Surface modifications of cellulose nanocrystals: Processes, properties, and applications

The interest in nanocellulose has recently increased in many fields due to its natural abundance, exceptional mechanical/optical properties, and better biocompatibility. During the production of cellulose nanocrystals (CNCs) via sulfuric acid hydrolysis, sulfate groups are introduced, which decrease the thermal stability of CNCs and, thus, have a profound negative effect on the potential application of CNCs in nanocomposites. Also, mechanical methods exhibit poor morphological properties in CNCs with a reduced degree of crystallinity. Therefore, to improve the processability and performance of CNCs and extend their industrial applications and quality, nano-cellulose undergoes surface modifications by physical and chemical means. Surface modifications of CNCs lower the surface energy, increase hydrophobicity, improve interfacial adhesion, enhance their compatibility between nanocomposite components, and improve their dispersion and interaction. Surface-modified CNCs have wide applications in medicine, catalysis, optics, remediation processes, electronics, textiles, pulp, paper, etc. Other applications of CNCs are: supporting catalysts and sensors, as diaphragms in earphones; for tissue engineering, scaffolds, for toughened paper, as polymer nanocomposites for developing membranes, in flexible panels for flat panel displays, in the optical application and biomimetic foams, and as rheology modifiers. This review provides the latest advances in surface modification of CNCs and the relevant processes, properties, and applications.