Rheological Properties of Cholesteric Liquid Crystal with Visible Reflection from an Etherified Hydroxypropyl Cellulose Derivative

Structural Color Out of the Blue: A Quantitative Framework for the Self-Assembly Kinetics of Cholesteric Cellulosic Mesophases

Cholesteric mesophases based on cellulose ethers, such as ethyl cellulose and hydroxypropyl cellulose, have been studied widely for their remarkable ability to display macroscopic structural color. However, the typical time scales involved in the multiscale self-assembly of cholesteric liquid crystals, from individual nanoscale helical arrangements to discrete microscopic domains, and their dependence on the gel's viscoelastic properties remain underexplored. Here, we establish a quantitative relationship between the kinetics of structural color formation after shear deformation and cholesteric order development at the nano- and microscales. Utilizing rheology in tandem with static and time-resolved reflectivity measurements, we underscore the strong influence of polymer diffusivity and chain elasticity on self-assembly kinetics in cholesteric cellulose ether gels. We show that our phenomenological model can be employed to assess the structure-property relationships of multiple polysaccharide systems, elucidating key design guidelines for the development and processing of structurally colored cholesteric mesophases.

Cholesteric Liquid Crystals with Thermally Stable Reflection Color from Mixtures of Completely Etherified Ethyl Cellulose Derivative and Methacrylic Acid

Cellulose derivatives have attracted attention as environmentally friendly materials that can exhibit a cholesteric liquid crystal (CLC) phase with visible light reflection. Previous reports have shown that the chemical structures and the degrees of substitution of cellulose derivatives have significant influence on their reflection properties. Although many studies have been reported on CLC using ethyl cellulose (EC) derivatives in which the hydroxy groups are esterified, there have been no studies on EC derivatives with etherified side chains. In this article, we optimized the Williamson ether synthesis to introduce pentyl ether groups in the EC side chain. The degree of substitution with pentyl ether group (DSPe), confirmed via 1H-NMR spectroscopic measurements, was controlled using the solvent and the base concentration in this synthesis. All the etherified EC derivatives were soluble in methacrylic acid (MAA), allowing for the preparation of lyotropic CLCs with visible reflection. Although the reflection peak of lyotropic CLCs generally varies with temperature, the reflection peak of lyotropic CLCs of completely etherified EC derivatives with MAA could almost be preserved in the temperature range from 30 to 110 °C even without the aid of any crosslinking. Such thermal stability of the reflection peak of CLCs may be greatly advantageous for fabricating new photonic devices with eco-friendliness.

Dominant Factors Affecting Rheological Properties of Cellulose Derivatives Forming Thermotropic Cholesteric Liquid Crystals with Visible Reflection

Hydroxypropyl cellulose (HPC) derivatives with alkanoyl side chains are known to form thermotropic cholesteric liquid crystals (CLCs) with visible reflection. Although the widely investigated CLCs are requisite for tedious syntheses of chiral and mesogenic compounds from precious petroleum resources, the HPC derivatives easily prepared from biomass resources would contribute to the realization of environment-friendly CLC devices. In this study, we report the linear rheological behavior of thermotropic CLCs of HPC derivatives possessing alkanoyl side chains of different lengths. In addition, the HPC derivatives have been synthesized by the complete esterification of hydroxy groups in HPC. The master curves of these HPC derivatives were almost identical at reference temperatures, with their light reflection at 405 nm. The relaxation peaks appeared at an angular frequency of ~10² rad/s, suggesting the motion of the CLC helical axis. Moreover, the dominant factors affecting the rheological properties of HPC derivatives were strongly dependent on the CLC helical structures. Further, this study provides one of the most promising fabrication strategies for the highly oriented CLC helix by shearing force, which is indispensable to the development of advanced photonic devices with eco-friendliness.