The Importance of Crystallite Bundles in The Chiral Self-Assembly of Cellulose Nanocrystals: Origin, Morphology and Function

Cellulose nanocrystals (CNCs) are elongated negatively-charged nanoparticles produced by acid hydrolysis of cellulosic biomass (e.g. cotton, wood pulp). CNC suspensions spontaneously form a left-handed cholesteric mesophase, which has a periodicity (pitch) determined by the characteristics of the individual CNCs (e.g. morphology, surface chemistry) and the suspension formulation (e.g. solvent, additives). This helicoidal arrangement of CNCs can be preserved as the suspension is dried, resulting in films with vibrant structural color. The chiral self-assembly of CNCs is thus a promising route to create materials with a tunable optical response from a sustainable biopolymer feedstock, but understanding how the properties of the large-scale structure emerge from the behavior of individual nanoparticles has been a persistent challenge.<br/><br/>First, by gradually tuning the size and shape of cotton CNCs using ultrasonication, and correlating the morphology of individual particles with their cholesteric mesophase behavior in suspension, we found that CNC “bundles” (i.e. clusters of laterally-bound elementary crystallites) are essential for the formation of a cholesteric phase. These bundles appear to act as chiral dopants, analogous to those used for molecular liquid crystals, whereby an increase in dopant concentration leads to a decrease in cholesteric pitch (and blueshifted film color). We then compared common ways to induce aggregation in CNC suspensions (e.g., post-hydrolysis centrifugation or excess ionic strength). This investigation revealed that the formation pathway significantly affects the morphology of the bundles and thus their effectiveness as chiral dopants. These findings demonstrate that the "making" and "breaking" of crystallite bundles offers a facile way to tailor the self-assembly behavior of CNC suspensions.