Electro -optical investigations of liquid crystalline mixtures

Doping a liquid crystal with an appropriate substance, which may or may not have mesogenic properties, can induce substantial changes in its physical properties. It may change the symmetry of the phase, it may induce new phases, and it may drastically change particular physical properties while leaving others unchanged or slightly altered. In consequence, doped liquid crystals and liquid crystalline mixtures are of great importance in industry. In this dissertation the results of several investigations of such systems are reported. The bend elasticity of a calamitic nematogen (4O.8), doped with a bent-core mesogen (P7-PIMB) was studied. Different electro-optical techniques, including the Frederiks transition technique, were used to monitor the bend elasticity of a set of doped samples as a function of the dopant concentration. The presence of bent-core compound in the sample monotonically reduced the bend elasticity. For the highest concentration sample (10.8% dopant by weight) the bend elastic constant decreased by 40%. In another investigation the influence of a bent-core dopant (same compound) on the nematic-smectic A pretransitional behavior of a calamitic liquid crystal (8OCB) was studied. The marked reduction of bend elasticity for a small amount of dopant holds true for the new mixtures in the temperature range close to nematic-smectic A phase transition temperature, a region that was not explored in the previous study. In addition, a substantial change (25%) in the smectic bare correlation length for a small concentration of dopant (6% by weight) was found. In the last chapter the results of an investigation of the stability of antiferroelectric phase in enantiomeric mixtures of an antiferroelectric liquid crystal are presented. Although the molecular origin of anticlinic structure, which results in antiferroelectricity when the mesogens are chiral, is not very well known, we may nevertheless introduce a phenomenological parameter, which in part may be due to the local dipole-dipole interactions. A set of experiments was performed for five different enantiomeric mixtures, and the interlayer interaction was deduced for each mixture. The interlayer interaction was found to decrease on decreasing the enantiomeric excess. A mean field theory was introduced to explain the observed behavior and it was found to be a viable approach to understanding the problem.