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.
Highlights
We go through the development of cholesteric nuclei from the isotropic phase and their potential impact on the final dry films; the spontaneous cellulose nanocrystals (CNCs) 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
While cellulose-based liquid crystals are not novel [1], there is without doubt a strong current trend of growing interest in cellulose nanocrystals (CNCs) and the cholesteric liquid crystal phases formed by suspensions of these particles
What is the reason for the long-range orientational order in nematic nanorod suspensions? This question was first answered by Lars Onsager in the late 1940s [55], in a landmark theoretical paper that was inspired by experimental observations of nematic ordering in suspensions of tobacco mosaic virus (TMV), rod-like viruses with dimensions not too different from those of CNCs
Summary
While cellulose-based liquid crystals are not novel [1], there is without doubt a strong current trend of growing interest in cellulose nanocrystals (CNCs) and the cholesteric liquid crystal phases formed by suspensions of these particles. A decade later, Marchessault et al [18] reported on the birefringent properties of aqueous CNC suspensions, connecting them for the first time to liquid crystal formation It was only in 1992 that Gray and co-workers concluded that the equilibrium liquid crystal phase is of cholesteric type and that the CNCs organize in a helically modulated fashion as a result [2]. Six years later his group demonstrated that films dried from CNC suspensions can show striking iridescent colors [3]. Rather than merely repeating the standard statements describing a helical director modulation in cholesterics, nematic elasticity, or tactoids nucleating from the isotropic precursor, we allow ourselves to dwell a bit longer than usual on these and other issues that are central to the review topic
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