Cellulose nanocrystals (CNCs) are bio-derived elongated nanoparticles that have attracted growing research interest over the past few decades. In particular, the spontaneous formation of a left-handed cholesteric phase in CNC suspensions is a promising way to create structurally colored films from this sustainable material. However, the inherent polydispersity of CNCs, and the associated challenges in characterizing CNC morphology, have made it difficult to establish clear structure-property relationships and have therefore hindered attempts to optimize the performance of cellulose-based materials.<br/>Here we present a workflow for CNC shape metrology that combines transmission electron microscopy (TEM) and atomic force microscopy (AFM), to provide morphological information on individual particles that is consistent with ensemble size measurements from dynamic light scattering, and UV-vis spectroscopy. As a demonstration of the relevance of this approach, we correlated the morphology of individual CNCs with their ensemble behavior in suspension and the color of photonic films. This analysis suggests that “bundles” (i.e. clusters of laterally-bound cellulose crystallites) are essential for the formation of a cholesteric phase. We propose that this sub-population can be interpreted as “chiral dopants”, analogous to those seen for molecular liquid crystals, and the color of CNC photonic films can therefore be tuned by controlling the proportion of bundles in the initial suspension.

Morphological analysis of cellulose nanocrystals: Insights into chiral self-assembly

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