Structural Characterization and Modifications of Surface-oxidized Cellulose Nanofiber

Plant cellulose has a hierarchical structure with hemicelluloses and lignin in cell walls, forming natural composites at the nanometer/micrometer levels. The smallest elements next to cellulose molecules are cellulose microfibrils ~3 nm and several microns in width and length, respectively (Fig. 1). Each cellulose microfibril consists of 30-40 fully extended cellulose chains, and forms crystalline structures with some disordered regions periodically present along the longitudinal direction of cellulose microfibril. Thus, cellulose microfibrils are unique bio-based and reproducible nanofibers most abundantly present and annually accumulating on earth. However, numerous inter-fibrillar hydrogen bonds are present between cellulose microfibrils, forming cellulose microfibril bunds, in plant cellulose fibers, which generally isolated and purified from wood chips by chemical pulping and subsequent multistage bleaching treatments. Hence, it has been difficult to efficiently separate and isolate individual cellulose microfibrils from wood cellulose fibers without serious damages, such as yield loss and reduction of molecular mass. In 1980s, Turbak et al. have developed a preparation method of aqueous microfibrillated cellulose (MFC) dispersions from wood cellulose fibers by repeated high-pressure homogenization in water1), and Daicel Corp. in Japan started commercial production of MFCs in 1980s to be used as fillers in composites and filters in food industry. MFCs consist of fibers and fibrils at the nanoand micro-meter levels. In 1990s, preparation and characterization of cellulose nanocrystals (CNCs) have been reported primarily in research groups in Canada and France2). CNCs are prepared from wood, cotton, and highly crystalline bacterial and algal celluloses generally by acid hydrolysis with aqueous ~64 % H2SO4 at high temperatures, and are obtained in the yields of ~40 %. The disordered regions present in cellulose microfibrils are preferentially hydrolyzed and removed as acidic water-soluble fractions to form spindle-like or rod-like CMCs with low average aspect (length/width) ratios. 365 Journal of the Japan Petroleum Institute, 58, (6), 365-375 (2015)