Twist-Bend Nematic Phase from the Landau-de Gennes Perspective.

Lech Longa,Wojciech Tomczyk

doi:10.1021/acs.jpcc.0c05711

Abstract

Generalized Landau–de Gennes theory is proposed that comprehensively explains currently available experimental data for the heliconical twist–bend nematic (NTB) phase observed in liquid crystalline systems of chemically achiral bent-core-like molecules. A bifurcation analysis gives insight into possible structures that the model can predict and guides in the numerical analysis of relative stability of the isotropic (I), uniaxial nematic (NU), and twist–bend nematic phases. An estimate of constitutive parameters of the model from temperature variation of the nematic order parameter and the Frank elastic constants in the nematic phase enables us to demonstrate quantitative agreement between the calculated and experimentally determined temperature dependence of the pitch and conical angle in NTB. Properties of order parameters also explain a puzzling lack of a half-pitch band in resonant soft X-ray scattering. Other key findings of the model are predictions of I–NTB and NU–NTB tricritical points and insight into biaxiality of NTB.

Highlights

The short-pitch heliconical structure formed by an ensemble of achiral bent-core-like mesogens and commonly referred to as the nematic twist−bend is one of the most astonishing liquid crystalline phases
We show how the problem can be solved in a systematic way if we start from a theory which holds without limitations for arbitrary one-dimensional periodic distortions of the alignment tensor that serve as ground states
We should mention that the fourth-order expansion, where c > 0 and d = e = f = 0, predicts that the NTB phase can be absolutely stable within the family of one-dimensional modulated structures,[11] but the theory does not give a quantitative agreement with the data for Sin the nematic phase of CB7CB unless an unphysically large value of ΔtNI is taken (Figure S1)

Summary

■ INTRODUCTION

The short-pitch heliconical structure formed by an ensemble of achiral bent-core-like mesogens and commonly referred to as the nematic twist−bend is one of the most astonishing liquid crystalline phases. We should mention that the fourth-order expansion, where c > 0 and d = e = f = 0, predicts that the NTB phase can be absolutely stable within the family of one-dimensional modulated structures,[11] but the theory does not give a quantitative agreement with the data for Sin the nematic phase of CB7CB unless an unphysically large value of ΔtNI is taken (Figure S1). Eq 10, with fQel containing only these two elastic terms accounts for absolutely stable NTB among one-dimensional modulated structures,[11] it is not sufficiently general to quantitatively reproduce, for example, elastic properties of bent-core systems in the parent nematic phase for it implies equality of splay and bend Frank elastic constants, which so far is not an experimentally supported scenario with stable NTB. To the best of our knowledge, the 2q0 signal has not been detected compounds.[44,53−55] so far in any of the examined

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES