Abstract
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.
Highlights
The ordered self-organization of rod-shaped has fascinated researchers since the first observation of liquid crystal (LC) formation in aqueous suspensions of tobacco mosaic virus in the 1930s1
By exploiting the spontaneous segregation that takes place according to the ratio L/d as the LC phase separates from the isotropic surroundings, to divide our Cellulose nanocrystals (CNCs) suspension into two fractions with predominantly long and short rods, we show that L/d has no impact on the onset of gelation, while it strongly favors liquid crystallinity
This LC-driven fractionation according to L/d can be confirmed experimentally by investigating the separated phases individually, as reported previously for carbon nanotubes[26] and CNCs27
Summary
The ordered self-organization of rod-shaped has fascinated researchers since the first observation of liquid crystal (LC) formation in aqueous suspensions of tobacco mosaic virus in the 1930s1. CNCs are often conveniently approximated as cylindrical rods of crystalline cellulose with a diameter d of a few nanometers and a length L on the order of a hundred nanometers, The LC phase formed by CNC suspensions is of the chiral nematic, or cholesteric, type It was Onsager who explained how liquid suspensions of nanorods can spontaneously develop long-range orientational order—the hallmark of nematics—as a result of the gain in translational entropy overcompensating the loss in rotational entropy, provided that a sufficiently high volume fraction φ of sufficiently anisotropic nanorods is available[16]. He found the stability limit of the isotropic phase (disordered rods) to be φ0 = 3.3d/L, and the nematic phase that
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