Mixing twist-bend and ferroelectric nematic liquid crystals

Twist-bend (${\mathrm{N}}_{\mathrm{tb}}$) and ferroelectric (${\mathrm{N}}_{\mathrm{F}}$) nematic liquid crystals exhibit several novel effects and new physical properties. Here, we report experimental studies on the phase diagram and some physical properties of binary mixtures of CB9CB and RM734 mesogens. Both N-${\mathrm{N}}_{\mathrm{tb}}$ and N-${\mathrm{N}}_{\mathrm{F}}$ phase transition temperatures and the corresponding enthalpies decrease significantly and, eventually, these transitions disappear at some intermediate compositions, stabilizing wide nematic phase (N). Temperature-dependent birefringence several degrees above the N-${\mathrm{N}}_{\mathrm{tb}}$ phase transition shows strong director tilt fluctuations. The critical range of the fluctuations increases with the nematic range and the critical exponent is consistent with the mean field. The spontaneous polarization of RM734 decreases drastically with the addition of CB9CB mesogen. The temperature dependence of the splay elastic constant of the mixtures' high-temperature nematic (N) phase strikingly differs from that of the pristine CB9CB and RM734 mesogens. The study shows that a small inclusion of either compound has a substantial effect on the phase diagram and physical properties.

Figure 1

(a) Molecular structures and phase transition temperatures of (a) CB9CB and (c) RM734 compounds. Textures of (b) ${\mathrm{N}}_{\mathrm{tb}}$ and (d) ${\mathrm{N}}_{\mathrm{F}}$ phases in planar cells (10 $\mu \mathrm{m}$). The width of the image is 0.7 mm. Inset shows texture in an untreated cell.

Figure 2

Representative polarizing optical microscope (POM) textures of a few mixtures at different temperatures. Double-headed yellow arrows indicate the rubbing direction. Wt. % of RM734 and the temperatures are marked on each image. (a)–(e) ${\mathrm{N}}_{\mathrm{tb}}$ phase, (f),(g) N to crystal transition, (h) ${\mathrm{N}}_{\mathrm{x}}$ phase, and (i) ${\mathrm{N}}_{\mathrm{F}}$ phase. Width of an image is 1.3 mm. Insets show textures in untreated cells.

Figure 3

(a) Phase diagrams of CB9CB and RM734 mixtures. Filled and open symbols represent transition temperatures obtained from DSC and POM, respectively. (b) Enthalpies of N-I transition (green circles), N-${\mathrm{N}}_{\mathrm{tb}}$ transition (red squares), and N-${\mathrm{N}}_{\mathrm{F}}$ or N-${\mathrm{N}}_{\mathrm{x}}$ transitions (stars). Dashed lines are drawn as a guide to the eye. Error bars represent the standard deviation of the mean. ${\mathrm{N}}_{\mathrm{x}}$ denotes a nematic phase whose texture is similar to ${\mathrm{N}}_{\mathrm{F}}$ without polarization.

Figure 4

Temperature dependent spontaneous polarization (${\mathrm{P}}_S$) for pristine RM734 and 90 wt. % mixture.

Figure 5

(a) Variation of birefringence $\mathrm{\Delta }n$ with shifted temperature with increasing wt. % of RM734. Inset shows the variation of $\mathrm{\Delta }n$ together with the Haller fit (red line) for 10 wt. % of RM734. (b) Mean-square fluctuations $\langle {\theta }^2\rangle$ of the heliconical tilt angle calculated from Eq. (2). Inset shows temperature variation of relative tilt fluctuations $\langle {\theta }_{N{N}_{tb}}^2\rangle -\langle {\theta }^2\rangle \sim t^{1-\alpha }$ with $1-\alpha$ close to 1. For clarity, the critical components of fluctuations are shifted by multiples of 10 along the Y axis. Cell thickness is 9.4 $\mu \mathrm{m}$.

Figure 6

Temperature (shifted) dependence of (a) Freedericksz threshold voltage $V_{th}$, (b) dielectric anisotropy ($\mathrm{\Delta }\epsilon$), and (c),(d) splay elastic constant $K_{11}$ at different wt. % of RM734. For clarity of different mixtures $K_{11}$ is shown in two different plots. Cell thickness is 10.5 $\mu \mathrm{m}$.