Cellulose Nanofibers: Recent Progress and Future Prospects

: Nanocelluloses are prepared by downsizing plant cellulose fibers, which are efficiently produced at the industrial level as paper and dissolving pulps from renewable wood biomass resources. The number of scientific publications and patents concerning nanocelluloses has been increasing every year, because nanocelluloses are expected to contribute to creation of a sustainable society partly in place of petroleum-based materials. Nanocelluloses are categorized as cellulose nanonetworks (CNNeWs), cellulose nanofibrils or nanofibers (CNFs), and cellulose nanocrystals (CNCs) depending on their morphologies, originating from crystalline cellulose microfibrils abundantly present in each plant cellulose fiber. When no chemical pretreatment is applied to plant cellulose fibers, only CNNeW-type nanocelluloses with heterogeneous morphologies are obtained even after harsh mechanical disintegration in water. In contrast, when position-selective chemical pretreatment is applied to plant cellulose fibers for introduction of a large amount of charged groups on the cellulose microfibril surfaces, CNFs and CNCs with homogeneous ~3 nm widths can be prepared from the chemically pretreated plant cellulose fibers by gentle mechanical disintegration in water. These charged groups are used as scaffolds to add diverse functionalities to nanocelluloses by simple ion exchange in water. Chemical modifications of nanocellulose surfaces, hydrogels, preparation of nanocellulose-containing composites with various organic and inorganic compounds, the fabrication processes from nanocellulose/water dispersions to dried films, fibers, and porous materials, as well as their versatile applications, have been extensively reported in the last few years. In this review, some research topics are selected from nanocellulose-related publications and briefly overviewed.