Abstract
The cholesteric liquid crystal self-assembly of water-suspended cellulose nanocrystal (CNC) into a helical arrangement was observed already more than 20 years ago and the phenomenon was used to produce iridescent solid films by evaporating the solvent or via sol-gel processing. Yet it remains challenging to produce optically uniform films and to control the pitch reproducibly, reflecting the complexity of the three-stage drying process that is followed in preparing the films. An equilibrium liquid crystal phase formation stage is followed by a non-equilibrium kinetic arrest, which in turn is followed by structural collapse as the remaining solvent is evaporated. Here we focus on the first of these stages, combining a set of systematic rheology and polarizing optics experiments with computer simulations to establish a detailed phase diagram of aqueous CNC suspensions with two different values of the surface charge, up to the concentration where kinetic arrest sets in. We also study the effect of varying ionic strength of the solvent. Within the cholesteric phase regime, we measure the equilibrium helical pitch as a function of the same parameters. We report a hitherto unnoticed change in character of the isotropic-cholesteric transition at increasing ionic strength, with a continuous weakening of the first-order character up to the point where phase coexistence is difficult to detect macroscopically due to substantial critical fluctuations.
Highlights
Functional materials produced in a sustainable manner from renewable resources are currently enjoying, rapidly increasing attention across the world, in recognition of their potential to deliver solutions to increasingly complex technological demands without exhausting our limited fossilebased resources
The relatively high aspect ratio rod shape promotes the long-range orientational ordering along a common orientation that is characteristic of nematic (N) liquid crystals (Onsager, 1949), and the chirality (*) of cellulose and chitin leads to a helical modulation of the director, with periods ranging from tens of micrometers down to some hundred nanometers
The conductometric titration confirmed that the final cellulose nanocrystal (CNC) surface charge was lower when less acid was used for the hydrolysis
Summary
Functional materials produced in a sustainable manner from renewable resources are currently enjoying, rapidly increasing attention across the world, in recognition of their potential to deliver solutions to increasingly complex technological demands without exhausting our limited fossilebased resources. Their surface chemistry can be readily tuned to allow easy suspension in aqueous solvents or for further chemical functionalization for incorporation in organic or inorganic matrices (Habibi, 2014), and the rods are mechanically strong, light-weight, transparent, and birefringent They have the attractive ability to self-organize into a long-range ordered, macroscopically anisotropic, liquid crystalline phase of chiral nematic (abbreviated N* and called cholesteric) type when suspended in a liquid host at volume fractions beyond a threshold value φ0 (Onsager, 1949; Marchessault et al, 1959; Revol et al, 1992; Revol and Marchessault, 1993; Lagerwall et al, 2014). The relatively high aspect ratio rod shape promotes the long-range orientational ordering along a common orientation (called the director) that is characteristic of nematic (N) liquid crystals (Onsager, 1949), and the chirality (*) of cellulose and chitin leads to a helical modulation of the director (de Gennes and Prost, 1993), with periods ranging from tens of micrometers down to some hundred nanometers
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