Bent-Shape Thermotropic Nematics with Negative Elasticity and Spontaneous Short-Pitch Modulation – BESTNEMATICS

With the constant progress of the information and communication technologies, enormous volumes of data are collected, treated, and finally delivered to the end-user as visual information. This last step requires high-quality display devices, with high resolution, contrast and luminosity. This multi-billion-euro market is now dominated by the liquid-crystal displays, based on electric field effects in the nematic (N) or chiral nematic (N*) phases, with relatively slow response times (> 1 ms). Achieving faster switching would be an extremely important breakthrough not only for better displays, e.g. for the modern 3D-display technologies, but also for fast components for the telecommunication industry - switches, couplers, filters.
The present project is based on the recently discovered new LC phases that are similar to the usual nematics but that are spontaneously distorted on a few nanometer scale, due to the bent shape of their molecules. These phases are fluid, like the N and N* phases, but with stiffer structure and with much shorter relaxation times (< 1 µs), opening the way for new types of fast-response displays and electro-optic devices. Recent observations confirm that bent-shaped molecules present a novel nematic phase, the “twist-bend” nematic, NTB. Unlike the uniformly oriented N-phase, in the NTB -phase the molecules are arranged on a conical helix, with extremely short pitch. Remarkably, this chiral organization takes place even for achiral molecules, with doubly-degenerated handedness in this case. The same structure and the main properties of the NTB -phase have been predicted in 2001 in a simple Landau-like model. As the bent-shaped molecules curve the director field around them, the bend elastic constant K3 should decrease and may even become negative, leading to a spontaneously bent state. However, because a pure bend cannot fill space continuously, additional twist or splay distortions are induced, leading to the predicted twist-bend, NTB, and “splay-bend”, NSB, nematic phases and even, possibly, to other phases, distorted in two or three dimensions.
In this project, we will investigate theoretically and experimentally the unique structure and properties of these spontaneously distorted nematics and we will prospect their potential technological applications. The negative-elasticity model will be developed in more detail, taking into account the surface alignment and the applied fields. A coarse-grained elastic model will be developed, by analogy with smectic mesophases. Experimentally, the phase diagrams of newly synthesized or already available bent-shaped mesogenic molecules will be explored in order to identify the NTB phase and the still unobserved NSB phase. The phase transitions, both upon temperature variation or under external fields, will be investigated experimentally and will be related to the molecular structure. X-ray scattering experiments will be used to confirm the nematic nature of the NTB and NSB phases and to investigate the short-range positional order of the molecules. Optic, electric and electro-optic techniques will be applied to measure the nematic order parameter S, the pitch of the structure, the heliconical tilt of the director, and other important characteristics of the bent-shape nematics.
Particular attention will be paid to characterize and to optimize the properties with a direct technological impact, e.g. elastic and dielectric constants, birefringence, flexoelectric polarization, viscosity, relaxation times, surface alignment, and anchoring energy. We will study in detail the electro-optic effects, both already reported and novel, in the spontaneously distorted phases, in order to understand their physical mechanisms. Finally, EVOSENS, a SME participating in the project, will further explore the possibilities for technological applications by manufacturing demonstrators of fast nematic devices for the display and telecommunication industries.