Twist-Bend Nematic phase- Behaviour of permittivity and Elastic Constants at atmospheric and at high pressure

Srividhya Parthasarathi, D. S. Shankar Rao, N. B. Palakurthy, C. V. Yelamaggad and S. Krishna Prasad,
J. Phys. Chem. B, 121, 896 (2017); J. Phys. Chem. B, 120, 5056 (2016)

We describe permittivity and Frank elastic constant measurements in the nematic phase of a binary system exhibiting a transition between the nematic (N) and the recently discovered twist-bend nematic (N_TB) phase at atmospheric as well as elevated pressures. Among the salient features observed at atmospheric pressure are (i) the existence of the N_TB phase even when the system is loaded with a high concentration of a rod-like component, (ii) Clear signature in permittivity at the N¬–N_TB transition, (iii) Bend elastic constant is lower than splay over a large phase space. Application of pressure initially decreases the magnitude ε_⊥ (permittivity perpendicular to the nematic director) and with further increase in pressure exhibits an increase in the value, feature similar to that obtained at room pressure when the rod like nematic (RLN) is doped at a high concentration. The dielectric anisotropy exhibits a trend reversal with temperature, the extent of which is affected at high pressures. Another salient feature is that application of pressure enhances bend elastic constant by a large factor of 5, whereas the splay constant (K₁₁) exhibits a much smaller change of only 70%. We propose that all these features can be understood in terms of the relative population of the more energetic horseshoe, and lower energy extended conformer adopted by the CB7CB molecule an argument validated by Xray diffraction experiment.