Fractionation of cellulose nanocrystals: enhancing liquid crystal ordering without promoting gelation

Colloids of electrically charged nanorods can spontaneously develop a fluid yet ordered liquid crystal phase, but this ordering competes with a tendency to form a gel of percolating rods. The threshold for ordering is reduced by increasing the rod aspect ratio, but the percolation threshold is also reduced with this change; hence, prediction of the outcome is nontrivial. Here, we show that by establishing the phase behavior of suspensions of cellulose nanocrystals (CNCs) fractionated according to length, an increased aspect ratio can strongly favor liquid crystallinity without necessarily influencing gelation. Gelation is instead triggered by increasing the counterion concentration until the CNCs lose colloidal stability, triggering linear aggregation, which promotes percolation regardless of the original rod aspect ratio. Our results shine new light on the competition between liquid crystal formation and gelation in nanoparticle suspensions and provide a path for enhanced control of CNC self-organization for applications in photonic crystal paper or advanced composites. The shape of nanoscale cellulose rods determines how they self-organize when suspended in water, show scientists in Luxembourg and the UK. Many interdependent factors determine whether the rods organize into a so-called liquid crystal state, dynamic and responsive, or if they are pinned in an immobile static gel. This makes it difficult to establish a clear understanding of the self-organization process. Jan Lagerwall from the University of Luxembourg and co-workers simplified the analysis by investigating a colloidal suspension of cellulose nanocrystals that spontaneously separates into rods of different length. They showed that colloids with a larger aspect ratio (the length of the nanorod divided by its diameter), favor a liquid-crystal state, but—contrary to expectation—they do this without promoting gelation. Such control of the self-organization of cellulose nanoparticles could enable more advanced composite materials. Fractionating nanorod suspensions by aspect ratio strongly affects liquid crystal formation without affecting gelation, in contrast to expectations from the classic percolation-based model. Gelation is rather triggered by increasing the counterion concentration until the CNCs lose colloidal stability. As a consequence, we significantly extend the regime where we can study equilibrium liquid crystal behavior without risking arrest into a gel state.