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Measurement Method of Dielectric Permittivities of Ferroelectric Liquid Crystal Based on Molecular Orientational Models
Abstract
The measurement method of the dielectric permittivities of FLC based on the molecular orientational models (MOM method) is presented. The dielectric permittivities are measured by using usual SSFLC cell directly, not from the helical structure or the achiral host material. The measurement results are reported and the propriety of MOM method is discussed.
IntroductionTheoretical Prediction of the Biaxial Nematic PhaseStructural FeaturesSynthesisCharacterization Methods
Concluding RemarksReferences
IntroductionNomenclatureSynthesis PrinciplesMesomorphic PropertiesThe Core, Paraffinic Chains, and Mesomorphic PropertiesLyotropic Metal-Containing Liquid CrystalsStructuresConclusion
References
Ferroelectric liquid crystals (FLC) are to conventional liquid crystal what Gallium Arsenide is to Silicon in the semiconductor area. The first generation of FLC displays in now present on the market and has some outstanding features based on the symmetric bistability which may be achieved in these materials. One of the greatest challenges for the next generation is to achieve an analog grey scale out of an essentially digital principle. We will analyze in some detail which major problems had to be solved to reach the present state and show how the final steps could be taken toward a new state-of-the-art level in liquid crystal devices. In the last decade university research and industrial R and D have almost equally contributed to treat the very serious complications caused by the so-called chevron structures We will review this important topic in particular detail.
The study of the molecular orientational states and the dielectric properties of surface stabilized ferroelectric liquid crystals (SSFLCs) was reviewed. The molecular orientational models were presented and they gave us a total understanding of the orientational states which appear in SSFLCs with parallel rubbing. Using the molecular orientational models, the dielectric properties of SSFLCs were studied. Three dielectric permittivities were measured for each of two samples, the permittivity of the homeotropic cell and the permittivity of the planar homogeneous cell with and without the DC bias, and then the dielectric tensor components were calculated based on the molecular orientational models. Results for Merck SCE-8 are reported. The novel frequency dependence of the dielectric biaxiality was found. The sign of the dielectric biaxiality inverted around 1kHz, being negative at low frequencies and positive at high frequencies. The relationship between the dielectric behavior of SSFLC cells and the dielectric biaxiality is discussed.
The study of ferroelectricity is a branch of solid state physics which has shown rapid growth during the recent years. Ferroelectric materials exhibit unusual electric properties which make them useful in modern (opto)electronic technology, esp. display technology. Ferroelectric and antiferroelectric liquid crystals, including also various polymer forms, are the hottest research topic today in liquid crystals. The field is at the very beginning of industrial exploitation - a sensitive phase in which a good reference work is needed and will have a broad spectrum of readers both at universities and in industry. © WILEY-VCH Verlag GmbH. D-69469 Weinheim (Federal Republic of Germany), 1999. All rights reserved.
The frequency dependence of the dielectric biaxiality of surface stabilized ferroelectric liquid crystals (SSFLCs) was studied. The principal values of the dielectric tensor ε1, ε2 and ε3 were measured by the MOM (molecular orientational model) method. Three dielectric permittivities were measured for each of two samples. These were the permittivity of the homeotropic cell and the permittivity of the planar homogeneous cell with and without the DC bias. Then the dielectric tensor components were calculated based on the molecular orientational models. We present the theory and experimental procedure of the MOM method. Measurements have been performed on Merck FLC compound SCE-8. The following novel dielectric behaviour was observed, as the DC bias voltage was increased the dielectric permittivity of the planar homogeneous cell decreased at the low frequencies (∼ 1 kHz) while increased at the high frequencies (10kHz ∼). The sign of the dielectric biaxiality ∂εε (= ε2 - ε1) inverted around 1 kHz, being negative at low frequencies and positive at high frequencies. The roles of the biaxiality on the dielectric behaviour of SSFLC cells are discussed.
