Recently, there has been a surge of interest in mesogens exhibiting the twist-bend nematic (N-TB) phase that is shown to be chiral even though formed by effectively achiral molecules. Although it now seems to be clear that the N-TB phase in the bulk is formed by degenerate domains having opposite handedness, the presence of a supramolecular heliconical structure proposed in the Dozov model has been contradicted by the Hoffmann et al. model in which the heliconical arrangement is replaced by a polar nematic phase. The evidence in support of this is that the quadrupolar splitting tensor measured in various experiments is uniaxial and not biaxial as expected for the twist-bend nematic structure. In this debate, among other evidence, the molecular translational diffusion, and its magnitude with respect to that in the nematic phase above the N-TB phase, has also been invoked to eliminate or to confirm one model or the other. We attempt to resolve this issue by reporting the first measurements of the translational self-diffusion coefficients in the nematic and twist-bend nematic phases formed 1 '',7 ''-bis-4-(4'-cyanobiphenyl-4'-yl) heptane (CB7CB). Such measurements certainly appear to resolve the differences between the two models in favour of that for the classic twist-bend nematic phase. [GRAPHICS] .

The self-assembly phenomena on a special class of poly-hydroxy sugar surfactant have been studied extensively. This class of material is classified as amphitropic liquid crystals since they exhibit both thermotropic and lyotropic liquid crystalline properties. Hence the potential applications of these non-ionic surfactants are more versatile than those from the conventional lyotropic liquid crystals including those from thermotropic phases, but the latters are yet to be realized. Unfortunately, due to the lack of interest (or even awareness), fundamental studies in thermotropic glycolipids are scanty to support application development, and any tangible progress is often mired by the complexity of the sugar stereochemistry. However, some applications may be pursued from these materials by taking the advantage of the sugar chirality and the tilted structure of the lipid organization which implies ferroelectric behavior. Here, we present our studies on the stereochemical diversity of the sugar units in glycosides that form the thermotropic/lyotropic phases. The structure to property relationship compares different chain designs and other popular polyhydroxy compounds, such as monooleins and alkylpolyglucosides. Different structural properties of these glycosides are discussed with respect to their self-assembly organization and potential applications, such as delivery systems and membrane mimetic study.

Here we report the chemical induction of the twist-bend nematic phase in a nematic mixture of ether-linked liquid crystal dimers by the addition of a dimer with methylene links; all dimers have an odd number of groups in the spacer connecting the two mesogenic groups. The twist-bend phase has been identified from its optical texture and x-ray scattering pattern as well as NMR spectroscopy, which demonstrates the phase chirality. Theory predicts that the key macroscopic property required for the stability of this chiral phase formed from achiral molecules is for the bend elastic constant to tend to be negative; in addition the twist elastic constant should be smaller than half the splay elastic constant. To test these important aspects of the prediction we have measured the bend and splay elastic constants in the nematic phase preceding the twist-bend nematic using the classic Frederiks methodology and all three elastic constants employing the dynamic light scattering approach. Our results show that, unlike the splay, the bend elastic constant is small and decreases significantly as the transition to the induced twist-bend nematic phase is approached, but then exhibits unexpected behavior prior to the phase transition.

Time-resolved NMR spectroscopy is a powerful method to investigate field-induced rotation of the director in a nematic liquid crystal. The method requires that the director does not rotate significantly during the acquisition of the free induction decay and hence the NMR spectrum. We have extended the method to systems where this is not the case and the observed NMR spectra are now found to contain novel oscillatory features. To understand these oscillations, we have developed a model combining both director and spin dynamics. In addition to increasing the information content of the time-resolved NMR spectra, it also proves possible to determine the field-induced relaxation time from a single spectrum.

The liquid-crystal dimer 1'',7''-bis(4-cyanobiphenyl-4'-yl)heptane (CB7CB) exhibits two liquid-crystalline mesophases on cooling from the isotropic phase. The high-temperature phase is nematic; the identification and characterization of the other liquid-crystal phase is reported in this paper. It is concluded that the low-temperature mesophase of CB7CB is a new type of uniaxial nematic phase having a nonuniform director distribution composed of twist-bend deformations. The techniques of small-angle x-ray scattering, modulated differential scanning calorimetry, and dielectric spectroscopy have been applied to establish the nature of the nematic-nematic phase transition and the structural features of the twist-bend nematic phase. In addition, magnetic resonance studies (electron-spin resonance and (2)H nuclear magnetic resonance) have been used to investigate the orientational order and director distribution in the liquid-crystalline phases of CB7CB. The synthesis of a specifically deuterated sample of CB7CB is reported, and measurements showed a bifurcation of the quadrupolar splitting on entering the low-temperature mesophase from the high-temperature nematic phase. This splitting could be interpreted in terms of the chirality of the twist-bend structure of the director. Calculations using an atomistic model and the surface interaction potential with Monte Carlo sampling have been carried out to determine the conformational distribution and predict dielectric and elastic properties in the nematic phase. The former are in agreement with experimental measurements, while the latter are consistent with the formation of a twist-bend nematic phase.

