Abstract
Sol-gel transition of carboxylated cellulose nanocrystals has been investigated using rheology, SAXS, NMR and optical spectroscopies to unveil the distinctive roles of ultrasound treatments and addition of various cations. Besides cellulose fiber fragmentation, sonication treatment induces fast gelling of the solution. The gelation is independent of the addition of cations, while the final rheological properties are highly influenced by the type, concentration and sequence of the operations since the cations must be added prior to sonication to produce stiff gels. The gel elastic modulus was found to increase proportionally to the ionic charge rather than the cationic size. In cases where ions were added after sonication, SAXS analysis of the Na+ hydrogel and Ca2+ hydrogel indicated the presence of structurally ordered domains in which water is confined, and 1H-NMR investigation showed the dynamics of water exchange within the hydrogels. Conversely, separated phases containing essentially free water were characteristic of the hydrogels obtained by sonication after Ca2+ addition, confirming that this ion induces irreversible fiber aggregation. The rheological properties of the hydrogels depend on the duration of the ultrasound treatments, enabling the design of programmed materials with tailored energy dissipation response.
Highlights
Nanocellulose (NC) is a renewable and biocompatible material with interesting and versatile properties which allow its integration in a huge number of applications, as has been extensively reviewed[1, 2]
In this regard NC behaves as a typical polysaccharide and its structure and dynamics in solution strongly depends on electrostatic bonds and on the surface available for their occurrence
Sol-gel transition in TEMPO oxidized cellulose nanocrystals aqueous solutions. 40 mL of aqueous slurry containing 6 mgmL−1 TOCs at pH 7 was sonicated for variable times
Summary
Nanocellulose (NC) is a renewable and biocompatible material with interesting and versatile properties which allow its integration in a huge number of applications, as has been extensively reviewed[1, 2]. The procedures to break natural cellulose and obtain nano-sized structures are usually based on the combination of chemical modification or enzymatic hydrolysis with mechanical refinement[3, 4] Fine changes of these procedures give rise to different nanostructure morphology: branched nanofibrils with amorphous regions and rod-like rigid nanocrystals[5]. The self-assembly of NC or NC-composites into soft hydrogels[13, 14] has been characterized in terms of macroscopic parameters such as mesh size, charge density, gelation rate, mechanical performances, or stability[15,16,17,18] In this context, rheology experiments have been performed and a gel-like behavior of NC suspensions with an elastic response even at a low concentration[19] has been reported. The stability of the hydrogel depends on the sonication treatment and this fact foresees the possibility of fabricating programmable gels that behave differently depending on the amount of energy they have to dissipate
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