We report the mechanical strength of native cellulose nanofibrils. Native cellulose nanofibrils, purified from wood and sea tunicate, were fully dispersed in water via a topochemical modification of cellulose nanofibrils using 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) as a catalyst. The strength of individual nanofibrils was estimated based on a model for the sonication-induced fragmentation of filamentous nanostructures. The resulting strength parameters were then analyzed based on fracture statistics. The mean strength of the wood cellulose nanofibrils ranged from 1.6 to 3 GPa, depending on the method used to measure the nanofibril width. The highly crystalline, thick tunicate cellulose nanofibrils exhibited higher mean strength of 3-6 GPa. The strength values estimated for the cellulose nanofibrils in the present study are comparable with those of commercially available multiwalled carbon nanotubes.

Plant holocelluloses were prepared from softwood, gymnosperm, hardwood, and herbaceous species, and subjected to TEMPO-mediated oxidation using the TEMPO/NaBr/NaOCl and TEMPO/NaOCl/NaO 2 Cl systems in water at pH 10 and 6.8, respectively. Weight recovery ratios of the water-insoluble TEMPO-oxidized holocellulose (TOH) fractions and their carboxylate contents, sugar compositions, and X-ray diffraction patterns were measured. When the oxidation at pH 10 was used, the carboxylate content of the TOHs increased up to 1.4-1.7 mmol g-1. The oxidation at pH 6.8 resulted in higher weight recovery ratios of TOHs and their lower carboxylate contents (0.8-1.2 mmol g-1) than those prepared by the oxidation at pH 10. Hemicelluloses in plant holocelluloses are preferentially degraded to water-soluble fractions and removed from TOHs in the oxidation at pH 10. In contrast, the TEMPO-mediated oxidation at pH 6.8 provides hemicellulose-rich TOHs in high weight recovery ratios, although their nanofibrillation yields were low. All TEMPO-oxidized holocellulose nanofibrils (TOHNs) obtained by mechanical disintegration treatment of TOHs in water had the same average widths of ~3 nm, when measured by atomic force microscopy in water, which were consistent with those of TOHs determined from X-ray diffraction patterns. The number-average lengths of TOHNs were 500-600 nm.

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Three holocelluloses (i.e., cellulose and hemicellulose fractions) are prepared from softwood and hardwood by the Wise method. These holocelluloses completely dissolve in 8% lithium chloride/1,3-dimethyl-2-imidazolidinone (LiCl/DMI) after an ethylenediamine (EDA) pretreatment. After diluting the holocellulose solutions to 1% LiCl/DMI, they are subjected to size-exclusion chromatography/multiangle laser-light scattering/photodiode array (SEC-MALLS-PDA) analysis. All holocelluloses exhibit bimodal molecular weight distributions primarily due to high-molecular-weight (HMW) cellulose and low-molecular-weight hemicellulose fractions. Plots of molecular weight vs root-mean-square radius obtained by SEC-MALLS analysis revealed that all the wood celluloses comprise dense conformations in 1% LiCl/DMI. In contrast, bacterial cellulose, which was used as a pure cellulose model, has a random coil conformation as a linear polymer. These results show that both softwood and hardwood HMW celluloses contain branched structures, which are probably present on crystalline cellulose microfibril surfaces. These results are consistent with those obtained by permethylation analysis of wood celluloses.