The static director distribution in thin nematic liquid crystal cells, subject to both electric and magnetic fields, has been investigated using a combination of deuterium nuclear magnetic resonance (NMR) spectroscopy and continuum theory in terms of the director distribution function, which gives the probability density for finding the director at a given orientation. A series of deuterium NMR spectra for the nematic liquid crystal, 4-pentyl-d(2)-4'-cyanobiphenyl deuteriated in the α position of the pentyl chain were acquired as a function of the applied electric field. This powerful experimental technique allowed us to observe uniform and nonuniform director alignment depending on the angle between the two fields and their relative strength. On the basis of the detailed experimental results, we have explored the factors that influence the nature of both the uniform and the nonuniform director distributions. We have discussed the questions that are raised by our attempt to understand the static director distribution as a function of the angle between the two fields. We have discovered that the alignment of the director at the surface of the Teflon spacers is essential in addition to the random variation in the cell thickness in order to account for the static director distribution determined from the NMR spectra.

Deuterium NMR spectroscopy has been employed to investigate the static and dynamic director alignment processes in a thin nematic film. One of the prime advantages in the use of deuterium NMR spectroscopy to determine the director orientation is that the form of the spectrum is influenced by the distribution of the director with respect to the magnetic field. This situation obtains because when the director is not uniformly aligned the observed spectrum is a weighted sum of the spectra from all director orientations. We have studied the uniform and non-uniform director distributions in a thin nematic liquid crystal cell using a combination of deuterium NMR spectroscopy, continuum theory, and the director distribution function [1], which gives the probability density for finding the director at an angle. The sample was confined between two glass plates and subject to magnetic and ac electric fields. In the absence of the electric field the director for 4-pentyl-d2-4'– cyanobiphenyl (5CB-d2), is aligned parallel to the magnetic field. When an electric field is applied then the director will be aligned with respect to the electric field (turn-on process). It has been found that for a range of cyanobiphenyls the sample moves as a monodomain during the turn-on process even when the angle, α, between the magnetic and electric fields, is as large as 89.0°. In marked contrast, when α is equal to about 90°, following the application of the electric field the director mainly moves away from being parallel to the magnetic field as the time-resolved NMR spectra recorded with α=89.7°. During the dynamic process, however, part of the director remains parallel to the magnetic field for a fairly long time and other parts show a broad director distribution in which it adopts a range of orientations between 0° and 90°. That is, the sample adopts non-uniform director states during the alignment process. A series of deuterium NMR spectra was also acquired as a function of the applied electric field to explore the director distribution. Uniform and non-uniform director alignments have been observed for the geometry of α=89.1°and α=89.9°, respectively. On the basis of detailed static experimental results we discuss the factors causing the uniform and non-uniform director distributions. When α is closed to right angle, a small variation of the film thickness over the entire cell causes a director distribution in a wide range of the angle around the threshold electric potential. If the probability for finding the director at each angle in a wide range over the entire film is constant, resultant director distribution will give a powder pattern of the spectrum, not the spectrum having two clear doublets. When the director is distributed in a small range of the angle, these essentially give the spectra with a line broadening so that with increasing electric field strength the director orientation changes continuously to the limiting value. It was found that the inhomogeneity of the thickness of the cell is a dominant factor to influence the director distribution, which is a factor dependent on the experimental condition. On the basis of detailed experimental results to explore the factors that influence the nature of the non-uniform director distribution, we discuss the unanswered questions created by the measurements of the director dynamics.

Certain physical properties of nematics can be obtained from the field‐induced static and dynamic director orientations in thin films. Here we describe how deuterium nuclear magnetic resonance (NMR) spectroscopy can be used to investigate the field‐induced director orientation in nematic liquid crystals. This powerful approach is illustrated with specifically deuteriated 4‐pentyl‐4′‐cyanobiphenyl (5CB) subject to the magnetic field of the spectrometer and an electric field applied at an angle to it. A series of deuterium NMR spectra was acquired as a function of the applied electric field, which can be used to explore the static director orientation. When the electric field is applied to the nematic, the director moves from being parallel to the magnetic field to being at an angle to it (the turn‐on process) because and are both positive for 5CB. After the electric field is switched off, the director relaxes back to being parallel to the magnetic field (the turn‐off process). Deuterium NMR spectra were recorded during the turn‐on and the turn‐off alignment processes as a function of time. Analysis of these results for the static and dynamic experiments, based on the predictions of continuum theory, provides the physical properties of the nematic.

