Wood nanocelluloses: fundamentals and applications as new bio-based nanomaterials

Nanocelluloses, which include nanofibrillated celluloses (NFCs) and cellulose nanocrystals (CNCs) with high and low aspect ratios, respectively, are promising new bio-based nanomaterials, prepared from wood and other plant celluloses by mechanical shearing in water with or without pretreatments. Low degrees of enzymatic hydrolysis, carboxymethylation, acetylation, oxidation, and other position-selective modifications on cellulose microfibril surfaces have been applied as pretreatments to wood celluloses to reduce energy consumption in the mechanical shearing process and to improve the nanofibrillation level of the obtained NFCs. NFCs are convertible to nanocellulose sheets, films, hydrogels, foams, and aerogels with fibril network structures or close-packing structures using coating on base films or filtration process like papermaking, which is advantageous for efficient removal of water predominantly present in the NFC/water dispersions. NFC-containing self-standing films, coated films, and NFC/matrix nanocomposites in most cases show explicitly high mechanical strength and ductility despite being lightweight and having optical transparency, thermal stability, and gas-barrier properties. Because NFCs have aspect ratios and molecular weights higher than those of CNCs, the most promising and challenging end products are NFC-containing nanocomposite materials having higher functionalities than those of the conventional fiber-reinforced composite materials.