Electric-field driven director oscillations in a nematic liquid crystal: a NMR investigation

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.     

Convective director structures upon continuous rotation of nematic side group polymers

We present the first study of convective director structures in nematic side group polymers. A thin liquid crystal cell (10-500 μm) was continuously rotated about an axis perpendicular to the field of a 7T NMR magnet. The director behaviour was followed by deuteron NMR as well as by polarization microscopy. While by optical studies the development of periodic director structures can be directly monitored, the analysis of the NMR lineshape gives detailed information about the director distribution in these structures. The development of the structures depends sensitively on the rotation frequency and is discussed in terms of non-linear amplification of long wavelength director fluctuations due to the coupling between director rotation and viscous flow of the nematic.     

Convective director structures upon continuous rotation of nematic side group polymers

Abstract

We present the first study of convective director structures in nematic side group polymers. A thin liquid crystal cell (10–500 μm) was continuously rotated about an axis perpendicular to the field of a 7T NMR magnet. The director behaviour was followed by deuteron NMR as well as by polarization microscopy. While by optical studies the development of periodic director structures can be directly monitored, the analysis of the NMR lineshape gives detailed information about the director distribution in these structures. The development of the structures depends sensitively on the rotation frequency and is discussed in terms of non-linear amplification of long wavelength director fluctuations due to the coupling between director rotation and viscous flow of the nematic.     

The elastic behaviour of liquid crystal dimers: an ESR investigation of their induced chiral nematic phases

We have attempted to investigate the dependence of the elastic behaviour of liquid crystal dimers on the parity of the spacer linking the two mesogenic groups by using an indirect method. In this the nematic phase formed by the dimers is converted to a chiral nematic by the addition of a chiral dopant. The uniform distribution of the director in the helical structure of the mesosphase is perturbed by the magnetic field of an ESR spectrometer. The extent of the field-induced perturbation is then determined from the appearance of the ESR spectrum of a nitroxide spin probe dissolved in the mesophase. The observed difference in the extent of the field-induced perturbation is analysed in terms of the higher orientational order of the even with respect to the odd dimer. However, the theoretical model fails to account for this difference in behaviour and a possible explanation is discussed.     

Magnetic-field induced biaxiality in nematic liquid crystals. Consequences for nuclear spin relaxation

When a uniaxial nematic liquid crystal is subjected to a magnetic field making a non-zero angle with the C_∞ axis, the uniaxial symmetry is broken. The principal effect is a field-induced biaxiality in the long-wavelength region of the director fluctuation spectrum. Whereas the induced biaxiality has little effect on the mean square director fluctuation amplitudes 〈n²_x〉 and 〈n²_y〉, which are dominated by short-wavelength modes, it can profoundly affect the nuclear spin relaxation behaviour, which is sensitive to long-wavelength modes. Motivated by the increasing number of nuclear spin relaxation studies of director fluctuations in thermotropic, amphiphilic, and polymeric nematic liquid crystals, we present here a theoretical analysis of the effects of field-induced biaxiality on nuclear spin relaxation.     

On the simulation of the director field of a nematic liquid crystal

A numerical method for the analysis of the static and dynamic behaviour of the director field, n, of a nematic liquid crystal is presented, where the equivalence of n and - n is taken into account. The occurrence of defects characteristic of nematics is studied for a hybrid cell, and for a material with a negative anisotropic susceptibility subject to an external field. Typical director configurations with defects are displayed graphically for some two dimensional arrangements and, in addition, the resulting pictures corresponding to an observation between crossed polarizer and analyser have been computed.     

Bifurcation and domain walls in thin films of liquid crystals

A perturbation method is proposed to determine the bifurcation diagram for instabilities in thin liquid-crystalline layers which are subject to an external magnetic or electric field. Different types of continuous and discontinuous field-induced director reorientations can be classified by using elementary bifurcation theory. When two differently oriented director positions are stable, domains may appear. For weakly distorted samples the structure and velocity of walls between stable differently oriented domains is described by a solitary wavy solution of the non-linear director equation. The results of the perturbation method are applied to nematic and smectic C films.     

Electric field effects on director pattern and disclinations in free standing nematic films of 8CB

Studies of electric field-induced orientational effects are carried out on substrate-free nematic 8CB films, held between parallel electrodes and containing a peripheral-1/2 loop. The loop separates the birefringent boundary from the central homeotropic zone. Static and dynamic characteristics of the splay-bend layers contiguous to the electrodes are investigated by polarizing microscopy. A model based on director field symmetry is proposed to explain (a) the growth of the transition layers independently of the associated singularities and (b) the formation or otherwise of static solitons (Brochard-Leger walls). Linear dependence of the growth rate of transition layers on field strength is compared with a similar behaviour for line defects. Bifurcation of a soliton at elevated voltages into a pair of - 1/2 lines, through a non-pincement process, is described.     

