From Equilibrium Liquid Crystal Formation and Kinetic Arrest to Photonic Bandgap Films Using Suspensions of Cellulose Nanocrystals

Abstract

The lyotropic cholesteric liquid crystal phase developed by suspensions of cellulose nanocrystals (CNCs) has come increasingly into focus from numerous directions over the last few years. In part, this is because CNC suspensions are sustainably produced aqueous suspensions of a fully bio-derived nanomaterial with attractive properties. Equally important is the interesting and useful behavior exhibited by solid CNC films, created by drying a cholesteric-forming suspension. However, the pathway along which these films are realized, starting from a CNC suspension that may have low enough concentration to be fully isotropic, is more complex than often appreciated, leading to reproducibility problems and confusion. Addressing a broad audience of physicists, chemists, materials scientists and engineers, this Review focuses primarily on the physics and physical chemistry of CNC suspensions and the process of drying them. The ambition is to explain rather than to repeat, hence we spend more time than usual on the meanings and relevance of the key colloid and liquid crystal science concepts that must be mastered in order to understand the behavior of CNC suspensions, and we present some interesting analyses, arguments and data for the first time. We go through the development of cholesteric nuclei (tactoids) from the isotropic phase and their potential impact on the final dry films; the spontaneous CNC fractionation that takes place in the phase coexistence window; the kinetic arrest that sets in when the CNC mass fraction reaches ∼10 wt.%, preserving the cholesteric helical order until the film has dried; the ’coffee-ring effect’ active prior to kinetic arrest, often ruining the uniformity in the produced films; and the compression of the helix during the final water evaporation, giving rise to visible structural color in the films.

Interrogating helical nanorod self-assembly with fractionated cellulose nanocrystal suspensions

“…Fractionation of cellulose nanocrystals. Because they are difficult to determine experimentally 1 , we generally do not work with the CNC volume concentrations ϕ 0 and ϕ 1 , instead using the corresponding critical mass concentrations w 0 and w 1 . With w 1 > w 0 the anisotropic phase is denser than the isotropic, hence tactoids sink by gravity in the biphasic regime, w 0 < W < w 1 , where W is the global CNC mass concentration of the sample.…”

“…If a biphasic sample is left undisturbed, the anisotropic phase will form a bulk sediment distinct from the isotropic supernatant, see Fig. 1d, e. We exploit this phase separation by filling the pristine CNC suspension (general characterisation in Supplementary Note 1), inherently highly disperse in length 1,17,21 , into a separatory funnel from which we can easily extract the sedimented anisotropic phase after 1-2 weeks. Significantly, a spontaneous fractionation of CNCs accompanies the phase separation [26][27][28] .…”

“…n a colloidal cholesteric (chiral nematic) liquid crystal phase 1 , the nanorods align in a direction n (the director) that is helically modulated along an axis m⊥n. Iridescent reflection, circularly polarised with the same handedness as the helix, arises in a narrow wavelength band if the helix pitch p is on the order of visible light wavelengths 2 .…”

“…In fact, cholesteric helical self-assembly can be found across a large range of biomaterials 7 , providing advantages beyond optics for, e.g., optimised packing of DNA/RNA [8][9][10] and spectacular mechanical properties of composites built around collagen 11,12 , amyloid 13 , chitin 14,15 or cellulose 16,17 . Attempts to mimic this structuring via self-assembly in cholesteric colloidal nanorod suspensions, for instance by drying cholesteric suspensions of cellulose nanocrystals (CNCs) 18 , often yield non-uniform films reflecting a range of colours with imperfect circular polarisation, and reproducibility and tunability are challenging 1,[19][20][21][22][23][24] .…”

“…Our fractionation method is based on the classic Onsager theory for explaining the spontaneous appearance of nematic long-range orientational order in non-disperse colloidal nanorod suspensions 25 , stating that the isotropic phase becomes unstable beyond a rod volume concentration ϕ = ϕ 0 = 3.3d/L, where d and L are the diameter and length, respectively, of the rods (the theory assumes cylindrical rods). Liquid crystalline nuclei-tactoids 1 -then separate out, with a larger ϕ = ϕ 1 = 4.5d/L, the lower stability limit of the anisotropic nematic phase. For a chiral system, like the CNC suspensions in this study, the anisotropic phase is chiral nematic, or cholesteric.…”