Akihiko Sugimura

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.

Nematic director deformation induced by a periodic surface anchoring strength

Abstract We have investigated the voltage dependence of the light transmittance for homogeneous thin nematic liquid crystal (NLC) cells composed of 4-pentyl-4′-cyanobiphenyl (5CB). The surfaces of a cell were coated with a thin polyimide film and also rubbed unidirectionally. We have also used a photo-alignment method to produce a planar director alignment. In order to produce one-dimensional periodical change of the surface anchoring strength, linearly polarized ultraviolet light, which passed through a chromium photo-mask with 5-μm periodical pitch, was irradiated on the photopolymer material. An NLC cell was set between polarizer and analyzer, which was crossed with the polarizer. When the voltage is applied to the NLC cell, the director for 5CB with its positive dielectric anisotropy rotates to orient parallel to the electric field. The voltage dependence of the light transmittance for the parallel setup shows an anomalous result. It has been interpreted from the experimental results that the anomalous optical leakage is induced by the director deformation resulting from a periodical change of the surface anchoring strength along the direction normal to the rubbing direction. The exact numerical analysis of the director deformation induced by the periodic change of the surface anchoring energy supports the appearance of the anomalous light leakage.

DEUTERIUM NMR SPECTROSCOPY AND FIELD-INDUCED DIRECTOR DYNAMICS IN LIQUID CRYSTALS

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), confined 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-d2.

Field-induced director dynamics of nematic 4-OCTYL-4′-cyanobiphenyl: a study by deuterium NMR spectroscopy

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.

Field-induced director alignment for 4-nonyl-4'-cyano-biphenyl near the smectic A - nematic transition

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.

Field-Induced Director Dynamics in the Nematic Phase of 4-Octyl-4″-Cyanobiphenyl. A Deuterium Nmr Investigation

The 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, n 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 n 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 45. 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.

Electric field‐driven director oscillations in nematic liquid crystals

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.

Weak surface anchoring energy of 4-cyano-4'-pentyl-1,1'-biphenyl on perfluoropolyether Langmuir-Blodgett films.

Functional director alignment layers are needed for high performance liquid crystal displays (LCDs). Reported herein is a novel polymer material for LC alignment, namely, perfluoropolyether (PFPE), which exhibits a weak surface anchoring energy for bend deformation and is amenable to simple fabrication of grooved surfaces by soft lithography, a surface topography desired for multistable LCDs. Liquid crystal optical cells fabricated using Langmuir-Blodgett films of PFPE (of variable thickness) exhibited weak surface anchoring energies on the order of 10(-5) Jm2 for the nematic liquid crystal 4-cyano-4-pentyl-1,1-biphenyl with no dependence on film thickness.

ANOMALOUS FIELD-INDUCED DIRECTOR DEFORMATION IN A HOMOGENEOUS THIN NEMATIC LIQUID CRYSTAL CELL

We have investigated the voltage dependence of the light transmittance for homogeneous thin nematic liquid crystal (NLC) cells composed of 4-pentyl-4′-cyanobiphenyl (5CB), which were confined between two glass plates. The substrate surfaces with polyimide layers were rubbed unidirectionally to make a uniform planar director orientation with a strong surface anchoring condition. An NLC cell was set between polarizer and analyzer. The He-Ne laser beam was passed through the NLC slab oriented with its optic axis parallel to the linearly polarized direction of the light beam. The light transmitted through the analyzer, which was crossed with the polarizer, was detected as a function of the applied voltage. In zero electric field it is expected that the director will have a uniform planar alignment and this orientation gives a minimum intensity of the light transmitted. When the voltage is applied to the NLC cell, the director for 5CB with its positive dielectric anisotropy rotates to orient parallel to the electric field. The voltage dependence of the light transmittance showed an anomalous result; that is, light leakage was observed for a certain voltage. This was interpreted by considering the director deformation at the boundaries between rubbed and non-rubbed regions of the polyimide surface.