Selective route for enhancing liquid crystal-based hydroxylpropyl cellulose by esterification

Abstract

Purpose This paper aims to study the effect of hydrolysis route of hydroxypropyl cellulose (HPC) on its esterification performance as liquid crystal material. The assessment was carried out from the data of spectra (Fourier-transform infrared analysis [FTIR] and 1H-nuclear magnetic resonance [1H-NMR]), thermal stability as well as optical properties via forming ordered mesophases at lower concentration than HPC. Design/methodology/approach The HPC was hydrolyzed by hydrochloric acid-methanol at times 9 and 18 h, and the products were esterified by decanoyl chloride. The products of hydrolysis and the esterification were characterized by FTIR, NMR, nonisothermal analysis, thermo-gravimetric analysis (TGA) and polarizing microscope to evaluate the role of degree of substitution of HPC as a result of hydrolysis, on esterification degree, thermal stability and thermal and liquid crystal behavior of the final esterified HPC. Findings The pretreatment by acid hydrolysis of HPC was successful for synthesizing novel cholesteric hydroxypropyl cellulose ester. The data of FTIR and TGA thermal analysis proved that hydrolysis and esterification of HPC with the decanoyl chain significantly enhanced crystallinity of this cellulose derivative from 0.57 to (1.7–1.9). Moreover, they provided products with superior thermal stability than pure HPC, as noticed from increasing the activation energy of degradation (Ea) from 514.3 to 806.2 kJ/mol. The NMR measurement proved that hydrolysis of HPC for 9 and 18 h decreased the degree of substitution from 3 to 2.1 and 1.3, respectively. Moreover, the esterified HPC showed a promising birefringence texture (chiral nematic) besides decreasing the critical concentration from 30% for HPC to 10% for the esterified unhydrolyzed HPC, while superior decreasing to 1–5% was observed for the esterified hydrolyzed HPC. Research limitations/implications There are two stages for preparation of decanoyl ester hydroxypropyl cellulose. At the first stage, HPC was treated by hydrochloric acid-methanol in ratio 1:10 at times 9 and18 h. At the second stage, HPC and hydrolyzed HPC were refluxed with decanoyl chloride (1:6) in presence of nitrogen atmosphere. The final product was precipitated by distilled water. Practical implications There are two stages for preparation of decanoyl ester hydroxypropyl cellulose. At the first stage, HPC was treated by hydrochloric acid-methanol in ratio 1:10 at times 9 and18 h. At the second stage, HPC and hydrolyzed HPC were refluxed with decanoyl chloride (1:6) in presence of nitrogen atmosphere. The final product was precipitated by distilled water. Originality/value The novelty of this work was focused on enhancing the crystallinity, thermal stability and liquid crystal behavior of esterified HPC, via decreasing the degree of substitution and consequently the type of OH group subjected to esterification. The decanoyl ester formation from the hydrolyzed hydroxypropyl cellulose is able to form ordered mesophases at even low concentration (promising birefringence texture at concentrations 1–5%). It is worthy to notice that the investigated route is able to omit the role of graphene oxide in promoting the liquid crystal behavior of HPC, as it hasn't any effect on critical concentration. This work will promote the use of HPC in technological applications, e.g. high modulus fibers and electronic devices.