Handedness Inversion in Chiral Nematic (Ethyl)cellulose Solutions: Effects of Substituents and Temperature

Abstract

Various acylates and phenylcarbamates of (ethyl)­cellulose (EC) were synthesized by acylation and carbanilation, respectively, of the residual hydroxyls of an EC (ethyl DS = 2.50). The acyl substituents adopted were propionyl, butyryl, cyclohexanoyl, and adamantoyl groups, and the phenylcarbamoyl substituents included 3-chlorophenylcarbamoyl, 4-chlorophenylcarbamoyl, 3-methylphenylcarbamoyl, and bare phenylcarbamoyl groups. Chiral nematic mesophases of the EC derivatives, formed in chloroform, acetic acid (AA), and dichloroacetic acid (DCA), were examined by various optical techniques for evaluation of the helical pitch (P) in a set of absolute value and twist sense. The lyotropic samples prepared with DCA (20 °C) always assumed a right-handed chiral nematic structure, regardless of the degree of acyl (or phenylcarbamoyl) substitution (DSAcyl (or DSPC), ≤0.50). The chiral nematic series with AA (20 °C) and chloroform (5 °C) showed an inversion of the twist sense from being left-handed to right-handed, when DSAcyl (or DSPC) of each derivative was increased. The critical DS value for the inversion varied depending on the chain length, bulkiness, and polar nature of the employed substituent. For all of the lyotropic series explored, it was generalized that a temperature elevation strengthens a left-handed twisting power in each chiral nematic mesophase. Reversal of the twist handedness was also observed in a cycle of heating and cooling of some chiral nematics in AA. This was interpreted as being due to compensation in the strength of the two chiral interactions, i.e., steric repulsion and dispersion interaction between mesogenic molecules, contributing in mutually opposite signs to the twisting power (2πP –1) of the mesophase.

Construction of Functional Materials in Various Material Forms from Cellulosic Cholesteric Liquid Crystals

“…In contrast to CNCs, left-and right-handed cholesteric helices have been observed for cellulose derivative-based ChLCs. Most recently, Nishio et al [30,31] proposed that the effects of the substituents and temperature on the helical handedness of cellulosic ChLCs can be systematically interpreted in terms of the balance between the dispersion interaction and steric repulsion, based on their experimental data and the theoretical work of Osipov [30,31].…”

Unraveling the Governing Mechanisms Behind the Chiral Nematic Self-Assembly of Cellulose-Based Polymers

“…68 Cholesteric liquid crystalline mesophases can display left-or right-handed helices, and the chirality may switch depending on the solvent or the DS, which influences what polarizations of light are reflected by the mesophase. 55 Twisting power is defined as the inverse of the pitch; it is negative for left-handed and positive for right-handed structures. 55 While EC mesophases tend to be left-handed, substituting acetyl, acyl, and phenylcarbamoyl groups for the hydroxyls above a critical DS can alter the chirality, leading to right-handed helices.…”

“…55 Twisting power is defined as the inverse of the pitch; it is negative for left-handed and positive for right-handed structures. 55 While EC mesophases tend to be left-handed, substituting acetyl, acyl, and phenylcarbamoyl groups for the hydroxyls above a critical DS can alter the chirality, leading to right-handed helices. 37,53,55 In these modified EC polymers, the left-handed chiral nematic mesophases are reported to have a pitch more strongly dependent on concentration than the right-handed ones.…”

“…37,55 Additionally, increasing the DS increases the righthanded twisting power of the mesophase. 37,55 Strengthening the right-handed twisting power will decrease the pitch, thereby blue-shifting the reflection peak. Past work suggests that derivatives with larger substituents tend to be righthanded and those with smaller side groups are left-handed.…”

Glucose‐sensitive structural color change of cholesteric liquid crystal formed by hydroxypropyl cellulose with phenylboronic acid moieties

“…The effect of the side‐chain structure of the cellulose derivatives on their liquid crystallinity has also been well inspected. These studies revealed that the reflection color of cellulosic ChLC is highly sensitive to diverse side‐chain parameters including side‐chain length, bulkiness, and degree of substitution 32–35 . Based on these characteristics, ChLC formed by cellulose derivatives with PBA‐side groups can be expected to achieve enzyme‐free color sensing of glucose.…”