Stimuli-responsive cellulose nanocrystals: From small molecule modification to controlled polymer grafting using...

Cellulose nanocrystals (CNCs) are made from naturally occurring cellulose. These nanocrystals exhibit exceptional mechanical, chemical, optical, renewable, and biocompatible properties, which have made them highly attractive for various applications. Despite their many advantageous features, CNCs are inherently hydrophilic, which limits their ability to incorporate into hydrophobic polymer matrices in high-performance nanocomposites. To address this limitation, surface functionalization methods are developed to tailor the properties of CNCs for specific applications. This review highlights various physical and chemical approaches for the modification of CNCs. Physical modification is typically achieved through electrostatic interactions, while chemical modification is conducted via two main strategies of small molecule modification and polymer grafting. The latter includes three approaches of "grafting from", "grafting onto", and "grafting through". In the "grafting from" technique, stimuli-responsive polymer chains capable of reacting to external stimuli grow directly on the surface of CNCs using different polymerization methods. Reversible deactivation radical polymerization (RDRP) techniques, such as atom transfer radical polymerization, reversible addition-fragmentation chain transfer polymerization, and nitroxide-mediated polymerization, are highly applicable in grafting reactions from the surface of CNCs. The "grafting onto" approach involves anchoring presynthesized polymers onto the surface of CNCs via coupling reactions. In the "grafting through" method, the surface of CNCs is functionalized using polymerizable groups (e.g., acrylic moieties) before in situ polymerization. Covalent grafting of stimuli-responsive polymers on CNCs aims to produce "smart" nanocrystals with tailored polymer chains on their surface. The RDRP methods help to manipulate the molecular weight of the grafted polymers and their dispersity, application of different functionalities, controlling the grafting density, and also site-specific modifications. These functionalized materials have diverse applications in drug delivery, antimicrobial systems, absorbents, Pickering emulsifiers, and biosensors for monitoring pH, temperature, bacterial growth, and glucose levels.