Strong aqueous gels of cellulose nanofibers and nanowhiskers isolated from softwood flour

Two nanocelluloses (cellulose nanofibers [CNF] and nanowhiskers [CNW]) were extracted from softwood flour using chemical refining followed either by mechanical fibrillation or acid hydrolysis. The CNF slurry formed an opaque gel that exhibited highly coiled and entangled long fibers with widths between 10 and 20 nm when studied using atomic force microscopy (AFM). The aqueous suspension of the CNW formed a transparent gel with unique morphology of rigid and uniform, whiskerlike structures with widths as low as 1.5-3 nm and lengths in micrometer levels. The viscoelastic properties of these hydrogels with solids content of 0.2 wt% were measured using dynamic rheology experiments. The elastic modulus (G') and viscous modulus (G '') were frequency independent in the low-frequency region. Furthermore, G' was almost 10-fold higher than G '', showing a typical elastic gel behavior. The lower crystallinity obtained from X-ray analysis indicated that the unique structure of CNW from wood could be attributed to the native cellulose being partly dissolved and regenerated during acid hydrolysis.