Interrogating helical nanorod self-assembly with fractionated cellulose nanocrystal suspensions

Abstract

The helical self-assembly of cholesteric liquid crystals is a powerful motif in nature, enabling exceptional performance in many biological composites. Attempts to mimic these remarkable materials by drying cholesteric colloidal nanorod suspensions often yield films with a non-uniform mosaic-like character, severely degrading optical and mechanical properties. Here we show—using the example of cellulose nanocrystals—that these problems are due to rod length dispersity: uncontrolled phase separation results from a divergence in viscosity for short rods, and variations in pitch can be traced back to a twisting power that scales with rod length. We present a generic, robust and scalable method for fractionating nanorod suspensions, allowing us to interrogate key aspects of cholesteric self-assembly that were previously hidden by colloid dispersity. By controlled drying of fractionated suspensions, we can obtain mosaic-free films that are uniform in colour. Our findings unify conflicting observations and open routes to biomimetic artificial materials with performance that can compete with that of nature’s originals.

Concentric chiral nematic polymeric fibers from cellulose nanocrystals

“…4), which occurred at 12 wt%. Previous reports have shown reducing the length of the CNC particles decreases the cholesteric pitch and increases the critical concentration required for cholesteric phase formation, in agreement with our result; 29,30 however, the observed difference between CNC batches may not be due solely to the particles size, 31 and instead could be caused by CNCs being obtained from different cellulose sources or due to variable reaction conditions during CNC extraction. The effect of salt addition was also tested by adding NaCl (0-12 mM) to a 9 wt% CNC suspension prior to capillary tube lling.…”

Modeling the cholesteric pitch of apolar cellulose nanocrystal suspensions using a chiral hard-bundle model

“…CNCs that are larger in both their length and width rather have a stronger helical twisting power, and their presence in the suspension can act as "chiral dopants" similar to the chiral molecules added to achiral molecular nematic liquid crystals to induce cholesteric order 64 . This echoes with the recent work of Honorato-Rios and Lagerwall, where they even defined a Helical Twisting Power (HTP) associated to the CNCs and in first approximation proportional to the CNC size 62 . In their work, they consider the approximate relationship 1/P = A(c eff − c 0 ), where A is identified as the HTP, and c 0 a constant that they proposed to interpret as the threshold mass fraction required to convey chiral interactions.…”

“…S4 of the supplementary material. We also plotted with a dashed line the approximate values of A and c 0 found by Honorato-Rios et al 62 .…”

“…Since then this dependence has been taken for granted in the literature 60,61 . However, Honorato-Rios and Lagerwall recently reported, using fractionated aqueous CNC suspensions, that actually longer CNCs lead to smaller pitch values 62 . This result disagrees with the scenario of FIG.…”

“…5(a), and suggests that the change in pitch length is not only determined by the length of the CNCs. 62 .…”