Cellulose nanocrystals (CNCs) are bio-sourced rod-like colloidal particles that can spontaneously form a cholesteric (chiral nematic) liquid crystal phase in suspension, leading to vivid structurally colored films in the solid state. However, despite intensive research on CNCs over the past few decades, the origin of their mesophase chirality remains unclear. While the underlying chiral crystalline structure of cellulose, mediated by the morphology of individual nano-rods, is widely believed to play a crucial role, most CNCs are not strongly twisted, and samples exhibit considerable polydispersity in both particle size and shape.<br/><br/>In this study, we used high-intensity ultrasound to gradually tune the size and shape of the CNCs, enabling direct comparison between their individual morphology and ensemble self-assembly behavior. By employing advanced statistical analysis of the particle shapes obtained from transmission electron microscopy and atomic force microscopy images, we established a quantitative relationship between the proportion of “bundles” (clusters of laterally-bound cellulose crystallites) and the cholesteric pitch, indicating that these bundles act as colloidal chiral dopants, analogous to those used in molecular liquid crystals. By uncovering this missing link in the transfer of chirality across length-scales, we explain the origin of the CNC cholesteric phase, and provide new insight into other recent achievements in the field, such as the effect of fractionation on CNC self-assembly.

Bundle abundance and chiral interactions in cellulose nanocrystal suspensions

ACS Spring 2022

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