V-shaped Switching Research Articles

Giant surface electroclinic effect in a chiral smectic A liquid crystal

We report the observation of a very large surface electroclinic effect in the smectic A* phase of a chiral liquid crystal. In planar-aligned cells of enantiomerically pure W415, the smectic A* phase grows in from the isotropic state with the layer normal rotated ψ = - 24° from the rubbing direction, a consequence of the surface electroclinic tilt θs of the director. The sign of θs depends on the molecular handedness, with θs ≡ 0 in the racemate, and increasing linearly with moderate enantiomeric excess before saturating as ee → 1. A uniform layer structure can be achieved using cross-rubbed alignment layers, in which case thin cells of W415 in the smectic C* phase display V-shaped (analogue) electro-optic switching.

A Generalized Hysteresis Model of Antiferroelectric Liquid Crystals

In this paper, we shall put forward a generalized irreversible rotation model which takes account of the long-range couplings between the adjacent dipoles in each layer to explain the experimentally observable hysteresis curves in ferro-, antiferro- and ferrielectric phases with the corresponding wave numbers of the ferroelectric orderings along the layer normal. It is found that the long-range interaction between the dipoles critically affects on the dynamic response under an alternating electric field. In the present model, it may be possible to explain the experimentally found subphases of the antiferroelectric materials in a unified framework. That is, the dynamic response of the possible phases between the ferroelectric (SmC*) phase (the upper temperature) and the antiferroelectric (SmCA*) phase (the lower temperature) will be substantially explained by a unified irreversible model under an alternating electric field. In addition, the hysteresis curve with the devil's staircase, as seen in the field-induced phase transitions, is found to be critically affected by the coupling strength between the spontaneous polarizations in each layer. We shall also show that the present model may be considered as a candidate to explain the thresholdless, hysteresis-free, the V-shaped switching as has been experimentally found.

On the V-shaped switching in antiferroelectric liquid crystals

The electrooptic response of a thin cell of antiferroelectric liquid crystal is investigated as functions of frequency and temperature. It was found, that under specific conditions the FLC cell exhibits the V-shaped switching, the parameters of which are strongly dependent on the frequency of applied electric field. The frequency dependence of electrooptic response has been modelled by taking in account the phenomenon of ion transport. The conditions for a hysteresis-free V-shaped switching were found from the model.

Electro-optical properties of thresholdless antiferroelectric liquid crystal and its application to high-resolution TFT-LCD

Thresholdless antiferroelectric liquid crystal (TLAF) characterized by V-shaped switching in a smectic liquid crystal is very attractive for active matrix driving, but the switching mechanism is not fully understood. We have studied electro-optical properties of the TLAF material MLC-0071, and developed a 15-inch TFT-LCD with XGA resolution using the material. Temperature-dependence of relative permittivity suggested the presence of a Goldstone mode. This result implies the V-shaped switching of MLC-0071 is based on a smectic C*-like phase. From the measurements of optical anisotropy etc., it was found that the TLAF had a chevron structure and the molecules oriented parallel to the substrate plane when gray-level voltage was applied. The 15-inch TLAF-TFT-LCD provided wide viewing angle, high contrast ratio and small color shift.

The hysteretic behavior of „V-shaped switching” smectic materials

We have investigated the electro-optic response below the bulk SmC* to SmCa* transition in materials exhibiting thresholdless, V-shaped switching in the SmC* phase. We are able to explain the very complicated hysteretic behavior observed in these materials and its dependence on frequency, temperature and field strength. Strong polar surface anchoring causes coexistence of stable or metastable synclinic (SmC*) domains and stable anticlinic (SmCa*) domains during switching. The resulting response has intermediate steps in the transmission-voltage curves which in the past may have been misinterpreted as ferrielectric states or phases.

V-shaped switching due to frustoelectricity in antiferroelectric liquid crystals

We have reinforced the assertion that the devil's staircase and the thresholdless, hysteresis free, V-shaped switching are two manifestations of the frustration between ferro-and antif-erro-electricity, emphasizing that the third nearest neighbor interaction parameter may become negative, i.e. J 3 < 0. Three important consequences are: (1) The subphase with q=1/6 becomes to exist stably even in the ground state. (2) A number of subphases emerge and may apparently look like a single subphase. (3) It is not easy to distinguish SC* from frustrated subphases with small q numbers. We have summarized compounds and mixtures for the Y-shaped switching that are described in Japanese Patent Gazettes open to public recently. The materials show an antiferroelectric, probably q=1/6, subphase or an apparently single subphase in a wide temperature range; some of them are characterized with novel chiral parts.

