STRUCTURE AND SOLUTION PROPERTIES OF SODIUM CARBOXYMETHYL CELLULOSE

We have investigated the macroscopic properties of eight commercial sa mples of sodium carboxymethyl cellulose (Na-CMC) with molecular weight s between 9000 and 360000 g mol(-1) and substitution degrees between 0 .75 and 1.47 in aqueous solutions. From rheological and electric biref ringence measurements (a.c. and d.c. methods) we distinguished four cr itical concentrations which depend on the molecular weight of the samp les, the charge density of the polyelectrolytes and the ionic strength of the solutions. For very low concentrations the polyelectrolytes ar e in their most extended conformation. The reduced Kerr constants and the relaxation times determined from the electric birefringence decay are independent of concentration and the viscosity is water-like. At a concentration c(0) the distance between the chains corresponds approx imately to their persistence length and the reduced Kerr constants dec rease for the lower molecular weight samples (up to 30000 g mol(-1). F or a concentration c(1) the extended chains start to overlap and all t he samples show an increase in viscosity which follows a scaling law o f (c/c(1))(1/2). Both concentrations are in good agreement with theore tical predictions by Dobrynin et al. With further increasing polyelect rolyte concentration the now coiled chains start to overlap and to ent angle. This concentration c(2) is characterized by a strong increase i n the viscosity with the same concentration scaling of (c/c(2))(5.5) a s for uncharged polymers. The relaxation times of all samples start to increase strongly. The polyelectrolytes now behave like neutral polym ers and form a transient network structure. For the higher molecular w eight samples a further concentration c(3) is observed at which the so lutions form a (thermoreversible) gel state. Any changes in the ionic strength of the polyelectrolyte solutions cause strong changes in the different concentration regions. The addition of salt or surfactant mo lecules as well as a change in the pH value of the solutions in genera l cause a decrease in the Kerr constants, the relaxation times and the viscosity.