Xanthan solutions, zero-shear viscosity

The zero-shear viscosity r 0 has been measured for isotropic solutions of various liquid-crystalline polymers over wide ranges of polymer concentration and molecular weight [70,128,132-139]. This quantity is convenient for studying the stiff-chain dynamics in concentrated solution, because its measurement is relatively easy and it is less sensitive to the molecular weight distribution (see below). Here we deal with four stiff-chain polymers well characterized molecu-larly schizophyllan (a triple-helical polysaccharide), xanthan (double-helical ionic polysaccharide), PBLG, and poly (p-phenylene terephthalamide) (PPTA Kevlar). The wormlike chain parameters of these polymers are listed in Tables... 

Fig. 19. Zero-shear viscosity of aqueous xanthan solutions at different sodium chloride concentration C, [140]...

These data are presented in Table 3. What is immediately evident from these data is that the closest correlation occurred when particle settling rates were compared to the apparent viscosity measured at shear rates of 1 sec . The xanthan polymer solution showed a greater ability to suspend the solids and had a higher apparent viscosity in the low shear rate range. In contrast the HEC solution had higher apparent viscosities at shear rates above 10 sec but exhibited lower particle suspension properties. The dependence on low shear rate apparent viscosity is not entirely unexpected. Roodhart has shown emperically that the zero shear viscosity must be factored into a Stokes law type calculation before settling velocities can be calculated for HPG solutions. Thus, reliance on viscosity measurements at the customary shear rates would not have selected the more efficient fluid for particle suspension. 

The viscosity is independent of the shear rate y at low shear rates as it can be clearly seen in Fig. 5.8. This viscosity is called the zero-shear viscosity /Jq. Above a critical shear rate Ycrit=80 s for the PAAm and Ycrit=5 s for the xanthan gum solution the viscosity decreases with the shear rate. Table 5.2 shows some examples for aqueous solutions of poly(acrylamide) of different molar masses and concentrations. The critical shear rate is generally shifted to lower values for higher molar masses. 

The sedimentation of particles in non-Newtonian fluids, such as aqueous solutions containing high molecular weight compounds (e.g. hydroxyethyl cellulose or xanthan gum), is not simple since these non-Newtonian solutions are shear thinning with the viscosity decreasing with increase in shear rate. As discussed above, these solutions show a Newtonian region at low shear rates or shear stresses, usually referred to as the residual or zero shear viscosity /(0). 

Solutions of welan are very viscous and pseudoplastic, ie, shear results in a dramatic reduction in viscosity that immediately returns when shearing is stopped, even at low polymer concentrations (230). They maintain viscosity at elevated temperatures better than xanthan gum at 135°C the viscosity half-life of a 0.4% xanthan gum solution is essentially zero, whereas a welan gum solution has a viscosity half-life of 900 minutes (230). The addition of salt to welan solutions slightly reduces viscosity, but not significantly. It has excellent stabiUty and theological properties in seawater, brine, or 3% KCl solutions... 

FIGURE 6.1 Apparent viscosity ( /a) of polymer solutions of various concentration (% w/w). Shear rate zero (extrapolated) for xanthan, about 100s 1 for the other solutions. (Approximate results after various sources.)... 

Viscosimetric determinations. The Newtonian intrinsic viscosity of the xanthan molecule was determined by measuring the viscosities of several dilute polymer solutions with a Contraves Low-Shear viscometer. Extrapolation at zero polymer concentration of the reduced specific viscosity gave the value of the intrinsic viscosity, and the Huggins constant was calculated from the slope of the curve. 

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