Structural Characterization and Modifications of Surface-oxidized Cellulose Nanofiber

Nanocelluloses with widths of <100 nm are prepared from abundant plant celluloses with or without chemical, enzymatic or physicochemical pretreatment by mechanical disintegration in water, and thus are bio-based and reproducible nanomaterials. In the 21st century, preparations and characterizations of various nanocelluloses and nanocellulose-containing composite materials have been extensively and intensively studied, and some of them have unique optical/mechanical/thermal/gas-barrier/catalytic properties applicable to high-tech material fields. In our laboratory, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-mediated oxidations of various polysaccharides including celluloses have been studied from 1995, and found to be the most efficient pretreatment for native plant celluloses to be converted to completely individual TEMPO-oxidized cellulose nanofibers (TOCNs) with homogeneous 3 nm widths and high aspect ratios. In this review paper, our recently reported papers concerning the following fundamental studies of fibrous TEMPO-oxidized celluloses (TOCs) and TOCNs are summarized and introduced: (1) determination of absolute molecular masses/molecular mass distributions of TOCs and TOCNs, (2) determination of aspect ratios of TOCNs from intrinsic viscosities of dilute TOCN/water dispersions, (3) simple and efficient surface hydrophobization of the originally hydrophilic TOCNs by counterion exchange treatments, and (4) preparation and characterization of transparent TOCN aerogels with low densities, high specific surface areas, low thermal conductivities and high mechanical performances.