We have used deuterium NMR spectroscopy to investigate the director dynamics and equilibrium behaviour in nematic liquid crystals (4‐pentyl‐ and 4‐octyl‐4′‐cyanobiphenyl, (5CB and 8CB), both specifically deuteriated) when subject to magnetic and a.c. electric fields. The angle between the magnetic and electric fields can be varied between 0 and 90° and the most common geometry we have used is for an angle of about 45°. For 5CB and 8CB (with positive and ) the director orientation was measured using time‐resolved NMR both when the electric field is applied and when it is turned off. In all cases it was found that the director alignment was uniform and the director relaxation follows closely the predictions of the torque‐balance equation given by the Leslie–Ericksen theory. In all these experiments we have employed a 10 kHz electric field; at such a relatively high frequency the director experiences an effectively constant value of the electric field. We have now investigated the behaviour of the nematic director for the two liquid crystals at much lower frequencies of the electric field: several Hz to about 1000 Hz. As before, the director orientation was measured using time‐resolved deuterium NMR spectroscopy. We have employed two geometries. In one, the electric and magnetic fields were inclined at ∼50°. We found that the director oscillates between two extreme orientations (determined by the frequency and the field strength) in a plane formed by the magnetic and electric fields. The oscillations were observed to continue for many cycles, indicating that the coherence in the director orientation was not lost during this motion. The director was found to remain uniformly aligned. The two extreme director orientations can also be determined from the NMR spectrum time‐averaged over many thousands of cycles of oscillations. At low frequencies (several Hz) these limiting angles are essentially independent of frequency but as the frequency increases so the two angles approach each other and become equal at high frequencies. More recently, we have used a geometry with the angle between the fields of ∼90°. A threshold behaviour is observed in this geometry for the director orientation as a function of the applied voltage. The time‐averaged spectra at low frequencies and at certain voltages showed unusual powder‐like features. Time‐resolved NMR measurements at 40 Hz and different voltages near the threshold value were carried out to understand the oscillatory behaviour which was also simulated. Turn‐on and turn‐off dynamics at high frequency were conducted revealing intriguing differences between the two pathways for the field‐induced relaxation. These results will be discussed and interpreted in terms of the torque‐balance equation with a time dependent electric field.

ABSTRACT Deuterium NMR spectroscopy has been used to investigate the director dynamics of deuteriated 4-α,α-d2-nonyl-4′-cyanobiphenyl, where the director was aligned by an electric field. The electric field direction made an angle of 47° with the magnetic field in order to provide a unique alignment pathway; accordingly the director is expected to rotate as a monodomain. The time dependence of the director orientation was investigated at different temperatures. At each temperature in the nematic phase, all of the deuterium NMR spectra indicate that the director was uniformly oriented as a monodomain. It was found that the relaxation time in the smectic A phase is about 1,000,000 times as large as that in the nematic phase. This also results in a strong pretransitional growth of the relaxation time as the transition to the smectic A phase is approached.

We have investigated the oscillatory behavior of the nematic director for 4-pentyl-4'-cyanobiphenyl (5CB) when it is subjected to a static magnetic field and a sinusoidal electric field. In these experiments the two fields were inclined at about 50 degrees and the frequency of the electric field was varied from several hertz to approximately 1000 Hz. The director orientation was measured using time-resolved deuterium NMR spectroscopy since this has the advantage of being able to determine the state of director alignment in the sample. In fact, for all of the frequencies studied the director is found to remain uniformly aligned. Since the diamagnetic and dielectric anisotropies are both positive the director oscillates in the plane formed by the two fields. These oscillations were observed to continue for many cycles, indicating that the coherence in the director orientation was not lost during this motion. The maximum and minimum angles made by the director with the magnetic field were determined, as a function of frequency, from the NMR spectrum averaged over many thousand cycles of the oscillations. At low frequencies (several hertz) these limiting angles are essentially independent of frequency but as the frequency increases the two angles approach each other and become equal at high frequencies, typically 1000 Hz. Our results are well explained by a hydrodynamic theory in which the sinusoidal time dependence of the electric field is included in the torque-balance equation. This analysis also shows that, for a range of frequencies between the high and low limits, these NMR experiments can give dynamic as well as static information concerning the nematic phase.

