Flexoelectric polarization studies in bent-core nematic liquid crystals

The flexoelectric polarization $\left(P_f\right)$ of four bent-core nematic liquid crystals (LCs) has been measured using the pyroelectric effect. Hybrid aligned nematic cells are fabricated for measuring the pyroelectric response over the entire range of the nematic phase. It is found that the magnitude of flexoelectric polarization $P_f$ and the sum of the flexoelectric coefficients $|e_1+e_3|$ for the bent-core LCs studied here are three to six times higher than for the calamitics. $P_f$ is found to depend on the transverse dipole moment of LC molecules. However, $|e_1+e_3|$ values are by no means giant as $|e_3|$ alone had been reported for a bent-core nematic system previously. The dependence of the sum of “splay and bend flexoelectric coefficients” is discussed in terms of the shape of the molecule and of the dipole moment directed normal to the molecular axis.

Figure 1

Molecular structure of (a) 5CB, (b) 4-cyanoresorcinol bisbenzoate homologues BC LC, and (c) 1, 2, 4-oxadiazole BC LC.

Figure 2

(a) The geometry of a HAN cell, the axes X-Z are shown to lie in the plane of paper, (b), (c) are the polarizing optical textures of the cell at 105 °C with the rubbing direction at an angle of ${45}^{\circ }$ and parallel to the polarizer. The domain boundary is shown as a defect line.

Figure 3

Temperature dependence of the flexoelectric polarization, $P_f$ (•) measured by the pyroelectric technique and the corresponding pyroelectric signal, $\gamma$ (○) of LC materials, 5CB $(t=8.2\mu \mathrm{m})$, BC4 $(n=4,t=3.4\mu \mathrm{m})$, BC7 $(n=7,t=4.3\mu \mathrm{m})$, and $\mathrm{BC}{7}_{\left(\mathrm{OOC}\right)}(R_1=R_2=\mathrm{OOC}{\mathrm{C}}_7{\mathrm{H}}_{15},t=5.7\mu \mathrm{m})$. Measurements were carried out using HAN cells with the cell thickness given in parentheses upon cooling process from the isotropic state. $t$ is the cell thickness.

Figure 4

Magnitude of flexoelectric coefficients $|(e_1+e_3)|$ (■) as a function of temperature for various liquid crystals, 5CB $(t=8.2\mu \mathrm{m})$, BC4 $(n=4,t=3.4\mu \mathrm{m})$, BC7 $(n=7,t=4.3\mu \mathrm{m})$, and $\mathrm{BC}{7}_{\left(\mathrm{OOC}\right)}(R=\mathrm{OOC}{\mathrm{C}}_7{\mathrm{H}}_{15},t=5.7\mu \mathrm{m})$; $t$ is the cell thickness. Thickness has been taken into account through equation (4) while calculating the flexoelectric coefficients.

Figure 5

Temperature dependence of (a) the $P_f$ (○) measured by the pyroelectric technique and corresponding $\gamma$ (•), and (b) $|(e_1+e_3)|$ (■) of BCLC material ODAZ-9. Measurements are carried out on a 7.8-μm-thick HAN cell while cooling down the sample from the isotropic state.