Electro-optic response of an antiferroelectric liquid crystal in submicron cells

The dynamics of chiral smectic phases of antiferroelectric liquid crystal MHPOBC in a confined geometry has been analysed. Using an electro-optic response technique, the temperature dependences of the relaxation rates and electro-optic strengths of the elementary excitations in thin, planar aligned, wedge-type cells of thickness from 0.3 to 4 μm have been measured and compared with those for a 50 μm hometropically aligned cell. The effects of the confined geometry are the following. (i) The smectic C^* _γ phase does not exist in planar aligned cells with thickness less than 4 μm. Instead of this phase, we have observed the coexistence of the ferroelectric smectic C^* phase and the antiferroelectric smectic C^* _A phase over a very wide temperature range. (ii) The smectic C^* _α phase is stable at all measured thicknesses down to 0.3 μm. (iii) We have observed a decrease of the smectic A-smectic C^* _α phase transition temperature, proportional to the inverse of the cell thickness. (iv) Additional, thickness-independent phase modes have been observed above some critical value of the measuring electric field in all tilted phases.     

Polarization and dielectric properties of an antiferroelectric liquid crystal

The spontaneous polarization and dielectric properties of a new antiferroelectric liquid crystal (AFLC) exhibiting several intermediate phases between the SmCA and SmC* phases are investigated. A low frequency ferri Goldstone mode and a higher frequency ferro Goldstone mode have been observed over a certain range of temperatures. The effect of d.c. bias voltage on these modes is examined. The results confirm the existence of an FiLC phase with q 1/2 between SmCgamma and SmC*. These also show the co-existence of the FiLC phase with SmC* over a narrow range of temperatures above the FiLC phase. The phase sequence for this material is found to contain SmCA, SmCgamma, FiLC, (FiLC coexisting with SmC*), SmC*, SmA phases on heating and SmA- SmC* SmCgamma- SmCA- phases on cooling.     

Dielectric spectroscopy of an antiferroelectric liquid crystal showing an antiferroelectric-ferrielectric transition

We report the dielectric relaxation behaviour in the antiferroelectric SmC_A* and ferrielectric SmC_γ* phases of the antiferroelectric liquid crystal 4-[5-(4-octloxyphenyl)-2-pyrimidinyl]phenyl 4,4,4-trifluoro-3-(methoxyphenyl)butanoate which shows an antiferroelectric transition at around 88±0.1°C. In the SmC_A* phase, two dielectric relaxation modes have been found, namely the usual antiferroelectric Goldstone mode and another arising from molecular rotation around its short axis. In the SmC_γ* phase, one dielectric relaxation mode has been observed due to the ferrielectric Goldstone mode. Dielectric increments and relaxation frequencies of the antiferroelectric and ferrielectric phases are estimated from the fits of the Cole-Cole function of the dielectric spectrum. The dependence of the bias field in the ferrielectric phase is also discussed.     

Investigations on the field-induced switching behaviour in an antiferroelectric liquid crystal

The switching currents and field-induced apparent tilt angles in an antiferroelectric liquid crystal, (R)-MHPOBC, were measured. The structural differences among different smectic C* subphases may sensitively reflect the field or temperature dependence of the apparent tilt angle. In a thin cell, the apparent tilt angle was found to change in two steps as a function of field strength in the SmC*alpha and SmC* phases: a steep increase at lower fields and a small linear increase at higher fields. The steep increase in apparent tilt angle is divided into two parts via a plateau in the ferrielectric SmC*gamma phase. Stepwise change with a plateau is also seen in the relation of apparent tilt angle versus temperature under various electric fields. Moreover, the apparent tilt angle in the vicinity of the plateau is almost temperature- and field-independent, implying a preferred orientation of the molecules in the ferrielectric state. The influence of the cell thickness on the structural changes was also investigated.     

Dopant effect on threshold electric field of antiferroelectric liquid crystal switching

The influence of dopants on the threshold electric field for switching anti-ferroelectric liquid crystals was determined from the optical response curve obtained by using a triangular wave. The effect of dopant concentration on the transition temperatures of the mixtures with a host material was also investigated. The threshold electric field was diminished depending on (i) the chemical structure of the additive and (ii) increasing amounts of the additive. The upper limits of the temperature region of the S*_CA phases also decreased with increasing amount of additive, and these phases disappeared at 40 mol% of additive in all cases. Compound (C) is the most effective with respect to the threshold electric field. It is a two ring compound and has a chiral part similar to that of the host antiferroelectric liquid crystal compound.     

