1 M NaCl solution the viscosity was barely below that of the aqueous solution, and decreased over the whole temperature range from 20 to 80 degrees C only by a mere 25%. The following structure-property relationship was established: eta(sp) = c center dot [eta] + 0.57 center dot (c center dot [eta])(2) + 0.15 center dot (c center dot [eta])(3.5) Viscometry ([eta] proportional to M-w(1.58)) resulted in a confirmation of the hydrodynamics of a Yamakawa-Fujii worm-like chain. Rheological measurements showed an extremely high viscosity yield: at c = 0.1 wt.-% and (<(gamma)over dot>) = 0.1 s(-1), eta was already higher than 1 000 mPa center dot s. In moderately concentrated solutions the relationship between the zero shear viscosity and molar mass was eta(0) proportional to M-w(5.5). Dynamic measurements showed that at c > 2 wt.-% the viscous component dominates over the elastic component (tan delta < 1), and that the Cox-Merz rule was not fulfilled.