In an attempt towards relating the dielectric biaxiality to the molecular structure, the dielectric anisotropy and biaxiality and their temperature dependence in the chiral smectic C phase have been determined for seven single compounds. It is found that, on a molecular level, compounds with a strong polar group connected with a cyclohexane ring exhibit a relatively large dielectric biaxiality. Among the substances with large dielectric biaxiality, it is noticed that attaching the polar group to the linking point of the cyclohexane ring and the alkyl chain containing the stereocenter results in the largest biaxiality. The studied compounds possess a smectic C phase preceded either by a smectic A or a nematic phase. As a general rule (with exceptions) we found that a large biaxiality is also coupled to a high value of the spontaneous polarisation. While the dielectric biaxiality for all studied materials is positive, the dielectric anisotropy (at 100 kHz) was found to be negative, especially at lower temperatures in the C* phase. The reported numerical data of the dielectric biaxiality were taken at 100 kHz, a frequency which is relevant for electro-optic device applications. Some recently synthesized structures have very large biaxialities, with δε = ε2 - ε1 ≈ 5. Control of both δε and δε are important for the switching characteristics of ferroelectric liquid crystals and adjustments in both parameters should be made when varying the addressing wave forms.
We realized a surface stabilized ferroelectric liquid crystal display with high contrast ratio and high transmittance using a conventional rubbing method. Such remarkable electrooptical properties are based on the switching between uniform states in a cell treated with an AC electric field in the SmC*. It was confirmed by X-ray diffraction that the treatment caused a permanent change in the smectic layer structure from a chevron to a quasi-bookshelf structure. An example of multiplexing is also displayed.
Characteristic features of zig-zag defects and disclinations in surface-stabilized ferroelectric liquid crystal cells are presented. It is pointed out that there exist two kinds of zig-zag defects, i.e., a lightning type and a hairpin type, which appear alternatively along the smectic layer normal. This fact indicates that two types of mismatching of a layer bend structure cause two kinds of zig-zag defects. Switching between two twisted states occurs with the formation of internal boat-shaped disclination loops in relation to the zig-zag defects. The nucleation process of the boat-shaped domains is quite characteristic; the bows appear on each side of the bright hairpin-type lines, while the sterns appear on both sides of the dark lightning-type lines. A model structure is proposed in order to interpret the experimental observation around the zig-zag defect lines on the basis of disclination dynamics.
On the basis of a smectic layer bend (chevron) structure confirmed recently, the absolute configuration of a zigzag defect and a boat-shaped disclination loop were investigated in surface-stabilized ferroelectric liquid crystal (SSFLC) cells. Comparing the color of the SSFLC state stabilized by a lateral electric field with the color calculated by a suitable model, the following configurations were clarified: 1) the apices of lightning and the thick lines of hairpin zigzag defects correspond to ≪l*≫g and ≫g*≪l, respectively, and 2) the bow and the stern of boat-shaped domains correspond to +2pi and -2pi wedge disclinations, respectively.
The method of determination of the three main components of the electric permittivity tensor, e, in chiral smectics C was proposed. To illustrate the method we measured the electric permittivity for the ferroelectric smectic DOBAMBC in the SmC and SmA phases and calculated the components of ∊-tensor as a function of temperature.
A method is presented allowing the determination of the three principal values (ε1,ε2,ε3) of the dielectric tensor for a chiral smectic-C liquid crystal in the case that the material has an uncompensated helix. Hence it is applicable for essentially all single substances and therefore suitable for developing correlations between molecular structure and dielectric properties of the smectic-C* materials. The method requires two samples and uses three different measurement geometries. The first sample has the smectic layers parallel to glass plates and is also used for the determination of the tilt angle theta needed in the evaluation of (ε1,ε2,ε3). The two other measurements are made on a sample with the smectic layers essentially perpendicular to the glass plates, allowing chevron or uniformly tilted layer structure in the sample. One measurement is taken in the presence of the helix and another in the nonhelical (unwound) state obtained by applying a bias field. Measurements have been performed on one ester compound exhibiting A* and C* phases. The dielectric tensor components have been calculated and are presented as functions of temperature and frequency.
Surface-stabilized ferroelectric liquid crystal with high-pretilt (about
15°) aligning film rubbed in parallel showed three types of states:
C1-uniform (C1U), C1-twisted (C1T), and C2 states. In the C1 state, the
uniform state (C1U) showing extinction positions and the twisted state
(C1T) showing no extinction position were observed. On the other hand,
the C2 state showed only one state showing extinction positions. The
properties of these three states were investigated and their
orientational model will be discussed in combination with the molecular
orientation on the surface of the aligning film.
