Grafting Polymers from Cellulose Nanocrystals: Synthesis, Properties, and Applications

Over the past 10 years, the grafting of polymers from the surface of cellulose nanocrystals (CNCs) has gained substantial interest in both academia and industry due to the rapidly growing number of potential applications of surface-modified CNCs, which range from building blocks in nanocomposites and responsive nanomaterials to antimicrobial agents. CNCs are rod-like nanoparticles that can be isolated from renewable biosources and which exhibit high crystallinity, tunable aspect ratio, high stiffness, and strength. Upon drying, the abundance of surface hydroxyl groups often leads to a degree of irreversible aggregation, as a result of strong hydrogen bonding. Moreover, their relatively hydrophilic character renders CNCs incompatible with hydrophobic media, e.g., nonpolar solvents and polyolefin matrices. By grafting macromolecules from their surface, CNCs can be imparted with surface characteristics and other physicochemical properties that are reminiscent of the grafted polymer. This has allowed the design of nanoscale building blocks whose readily tunable properties are useful for the formation of both colloidal dispersions and solid state materials. In this Perspective, we provide an overview of the morphology and surface chemistry of CNCs and detail various techniques to manipulate their surface chemistry via polymer grafting from approaches. Moreover, we explore the most common polymerization techniques that are used to graft polymers from the surface and reducing end groups of CNCs, including surface-initiated ring-opening polymerization (SI-ROP), surface-initiated free (SI-FRP), and controlled (SI-CRP) radical polymerization. Finally, we provide insights into some of the emerging applications and conclude with an outlook of future work that would benefit the field.