The field-induced alignment of a smectic-A phase is, in principle, a complicated process involving the director rotation via the interaction with the field and the layer rotation via the molecular interactions. Time-resolved nuclear magnetic resonance spectroscopy has revealed this complexity in the case of the director alignment, but provides no direct information on the motion of the layers. Here we describe a time-resolved x-ray diffraction experiment using synchrotron radiation to solve the challenging problem of capturing the diffraction pattern on a time scale which is fast in comparison with that for the alignment of the smectic layers. We have investigated the alignment of the smectic-A phase of 4-octyl-4(')-cyanobiphenyl by a magnetic field. The experiment consists of creating a monodomain sample of the smectic-A phase by slow cooling from the nematic phase in a magnetic field with a flux density of 7 T. The sample is then turned quickly through an angle phi(0) about an axis parallel to the x-ray beam direction but orthogonal to the field. A sequence of two-dimensional small angle x-ray diffraction patterns are then collected at short time intervals. Experiments were carried out for different values of phi(0), and at different temperatures. The results show that the alignment behavior changes fundamentally when phi(0) exceeds 45 degrees, and that there is a sharp change in the alignment process when the temperature is less than 3 degrees C below the smectic-A-nematic transition. The results of the x-ray experiments are in broad agreement with the NMR results, but reveal major phenomena concerning the maintenance of the integrity of the smectic-A layer structure during the alignment process.

The rotational viscosity coefficient of a nematic liquid crystal with a positive diamagnetic susceptibility anisotropy can be determined by monitoring the time dependence of the director orientation as it is rotated by a field from a nonequilibrium to the equilibrium state parallel to the field. A variety of techniques is available using different properties to monitor the director orientation as a function of time. Normally these experiments are designed so that the property used to determine the director orientation does not change during the time taken for its measurement. Here using ESR spectroscopy, we explore the benefits of exploiting exactly the opposite situation. That is during the time taken to record the ESR spectrum the director orientation is allowed to change. We have developed both semi-quantitative and quantitative models to allow us to simulate how the form of the spectrum depends on experimental conditions such as the field scan rate. These models have also proved to be valuable in designing the experiment and in analysing the spectra. It seems that this novel ESR experiment provides a valuable route to the field-induced relaxation time and hence to the rotational viscosity coefficient.

Studies of the field-induced alignment of the SmA phase using deuterium NMR spectroscopy have revealed a complex pattern of behaviour when the director is initially orthogonal to the aligning field. Here we report the electric field-induced alignment of the SmA director using time-resolved deuterium NMR when the aligning electric field E is at an angle with the magnetic field B of the spectrometer which is considerably smaller than 90degrees; here the director is initially aligned parallel to the magnetic field. The dynamics of the electric field-induced alignment of the director for the smectic phase of 4-alpha,alpha-d(2)-octyl-4'-cyanobiphenyl (8CB-d(2)) was investigated at two angles between B and E of roughly 45degrees and 54.5degrees the so-called magic angle, at different electric field strengths and also at two temperatures, 305.1 K and 302.6 K The dynamics of the SmA director alignment of 8CB-d(2) was monitored by measuring the deuterium NMR spectrum as a function of time. The results for the 45degrees and 54.5degrees geometries revealed, in contrast to the complex dynamics of alignment of the SmA director observed for the 90degrees geometry, a much simpler pattern of relaxation. Here the director appeared to be aligned almost as a monodomain. In general, for either of the two geometries employed here, the ultimate angle of alignment the director achieves relative to E depends on the electric field strength. Lowering the temperature by just 2.5degreesC from 305.1 K to 302.6 K has a dramatic effect on the rate of director relaxation presumably because of the large increase in the combined rotational viscosity and the elastic energy effects of the SmA phase with decreasing temperature. Furthermore for the 45 geometry at 302.6 K, the SmA sample separates on relaxation ultimately into two domains with different alignment angles.

Deuterium NMR spectroscopy has been used to investigate the director dynamics in the nematic phase of perdeuteriated 4-octyl-4'-cyanobiphenyl-d(25) (8CB-d(25)) When the electric field is applied to the nematic film, the director moves from being parallel to the magnetic field to being at an angle with respect to it. After the electric field is switched off, the director relaxes back to being parallel to the magnetic field. Deuterium NMR spectra were recorded during the, turn-on and the turn-off alignment processes as a function of time. This particular technique was chosen because the spectral peaks associated with each rigid group in the molecule are clearly resolved and of comparable intensity. For all of the experiments at different temperatures in the nematic phase of 8CB-d(25) we find that the field-induced relaxation times are independent of the group used to determine the director orientation during the alignment process.

Deuterium NMR spectroscopy together with spectral simulations have been used to investigate the field-induced director dynamics in a nematic liquid crystal, 4-pentyl-4'-cyanobiphenyl (5CB), conned in a slab between two electrodes. The NMR spectra have been measured when turning the electric field on and turning it off. Measurements were also made at different temperatures to explore how the temperature effects the director relaxation. At higher temperatures, some complications arise as peculiar oscillations are observed in the spectra. With spectral simulation this phenomena is shown to result from the relaxation of the director on a timescale comparable to that of the experiment which is the effective spin-spin relaxation time. The simulated spectra are compared with the experimental spectra for the specifically deuteriated 5CB-d(2).