Mechanical and Electro-Optical Properties of Unconventional Liquid Crystal Systems
Kent State University
Four types of unconventional liquid crystal systems- amphotropic glycolipids, novel bent-core liquid crystals, bent-core liquid crystal and glycolipid mixtures, and colloidal crystal-liquid crystal systems were studied and characterized by polarizing microscopy, electrical current, digital scanning calorimetry, and dielectric spectroscopy. Thermotropic properties of glycolipids show a number of unusual properties, most notably high (60-120) relative dielectric constants mainly proportional to the number of polar sugar heads. The relaxation of this dielectric mode is found to be governed by the hydrogen bonding between sugar heads. Studies on novel bent-core liquid crystals reveal a new optically isotropic ferroelectric phase, molecular chirality-induced polarity, and transitions between molecular chirality and polarity driven phases. Mixtures of several bent-core substances with nematic, polar SmA and SmC phases, and a simple amphiphilic sugar lipid with SmA mesophase found to obey the well known miscibility rules, i.e. the sugar lipid mixes best with the polar SmA bent-core material. In addition, the chiral sugar lipid was found to induce tilt to the non-tilted polar SmA phase, which represents a new direction among the chirality ĘC polarity ĘC tilt relations. The effects of the surface properties and electric fields were studied on various colloid particles-liquid crystal systems. It is found that the surface properties (hydrophobicity, roughness, rubbing) of the substrates are important in determining the size and symmetry of colloidal crystals. The director field of the liquid crystal infiltrated in the colloid crystals can be rendered both random and uniform along one of the crystallographic axis. We present the first observations of DC electric-field-induced rotational and translational motion of finite particles in liquid crystals. The electrorotation is essentially identical to the well- known Quincke rotation, which in liquid crystals triggers an additional translational motion at higher fields. Analysis of the electro-rotation and translations provides new ways to probe local rheological properties of liquid crystals.