Electric field effects on director pattern and disclinations in free standing nematic films of 8CB

Studies of electric field-induced orientational effects are carried out on substrate-free nematic 8CB films, held between parallel electrodes and containing a peripheral-1/2 loop. The loop separates the birefringent boundary from the central homeotropic zone. Static and dynamic characteristics of the splay-bend layers contiguous to the electrodes are investigated by polarizing microscopy. A model based on director field symmetry is proposed to explain (a) the growth of the transition layers independently of the associated singularities and (b) the formation or otherwise of static solitons (Brochard-Leger walls). Linear dependence of the growth rate of transition layers on field strength is compared with a similar behaviour for line defects. Bifurcation of a soliton at elevated voltages into a pair of - 1/2 lines, through a non-pincement process, is described.     

Manipulation of the director in bicellar mesophases by sample spinning: a new tool for NMR spectroscopy.

It is shown that bicellar nematic liquid-crystalline phases can be oriented with the director (the normal to the bicellar plane) at an arbitrary angle to the applied magnetic field by sample rotation around one axis (variable-angle sample spinning) or around two axes successively (switched-angle spinning). This promises to open novel possibilities for NMR studies of bicelles and proteins incorporated into bicelles or dissolved in a solution containing bicelles, including the correlation of several orientations in a two-dimensional NMR experiment.     

Optical studies of dynamic director configurations in ferroelectric liquid crystal devices

An investigation into the transmission spectrum of a ferroelectric liquid crystal device is undertaken. This is done both for an initial static state and during a switching process. Comparisons are made between experimental data and theoretical predictions. The dynamic internal director configurations in the device is shown to be consistent with a simple model during both monopolar and bipolar addressing pulses.     

Field-induced reorientation dynamics of a surface stabilized ferroelectric liquid crystal mixture

Time-resolved measurements of electro-optical response and Fourier-transform infrared absorption have been employed to study the submolecular motion of a surface stabilized ferroelectric liquid crystal (SSFLC) mixture during a field-induced reorientation process. All observed submolecular groups were found to rotate in unison about the layer normal in a steady d.c. field. In a transient situation, the FLC director can respond rapidly to a pulsed driving field. However, the orientation distribution was found to spread at first and then slowly converge to the new direction of the FLC director. When the field is switched off, all the core groups relax to a steady-state direction with varying relaxation times. The results reveal that submolecular fragments of different molecular species in the FLC mixture move correlatively in a steady d.c. field and during a field-induced reorientation process.     

Automated analysis of high-resolution NMR spectra III—the nematic phase spectra of three allyl halides

In recent publications we have described a novel method of analysis for high-resolution NMR spectra of compounds in isotropic solution which differs from conventional procedures in that it exploits the full information content of a spectrum and does not require assignments of energy levels or transitions. We now present a further development of this technique which also makes it applicable to the NMR spectra of molecules partially oriented in liquid crystal solvents. Illustrative applications are reported for the nematic phase spectra of allyl fluoride, allyl chloride and allyl bromide. The results are discussed in terms of various theoretical models of the interplay between the internal and overall rotations of the conformationally mobile solutes.     

Optical studies of dynamic director configurations in ferroelectric liquid crystal devices

An investigation into the transmission spectrum of a ferroelectric liquid crystal device is undertaken. This is done both for an initial static state and during a switching process. Comparisons are made between experimental data and theoretical predictions. The dynamic internal director configurations in the device is shown to be consistent with a simple model during both monopolar and bipolar addressing pulses.     

Azimuthal director gliding at a strongly anchoring interface of polyimide

A gliding of the director at the interface between a nematic liquid crystal and a solid medium is generally observed at many interfaces giving weak or moderately strong anchoring. This phenomenon is characterized by strongly non-linear dynamics and very long relaxation times (hours-days). The gliding of the director has also been observed very recently at the interface between a rubbed polyimide layer and a nematic liquid crystal which gives strong azimuthal anchoring. However, due to the weak nature of the experimental signals that characterizes the strong anchoring, this latter measurement was appreciably affected by thermal drift. In this paper, we develop a new experimental reflectometric method whereby the thermal drift is appreciably reduced. The method allows us to obtain more accurate signals and to investigate their time dependence. It is shown that the director gliding is well represented by a stretched exponential, as well as in the case of weak anchoring substrates. These measurements confirm that the gliding of the director is a universal phenomenon characterizing any kind of substrate with either weak and strong anchoring.     

