Self-aligned integration of native cellulose nanofibrils towards producing diverse bulk materials

Never-dried and once-dried hardwood celluloses were oxidized by a 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated system, and highly crystalline and individualized cellulose nanofibers, dispersed in water, were prepared by mechanical treatment of the oxidized cellulose/water slurries. When carboxylate contents formed from the primary hydroxyl groups of the celluloses reached approximately 1.5 mmol/g, the oxidized cellulose/water slurries were mostly converted to transparent and highly viscous dispersions by mechanical treatment. Transmission electron microscopic observation showed that the dispersions consisted of individualized cellulose nanofibers 3-4 nm in width and a few microns in length. No intrinsic differences between never-dried and once-dried celluloses were found for preparing the dispersion, as long as carboxylate contents in the TEMPO-oxidized celluloses reached approximately 1.5 mmol/g. Changes in viscosity of the dispersions during the mechanical treatment corresponded with those in the dispersed states of the cellulose nanofibers in water.

Cellulose nanofibrils offer interesting potential as a native fibrous constituent of mechanical performance exceeding the plant fibers in current use for commercial products. In the present study, wood nanofibrils are used to prepare porous cellulose nanopaper of remarkably high toughness. Nanopapers of different porosities and from nanofibrils of different molar mass are prepared. Uniaxial tensile tests are performed and structure-property relationships are discussed. The high toughness of highly porous nanopaper is related to the nanofibrillar network structure and high mechanical nanofibril performance. Also, molar mass correlates with tensile strength. This indicates that nanofibril fracture controls ultimate strength. Furthermore, the large strain-to-failure means that mechanisms, such as interfibril slippage, also contributes to inelastic deformation in addition to deformation of the nanofibrils themselves.