Surface-stabilized ferroelectric liquid crystals (SSFLC) used with
high-pretilt aligning film rubbed in a parallel direction showed three
states, C1U, C1T and C2, which exhibited different optical properties.
The temperature dependence of the memory angles of these states was
measured for eight kinds of SSFLC sample cells which were prepared using
two FLC materials and four aligning films. The increase in the surface
pretilt angle made the memory angle of the C1U state greater, but did
not affect that of the C2 state. The experimental results were quite
consistent with the calculated relationships between the memory angle
and the surface pretilt angle.
Two new effects specific to ferroelectric liquid crystals are described: (a) ferroelectric polar surface interactions and (2) spontaneous splay of the polarization. It is shown that a consequence of these effects is the possibility of first-order reorientation transitions induced by applied electric fields.
Surface-stabilized ferroelectric liquid crystals (SSFLC) with high-pretilt (about 15°) aligning film showed three types of states (C1U, C1T and C2) with different optical properties. The transmitted light of these states was calculated using the 4× 4 matrix method, and the optical memory angle and transmission spectra were compared with the experimental observations. The experimental characteristics of these states were consistent with the calculated results, indicating the validity of the proposed molecular orientational models of these states. In addition, we simulated the effect of the pretilt angle of the aligning film on the optical properties of the SSFLC device.
The orientational states of surface-stabilized ferroelectric liquid crystals with parallel rubbing were investigated by changing the pretilt angle of the aligning film. Four states, C1-uniform (C1U), C1-twisted (C1T), C2-uniform (C2U), and C2-twisted (C2T) states, were observed. The C1U and C2U states showed extinction positions, whereas the C1T and C2T states did not. The memory angle of the C1U state was larger than that of the C2U state for the respective cells. The molecular orientational model of these states is discussed with regard to the experimental results.
Smectic layer structures of some surface-stabilized ferroelectric liquid crystal cells were studied using high-resolution X-ray analysis. The cells were constructed of two glass plates coated with a polymer aligning film rubbed in parallel and exhibited a typical chevron layer structure. The correlation between the layer tilt angle and the molecular tilt angle was investigated in detail, taking into account the differences between the molecular tilt and apparent tilt angles. It was confirmed that the layer tilt angle correlated with the molecular tilt angle though it was slightly smaller, and the ratio of the layer tilt angle to the molecular tilt angle gradually decreased as the molecular tilt angle increased.
The relationship between the molecular orientation and the optical memory angle of the surface-stabilized ferroelectric liquid crystals was investigated. We have numerically calculated the optical memory angle based on two models of director orientations, the \mbi{n}-director continuous and \mbi{c}-director continuous models, varying the surface pretilt and nonlinearity of deformation of the azimuthal angle and compared the calculation results with the measured memory angle of some FLC samples.
Splayed states in ferroelectric liquid crystal (FLC) cells are seen in
general if the elastic distortion energy involved is less than the
surface polar interaction energy. Here such states are examined by the
propagation of optic modes along a thin layer of FLC. This allows the
examination of the optic tensor configuration and indicates the
existence of a pinned point at the chevron interface in the smectic
layering. This leads to the direct observation that in a chevron type
cell these are actually half splayed states, where one half of the cell
is uniform, and the other half is splayed. It is seen that the splayed
part can be switched between the two halves of the cell.
The switching characteristics of a ferroelectric liquid crystal with negative dielectric anisotropy were studied. An anomalous behavior has been found in the applied voltage dependence of the switching time, i.e., with increasing applied voltage in the low voltage region the switching time decreases monotonically, while in the high voltage region it attains a minimum and then increases.
The molecular orientational states of homogeneously aligned, helix unwinding, chiral smectic C liquid crystals placed in a thin cell (surface-stabilized ferroelectric liquid crystals [SSFLC]) were studied. They were classified by the optical viewing conditions and the relationship between the directions of the chevron layer structure and the surface pretilt. The molecular orientational models of the states were considered and illustrated with regard to the experimental results. The models of molecular orientation give us a total understanding of the orientational states which appear in SSFLCs with parallel rubbing. Furthermore, the effect of the surface pretilt angle on the orientational and optical properties of SSFLCs is discussed.
