Sodium Carboxymethyl Cellulose (Na CMC) is used for its thickening and swelling properties in a wide range of applications including pharmaceutical, food, home and personal care products, as well as in paper, water treatment and mineral processing industries. Despite its broad industrial use, Na CMC structure-property relationships in solution are known to be highly complex and influenced by a number of parameters. The focus of this thesis was to expand fundamental understanding of its solution characteristics.
The behaviour of salt-free semi-dilute aqueous Na CMC solutions was investigated over a wide range of concentrations using rheology as well as Light Scattering (LS). The concentration dependence of the solution specific viscosities could be described using an approach which interpolates between two power law regimes and introduces one characteristic crossover concentration. This behaviour was interpreted as a transition from the semi-dilute non-entangled to the entangled concentration regimes, and was not observed in the solution structure, as determined using Static Light Scattering (SLS). Dynamic Light Scattering (DLS) revealed three relaxation modes. The two faster relaxations were assigned as the ‘fast’ and ‘slow’ relaxation modes typically observed in salt-free polyelectrolyte solutions within the semi-dilute concentration range. The third, typically weak mode, was attributed to the presence of a small amount of poorly dissolved cellulose residuals. Since filtration altered the solution behaviour, without sufficiently removing the residuals, data collection and processing were adapted to account for this, which facilitated a detailed light scattering investigation of the original solutions. The relaxation time of the slow mode demonstrated a similar crossover behaviour as observed for the specific viscosity, further demonstrating the crossover dynamic nature.
Sonication led to a decrease in the solution shear-thinning behaviour and viscosity. The relaxation time of the slow relaxation was also found to decrease, consistent with the previously established correlation between solution viscosity and the slow relaxation mode. SLS did not probe any change in the structure. Additional LS and Size Exclusion Chromatography (SEC) suggested that there was no significant Na CMC chain breakage, if any, and that the proportion of Na CMC chains involved in the domains decreased. Hence, it was proposed that the shear forces applied during sonication were strong enough to remove some Na CMC chains from the domains, while the domain sizes remained constant. These looser domains would be more deformable under flow, leading to the observed decrease in the viscosity and shear-thinning behaviour. A partial or full recovery of the slow relaxation mode was also observed after a month, supporting the hypothesis that domains could correspond to a metastable equilibrium, while sonication had no effect on the solutions with added salt, suggesting that electrostatic forces could be responsible for the cohesion of the domains.
