The Impact of Surface Charges of Carboxylated Cellulose Nanofibrils on the Water Motions in Hydrated Films

Abstract

Cellulose nanofibrils (CNFs) with carboxylated surface ligands are a class of materials with tunable surface functionality, good mechanical properties, and bio-/environmental friendliness. They have been used in many applications as scaffold, reinforcing, or functional materials, where the interaction between adsorbed moisture and the CNF could lead to different properties and structures and become critical to the performance of the materials. In this work, we exploited multiple experimental methods to study the water movement in hydrated films made of carboxylated CNFs prepared by TEMPO oxidation with two different surface charges of 600 and 1550 μmol·g –1 . A combination of quartz crystal microbalance with dissipation (QCM-D) and small-angle X-ray scattering (SAXS) shows that both the surface charge of a single fibril and the films’ network structure contribute to the moisture uptake. The films with 1550 μmol·g –1 surface charges take up twice the amount of moisture per unit mass, leading to the formation of nanostructures with an average radius of gyration of 2.1 nm. Via the nondestructive quasi-elastic neutron scattering (QENS), a faster motion is explained as a localized movement of water molecules inside confined spheres, and a slow diffusive motion is found with the diffusion coefficient close to bulk water at room temperature via a random jump diffusion model and regardless of the surface charge in films made from CNFs.

Understanding Nanocellulose–Water Interactions: Turning a Detriment into an Asset

“…Guccini et al were able to detect two types of motions for the water inside a TOCNF network by QENS: one with fast, localized movement of water molecules and the other with slower, diffusive motion. The slower motion was independent of the surface charge of the TOCNFs …”

Protonation of Surface Carboxyls on Rice Straw Cellulose Nanofibrils: Effect on the Aerogel Structure, Modulus, Strength, and Wet Resiliency

“…1 To process or convert TEMPO-oxidized CNFs (TO-CNFs) into solids, a significant quantity of water must be removed where the concentration, 2−4 degrees of oxidation, 3,5 surface charge, 3,5 and protonation 5−7 have been shown to influence their structures and, in particular, their wet mechanical properties and performances. 5,8,9 TEMPO-mediated oxidation of purified rice straw (RS) cellulose, followed by blending, has produced TO-CNFs in reduced dimensions and increased surface charges with increasing oxidation levels. 10 From optimal TEMPO oxidation (5 mmol NaClO/g cellulose) and blending (37k rpm, 30 min), RS cellulose was readily fibrillated into ca.…”

“… 1 To process or convert TEMPO-oxidized CNFs (TO-CNFs) into solids, a significant quantity of water must be removed where the concentration, 2 − 4 degrees of oxidation, 3 , 5 surface charge, 3 , 5 and protonation 5 − 7 have been shown to influence their structures and, in particular, their wet mechanical properties and performances. 5 , 8 , 9 …”

“…In contrast, slowly freezing (−20 °C, 15 h) TO-CNFs at higher concentrations (0.2–0.6 w/v %) nucleated much larger ice crystals to further concentrate TO-CNFs toward extensive association and, upon lyophilization, produced an interconnecting-thin-film-like cellular wall structure surrounding large pores vacated from the sublimation of ice crystals in the form of ultra-low-density aerogels. 2 While surface charges have affected TO-CNF organization in films due to varying oxidation levels, 5 or different levels of protonation under the same oxidation, 7 how surface charges, or the lack thereof, affect the self-assembly of TO-CNFs in aerogels via slow ice-templating and lyophilization has not been elucidated.…”

“…The presence of charges in the fabrication and dispersion of nanocelluloses is well recognized and has been extensively studied in CNFs by 2,2,6,6-tetramethyl piperidine-1-oxyl (TEMPO)-mediated oxidation . To process or convert TEMPO-oxidized CNFs (TO-CNFs) into solids, a significant quantity of water must be removed where the concentration, − degrees of oxidation, , surface charge, , and protonation − have been shown to influence their structures and, in particular, their wet mechanical properties and performances. ,, …”