Dopant effect on threshold electric field of antiferroelectric liquid crystal switching

Abstract

The influence of dopants on the threshold electric field for switching anti-ferroelectric liquid crystals was determined from the optical response curve obtained by using a triangular wave. The effect of dopant concentration on the transition temperatures of the mixtures with a host material was also investigated. The threshold electric field was diminished depending on (i) the chemical structure of the additive and (ii) increasing amounts of the additive. The upper limits of the temperature region of the S*_{C A} phases also decreased with increasing amount of additive, and these phases disappeared at 40 mol% of additive in all cases. Compound (C) is the most effective with respect to the threshold electric field. It is a two ring compound and has a chiral part similar to that of the host antiferroelectric liquid crystal compound.     

The dielectric relaxation processes in an antiferroelectric liquid crystal under cylindrical confinement

In the cylindrical pore geometry of inorganic Anopore membranes the collective relaxation processes observed in a bulk antiferroelectric liquid crystal change considerably under confinement. The frequency degeneration of the soft and Goldstone modes present at the smectic A* (SmA*)-chiral smectic C (SmC*) phase transition in the bulk phase is removed under geometrical restrictions. The relaxation rate of the soft mode is strongly modified due to the deformation of the smectic layers in the curved geometry of the pores and is superimposed by the molecular relaxation process in the SmA* and SmC* phases. The soft mode in confinement splits into two relaxation processes, which are present through all other mesophases (SmC* and SmC_a*). One of them is nearly temperature independent and slightly decreases in frequency in the SmC_a* phase. This Goldstone-like process can be assigned to the highly deformed helical structure fluctuations. The second one exhibits the characteristic features for the molecular and soft mode relaxation processes depending on the temperature range. The biquadratic and the piezoelectric coupling between the tilt angle and spontaneous polarization are revealed in their temperature dependence.     

Generation of frustrated liquid crystal phases by mixing an achiral nematic-smectic-C mesogen with an antiferroelectric chiral smectic liquid crystal

By mixing the achiral liquid crystal HOAB, exhibiting a nematic (N)-smectic-C (SmC) mesophase sequence, with the chiral antiferroelectric liquid crystal (AFLC) (S,S)-M7BBM7, forming the antiferroelectric SmC(a)(*) phase, at least seven different mesophases have been induced which neither component forms on its own: a twist-grain-boundary (TGB(*)) phase, two or three blue phases, the untilted SmA(*) phase, as well as all three chiral smectic-C-type "subphases," SmC(alpha)(*), SmC(beta)(*), and SmC(gamma)(*). The nature of the induced phases and the transitions between them were determined by means of optical and electro-optical investigations, dielectric spectroscopy, and differential scanning calorimetry. The induced phases can to a large extent be understood as a result of frustration, TGB(*) at the border between nematic and smectic, the subphases between syn and anticlinic tilted smectic organization. X ray scattering experiments reveal that the smectic layer spacing as well as the degree of smectic order is relatively constant in the whole mixture composition range in which AFLC behavior prevails, whereas both these parameters rapidly decrease as the amount of HOAB is increased to such an extent that no other smectic-C-type phase than SmC/SmC(*) exists. By tailoring the composition we are able to produce liquid crystal mixtures exhibiting unusual phase sequences, e.g., with a direct isotropic-SmC(a)(*) transition or a temperature range of the SmC(beta)(*) subphase of about 50 K.     

Statistical model of greyscale in antiferroelectric liquid crystal cells

Surface stabilized antiferroelectric liquid crystals are known to give multiplex-compatible greyscales by applying simple waveforms. Statistical variations of cell parameters are believed to be at the origin of this greyscale. In this work, an antiferroelectric model is applied to the study of cell statistical variations, aiming to identify the parameters whose variations may account for the experimental results obtained with these cells. It has been found that moderately small parameter variations, well within manufacturing tolerances, may lead to greyscales whose voltage range and shape are similar to those observed experimentally.     

