Structure and solution properties of sodium car☐ymethyl cellulose

Abstract

We have investigated the macroscopic properties of eight commerical samples of sodium car☐ymethyl cellulose (Na-CMC) with molecular weights between 9000 and 360 000 g mol −1 and substitution degrees between 0.75 and 1.47 in aqueous solutions. From rheological and electric birefringence measurements (a.c. and d.c. methods) we distinguished four critical concentrations which depend on the molecular weight of the samples, the charge density of the polyelectrolytes and the ionic strength of the solutions. For very low concentrations the polyelectrolytes are 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 approximately to their persistence length and the reduced Kerr constants decrease for the lower molecular weight samples (up to 30 000 g mol −1). For a concentration c 1 the extended chains start to overlap and all the samples show an increase in viscosity which follows a scaling law of( c/c 1) 1/2. Both concentrations are in good agreement with theoretical predictions by Dobrynin et al. With further increasing polyelectrolyte concentration the now coiled chains start to overlap and to entangle. This concentration c 2 is characterized by a strong increase in the viscosity with the same concentration scaling of( c/c 2) 5.5 as for uncharged polymers. The relaxation times of all samples start to increase strongly. The polyelectrolytes now behave like neutral polymers and form a transient network structure. For the higher molecular weight samples a further concentration c 3 is observed at which the solutions 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 molecules as well as a change in the pH value of the solutions in general cause a decrease in the Kerr constants, the relaxation times and the viscosity.