Ellipsometric study of freely-suspended films of an antiferroelectric liquid crystal mixture showing V-shaped switching

Ellipsometric studies have been conducted on freely-suspended films of an antiferroelectric liquid crystal mixture showing V-shaped switching. We observed a discontinuous change of the phase difference between s- and p- components of transmitted light Δ, at 46 °C indicating a phase transition from smectic-C* to the ferrielectric phase, although this transition is continuous in conoscopic observation. We also discussed the details of A as a function of an applied electric field in each phase.

Alignment instability caused by anchoring of the in-plane directors to the rubbing direction in the V-shaped switching

In order to investigate the molecular alignment in the smectic liquid crystals showing the V-shaped switching behavior, two types of the liquid crystal cells, a sandwiched cell and an open cell, were prepared. Homogeneous alignment with the smectic layer normal formed at ∼12° with respect to the rubbing direction r is obtained in the Sm-A phase in both cells. However, in the open cell, the director n is rotated towards r when the Sm-A sample is cooled into the tilted Sm-X* phase. In addition, its maximum rotation angle is ∼12°. In this configuration, the molecules lie along r. Meanwhile, the sandwiched cell with the one-side rubbing treatment exhibits the averaged optical axis almost parallel to the layer normal in the Sm-X* phase. In this paper, a simple small-twisted state in the thin sandwiched cell is suggested to account for the experimental results.

PAb12 V字スイッチングを示す「フェリ誘電相」の構造

PAb12 V字スイッチングを示す「フェリ誘電相」の構造

Frustration between Ferroelectricity and Antiferroelectricity in Extremely Soft Chiral Smectic-C Like Phases of Liquid Crystals

In a group of compounds that exhibit antiferroelectric and/or ferrielectric liquid crystal phases, the bent molecular shape has been considered to cause unique interactions among smectic layers. The interactions are responsible for the successive phase transitions as described by the devil's staircase and the V-shaped switching, which are related with the interlayer ordering and the diminished tilting correlation, respectively. These two are apparently contradictory each other, but result from the same cause, i.e. frustration between ferroelectricity and antiferroelectricity in the Sm-C* like phase. The ANNNI model Yamashita and Miyazima introduced has been shown to describe the frustration appropriately provided that we undertake some refinements: the Ising spin corresponding to the tilting sense of the local in-plane director, the X-Y character of the spin, the tilt angle decrease toward Sm-A, and most importantly, the reassignment of the pseudo-spin to the biasing direction of the molecular rotation about their long axis.

Molecular motion in a smectic liquid crystal showing V-shaped switching as studied by optical second-harmonic generation

Molecular motion in a smectic liquid crystal showing V-shaped switching as studied by optical second-harmonic generation

Thresholdless Antiferroelectricity in a Novel Chiral Swallow-Tailed Liquid Crystal

A novel material, 1-ethylpropyl (S)-2-{6-[4-(4‘-decyloxyphenyl)benzoyloxy]-2-naphthyl}propionate, has been designed and synthesized for the study of mesomorphic properties. The results indicated that this material displays a phase sequence of Iso−Bp−N*−TGBA−SA−SCA*−Cr. Further investigation of the antiferroelectric SCA* phase by measurement of the transmittance vs electric field by applying a field of triangular waveform in a 5 μm homogeneously aligned cell showed a typical V-shaped switching at frequency of 0.5 Hz, demonstrating that this phase possesses thresholdless antiferroelectricity.

Collective Molecular Motion during V-Shaped Switching in a Smectic Liquid Crystal

The molecular motion during V-shaped switching in a homogeneously aligned smectic C*-like liquid crystal (LC) cell has been investigated by measuring the effective optical anisotropy Δneff, apparent tilt angle θapp, switching current, and second-harmonic generation and comparing them with the simulated results based on two extreme models, i.e., random model and collective model, where molecules switch randomly and collectively, respectively. The comparison revealed that the collective switching motion of LC molecules is more reasonable than the random motion. Moreover, it was also confirmed that the observed infrared absorption anisotropy of the phenyl stretching mode due to LC molecular distributions strongly supports the collective model. From these results, it was demonstrated that LC molecules do not switch randomly but all the LC molecules rotate collectively on a cone under the driving field.

