Directed Aggregation of Cellulose Nanocrystals to Enhance Chiral Twist

Abstract: Cellulose nanocrystals (CNCs) are bioderived nanoparticles that can be isolated from any source of natural cellulose via sulfuric acid hydrolysis. Arising from a combination of the negatively-charged sulfate half-ester groups grafted during this process and their elongated morphology, CNCs typically form colloidal cholesteric liquid crystalline phases in aqueous suspension. Recently, the chiral strength of such a CNC mesophase was correlated to the presence of CNCs with a 'bundle' morphology, analogous to the case of chiral dopants in molecular liquid crystal systems. This indicates the central role these composite particles play in the chiral behavior of CNCs, however the origin and formation pathway of the CNC bundles remains elusive. In this study, we systematically explore how different post-hydrolysis treatments alter the morphology of the CNCs (using electron microscopy, viscosimetry, and electron diffraction) and correlate this to changes in the observed liquid crystalline behavior. We found that the centrifugation step applied during CNC purification favors the formation of bundles of aligned crystallites, attached preferentially on their hydrophobic faces. This is in stark contrast to ionic treatments, where uncontrolled aggregation dominates. This reveals the importance of these often-disregarded purification steps on the final chiral and liquid crystalline properties of CNCs and promotes routes to tailor them towards a variety of applications.