Thermo-optical methodology of phase transitions of eutectic liquid crystal mixtures

Liquid crystals are one of the most used materials in today's display technology and other electro-optical applications due to their optical and electrical properties, which change depending on orientation. The fact that these properties are directionally related entails the necessity to control the orientation of liquid crystal molecules. Liquid crystals have low viscosity in the nematic phase. Low viscosity allows molecules to easily change their orientation with an external factor. Mechanical, electrical and magnetic fields are suitable methods for controlling the orientation of molecules. However, these methods can lead to the deformations in the formation of molecules. For this reason, confinement method is used to control the orientation of the molecules more effectively. When we look at liquid crystal applications (LCD, sensor, optical switching, thermometer), we see that the confinement effect is an effective method. For this reason, the physical properties of the molecules become important in confinement areas. In this thesis study, the effect of confinement on the thermal, structure and molecular dynamic properties of the liquid crystal mixture was investigated by using eutectic E7 liquid crystal mixture, nano sized alumina (AAO) membranes with hard surface cylindrical pores. Characterization of the mixture was investigated by differential scanning calorimetry (DSC), X-ray diffraction (XRD), dielectric spectroscopy (DS). When the phase transition temperatures obtained by DSC were examined, it was found that the nematic-isotropic phase transition temperature for the confined E7 liquid crystal was independent of pore diameter for 65 nm, 35 nm and 25 nm diameters. In addition, a new phase transition which is different from nematic phase for diameters of 25 nm and 35 nm was observed in the DSC results and XRD patterns due to size effect. When the molecular dynamic properties investigated by dielectric spectroscopy, dynamics of liquid crystal molecules confined into the pores accelerate and the alpha relaxation process becomes dominate. When the glassy transition temperatures investigated with DSC and DS, it has been found that the glassy transition temperature of the confined E7 liquid crystal decreases as the pore diameter decreases. The results of this thesis work are important in terms of the characteristics of the eutectic E7 liquid crystal mixture which is suitable for technological applications because of its wide nematic phase range. Keywords: E7 eutectic liquid crystal, Confinement effect, Alumina porous membrane, Phase transition temperature, Molecular dynamics.