New insights into the liquid crystal behaviour of hydrogen-bonded mixtures provided by temperature-dependent FTIR spectroscopy

Abstract

The phase behaviour of equimolar mixtures of 6-(4ʹ-cyanobiphenyl-4-yl)hexyloxybenzoic acid (CB6OBA) with either 1-(4-butylazobenzene-4ʹ-oxy)-5-(4-oxypyridine)pentane (BuABO5OPyr) or 1-(4-butylazobenzene-4ʹ-oxy)-6-(4-oxypyridine)hexane (BuABO6OPyr) is reported. CB6OBA shows a monotropic twist-bend nematic phase and an enantiotropic nematic phase whereas the two pyridyl-based components do not exhibit liquid crystallinity. Both equimolar mixtures (CB6OBA/BuABOnOPyr) show enantiotropic nematic phases. The nematic-isotropic transition temperature and associated entropy change are higher for the CB6OBA/BuABO6OPyr mixture than for the CB6OBA/BuABO5OPyr mixture. This may be accounted for in terms of the average shapes of the hydrogen-bonded 1:1 complexes formed between the two differing components in the mixtures. However, Fourier transform infrared spectroscopy reveals that this complex is not formed quantitatively, but instead a complex mixture exists over the complete temperature range studied, involving the 1:1 complex, both cyclic and open acid dimers, free acid and hence, free BuABOnOPyr molecules.