Splay-bend elasticity of a nematic liquid crystal with T-shaped molecules

Interest in the synthesis and physical characterization of liquid crystals with chemical structure similar to the shapes of many English letters such as V, T, H, W has increased after the discovery of electro-optical switching in bent-core liquid crystals 1–3. The structure-property relationship of such unconventional liquid crystals has always been interesting from both technological and fundamental points of view. It is observed that the physical properties and, in particular electro-optical responses, in these unconventional liquid crystals are very different compared with the calamitic liquid crystals, and the molecular shape has significant effect on phase structure and macroscopic physical properties 4. For investigating the structure-property relation, nematic phase N has been chosen due to its simplicity. There are many interesting physical properties reported in the nematic phase of bent-core liquid crystals among the unconventional liquid crystal molecules mentioned above. It has been found that bent-core nematic has a number of extraordinary properties such as the sign inversion of elastic anisotropy K33�K11 5, unusual electroconvection 6, giant flexoelectricity 7, and large viscosity 8. In this paper we report measurement of order parameter, splay, and bend elastic constants of a liquid crystal comprising of T-shaped molecules. These are found to be significantly different from the conventional calamitic and bent-core liquid crystals. Further, from the x-ray diffraction measurements we propose a possible orientation of these T-shaped molecules in the Sm-A layers. The compound used in the present study was synthesized in our laboratory and the details will be reported elsewhere. The chemical structure of the compound is shown in Fig. 2a. It has the following phase sequence: Cr 90 ° C Sm-A 108 ° C N 128 ° C I. Planar aligned cells were prepared by using two glass plates coated with indium tin-oxide. The plates were treated with polyimide and cured at 180 ° C for one hour and rubbed antiparallel to prepare cells with planar alignment. For homeotropic cells, JALS-204 was used and cured at 200 ° C for 1 h. Typical cell thickness used for the experiments was d 5 m. The parallel and perpendicular components of the dielectric constant were measured as a function of temperature in homeotropic and planar cells, respectively, with a LCR meter Agilent 4980A at 4111 Hz, applying a voltage of 0.3 V. The optical phase retardation was measured by using the phase modulation technique 9. The setup comprises a Helium-Neon laser = 632.8 nm, photoelastic modulator PEM-100 and a lock-in amplifier SR-830. The retardation and sample capacitance were measured simultaneously as functions of temperature and voltage up to 20 V in steps ranging from 0.05 V to 0.5 V. The x-ray diffraction measurements were made using Cu K radiation from a rotating anode. The splay elastic constant K11 was measured directly from the Freedericksz threshold voltage Vth of the voltage dependent retardation data using the equation K11=oVth/ 2 , where = � is the dielectric anisotropy. The voltage dependent optical phase difference , and voltages above the threshold voltage Vth, are given by following parametric equations