Temperature Dependence of the Electroclinic Effect in the Twist-Bend Nematic Phase

Funding Information: This research was funded by the Croatian Science Foundation (Grant No. IP-2019-04-7978); by the Agence Nationale pour la Recherche ANR (France) through Grant BESTNEMATICS, No. ANR-15-CE24-0012; by the French-Croatian bilateral program COGITO; by the Université de Picardie Jules Verne, Amiens, France. Publisher Copyright: © 2023 by the authors.Peer reviewedPublisher PD

Twist-bend nematics, liquid crystal dimers, structure–property relations

<p>One of the current challenges in liquid crystal science is to understand the molecular factors leading to the formation of the intriguing twist-bend nematic phase (N_TB) and determine its properties. During our earlier hunt for the N_TB phase created on cooling directly from the isotropic phase and not the nematic phase, we had prepared 30 symmetric liquid crystal dimers. These had odd spacers and methylene links to the two mesogenic groups; desirable but clearly not essential features for the formation of the N_TB. Here, we report the phases that the dimers exhibit and their transition temperatures as functions of both the lengths of the spacer and the terminal chains. In addition we describe the transitional entropies, their optical textures, the X-ray scattering patterns and the ²H NMR spectra employed in characterising the phases. All of which may lead to important properties of the twist-bend nematic phase.</p

Understanding the twist-bend nematic phase: the characterisation of 1-(4-cyanobiphenyl-4'-yloxy)-6-(4-cyanobiphenyl-4'-yl) hexane (CB6OCB) and comparison with CB7CB

Acknowledgements The FFTEM data were obtained at the (Cryo) TEM facility at the Liquid Crystal Institute, Kent State University, supported by the Ohio Research Scholars Program Research Cluster on Surfaces in Advanced Materials. ODL acknowledges the support of NSF DMR-1410378 grant. The authors are grateful for financial support from MINECO/FEDER MAT2015-66208-C3-2-P and from the Gobierno Vasco (GI/IT-449-10) OA via RSC Gold4GoldPeer reviewedPublisher PD

Biaxiality-induced magnetic field effects in bent-core nematics: molecular field and Landau theory

Nematic liquid crystals composed of bent-core molecules exhibit unusual properties, including an enhanced Cotton-Mouton effect and an increasing isotropic (paranematic)-nematic phase transition temperature as a function of magnetic field. These systems are thought to be good candidate biaxial liquid crystals. Prompted by these e xperiments, we investigate theoretically the effect of molecular biaxiality on magnetic field-induced phenomena for nematic liquid crystals, using both molecular field and Landau theory. The geometric mean approximation is used in order to specify the degree of molecular biaxiality using a single parameter. We reproduce experimental field-induced phenomena, and predict also an experimentally accessible magnetic critical point. The Cotton-Mouton effect and temperature dependence of the paranematic-nematic phase transition are more pronounced with increased molecular biaxiality. We compare our theoretical approaches and make contact with recent relevant experimental results on bent-core molecular systems

A pairwise additive potential for the elastic interaction energy of a chiral nematic

We have developed a pairwise additive potential model to describe the macroscopic elastic interactions of a chiral nematic liquid crystal. The potential is obtained from the expression for the elastic free energy density discretized onto a cubic lattice and mapped onto a suitable expansion in scalar invariants. ne value of the pair potential is explored by means of a Monte Carlo lattice computer simulation. It allows, in a simple and efficient way, the simulation of liquid crystal devices and samples in confined geometries, taking into account the effect of the temperature of the sample and of the thermal fluctuations in the director distribution, as well as the elastic interactions

A molecular field theory for biaxial nematics composed of molecules with C_{2h} point group symmetry

The biaxial nematic phase is generally taken, either explicitly or implicitly, to have D2h point groupsymmetry. However, it is possible for the biaxial phase to have a lower symmetry depending on that of its constituent molecules. Here we develop a molecular field theory for a nematogen composed of C2h molecules in terms of the nine independent second rank orientational order parameters defining the C2h biaxial nematic. In addition there is a rank one order parameter constructed from two pseudovectors which is only non-zero in the C2h phase. The theory is simplified by removing all but the three dominant order parameters. The predicted phase behaviour is found to be rich with three possible biaxial nematic phases and with the transitions involving a biaxial nematic phase exhibiting tricritical points

Biaxial nematics: computer simulation studies of a generic rod-disc dimer model

One possible route to the elusive biaxial nematic phase is through rod–disc dimers in which the rod and disc mesogenic units are linked via a flexible spacer. We have developed a continuous generic model of such rod–disc dimers in which neighbouring like groups tend to align parallel to each other while unlike groups tend to be orthogonal. A torsional potential controls the relative orientations of the groups within a single dimer; depending on the strength of the torsional potential, the groups may be orthogonal or parallel in the conformational ground state. Monte Carlo simulations show that a rigid rod–disc dimer is most likely to form a biaxial nematic phase if the anisotropies of the two groups are the same. Introduction of flexibility is found to have little effect on the qualitative behaviour of the dimer as the relative anisotropy of the two mesogenic groups is changed. However, when the torsional potential strongly favours the alignment of the rod and disc within a single molecule with their symmetry axes parallel there is a dramatic change. The system then exhibits a strong hysteresis in the molecular shape and biaxiality and the biaxial nematic–isotropic transition becomes strongly first order, in marked contrast to the second-order character usually found for this transition. This first-order transition is observed to occur for a range of relative anisotropies of the two groups rather than at a single point