Effects of plant source selection and chemi-mechanical treatment on the fiber microstructures and mechanical behaviors of nanocellulose films

Cellulose nanofibers (CNFs) were isolated and prepared from six different plant sources (Nordic pine, poplar, cotton, flax, bamboo, and pineapple leaf fibers) through a carboxymethylation-homogenization treatment. The surface morphologies, size distributions, and chemical structures of the CNFs and their microfibers were investigated in detail. Atomic force microscopy (AFM) analysis showed that all kinds of CNFs had uniform diameters of less than 10 nm. However, the length and aspect ratio of CNFs exhibited significant differences due to the differences of anatomical characteristics from pulp species. Among these six nanofibers, the pineapple leaf-based nanofibers had the highest length of ca. 2.21 μm and aspect ratio of ca. 1263. Meanwhile, the resulting pineapple leaf-based nanocellulose film possessed the strongest tensile strength (229.0 ± 9.8 MPa) and toughness (33.9 ± 2.9 MJ/m³). Interestingly, the aspect ratio of cotton nanofibers was only 556, lower than that of bamboo, Nordic pine, and flax nanofibers, but the tensile strength (210.6 ± 4.8 MPa) and toughness (22.4 ± 0.6 MJ/m³) of cotton-based nanocellulose film were second only to the pineapple leaf-based nanocellulose film. The critical reason is that the cotton-based nanocellulose exhibited the highest crystallinity index (76.6%), superior to the other source-based nanocellulose. These results suggested that the high aspect ratio or high crystallinity are responsible for the excellent mechanical strengths of the nanocellulose film. This work sheds light on the preparation and selection of highly spindly or crystalline nonwood nanofibrils, suggesting that the pineapple leaf or cotton nanofibers have great potential as strength additives for nanocomposites.