Twist-Bend Nematic Phase: Role of Third-Order Legendre Polynomial Term in Chiral Interaction Potential (original) (raw)

Twist-bend nematic phase in the presence of molecular chirality

Liquid Crystals, 2018

One of the interesting questions related to bent-shaped materials is how structural chirality of the twistbend nematic (N TB) phase can respond to the presence of molecular chirality, with later being introduced through chiral centers incorporated in the molecules or chiral dopants. We study this problem using minimal coupling Landau-de Gennes theory of N TB , supplemented by a term representing intrinsic molecular chirality. Relative stability and properties of cholesteric (N Ã), chiral twist-bend nematic with heliconical structure (N Ã TB), and other homogeneous phases are studied. Besides nematic, chiral twistbend nematic and cholesteric phases of vanishing global polarization, we find their polar analogues. In particular, a coupling between molecular chirality, alignment tensor and polarization fields can promote a globally polar and chiral twist-bend nematic (N Ã TBp) with period comparable to that of N Ã .

Twist–Bend Nematic Phase from the Landau–de Gennes Perspective

The Journal of Physical Chemistry C

Generalized Landau−de Gennes theory is proposed that comprehensively explains currently available experimental data for the heliconical twist−bend nematic (N TB) 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 (N U), 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 N TB. 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−N TB and N U −N TB tricritical points and insight into biaxiality of N TB .

On the twist-bend nematic phase formed directly from the isotropic phase

Liquid Crystals, 2016

The intriguing twist-bend nematic phase (NTB) is formed, primarily, by liquid crystal dimers having odd spacers. Typically the phase is preceded by a nematic phase (N) via a weak first-order transition. Our aim is to obtain dimers where the NTB phase is formed directly from the isotropic phase via a strong first-order phase transition. The analogy between such behaviour and that of the smectic A (SmA)-N-I sequence suggests that this new dimer will require a short spacer. This expectation is consistent with the prediction of a molecular field theory since the decrease in the spacer length results in an increase in the molecular curvature. A vector of odd dimers based on benzoyloxybenzylidene mesogenic groups with terminal ethoxy groups has been synthesised with spacers composed of odd numbers of methylene groups. Spacers having 5, 7, 9, and 11 methylene groups are found to possess the conventional phase sequence NTB-N-I; surprisingly for the propane spacer the NTB phase is formed directly from the isotropic phase. The properties of these dimers have been studied with care to ensure that the identification of the NTB phase is reliable.

Twist-bend nematic phases of bent-shaped biaxial molecules

Soft Matter, 2016

How change in molecular structure can affect relative stability and structural properties of the twist-bend nematic phase (N TB)? Here we extend the mean-field model 1 for bent-shaped achiral molecules, to study the influence of arm molecular biaxiality and the value of molecule's bend angle on relative stability of N TB. In particular we show that by controlling biaxiality of molecule's arms up to four ordered phases can become stable. They involve locally uniaxial and biaxial variants of N TB , together with the uniaxial and the biaxial nematic phases. However, the Vshaped molecule show stronger ability to form stable N TB than a biaxial nematic phase, where the latter phase appears in the phase diagram only for bend angles greater than 140 • and for large biaxiality of the two arms.

Nematic twist–bend phase in an external field

Proceedings of the National Academy of Sciences

The response of the nematic twist–bend (NTB) phase to an applied field can provide important insight into the structure of this liquid and may bring us closer to understanding mechanisms generating mirror symmetry breaking in a fluid of achiral molecules. Here we investigate theoretically how an external uniform field can affect structural properties and the stability of NTB. Assuming that the driving force responsible for the formation of this phase is packing entropy, we show, within Landau–de Gennes theory, that NTB can undergo a rich sequence of structural changes with the field. For the systems with positive anisotropy of permittivity, we first observe a decrease of the tilt angle of NTB until it transforms through a field-induced phase transition to the ordinary prolate uniaxial nematic phase (N). Then, at very high fields, this nematic phase develops polarization perpendicular to the field (Np+). For systems with negative anisotropy of permittivity, the results reveal new mod...

Coarse-grained model of the nematic twist-bend phase from a stable state elastic energy

Physical Review E, 2020

The twist-bend nematic (N TB) phase is a doubly degenerated heliconical structure with nanometric pitch and spontaneous bend and twist deformations. It is favored by symmetry-breaking molecular structures, such as bent dimers and bent-core molecules, and it is currently one of the burgeoning fields of liquid-crystal research. Although tremendous advances have been reported in the past five years, especially in molecular synthesis, most of its potential applications are held back by the lack of a proper and definitive elastic model to describe its behavior under various situations such as confinement and applied field. In this work we use a recently proposed stable state elastic model and the fact that the mesophase behaves as a lamellar structure to propose a mesoscopic or coarse-grained model for the N TB phase. By means of standard procedures used for smectic and cholesteric liquid crystals, we arrive at a closed-form energy for the phase and apply it to a few situations of interest. The predicted compressibility for several values of the cone angle and the critical field for field-induced deformation agree well with recent experimental data.

Elastic continuum theory: Towards understanding of the twist-bend nematic phases

The twist-bend nematic phase, N TB , may be viewed as a heliconical molecular arrangement in which the director n precesses uniformly about an extra director field, t. It corresponds to a nematic ground state exhibiting nanoscale periodic modulation. To demonstrate the stability of this phase from the elastic point of view, a natural extension of the Frank elastic energy density is proposed. The elastic energy density is built in terms of the elements of symmetry of the new phase in which intervene the components of these director fields together with the usual Cartesian tensors. It is shown that the ground state corresponds to a deformed state for which K 22 > K 33 . In the framework of the model, the phase transition between the usual and the twist-bend nematic phase is of second order with a finite wave vector. The model does not require a negative K 33 in agreement with recent experimental data that yield K 33 > 0. A threshold is predicted for the molecular twist power below which no transition to a twist-bend nematic may occur.

Nematic twist-bend phase with nanoscale modulation of molecular orientation

Nature communications, 2013

A state of matter in which molecules show a long-range orientational order and no positional order is called a nematic liquid crystal. The best known and most widely used (for example, in modern displays) is the uniaxial nematic, with the rod-like molecules aligned along a single axis, called the director. When the molecules are chiral, the director twists in space, drawing a right-angle helicoid and remaining perpendicular to the helix axis; the structure is called a chiral nematic. Here using transmission electron and optical microscopy, we experimentally demonstrate a new nematic order, formed by achiral molecules, in which the director follows an oblique helicoid, maintaining a constant oblique angle with the helix axis and experiencing twist and bend. The oblique helicoids have a nanoscale pitch. The new twist-bend nematic represents a structural link between the uniaxial nematic (no tilt) and a chiral nematic (helicoids with right-angle tilt).