Nastyshyn Y. Optical anisotropy of distorted and defective structures in liquid crystals

Object:Peculiarities of the optical anisotropy in liquid crystal samples with distorted and singular director field. Experimental techniques: Optical polarization microscopy has been used to control the optical quality of the samples, for the identification of the mesophases, for the studies of defects and for the measurement of the optical anisotropy. Crystal Rotation Technique, Magnetic Null and Conoscopic Magnetic Null have been employed for the determination of the director pretilt angle. Optical Conoscopy has been used for the detection of distortions in the nematic cells. Anchoring coefficient was measured by the following techniques: High field techniques proposed by Yokoyama and van Sprang, by RV-technique, by the technique of the Frederiks threshold. Total internal reflection technique and Abbe refractometry have been used to measure refractive indices of the nematics. Optical Spectroscopy has been employed for the determination of optical anisotropy (birefringence and light absorbtion) of the samples. Senarmont technique was used to measure optical phase retardation of the nematics distorted by the electric field (at the determination of the anchoring coefficient) and of the smectic Q. Fluorescent Confocal Polarization Microscopy gave a possibility to visualize the structure of the hybrid aligned smectic A films in the plane perpendicular to the plane of the bounding substrates. Atomic Force Microscopy and Scanning Electron Microscopy have been employed for the visualization of the surface profile of the cell substrates and of the optical diffusers, which are used to produce a diverging laser beam. Computer Rheometry gave the information on the rheological properties of the isotropic phase of a lyotropic chromonematic and of a liquid crystals exhibiting the cholesteric-smectic A phase transition. Optical anisotropy of distorted and defective liquid crystals is considered in the dissertation. It is shown theoretically and experimentally that due to a high sensitivity of optical anisotropy to thepresence of distortions and defects in the director field the optical conoscopy can be effectively applied to detect and characterize distortions of the director field in flat nematic cells; computer simulated and experimentally obtained conoscopic patterns for distorted nematic cells are presented; so-called Conoscopic Magnetic Null Technique for the determination of the nematic pretilt angle and for the detection of hybridity in the director field is proposed. Theoretical and experimental studies of optical phase retardation R as a function of voltage V applied to a planar cells well above the Frederiks transition result in a so called RV-technique for the determination of nematic polar anchoring coefficient W. Special attention is paid to the validity of the W values obtained using the high field techniques. It is shown that unphysical (voltage dependent and negative) W values obtained for some cells (a problem reported in the literature and documented by us) are caused by in-plane inhomogeneities of the cell substrates. A protocol that allows one to check if the cell can be used to measure W by the high field techniques is suggested. Well aligned cells of lyotropic chromonematics: cromolyn, which is transparent and two light absorbing materials, Blue 27 and Violet 20 in the visible light wave length region are characterized for the first time. Temperature, concentration and wavelength dependencies of the birefringence and absorbing coefficients (for light absorbing materials) are presented; data for absorbing coefficients of Blue 27 have been used to determine the temperature behavior of the nematic scalar order parameter. The results of static and dynamic light scattering in the isotropic phase of the chromonematic indicate that the length of the chromonic aggregates increases approaching the phase transition to the nematic phase. Studies of optical anisotropy for frustrated liquid crystal phases: blue phases and smectic Q are performed. For blue phases and pretransitional region of the isotropic phase the temperature behavior of the azimuth and ellipticity of the light waves exiting from the samples indicate the presence of optical anisotropy. It is concluded that the surface influence is one of the possible mechanisms of optical anisotropy. Measurements of the birefringence for another frustrated liquid crystal phase, smectic Q together with the X-ray results allows one to establish the structure of this phase. Optical characterization of the defects is represented by the investigations of the imperfections in Focal Conic Domains (FCD) in the Smectic A phase, by the defects transformations at the transition from the cholesteric to the TGBA phase and by the textural analysis of the B7 banana phase at the nucleation of this phase from its isotropic melt; models for defects and their transformations are proposed.