Dielectric modes in antiferroelectric liquid crystal observed at low temperatures

A broad temperature antiferroelectric binary mixture has been investigated by means of dielectric spectroscopy. The sample was cooled down to –70°C. It was found that the sample was still in antiferroelectric phase. This is the widest antiferroelectric mixture ever seen (～170°) in which three well-separated modes have been detected at room temperature. In addition, the bias field influence on existing modes has been observed. All modes change their strengths with bias field. Results show that the fastest mode, called X mode previously, gradually disappears around –30°C. When the temperature decreases below –40°C, one can indisputably observe additional mode, faster than X mode. This mode (named as Y mode) observed for extra low temperatures is bias independent. It can be the molecular mode, connected with rotation around long molecular axis. The rotation around short molecular axis seems to be blocked in antiferroelectric packing. To calculate parameters of observed modes, Cole–Cole model was used. The parameters of Y mode are discussed in this article.     

Analytical approach to V-shaped characteristics in antiferroelectric liquid crystal displays

Not only in ferroelectric liquid crystal displays (LCDs), but also in antiferroelectric LCDs, grey levels are possible by actively addressing the ‘continuous director rotation mode’. For ferroelectric LCDs this was shown qualitatively and quantitatively in previous articles. In this article it is shown that an exact analytical approach is also possible for antiferroelectric LCDs. In two consecutive layers the director orientations are symmetric, and at zero voltage they are in a splayed state. The conditions on alignment layer thickness and interaction coefficients are related to those of ferroelectric liquid crystal but are easier to fulfill.     

Analytical approach to V-shaped characteristics in antiferroelectric liquid crystal displays

Not only in ferroelectric liquid crystal displays (LCDs), but also in antiferroelectric LCDs, grey levels are possible by actively addressing the 'continuous director rotation mode'. For ferroelectric LCDs this was shown qualitatively and quantitatively in previous articles. In this article it is shown that an exact analytical approach is also possible for antiferroelectric LCDs. In two consecutive layers the director orientations are symmetric, and at zero voltage they are in a splayed state. The conditions on alignment layer thickness and interaction coefficients are related to those of ferroelectric liquid crystal but are easier to fulfill.     

A bent-shape liquid crystal compound with antiferroelectric triclinic-monoclinic phase transition

Electro-optical and polarization current measurements on 1,3-phenylene-bis[4-(3-fluoro-4-decyloxyphenyliminomethyl) benzoate] (3F-10-O-PIMB) revealed a second order phase transition between two antiferroelectric 'smectic banana' phases. The observations show that the switching between the ferroelectric states in the higher temperature (HT) phase requires higher thresholds than in the lower temperature (LT) phase. It is hypothesized that the HT phase has a lower (triclinic, C 1 ) symmetry, than that of the LT phase (monoclinic, C 2 ). It is also shown that electric fields can induce transitions between different 'smectic banana' phases.     

A bent-shape liquid crystal compound with antiferroelectric triclinic-monoclinic phase transition

Electro-optical and polarization current measurements on 1,3-phenylene-bis[4-(3-fluoro-4-decyloxyphenyliminomethyl) benzoate] (3F-10-O-PIMB) revealed a second order phase transition between two antiferroelectric ‘smectic banana’ phases. The observations show that the switching between the ferroelectric states in the higher temperature (HT) phase requires higher thresholds than in the lower temperature (LT) phase. It is hypothesized that the HT phase has a lower (triclinic, C₁) symmetry, than that of the LT phase (monoclinic, C₂). It is also shown that electric fields can induce transitions between different ‘smectic banana’ phases.     

Preliminary communication Bi-mesogenic organosiloxane liquid crystal materials exhibiting antiferroelectric phases

The first antiferroelectric low molar mass organosiloxane liquid crystal materials are presented. The molecules are bi-mesogens consisting of two chiral mesogenic units attached, via methylene spacers, symmetrically to a siloxane group. Three series of compounds with different laterally substituted halogens on the phenyl ring nearest to the chiral centre have been studied. For siloxane groups containing three silicon atoms all of the materials exhibit antiferroelectric phases over a 60 C temperature range. The tilt angle in the SmC* A is, within experimental uncertainties, independent of temperature and close to 45 degrees for the three series with a spontaneous polarization greater than 110nC cm -2.     

