Effect of end groups with aliphatic chain or ring structures on the thermal properties and optical textures of achiral and chiral liquid crystal compounds

Abstract

Thermal properties and orientation in optical textures of liquid crystal (LC) are two crucial factors for LC application. Here, a series of achiral and chiral LC compounds (Cn) with different end groups were prepared. The chemical structures were investigated by Fourier transform infrared (FT-IR) and proton nuclear magnetic resonance (1H NMR). The thermal properties and optical textures were studied by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (XRD). The results showed that all the compounds possessed good thermal properties and enantiotropic mesomorphism. For achiral compounds C1C4, droplets and schlieren textures of nematic phase (N) and broken fan-shape textures and dull grayish/cloudy textures of smectic phase (Sc) were seen. Oily streak and focal conic textures of chiral nematic phase (N*), fan-shape textures and dull grayish/cloudy textures of chiral smectic phase (Sc*) were observed for chiral compounds C5C7. For C1, C2, C3 and C5 with aliphatic chain in the end groups, it was found that lateral substitution in the end groups had a greater effect on the clearing point than that of the increase in the length of end groups. Moreover, due to the increasing free volume for the increase of branch chain on six-membered ring structure, the clearing point (Ti) of C4 was higher than those of C6 and C7.