The twist-bend nematic phase: translational self-diffusion and biaxiality studied by 1H nuclear magnetic resonance diffusometry (original) (raw)
Related papers
Physical Review E, 2015
The nature of the nematic-nematic phase transition in the liquid crystal dimer 1 ,9-bis(4-cyanobiphenyl-4-yl) nonane (CB9CB) has been investigated using techniques of calorimetry, dynamic dielectric response measurements, and 2 H NMR spectroscopy. The experimental results for CB9CB show that, like the shorter homologue CB7CB, the studied material exhibits a normal nematic phase, which on cooling undergoes a transition to the twist-bend nematic phase (N TB), a uniaxial nematic phase, promoted by the average bent molecular shape, in which the director tilts and precesses describing a conical helix. Modulated differential scanning calorimetry has been used to analyze the nature of the N TB-N phase transition, which is found to be weakly first order, but close to tricritical. Additionally broadband dielectric spectroscopy and 2 H magnetic resonance studies have revealed information on the structural characteristics of the recently discovered twist-bend nematic phase. Analysis of the dynamic dielectric response in both nematic phases has provided an estimate of the conical angle of the heliconical structure for the N TB phase. Capacitance measurements of the electric-field realignment of the director in initially planar aligned cells have yielded values for the splay and bend elastic constants in the high temperature nematic phase. The bend elastic constant is small and decreases with decreasing temperature as the twist-bend phase is approached. This behavior is expected theoretically and has been observed in materials that form the twist-bend nematic phase. 2 H NMR measurements characterize the chiral helical twist identified in the twist-bend nematic phase and also allow the determination of the temperature dependence of the conical angle and the orientational order parameter with respect to the director.
Liquid Crystals
On orientational order in nematic and twist-bend nematic phases: A 2 H-NMR study of binary mixtures of the odd dimer,1″,9″-bis(4-cyanobiphenyl-4′-yl) nonane, (CB9CB), and the monomer, 4-pentyl-4′-cyanobiphenyl, (5CB-d2)
Liquid Crystals
Twist-bend nematics and heliconical cholesterics: a physico-chemical analysis of phase transitions and related specific properties Certain nematic liquid crystals, e.g., those formed by banana-shaped molecules, can exhibit a low-temperature twist-bend nematic (Ntb) phase. Upon addition of chiral dopants, a chiral version of the twist-bend phase (N * tb) can be observed below the conventional chiral nematic (N *) phase, while under electric field the N * phase is transformed into a specific state known as "heliconical cholesteric". In this work, we have studied the well-known Ntb-forming system CB7CB:CB6OCB with addition of 5CB and chiral dopants (ChD) using optical microscopy, differential scanning calorimetry (DSC), and measurements of temperature-dependent optical transmission using optical microscopy. Effects of 5CB and different chiral dopants (of steroid or non-steroid nature) upon thermal stability of Ntb phase were analyzed, and selective reflection in the visible range could be observed in the N * phase with a sufficiently high ChD content. As a peculiar feature, sharp unwinding of the cholesteric helix was observed at lower temperatures when approaching the N * tb phase, which appears to be similar to the helix unwinding close to the smectic-A transition.
Physical Review E
An analysis of the IR absorbance for the segmented functional groups of liquid crystal dimers mesogen and linker enabled the orientation order to be determined and information about the dipole interactions in the nematic and twist-bend nematic phases to be obtained. The long axis orientational order increases as the temperature decreases in the nematic phase, although much more slowly than for the classical nematics, and then reverses this trend in the twist-bend nematic phase due to the tilt of the molecules. In the nematic phase, the short axis of the molecule performs an isotropic uniform rotation and has a uniaxial alignment. In the twist-bend nematic phase, however, biaxial ordering occurs and grows significantly in accordance with the helical deformation of the director. Changes in the mean absorbance in the twist-bend nematic phase were observed: a decrease for the longitudinal dipole at the nematic-twist-bend nematic phase transition, thus emphasizing the antiparallel axial interaction of the dipoles, while the absorbance of the transverse dipoles remains unchanged up to 340 K, and then the latter become parallelly correlated.