Electrooptic investigations of enantiomeric mixtures of the antiferroelectric liquid crystal TFMHPOBC

Measurements are reported for the polarization in the ordered phase, and for the tilt susceptibility in the 'disordered' smectic A phase, of mixtures of left- and right-handed enantiomers of TFMHPOBC. The tilt susceptibility was found to exhibit a critical exponent gamma 1.20 0.05, and both its magnitude and the polarization were found to be nonmonotonic in enantiomeric excess X of S-TFMHPOBC. Their ratio, however, was found to be approximately constant with X. Several possible explanations are examined.     

Bulk optical properties in binary mixtures of antiferroelectric liquid crystal compounds showing V-shaped switching

In order to clarify the origin of the V-shaped switching observed in thin cells ofantiferroelectric/ ferrielectric liquid crystals, bulk properties have been studied by means of helical pitch and conoscope measurements using thick free-standing films of binary mixtures with various mixing ratios. In the temperature range showing V-shaped switching in thin cells, helical structure clearly exists, indicating the existence of ordered phases. Some indistinct phase changes with temperature, coexistence of phases and quasi-continuous phase changes with an applied electric field were observed, suggesting a system with weak inter-layer correlation. By comparing the phase diagrams made using thin homogeneous cells and thick free-standing films, it was found that V-shaped switching occurs in the region where various subphases exist in the bulk. The appearance of many indistinct phases is consistent with the weak interlayer correlation. In this way, it was concluded that the V-shaped switching occurs in tilted smectic layers, in which the tilt direction is weakly correlated along the layer normal.     

V-shaped switching in binary mixtures of an achiral swallow-tailed material with the antiferroelectric liquid crystal (S)-MHPOBC

An achiral swallow-tailed material, 2-propylpentyl 4-(4'-decyloxybiphenyl-4-carbonyloxy)benzoate, p, showing SmA and SmC_alt phases was prepared for mixing (by weight percentage) with an antiferroelectric liquid crystal, (S)-MHPOBC, m, for the study. The binary mixture p15/m85 using (S)-MHPOBC (85%) as a host doped with achiral material (15%) resulted in a phase sequence SmA-SmC*-SmC*_A. The electro-optic response of this mixture in the ferroelectric SmC* phase displayed V-shaped switching, while that in the antiferroelectric SmC*_A phase displayed a double hysteresis switching. The mixture p85/m15 possessed SmA* and SmC*_A phases; V-shaped switching was found in the antiferroelectric SmC*_A phase of this mixture. These optical phenomena implied that a binary mixture containing a larger amount of achiral swallow-tailed material and/or possessing relatively lower polarization favours the occurrence of V-shaped switching in the antiferroelectric phase. The results of this work also suggested that thresholdless V-shaped switching in chiral smectic liquid crystals can be achieved by mixing an achiral swallow-tailed material with an antiferroelectric liquid crystal.     

V-shaped switching in binary mixtures of an achiral swallow-tailed material with the antiferroelectric liquid crystal (S)-MHPOBC

An achiral swallow-tailed material, 2-propylpentyl 4-(4′-decyloxybiphenyl-4-carbonyloxy)benzoate, p, showing SmA and SmC_alt phases was prepared for mixing (by weight percentage) with an antiferroelectric liquid crystal, (S)-MHPOBC, m, for the study. The binary mixture p/15/m85 using (S)-MHPOBC (85%) as a host doped with achiral material (15%) resulted in a phase sequence SmA-SmC*-SmC*_A. The electro-optic response of this mixture in the ferroelectric SmC* phase displayed V-shaped switching, while that in the antiferroelectric SmC*_A phase displayed a double hysteresis switching. The mixture p85/m15 possessed SmA* and SmC*_A phases; V-shaped switching was found in the antiferroelectric SmC*_A phase of this mixture. These optical phenomena implied that a binary mixture containing a larger amount of achiral swallow-tailed material and/or possessing relatively lower polarization favours the occurrence of V-shaped switching in the antiferroelectric phase. The results of this work also suggested that thresholdless V-shaped switching in chiral smectic liquid crystals can be achieved by mixing an achiral swallow-tailed material with an antiferroelectric liquid crystal.