Distortions in structures of the twist bend nematic phase of a bent-core liquid crystal by the electric field

Abstract

The dielectric spectra of the twist bend nematic phase (N_{TB}) of an achiral asymmetric bent-core liquid crystalline compound are studied for determining the various relaxation modes. Dielectric measurements are also carried out under the bias field E up to 8 V/µm. Two molecular and two collective relaxation processes are observed. The orientational order parameters with respect to the local and the main directors determined using molecular modes are used to find the heliconical angle. The results also show that the order parameter with reference to the main director reverses its trend from increasing to decreasing at temperatures of a few degrees above the N_{TB} to N transition. The collective relaxation modes are assigned to (a) distortions of the local director by the electric field at a frequency of ∼100kHz while the periodic helical structure remains unaltered (mode attributed to flexoelectricity); (b) changes in the periodic structure arising from a coupling of the dielectric anisotropy with the electric field at the lowest frequency in the range of 100 Hz-10 kHz. Frequency of the higher frequency collective mode (∼100kHz) depends primarily on the heliconical angle and has anomalous softeninglike behavior at the N-N_{TB} transition. The lowest frequency mode is studied under the bias field E; the modulus of the wave vector gradually vanishes on increasing E (except for an initial behavior, E^{2}<0.1V^{2}/μm^{2}, which is just the opposite). The transition from the twist bend to splay bend structure is observed by a sudden drop in the frequency of this mode, followed by a linear decrease in frequency by increasing E. The results agree with the predictions made from the currently proposed models for a periodically distorted N_{TB} phase.

The Beauty of Twist-Bend Nematic Phase: Fast Switching Domains, First Order Fréedericksz Transition and a Hierarchy of Structures

“…As the electric field is applied across the parallel-rubbed planar-aligned cell, the N TB phase is converted to a splay-bend for fields larger than a few V/µm depending on the ∆ε and optical biaxiality. This was observed for the first time in [30] especially for a bent-core LC, and was theoretically predicted by Longa et al [31,32] soon after. This has recently been verified by Meyer et al [33] through birefringence measurements, which suggest that molecular biaxiality plays a key role in not only stabilising the N TB phase but also in producing a field-induced splay-bend phase.…”

Biaxiality-driven twist-bend to splay-bend nematic phase transition induced by an electric field

“…If this biaxiality is neglected, then no N TB -N SB phase transition is expected when E ∥ N . In contrast, the transition was previously predicted and also reported in a completely different geometry where the field was applied perpendicular to the macroscopic symmetry axis, E ⊥ N ( 31 – 33 , 44 ). However, the phase transition is then intuitively expected because the uniaxial symmetry of the system is actually broken by the field.…”

“…A similar effect is also expected ( 31 , 32 ) when a strong electric field, E , is applied parallel to h in the N TB phase of a compound with positive dielectric anisotropy, ∆ε > 0. So far, the estimates of the field strength required for a measurable transition shift range from a few ( 31 , 33 ) to 100 V/μm ( 32 ). For ∆ε < 0 and/or E ⊥ h , the uniaxial symmetry of the N TB phase is broken and the N TB cone becomes elliptical, leading eventually, as expected, to an N TB to N SB transition ( 31 – 33 ).…”

Methylene- and thioether-linked cyanobiphenyl-based liquid crystal dimers CBnSCB exhibiting room temperature twist-bend nematic phases and glasses

“…33 Scanning electron microscopy observations of the photo-crosslinked samples of the N TB phase revealed a helical organization of the nanostructures. [34][35][36] In addition to the N TB phase, other structurally associated mesophases, such as heliconical smectic (Sm) phases based on bent-core molecules 37,38 and bent LC dimers, 39,40 the twist grain-boundary-twistbend nematic phase, 41 and the splay-bend N (N SB ) phase electrically induced from the N TB phase [42][43][44] have been experimentally verified. Furthermore, the splay N (N S ) phase with the modulation rotated 90° with respect to the N SB phase was discovered in some rod-like mesogens.…”