Splay bend elasticity of a bent-core nematic liquid crystal

We measured the splay $\left(K_{11}\right)$ and bend $\left(K_{33}\right)$ elastic constants in the nematic phase of a bent-core liquid crystal. In the vicinity of the nematic-isotropic transition temperature $K_{33}$ is proportional to the square of the order parameter. In the nematic range $K_{11}$ increases monotonically with decreasing temperature, whereas $K_{33}$ is practically independent of temperature and is smaller than $K_{11}$. $K_{33}$ exhibits a pretransitional slow divergence toward the transition temperature to the smectic phase and becomes slightly larger than $K_{11}$. The small $K_{33}$ is explained on the basis of strong coupling of the bent shape of the molecules with the bend distortion.

Figure 1

(Color online) (a) Variation of birefringence and order parameter as a function of reduced temperature. Chemical structure of the compound used in the experiment is also shown in the inset. (b) Variation of parallel ${ϵ}_{\parallel }$ and perpendicular ${ϵ}_{\perp }$ components of dielectric constant as a function of temperature.

Figure 2

(Color online) Retardation as a function of applied electric field at $157°\text{C}$. The best fit to the theoretical retardation obtained by using Eq. (2) is given by a solid line. $K_{33}$ is determined by best fit.

Figure 3

(Color online) Variation of splay $\left(K_{11}\right)$ and bend $\left(K_{33}\right)$ elastic constants as a function of reduced temperature. Temperature-dependent $K_{11}$ and $K_{33}$ are shown against $\Delta n$ in the inset. Solid lines are best fit to $K_{11}$ and $K_{33}$ vs $\Delta n^x$. $x=2.9$ and 2 in cases of $K_{11}$ and $K_{33}$, respectively.

Figure 4

Schematic representation of (a) splay and (b) bend distortion in bent-core nematic liquid crystals.