Recent Advances and Opportunities for Cellulose Nanocrystal-Based Liquid Crystalline Polymer Hybrids and Composite Materials

Cellulose nanocrystals (CNCs) are one-dimensional, rod-like, anisotropic, renewable, sustainable, and degradable polysaccharide-based materials with a high aspect ratio and tunable surface chemistry. The intriguing ability of CNCs to self-assemble into a lyotropic chiral nematic or cholesteric liquid crystal phase with a helical arrangement that exhibits birefringence, iridescence, and pitch-dependent 1D photonic properties has attracted significant research interests. However, there are several roadblocks in the application of CNCs as sustainable and degradable liquid crystal (LC) materials. These include (i) problems with aggregation of CNCs that prevent solubility in different solvents, (ii) difficulties in processing, aligning, and spatial and temporal control of lyotropic gels and dried films of CNCs, and (iii) poor mechanical and viscoelastic properties of these gels and films. This Perspective describes the synthetic methods used to functionalize CNCs with LC pendants or liquid crystalline polymers (LCPs) by chemical conjugation or physical self-assembly. These CNC hybrid materials showed improved solubility in a variety of solvents. The interplay between the liquid crystalline CNCs and liquid crystalline LCs or LCPs during self-assembly can be tailored by selecting LCs or LCPs of a diverging orientation order. The final self-assembled hierarchical structure with or without a field-directed assembly can be used to fabricate stimuli-responsive CNC hybrid materials. Furthermore, kinetic and thermodynamic control to align LC domains and prepare materials with long-range order are also discussed. Finally, spatial–temporal control, viscoelastic properties, and degradation of the CNC hybrid materials and their applications are also reported.