Effect of alignment layer on V-shaped switching in a chiral smectic liquid crystal

The influence of the alignment layer on V-shaped switching in thin homogeneous cells of a chiral smectic liquid crystal were studied by means of electro-optic and switching current measurements.Severalpolyimides withdifferent chemicalstructures wereusedas thealignment layer in thin homogeneous cells; V-shaped switching was observed with some of them and W-shaped switching with others. It was also shown that the switching current constantly flows during the V-shaped transmittance change. In order to clarify this effect of the alignment layer on V-shaped switching, polyimide films with various polarity and thickness were examined. It is concluded that thick alignment layers assist liquid crystal materials in realizing V-shaped switching, even if the materials are not ideal for this process.

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.

A novel property caused by frustration between ferroelectricity and antiferroelectricity and its application to liquid crystal displays-frustoelectricity and V-shaped switching

We have studied the frustration between ferro- and antiferro-electricity in chiral smectic C like liquid crystalline phases, which is not only fundamentally interesting but also very attractive from an application point of view. It causes temperature induced successive phase transitions as characterized by a devil's staircase and the thresholdless, hysteresis-free, V-shaped switching induced by an applied electric field. The devil's staircase indicates some type of interlayer ordering, while the V-shaped switching suggests considerably diminished tilting correlation. These two are apparently contradictory to each other, but result from the same cause, i.e. the frustration. We have first summarized experimental facts regarding subphases and successive phase transitions observed in many compounds and mixtures, which we believe are related to one another and result from the frustration. We have introduced several different theoretical explanations for these observed facts, and shown that only the axial next nearest neighbor Ising (ANNNI) model can explain almost all of the facts, provided that it is unified with the XY model appropriately. The unified model can make a comprehensive explanation in the most natural way based on the most probable molecular interactions. We have then emphasised that there are several modes regarding the V-shaped switching, because the system becomes so soft with respect to the tilting direction and sense that any additional external or internal force modifies the in-plane local director alignments. For the practically usable ones, we have emphasised the need for some type of randomization in the molecular alignment at the tip of the V and/or the switching process. In particular, the two dimensional (ideally, cylindrically symmetric) azimuthal angle distribution of local in-plane directors around the smectic layer normal is most attractive. Such a randomized state at the tip of the V is thermodynamically unique under a given condition imposed by interfaces. It stays stable even when the smectic layer structure, such as a chevron, changes with temperature. Finally, we have summarized the so-far reported compounds and mixtures for the V-shaped switching and introduced some prototypes of LCDs using them.

The case of thresholdless antiferroelectricity: polarization-stabilized twisted SmC* liquid crystals give V-shaped electro-optic response

We have studied the three-component liquid crystal mixture reported to exhibit ‘thresholdless antiferroelectricity’ [Inui et al., J. Mater. Chem., 1996, 6, 671]. We find that the thresholdless or V-shaped switching is obtained in the absence of antiferroelectricity. This analog electro-optic response is due to the field-induced switching of a twisted smectic C* structure stabilized by polar surface interactions and by electrostatic bulk polarization charge interactions. The latter confine the director twist to thin surface regions leaving the bulk of the cell uniform, which gives good extinction at zero field. In sufficiently thin cells, such thresholdless switching can in fact be followed down to much lower temperatures, where the bulk would be antiferroelectric, but is maintained in the cells in the ferroelectric state by hysteresis from surface action.

Surface Dependent V-Shaped Switching in a Chiral Smectic Liquid Crystal

We have extensively studied the influence of the substrate surface on the V-shaped switching in thin planer cells of a chiral smectic liquid crystal and found that the substrate surface plays a major role in the emergence of V-shaped switching: namely, thick alignment layers and less polar surfaces are ideal for V-shaped switching. It was also found that, instead of using a thick alignment layer, ideal V-shaped switching could be achieved by using a thin alignment layer with a less polar insulating layer.

1-6b V字型スイッチングに対する配向層の効果

1-6b V字型スイッチングに対する配向層の効果

Thresholdless Switching in Homogeneous Cells and Antiferroelectric Conoscopic Figures in Free-Standing Films as Observed in A Binary SC*-Like Mixture

To elucidate the V-shaped switching reported by Inui et al., we have studied another binary mixture. In its free-standing film, several subphases were observed in the ordinary sequence, indicating frustration between ferro- and antiferro-electricity; in particular, the antiferroelectric subphase exists in a temperature range as wide as 22 °C and the helical pitch is longer than 2. 5 μm. In a homogeneous cell, however, only two liquid crystal phases were distinguished; one is SA and the other is a phase characterized by the nearly V-shaped switching. We have concluded that the substrate interfaces induce tilting randomization, breaking the reduced intrinsic tilting correlation in adjacent layers.