Liquid crystallinity of aqueous TEMPO-oxidized hydroxypropyl cellulose solutions: effects of main-chain and side-chain substituents

Hydroxypropyl cellulose (HPC) is a stable molecule that presents cholesteric liquid crystal (ChLC) behavior in aqueous systems. However, this ChLC nature has not been fully utilized. In the present study, carboxy groups were introduced to HPC as a means to modify its aqueous ChLC characteristics. 2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO)-oxidation was performed for a HPC with degrees of substitution (DS) and molar substitution of the hydroxypropyl groups of 2.19 and 4.25, respectively. The oxidation reaction not only introduced carboxy groups to the C-6 position of the main chain, but also produced ketone groups by oxidizing the hydroxy groups at the side-chain termini. These ketone moieties could be reduced selectively under mild conditions. The degree of oxidation (DO; ≤ 0.37) and degree of ketone substitution (DS ketone ; ≤ 0.20) were determined by conductivity titration and NMR, respectively. The effects of substituents on the ChLC properties were determined by multiple regression analysis as well as empirical and theoretical analyses. An increase in DO decreased the twisting of the ChLC structure, while an increase in DS ketone had little effect on twisting but increased the distance between polymer molecular chains and decreased the rigidity of the molecule as a whole. The interaction of the carboxy groups with the solvent water appeared to be shielded when the side-chain ends were ketonized. These findings provide information on the effects of backbone and side-chain substituents on the supramolecular structures of water-soluble ChLC as well as give the possibility of controlling ChLC via ionic functional groups.