DILUTE-SOLUTION DYNAMIC VISCOELASTIC PROPERTIES OF XANTHAN POLYSACCHARIDE

The frequency dependences of the storage and loss shear moduli, G' and G'', of dilute solutions of highly purified xanthan polysaccharide were measured at 20.0-degrees-C using the Birnboim-Schrag multiple-lumped resonator. The frequency range was 150-8000 Hz and the concentration range was 0.1-0.4 g/l. Three solvents were used, one of which contained 75% by weight glycerol to increase the solution viscosity. Measurements of oscillatory flow birefringence were also made in one solvent over the frequency range from 1-630 Hz and agreed well with the viscoelastic data in the range of overlap. The intrinsic viscosity in both water and 25%/75% water/glycerol (with 0.085 mol/l sodium chloride) was determined as 5200 ml/g. The frequency dependences of G' and G'', extrapolated to infinite dilution, could be fitted to a hybrid model for semiflexible rods which was modified to take into account a moderate degree of molecular weight distribution (M(w)/M(n) = 1.4). From the experimental data the persistence length of the native xanthan was estimated at 500-800 nm and Young's modulus of the rod, modeled as a homogeneous body, was calculated to be about 2 X 10(9) dyn/cm2.