Using a simple uniform switching model, we investigate the behaviour of the voltage dependent switching time of surface stabilized ferroelectric liquid crystals as a function of the biaxial permittivity tensor and the layer tilt angle. We show that the dielectric biaxiality can markedly effect the response time (τ) of the device and is the origin of the minimum in τ as a function of voltage (V) in tilted layer systems. The dielectric biaxiality should, therefore, be optimized for multiplexing schemes which use the τ—V minimum.
We present results of theoretical modelling and experimental study of director distributions and the associated optical properties of chevron surface stabilized ferroelectric liquid crystal (SSFLC) cells. Chevron cells are modelled as being two stacked FLC slabs, described by distinct orientation distributions of the director [ncirc]-polarization P([ncirc] – P) couple. In each slab the [ncirc] – P distribution is governed by bulk Frank elastic and electric field-induced torques and by surface torques at the FLC-solid interface and at the chevron interface, where the two distributions are coupled. The optical properties of such structures are calculated numerically and the results tested using polarized light microscope-based spectrophotometric measurements of the wavelength dependent transmission of chevron SSFLC cells with varying sample orientation and applied field. Measured transmission spectra agree quantitatively with the model and provide evidence for the constraint of the ň – P field at the chevron interface, particularly in the case when the chevron interface is displaced from the cell midplane.
AC field stabilisation of ferroelectric SC* liquid crystals with the chevron layer geometry requires that there is a significant dielectric biaxiality., To investigate this, the electric permittivities of an SC host mixture based on the phenyl pyrimidines and exhibiting an N-SA-SC phase sequence are reported. A large relaxation of ε∥ occurred, which caused a sign reversal of the dielectric anisotropy with a crossover frequency of 25 kHz at 25°C. The host electro-optic behaviour was then characterised using extinction angle measurements as a function of applied voltage at frequencies corresponding to Δ≈ > 0, Δε ≈ 0 and δε < 0, and the results explained using a tilted layer model with a biaxial dielectric tensor. The magnitude of the SC dielectric biaxiality, the layer tilt angle in the planar homogeneous geometry and the cone angle are determined. Measurements for several other hosts commonly used in commercial FLC mixtures are reported and it is shown that in each case the dielectric biaxiality is sufficiently large to cause AC field stabilisation in ferroelectric liquid crystals.
A new set of matrix addressing schemes for ferroelectric liquid crystal displays is reported. The schemes use the minimum in the response time-voltage characteristic found in certain mixtures and deliver improved operating speed and contrast ratio compared with previously reported schemes operating in this mode.
The dielectric biaxiality of SCE13, a commercially available ferroelectric liquid crystal (FLC) mixture, has been measured. The results have been used to compare theoretically predicted AC field stabilization effects and the voltages for response time minima with experiment.
The temperature Tm at which the maximum dielectric constant εEm is observed, decreases with decreasing cell thickness. However, in the cell thinner than 10 μm, Tm again increases to higher temperatures and Em decreases remarkably. These behaviors can be explained in terms of free energy theory by taking effects of the thickness on the relaxation time into consideration for thicker cells and by taking the hindrance of the molecular motion due to the wall effect into consideration for thinner cells.Characteristics of electro-optic effects of ferroelectric liquid crystals are discussed as a function of the cell thickness and in comparison with the thickness dependence of dielectric properties.
A high resolution (5 lines/mm) large area(l4inch) FLCD has been developed for use in various personal and office equipment like computer, workstation, wordprocessor and desktop-publishing systems etc. The 1280 × 1120 pixel screen is simply multiplexed, thus working at a 1/1120 duty rate.
X-ray scattering and optical studies reveal two key features of Surface Stabilized Ferroelectric Liquid Crystal (SSFLC) cell structure: (1) The temperature dependence of the layer thickness is an important effect in determining both the layer and director-polarization structure; and (2) There exists in some SSFLC cells an internal FLC—FLC interface, where surface stabilization of the director-polarization field occurs in addition to that at the FLC—substrate interface.
