DIELECTRIC RELAXATION SPECTROSCOPY AND ELECTRO-OPTICAL STUDIES OF ANTIFERROELECTRIC AND FERROELECTRIC LIQUID CRYSTALS AND LIQUID CRYSTAL NANO-COMPOSITES

Blending nanomaterials with liquid crystal (LC) is considered to be a prospective method to enhance the electro-optical properties of the host. Threshold voltage, driving voltage, residual dc, response time and rotational viscosity etc. are found to decrease in nano doped LC system and in turn the anisotropic order of the liquid crystalline host imparts order in the nano-sized guest particles. Proper selection of size, shape and crystallographic phase of nanomaterial is important to achieve desired improvements of the host. This dissertation presents results of doping different type of nanomaterials in nematic, ferro- and antiferroelectric liquid crystal medium. Conducting polymer nanotubes in nematic LC exhibits remarkable reduction in the threshold and driving voltage which is good from application point of view. The residual dc is also reduced significantly in the doped cell and the reduction is even more than that observed in the carbon nanotube doped same LC system. The influence of multiferroic bismuth ferrite nanoparticles on the electro-optical and dielectric properties of an antiferroelectric LC mixture is investigated in planar cells. It is shown that dispersion of nanoparticles lead to modifications of response time, spontaneous polarization, rotational viscosity and voltage required for switching the molecules between two ferroelectric states in the antiferroelectric phase. The large electric field exerted by the nanoparticles on LC molecules and probable charge transfer among them, causes weakening of the interlayer and intermolecular interactions of LC molecules in the antiferroelectric and ferroelectric phases, respectively. In a ferroelectric LC/ conducting polymer nanotubes composite system electrooptic study reveals a lower electrical response time, rotational viscosity and spontaneous polarization. By fitting the capacitance with voltage in a Preisach model, four dipolar species in both pure and nanocomposite system have been obtained. The orientation of the four dipolar species in the composites system is such that the effective dipole moment in the transverse direction of the FLC molecule is less than that in FLC compound. Another aspect of this dissertation is to study a newly synthesized orthoconic antiferroelectric liquid crystal which provides an excellent dark state because of its high tilt. A detailed investigation of the dielectric, electro-optical properties and X-ray studies of the partially fluorinated high-tilt antiferroelectric LC material revealed that the SmA* phase of this material is of de Vries type. Double-peak polarization current response and tristable-optical-switching studies have revealed an antiferroelectric molecular ordering in the de Vries SmA* phase of this material. Two distinct dielectric relaxation peaks in the SmA* phase also complement the antiferroelectric-like ordering of the molecule in the SmA* phase.The research was conducted under the supervision of Prof. Subir Kumar Roy of the Spectroscopy division under SPS [School of Physical Sciences]The research was carried out under IACS fellowship and DST research gran