Effect of Chemical Functionality on the Mechanical and Barrier Performance of Nanocellulose Films

In the present work, we have partially modified fibrils chemically to create a shell of derivatized cellulose that surrounds the crystalline core of native cellulose. Through the different modifications, we aimed at creating a toolbox to enable the properties of CNF materials and materials containing CNFs to be tuned to meet specific material demands. In total, nine different chemical modifications using different aqueous-based procedures were used as chemical pretreatments before CNF production through homogenization. Eight of these modifications included periodate oxidation with an average of 27% of the anhydroglucose units in the cellulose chain being cleaved into dialdehydes. The presence of aldehydes then facilitated a conversion to other functional groups. TEMPO oxidation was also performed, alone or combined with either periodate oxidation or periodate oxidation followed by borohydride reduction or chlorite oxidation. The nine different CNFs produced after chemical modifications were characterized by size, charge density, and colloidal stability. Free-standing films were subsequently fabricated form the fibrils and characterized by mechanical, moisture adsorption, and barrier properties, as well as by their degradation upon heating. The properties of the CNFs achieved through the different modification methods varied depending on the functional group present in the modified CNF shell. After periodate oxidation, no colloidal stability was observed. Interestingly, the periodate-oxidized fibers could be fairly easily homogenized at a concentration of 4 wt %, in contrast to 2 wt % for the other fibers. When charges were introduced onto the periodate-oxidized fibers, CNFs with a higher colloidal stability were produced, but to facilitate CNF production, the mass concentration during this step had to be decreased. After the sequential modification by periodate oxidation and borohydride reduction, a great increase in ductility in the films was observed, but interestingly, when more charges were introduced, this increase in ductility was lost, and the mechanical strength increased. CNFs without further modification after the periodate oxidation resulted in cross-linked films with low moisture sorption and a low oxygen permeability: 0.2–0.7 mL·μm/(m2·24h ·kPa) at 50% RH and 0.3–14 mL·μm/(m2·24h·kPa) at 80% RH depending on the modification performed.