Dielectric biaxiality plays an important role in the electric field behaviour of ferroelectric liquid crystal devices, providing the necessary driving force for AC field stabilization and playing an important role in device dynamics. The static electric permittivities have been measured as a function of temperature for several mixtures with an N‒SA‒SC phase sequence. Three permittivities are defined for the biaxial SC phase, and were determined from measurements of homeotropic and planar cells and from the average permittivity extrapolated from the uniaxial nematic and smectic A phases. The SC cone angle is assumed to be equivalent at dielectric and optical frequencies. Results are reported for five common ferroelectric liquid crystal hosts and for two racemic mixtures, including the racemic analogue of the commercially available SCE 13. The dielectric biaxiality is sufficiently large below the SA to SC transition in each of the studied mixtures to have a strong influence on the electro-optic behaviour. This is consistent with the occurrence of AC stabilization in surface stabilized ferroelectric liquid crystal devices with the chevron layer configuration.
Ferroelectric liquid crystal displays offer an attractive matrix addressed alternative for complex displays. However, their potential can only be realised by using novel addressing schemes suitable for the fast responding, DC sensitive, ferroelectric electro-optic effect.Over a period of several years, a high performance, novel, ferroelectric drive scheme has been developed in a UK, JOERS/Alvey funded, collaborative programme. The JOERS/Alvey drive scheme is based on monopulse strobe waveforms and utilises the response time - voltage minimum found in certain materials. Using this novel drive scheme, complex ferroelectric displays with a high contrast ratio, wide operating range and fast addressing times have been made The JOERS/Alvey drive scheme is described, compared with other published drive schemes, and the operating principles discussed.
The surface-stabilized ferroelectric liquid crystal (SSFLC) effect is a serious candidate for application in television displays, due to its high switching speed and viewing angle independence. For the television application, however, several severe requirements must be fulfilled, of which the demand for a large amount of gray levels is very crucial in view of the bistable character of the SSFLC effect. A review and discussion of the different approaches to realize gradation in SSFLC structures is given. The so-called dither techniques yield discrete gray levels but become complex when many levels are required. Analogue gradation methods are based on in-pixel domain switching but require an accurate control of the multidomain structures. Active-matrix addressing of the SSFLC effect enhances the reproducibility of the multidomain gray levels via the charge-control process, resulting in a fast-switching, viewing-angle-independent video display. Passive-matrix addressing (multiplexing) takes advantage of the bistability and the switching speed of the SSFLC effect. One approach to control the multidomain gray levels in passive-matrix addressing is extensively dealt with; the “texture-method”. The mcchanism, the addressing scheme, the obtainable contrast in the different textures and the applicability of various FLC mixtures are discussed. The resulting performance of test displays, operating at video rate and possessing full gradation, is evaluated. Finally, some indications will be given of the remaining problems, which need further improvement before FLC displays can be successfully applied for television.
Ferroelectric smectic C (FSC) liquid crystals are used in a simple new geometry that allows the spontaneous formation of either of two surface‐stabilized smectic C monodomains of opposite ferroelectric polarization. These domains are separated by well‐defined walls which may be manipulated with an applied electric field. The resulting electro‐optic effects exhibit a unique combination of properties: microsecond dynamics, threshold behavior, symmetric bistability, and a large electro‐optic response.
A new method of measuring the low tilt bias angles of liquid‐crystal displays is proposed. It is based on the threshold characteristics of magnetocapacitance, which vary according to the relative direction of the magnetic field and the directors. The tilt bias angle is determined completely by geometrical configurations and without any fitting procedure. The method ensures accuracy up to 0.1° in the present example. Threshold field variations due to sample rotation in the magnetic field are calculated from the theoretical magnetocapacitive curves, and experimental results are in good agreement with them.
The recent discovery of ``chevron'' structured smectic-C (SC) layers in surface-stabilized ferroelectric liquid-crystal (SSFLC) cells [T. P. Rieker, N. A. Clark, G. S. Smith, D. S. Parmar, E. B. Sirota, and C. R. Safinya, Phys. Rev. Lett. 59, 2568 (1987)] enables the understanding of many commonly observed features of SSFLC director and layer structure. We present the full three-dimensional layer structure of zigzag walls, the predominant SSFLC defect, which we find to mediate change in chevron direction. We show that stabilization of the director field in SC chevron cells occurs at the chevron interface, so that SC chevron cells behave as two nearly independent cells optically and electrically in series.