Surface modifications of nanocellulose: From synthesis to high-performance nanocomposites

Nanocellulose is at the cutting edge of current research owing to its highly useful features, such as abundance and renewability of its source, biocompatibility, nanoscale dimension, high specific surface area, and tunable surface chemistry. Its multifunctional characteristics present opportunities for novel applications across a wide range of emerging fields. However, despite of its many beneficial features, its inherent hydrophilicity is posing a substantial challenge for the application of nanocellulose as reinforcing filler in conventional plastics as it complicates its dispersion in hydrophobic host polymers and often results in aggregated structures that detriment properties. The formation of aggregates is a significant barrier to achieving outstanding mechanical performance in composites. Manipulation and fine-tuning of the interfacial properties of nanocellulose has thus been recognized as a crucial step in exploiting its full potential in the development of new materials. Diverse surface-modification routes are now being developed to improve the miscibility and interfacial compatibility of nanocellulose with hosting matrices and also to confer new functionalities. We present a comprehensive overview of the fundamental aspects of a broad range of surface-modification strategies, from basic grafting of small molecules to sophisticated development of responsive platforms. We have emphasized the quantification of these modifications in terms of degree of substitution and their possible relationship with level of dispersion in organic solvents. We also reviewed the polymer reinforcement as it is one of the most prominent areas of nanocellulose research. We cover various domains of nanocellulose-reinforced polymer composites that are reported in the current literature and provide insights into technical breakthroughs and the scientific underpinnings of reinforcement mechanisms of nanocellulose in various polymer matrices with the aid of easy-to-understand illustrations. Finally, we critically evaluate the challenges encountered in the current research and highlight future opportunities for research to enable tunable and high-performance nanostructured cellulosic materials for applications across engineering and biomedical sciences.