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Electric-field driven director oscillations in nematic liquid crystals
No full textWe 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 ac 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 10kHz 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 fields: several Hz to about 1000Hz. 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 ~90º. 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 40Hz 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
H-2 NMR spectroscopic investigation of para-nitroazobenzene liquid crystals
No full textThe orientational order of two liquid crystals, namely, 6-[4-(4-nitrsphenylazo])phenyloxy] hexyl diethanolamines (C6) and 10-1-bromo[4-(nitrophenylazo)phenyloxy]alkane (B10) was studied by means of H-2 NMR spectroscopy with hexamethylbenzene-d(18) as the probe molecule. The results show that the directors in the smectic a phase of C6 and the nematic phase of B10 could be aligned, which was parallel to the magnetic field. The orientational order parameter of the solute molecules in C6 was about 0.2, while it is only 0.1 in B10, which is expected because the more ordered smectic phase tends to align solute molecules to a high level. Compared to the orientational order parameter of the solute in the SmC phase of 4[3,4,5-tris(4-dodecyloxybenzyloxy)-benzoyloxy]-4-(4 ' -dodecyloxybenzoyloxy)biphenyl (I) (P-2 = 0.14), it is larger in SmA. phase of C6. The relatively higher orientational order parameter of the solute in C6 is attributed to the formation of intermolesular H-bonds in the SmA phase of C6
Field-induced director dynamics in the nematic phase of 4-octyl-4'-cyanobiphenyl. A deuterium NMR investigation
No full textThe response times of liquid crystal display devices are determined by a range of factors but the most important of these is usually the rotational viscosity coefficient, gamma(1). In order to understand the relationship between molecular structure and viscosity it is of considerable interest to measure this viscosity coefficient for a variety of nematogens. Here we report the determination of gamma(1) for 4-octyl-4'-cyanobiphenyl-d(2) at two temperatures using deuterium NMR spectroscopy. In these experiments the time taken for the alignment of the director can be changed by a field, either magnetic or electric. To do this the liquid crystal film was enclosed in a cell which allows the application of an electric field within the NMR spectrometer. The rate of director relaxation was followed by recording the deuterium NMR spectrum as a function of time during the process of turning the electric field on or off. We have carried out the experiments for a geometry in which the director orientation with respect to the magnetic field of the spectrometer does not exceed 45degrees. The alignment of the director throughout the relaxation process was observed to be uniform. The director relaxation was found to follow closely the predictions of the torque-balance equation given by the Leslie-Eriksen theory. The relaxation times for the turn-on and turn-off processes were determined from this equation and found to be of the order of 1-2 ms. A knowledge of the anisotropic electric and magnetic susceptibilities then allows the determination of the rotational viscosity coefficient
NMR determination of the physical properties of nematics
No full textCertain 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 Delta epsilon and Delta chi 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
Field-induced director dynamics in a nematic liquid crystal: a molecular site dependence?
No full textStudies of the field-induced alignment of the nematic director for low molar mass materials using vibrational spectroscopy have revealed some fascinatingly unexpected results. Several studies have found that the relaxation time for the director alignment is dependent on the group in the molecule used to monitor the director orientation. Seemingly this undermines the basic concept of the Leslie-Ericksen hydrodynamic theory of nematics. Here we report complementary studies using deuterium NMR spectroscopy of perdeuteriated 4-pentyl-4'-cyanobiphenyl where the director is aligned by a magnetic or an electric field. This particular technique was chosen because the spectral peaks associated with each rigid group in the molecule are clearly resolved and of comparable intensity. We have investigated the director alignment using different temperatures, electric field strengths and angles between the magnetic and electric fields, each of which influences the director relaxation time. For all of the experiments we find that the relaxation times are independent of the group used to determine the director orientation during the alignment process
Director reorientation processes in a monodomain thin nematic liquid crystal film: a deuterium NMR spectroscopy study
No full textDeuterium nuclear magnetic resonance (NMR) spectroscopy has been used to investigate the director dynamics in the nematic liquid crystal, 4-pentyl-4'-cyanobiphenyl (5CB), confined between two glass plates and subject to magnetic and electric fields. The nematic cell was held in the NMR probe head so that the electric field, whose direction is normal to the substrate surface, makes an angle of about 45° with the magnetic field. This experimental geometry avoids the degeneracy in the field-induced alignment pathway for the director found for larger angles. A series of deuterium NMR spectra, obtained using a quadrupolar echo sequence, was acquired as a function of time. When the electric field, whose intensity is controlled so that the director makes an angle with the magnetic 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 the magnetic field because ?(?)over tilde and ?(?)over tilde are both positive for 5CB. 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. The realignment pathway of the director was monitored by measuring ?(?)over tilde, the deuterium quadrupolar splitting. We have studied the time dependence of the director orientation for the turn-on and turn-off processes at different temperatures in the nematic phase. The temperature independence of ?(?)over tilde/?(?)over tilde is also discussed on the basis of experiment and theory. The diamagnetic anisotropy and the rotational viscosity coefficient were also determined as a function of temperature. The deuterium NMR spectra corresponding to the field-induced director dynamics were predicted by an analysis based on hydrodynamic theory
The twist-bend nematic phase: translational self-diffusion and biaxiality studied by 1H nuclear magnetic resonance diffusometry
No full textRecently, there has been a surge of interest in mesogens exhibiting the twist-bend nematic (NTB) phase that is shown to be chiral even though formed by effectively achiral molecules. Although it now seems to be clear that the NTB 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 NTB 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.<br/
On the investigation of field-induced director dynamics: a novel ESR experiment
No full textThe 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 non-equilibrium 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
Electric-field driven director oscillations in a nematic liquid crystal: a NMR investigation
No full textWe 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 50degrees and the frequency of the electric field was varied from several hertz to approximate to1000 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
Field-induced alignment of a smectic-A phase: a time-resolved x-ray diffraction investigation
No full textThe 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(o) 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(o), and at different temperatures. The results show that the alignment behavior changes fundamentally when phi(o) exceeds 45degrees, and that there is a sharp change in the alignment process when the temperature is less than 3 degreesC 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
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