Proton NMR investigation of a hydrogen-bonded liquid crystal gel

Proton NMR is employed to determine director distributions in a hydrogen-bonded liquid crystal gel in the presence of a magnetic field. The samples consist of the mesogen 8CB mixed with small percentages of gelator, which forms a hydrogen-bonded network when the sample is cooled below the gelation point in the isotropic liquid. Since the gelation occurs above the clearing point, a non-oriented random director configuration is frozen in. The configuration of the hydrogen-bonded network is found to be isotropic. It fixes a preferential random local orientation of the nematic director, even in the presence of an external magnetic field of a few Tesla. The NMR spectra of the samples give information on orientation and order in such systems. A simple model for the director field is provided.     

Proton NMR investigation of a hydrogen-bonded liquid crystal gel

Proton NMR is employed to determine director distributions in a hydrogen-bonded liquid crystal gel in the presence of a magnetic field. The samples consist of the mesogen 8CB mixed with small percentages of gelator, which forms a hydrogen-bonded network when the sample is cooled below the gelation point in the isotropic liquid. Since the gelation occurs above the clearing point, a non-oriented random director configuration is frozen in. The configuration of the hydrogen-bonded network is found to be isotropic. It fixes a preferential random local orientation of the nematic director, even in the presence of an external magnetic field of a few Tesla. The NMR spectra of the samples give information on orientation and order in such systems. A simple model for the director field is provided.     

Generalised technique for calculation of plane director profiles in bounded nematic liquid crystals

We propose an improved conformal mapping technique for analytical calculation of two-dimensional profiles of the nematic liquid crystal (NLC) director in a cell with a simply connected cross-sectional region. We consider the case of the strong anchoring and the piecewise constant director pretilt on the piecewise smooth curve which bounds the region. Obtained expressions for the director profile explicitly depend on the conformal mapping which maps the region onto the upper half plane of a complex plane. An advantage of our method in comparison with the standard conformal mapping technique is that it does not require the knowledge of the inverse mapping and the calculation of the integral in the Poisson formula. Proposed technique allows to take into account topological defects in the bulk of the NLC on the symmetry axis of the region. As an example of how the method can be used, we find an analytical expression for the director profile in a horizontal cylindrical groove partly filled with the NLC. We consider the case where a disclination line parallel to the axis of the groove occurs in the bulk of the NLC. The equilibrium position of the disclination line is found.     

The influence of director fluctuations on molecular reorientation of a small probe molecule in a liquid-crystalline environment

The influence of director fluctuations on a probe molecule dissolved in a nematic liquid crystal is calculated to all orders using the gaussian properties of the director field. Consequences for the N.M.R. spectrum and relaxation through quadrupolar interaction of the probe molecule are indicated. It is found that the equilibrium distribution of molecular orientations, and consequently quadrupolar spectral splittings are virtually unchanged by fluctuations in the director field. The spectral densities are also obtained and it is shown that in addition to J₀₁ also J₀₀ and J₀₂ become frequency dependent in the N.M.R. frequency range. The frequency dependence of J₀₀ and J₀₂ has a logarithmic contribution; the magnitude of the frequency dependent contribution to J₀₂ is, however, insufficient to explain experimental results.     

Resolving the problem of chromatographic overlap by 3D cross correlation (3DCC) processing of LC, MS and NMR data for characterization of complex glycan mixtures

Chromatographic overlap is a common problem in the analysis of complex mixtures. As a result, it is not possible to identify the components because each resulting NMR or MS spectrum contains multiple components. We introduce three-dimensional cross correlation (3DCC) that dissects NMR spectra of a mixture into spectra of the individual components without actually separating them. Correlation of peaks from MS and NMR profiles along a common LC time domain yields 3DCC NMR spectra of pure components correlated with a mass and a retention time. The method requires an LC run followed by fractionation and recording of MS and NMR spectra. The method is applicable to mixtures of any classes of molecules. Here, we demonstrate its application to a mixture of complex glycans obtained from a glycoprotein. Fourteen glycans eluting within only 3 min showed heavy overlap in the chromatographic run. 3DCC allowed their direct characterization without separation. Some of these structures from the glycoprotein bovine fibrinogen had not previously been described. The 3DCC procedure has been implemented in standard software. Actually, 3DCC can be used for any combination of separation techniques, like LC or GC, combined with two characterization methods like UV, IR, Raman, NMR or MS.