Influence of the chain length on the thermal and dielectric properties of thermotropic liquid crystals

Abstract

A novel series of liquid crystals was obtained using hydrogen-bonded 2:1 complexes of 4- n -alkoxy-2-fluorobenzoic acid (nOBAF) and succinic acid. The formation of these complexes was confirmed by infrared spectroscopy. Their thermal behavior was investigated by differential scanning calorimetry and polarized optical microscopy. All of the synthesized complexes exhibited smectic A (SmA) and nematic mesophases. The temperature range of the nematic phase was wider for the compounds with shorter alkoxy chain. The SmA phase became more prevalent as the alkoxy chain was lengthened. The complex dielectric permittivity in the frequency range from 1 Hz to 10 MHz showed a relaxation at 4 MHz coming from the rotations of molecules around their main axis and a low frequency relaxation due to the presence of ionic species. The latter was stronger for short-chained mesogens and caused a strong electroconvection phenomenon. • New hydrogen-bonded liquid crystals with succinic acid spacer are investigated. • They form smectic A and nematic phases for all chain lengths of 7–16 carbons. • Long-chained mesogens favor the SmA phase at the expense of the nematic. • An electroconvection phenomenon is observed for short-chained mesogens.