Viscoelastic properties of nanocelluloses in liquid and dried states: Comparison of core-shell vs. surface-charged structures

The influence of the electrostatic or steric stabilization of nanocelluloses in water on their viscoelastic properties as dispersions and assembled films was investigated. Sterically stabilized nanocelluloses were extracted from Japanese persimmons via acid treatment, purification, and bleaching. The obtained nanocelluloses were composed of cellulose, hemicelluloses and pectin. Hemicelluloses and pectin were organized as steric layers on the cellulose nanofibril surfaces, forming core-shell structured nanocelluloses with carboxylate content of 0.2 mmol/g. Electrostatically stabilized nanocelluloses, namely surface-charged nanocelluloses, with higher carboxylate content of 0.9 mmol/g were prepared from softwood bleached kraft pulp by 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-mediated oxidation. Both the core-shell structured and surface-charged nanocelluloses were prepared as dispersions at the individual nanofibrils level in water. Viscoelastic properties of the obtained dispersions and their assembled films prepared by spin-coating method were estimated using a rheometer and a quartz crystal microbalance with dissipation (QCM-D) instrument, respectively. The influence of the pH and salt concentration on these properties were assessed. When the pH and salt concentration of the nanocellulose dispersions were changed, the core-shell structured nanocellulose dispersions showed smaller variation in viscoelastic properties than the surface-charged ones. The core-shell structured nanocellulose dispersions were more stable against changes of the pH and salt concentration in terms of their viscoelastic properties. The previous statement was then confirmed by the QCM-D measurement of the assembled films; the core-shell structured nanocelluloses showed lower storage and loss moduli.