Non‐Newtonian viscosities in dilute aqueous solutions of sodium carboxymethylcellulose

Abstract

The reduced viscosity vs. concentration relation was measured for sodium chloride solutions of sodium carboxymethylcellulose (Na‐CMC of DP ≓ 400 and DE ≓ 0.7) in a range of low concentrations (0.003 to 0.09 g./100 ml.), and the data obtained were analyzed following the procedure of Pals and Hermans. The results confirmed in many aspects findings which had previously been made in the literature, and supplied values of the necessary molecular quantities for our own sample. They indicated that our Na‐CMC molecule was relatively uncoiled even in solutions of high ionic strength. The Huggins slope constant was found to increase approximately linearly with increasing reciprocal of the ionic strength of the solution. Next, the shear dependence of reduced viscosity in dilute solutions of this polyelectrolyte was investigated by the use of a viscometric apparatus which was especially designed for accurate measurements at low shear rates. The shear rate region investigated was from about 50 to 1000 sec.⁻¹. The measurements were made on salt‐free solutions and on solutions of constant ionic strength; in both cases only the regions of sufficiently low concentrations were treated. In the salt‐free solutions the η_sp/C vs. q (q is shear rate) curves showed an upward curvature rather than a quadratic dependence at the limit of shear rate. In this case, the initial slope of each curve increased markedly with increasing dilution. The plots of η_sp/C vs. q for solutions of constant ionic strength (ca. 0.0015 mole/l.) were approximately represented by straight lines within the concentration range investigated. The slope of the straight lines decreased regularly with increasing dilution. Some evidence that these plots have a horizontal initial tangent was observed, but this was never conclusive due to the paucity and inaccuracy of exprimental data at shear rates smaller than 100 sec.⁻¹. The intrinsic viscosity obtained from η_sp/C vs. C curves at constant shear rates for the isoionic system was independent of q. From these findings it was realized how markedly the viscous behavior in dilute solutions of the polyelectrolyte studied was influenced by the addition of small amounts of neutral salt, not only in viscosity value itself but also in its shear rate dependence. Our data and results herein presented were concerned only with a sample of one (average) molecular weight and one degree of etherification. More data, therefore, are needed on samples of varying degrees of polymerization and etherification before a more definite understanding of the problem herein treated is possible.