Investigation of mesogenic properties of hydrogen bonded 5BA/nCB (n=5-8) liquid crystal mixtures

In this thesis, homogeneous liquid crystal mixtures of equal mass 5BA/5CB, 5BA/6CB, 5BA/7CB and 5BA/8CB were produced from 4-pentylbenzoic acid (5BA), 4-pentyl-4'-cyanobiphenyl (5CB), 4-hexyl-4'-cyanobiphenyl (6CB), 4-heptyl-4'-cyanobiphenyl (7CB) and 4-octyl-4'-cyanobiphenyl (8CB) mesogenic chemicals. Thermal analyzes of the produced mixtures were performed with a Differential Scanning Calorimetry (DSC) device. Liquid crystalline properties such as phase transition temperature, enthalpy, entropy and phase transition order of liquid crystal mesogenic phases were investigated using DSC data. DSC results showed that phase transition peaks indicating crystal-smectic, smectic-nematic and nematic-isotropic phase transformations were formed during continuous heating and cooling cycle. It has been observed that mesogenic phase transformations generally occur in the 1st order and the transformations that occur during cooling are more regular. It was found that the mesophase range observed during cooling was wider than the mesophase range observed during heating, and the mesophase area of the 5CB/8CB liquid crystal mixture was higher at approximately 102 oC compared to the other mixtures. The morphological phase structures of the produced mixtures were investigated with Polarized Optical Microscope (POM) and it was seen that the phase transition temperature values were compatible with the phase transition temperature values obtained with DSC. Fourier Transform Infrared Spectroscopy (FTIR) analysis results of binary mixtures of 5BA/5CB, 5BA/6CB, 5BA/7CB, 5BA/8CB taken in equal mass showed that intramolecular bonds were preserved and hydrogen bonds were formed between the molecules of the components forming the mixture. The crystalline microstructure properties of the synthesized liquid crystal mixtures were investigated by X-ray diffraction (XRD). Basal intermolecular distance values were calculated using XRD results. It was found that the molecular arrangement in the crystalline structures of the liquid crystalline mixtures produced was monoclinic structure.