RHEOLOGY OF XANTHAN AND SCLEROGLUCAN IN SYNTHETIC SEAWATER

Rheological characterization of xanthan from three different sources and scleroglucan from two different sources has been carried out using synthetic seawater as solvent. The measurements spanned temperatures from 20-degrees-C to 70-degrees-C, shear rates from 0.5 s-1 to 100 s-1 and polymer concentrations from 30-mu-g/ml to 1 mg/ml. The observed difference in effectiveness of increasing solution viscosity at low shear is quantitated in terms of the intrinsic viscosity [eta]. Scleroglucan A was found to be the most effective with [eta] = 14 000 ml/g and xanthan B the least effective with [eta] = 5060 ml/g. both at T = 20-degrees-C. The temperature coefficients of the intrinsic viscosities, partial derivative ln[eta]/partial derivative T, was found to lie in the range partial derivative ln [eta]/partial derivative T = -(9-19) 10(-3) K-1 for the xanthans and partial derivative ln[eta]/partial derivative T = -1.9 10(-3) K-1 for the scleroglucans. These findings were interpreted in terms of a decrease in chain stiffness with temperature. Assuming a worm-like chain model the temperature coefficients of the persistence length q were partial derivative lnq/partial derivative T = -(9.4-21) 10(3) K-1 and -2.1 10(-3) K-1 for xanthans and scleroglucans, respectively. The viscosity at low shear rate in the semi-dilute concentration range is accounted for in terms of entanglement of overlapping chains. The lowering of viscosity in the semi-dilute concentration range associated with increasing shear rate is observed for smaller shear rates than in the dilute regime. Thus the decrease in entanglement starts at lower shear rate than orientational effects at the molecular level.