Liquid crystal is a common material used for liquid crystal display (LCD) in our mobile phones, TVs and other electronic devices. It not only shows anisotropic property usually seen in crystalline materials, but simultaneously exhibits fluidity. The nematic (N) phase is the most common liquid crystalline phase, where the long molecular axis of the rod-like molecules orients more or less parallel to each other, while their centers of mass are distributed in an isotropic manner. When the molecular symmetry is reduced, like dimer formed by covalently linking two mesogenic cores via an oligomethylene spacer of varying length and parity, might cause a peculiar nematic phase—Twisted-Bend Nematic (N_TB) from a theoretical perspective. Indeed, in the last decade, the existence of the N_TB phase was confirmed experimentally. Therein, molecules form nanoscopic chiral helix structure irrespective of the molecular chirality with the nematic order remained. Though many researches on the phase structure have been intensively made, quantitative determination of the liquid crystalline elastic moduli in the N_TB phase had not been well discussed due to many experimental difficulties. Some works studied the pretransitional behavior of the elastic moduli in the N phase sitting at higher temperature than the N_TB phase, and extrapolated the values to estimate elastic moduli in the N_TB phase.

Recently, Satoshi Aya group showed an experimental determination of compressive and effective splay elastic moduli B and K₁₁, as well as the rotational viscosity γ₁ in the both the N and N_TB phase for a thioether-linked cyanobiphenyl dimer with a heptamethylene spacer (CBS7SCB), leading to a possible discussion of the viscoelastic properties between the N and N_TB phases. The elucidation of the intrinsic viscoelastic properties of the N_TB phase offers better understanding of the topology and flow properties in the N_TB phase.

Figure 1 (a) Temperature dependence of the effective splay elastic modulus K₁₁ in N phase and N_TB phase, (b) SEM image of photopolymerised film in N_TB phase and schematic of the structure originated from pseudo-layer buckling instability 

We tested the phase transition temperature of CBS7SCB by DSC, and observed the topological rope texture of the N_TB phase through a polarized light microscope. We found that the periodicity of the director oscillation was linear to the sample thickness. When the thickness exceeds 40 µm, the stripe texture was replaced by focal conic texture. According to Helfrich-Hurault instability model, the observation indicates that the formation of the stripe texture originates from the imbalance between the splay and compressive elasticity in the N_TB phase. To know the quantitative values of the moduli, we measured the compressive modulus B through a customized compressive rheometer at variable temperatures. Subsequently, the effective elastic moduli K₁₁ was calculated according to the Helfrich-Hurault instability model in the N_TB phase. The effective elastic moduli K₁₁ is about a thousand times larger than that of the N phase. Similarly, the rotational viscosity γ₁ of the N_TB phase is 300 times larger than that of the N phase. In addition, to know the orientational field of the N_TB phase, we photocured the material in the N_TB phase and observed the molecular orientation of the coarse-grained director (parallel to the N_TB helix axes) by using a scanning electron microscope. The maximum splay angle of the N_TB helix axes was about 40˚.

Viscoelastic properties of a thioether-based heliconical twist–bend nematogen

Junchen Zhou, Wentao Tang, Yuki Arakawa*, Hideto Tsuji and Satoshi Aya*

https://doi.org/10.1039/C9CP06861A

Thesis link：

https://pubs.rsc.org/en/Content/ArticleLanding/2020/CP/C9CP06861A#!divAbstract