Biaxial Nematic Mesophases from Shape-Persistent Mesogens with a Fluorenone Bending Unit
Chemistry - A European Journal, 2010
Nematic phases, applied in almost all commercial liquidcrystal (LC) displays, are LC phases with mesogens isotropically distributed and exhibiting orientational long-range order of one preferred molecular axis along a common direction defined by the so-called director. These are the phases with the lowest viscosity and besides the anisotropic properties, are most similar to isotropic liquids. Biaxial nematics should realise a long-range orientational order of all three molecular axes along three mutually perpendicular directors, but maintain the isotropic distribution of the molecular centres of gravity and their molecular mobility. Such phases are not only appealing from the viewpoint of basic research and theoretical modelling, but are also of technological interest to speed up the switching of LC displays. After the prediction by Freiser, Saupe and Yu were the first to discover such mesophases in a narrow region of the phase diagram of a lyotropic LC. The much-pursued synthesis of molecules for board-shaped mesogens, though suitable for forming thermotropic biaxial nematic phases, did not, however, reveal a widely accepted biaxial mesophase. Only recently, biaxial nematic phases of multipodes [7] and polymers [8] with side-on attached nematogens, the rotation of which about the long axis is hindered, could be confirmed. In the area of low-molecular-weight mesogens, theory predicted V-shaped mesogens and mixtures of rod-and disc-shaped molecules as potential candidates for biaxial nematic phases. Indeed, the biaxial nematic phase at high temperatures, above 100 8C, was found in a series of oxadiazole derivatives, the results of which were most widely accepted and confirmed. We were interested in parameters controlling biaxiality in such phases and focused therefore on shape-persistent molecules 1 with a well-defined bending angle (Scheme 1), which is supposed to be a principal parameter in theoretical predictions. They were designed to self-assemble exclusively in nematic phases owing to the substitution pattern of alkoxy chains. However, only planar bending units with a dipole along the bisector revealed nematic phases. [15] [a] Prof.
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 .
Liquid Crystals, 2018
The appearance of some special features in 2 H-NMR spectra, which could not be explained by the conventional nematic order, was crucial for the discovery of the twist-bend nematic (N TB) phase. Such features reflect the order and symmetry of the local environment of a probe and in principle their analysis can provide quantitative insights into the structural parameters of the phase. We have developed a methodology for the analysis of experimental data, different from the approaches that are normally used for two main reasons: (i) the structural features of the N TB phase, specifically its local polarity, biaxiality and chirality, are consistently taken into account; (ii) the molecular features are introduced in a detailed way, through molecular modelling. We have applied this methodology to analyse 2 H-NMR quadrupolar splittings for two representative systems, CB7CB and CB6OCB. The integration of theory, computational modelling and experiment allows us to extract quantitative information from the experimental data and to predict various properties, even not directly probed by measurements. We discuss how the molecular differences between CB7CB and CB6OCB translate into a different phase behaviour and we address issues concerning the distinguishing features of the N TB phase, such as the definition of a local tilted director.
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.
The twist-bend nematic phase of bent mesogenic dimer CB7CB and its mixtures
arXiv (Cornell University), 2015
Binary mixtures of the twist-bend nematic-forming liquid crystal CB7CB with the prototypical rodlike liquid crystal 5CB exhibit a twist-bend nematic (NTB) phase with properties similar to those reported for neat CB7CB. The mixtures appear homogeneous, with no micron-or nano-scale segregation evident at any concentration. The linear dependence of the phase transition temperature on concentration indicates that these binary mixtures are nearly ideal. However, a decrease in the viscosity with the addition of 5CB allows the characteristic twist-bend stripe textures to relax into a state of uniform birefringence. We confirm the presence of nanoscale modulations of the molecular orientation in the mixtures by freeze-fracture transmission electron microscopy (FFTEM), further evidence of their twist-bend nature. We devise and implement a statistical approach to quantitatively measure the ground state pitch of the twist-bend phase and its mixtures using FFTEM. The addition of 5CB generally shifts the measured ground-state pitch distributions towards larger pitch. Interestingly, the pitch appears to increase discontinuously by ~10 nm at the 50 wt% concentration of 5CB, indicating that the twist-bend phase undergoes a structural transition at higher 5CB concentrations.