TY  - THES
AU  - Dey, Kartick Ch.
TI  - INVESTIGATION ON SOME ANTIFERROELECTRIC LIQUID CRYSTALS AND THEIR MIXTURES
PB  - University of North Bengal
VL  - Dissertation
M1  - PUBDB-2020-04474
SP  - 222
PY  - 2020
N1  - Dissertation, University of North Bengal, 2020
AB  - Liquid crystals are fascinating self–assembled soft materials in which the molecules are orientationally ordered and partially positionally ordered. Liquid crystals easily response to small perturbations like the electric field, magnetic field, surface effect, etc. for which they find applications in diversified fields along with display technology.At present high definition displays (AMLCDs) with a viewing angle of  178 degrees are available in the market. Twisted nematic liquid crystals are generally used in such displays. The high value of response time ( ms) governing the frame rate, ghost effect and contrast ratio is still a matter for further improvement. As a result, ferroelectric liquid crystal (FLC) and antiferroelectric liquid crystal (AFLC) draw a special interest from the last decades of the 20th century due to their low response time (µs). But manifold problems were faced in the development of displays based on FLCs. Some of them are small cell spacing (1–2 µm) to unwind the helix, the problem of mechanical shock due to unstable molecular anchoring at the surface, contrast ratio, etc. The AFLCs are promising materials to solve the issues because of their fascinating properties – tristable switching behavior, micro-second order response time, intrinsic analog gray-scale capability, hemispherical viewing angle (in-plane switching geometry) high contrast ratio and no-ghost effect, etc. AFLCs are also interesting for the basic studies in soft condensed matter field as these materials show various sub-phases with distinct macroscopic properties. So enough scope is here for the improvement of the display industry in the coming years with fascinating properties of antiferroelectric liquid crystals.Physical properties of six antiferroelectric liquid crystals and their three mixtures have been studied in this dissertation using different experimental techniques viz. polarizing optical microscopy, differential scanning calorimetry, synchrotron X-ray diffraction, dielectric spectroscopy, electro-optic study. The first four compounds (DM0, DM1, DM2, and DM3) are biphenyl benzoate core-based, in which the number and position of lateral fluorination in the core differ. The first compound DM0 is a core protonated compound but in DM1 one fluorine atom is introduced at ortho position of the benzoate core, while it is introduced at position meta in DM2, but in DM3 fluorine atoms are introduced both at ortho and meta positions of the benzoate core structure. In chapter 3, the effect of lateral fluorination on the mesogenic behaviour of these four compounds is discussed. All the four compounds exhibit Cr–SmCA*–SmC*–SmA*–Iso phase sequence but in different temperature ranges. Due to lateral fluorination, the clearing point decreases in all the fluorinated compounds but the melting point and the stability of SmCA* phase decrease or increase depending upon the position of lateral fluorination. Layer spacings increase in DM1 and DM2 whereas it is found to decrease in DM3. All the compounds show a tricritical nature of SmC*-SmA* transition and orthoconic nature of tilt. They also exhibit very large dielectric increments in SmC* phase but in singly fluorinated DM1 and DM2 its value is less and in doubly fluorinated DM3 it is more than the core protonated compound. Four relaxation modes (PL, PH, GM, and SM) are observed and their critical frequencies are found to decrease in all fluorinated compounds. Domain mode is also observed. Fluorination results in the slower response under a square pulse, of the order of a few hundred microseconds.Properties of a binary eutectic antiferroelectric mixture, formed mixing 50 wt.
LB  - PUB:(DE-HGF)11
DO  - DOI:10.3204/PUBDB-2020-04474
UR  - https://bib-pubdb1.desy.de/record/452033
ER  -