Properties Of Antiferroelectric Liquid Crystals Research Articles

Electro-Optic Effect of Laser Photobleaching on Viscoelastic Properties of Chiral Liquid Crystals

Viscoelastic properties are one of the most fundamental properties of chiral liquid crystals. In general, their determination is not a straightforward task. The main problem is the multitude of physical parameters needed to determine the value of the elasticity and viscosity constants. It is also necessary to consider the character of a respective phase. This problem is particularly important in the case of chiral phases such as ferroelectric and antiferroelectric phases or in the blue phases. There are several experimental methods to measure viscosity and elasticity constants in chiral phases. These methods use various phenomena to detect deformation, e.g., light transmission, polarization current, light modulation, dielectric constant and helix deformation or helix unwinding. Commonly, an external electric field is used to induce deformation, the homogeneity of which inside the cell is essential. This study is focused on the analysis of the effect of laser photobleaching on the electro-optic properties of the antiferroelectric liquid crystal and on the homogeneity of the electric field. The results obtained by confocal microscopy as a function of the cell depth are presented. The influence of the stabilization procedure of the isolated region performed by controlled laser photobleaching on the electro-optic properties has been studied. The observation was conducted using a polarizing microscope, and numerical analysis of two-dimensional colored textures was performed. The obtained results suggest that laser photobleaching can produce an anchoring effect, which has a positive effect on the electro-optic properties of antiferroelectric liquid crystal.

Ferroelectric and Antiferroelectric Liquid Crystals

Ferroelectric liquid crystals have been a major research topic since 30 years. However, when it comes to liquid crystals, the term “ferroelectric” is strongly ambiguous and frequently not only leads to confusion and misunderstanding but also obscures the basic concepts. The property of ferroelectricity in liquid crystals was first claimed in 1975. Five years later so-called surface-stabilized ferroelectric liquid crystals were described, which caused a surge in industrial interest because of their promising electro-optical applications, in particular for high-resolution liquid crystal displays. The industrial interest led to a considerable activity in synthesizing new compounds with the desired properties. In the course of this materials development antiferroelectricity in liquid crystals was then also sought for and was finally identified in 1989. At the same time as the first industrial ferroelectric devices (the Canon displays) came on the market in 1995, the antiferroelectric liquid crystals were being considered as even more promising for applications. In parallel, ferrielectric liquid crystal materials were announced. Moreover, in 1995, a new phenomenon, the “thresholdless antiferro-electricity” was advertised. I believe that these names indicate a certain lack of comfort in adopting terms and definitions taken from the field of solid state phenomena. Therefore, it might be adequate to question whether liquid crystal materials really qualify to be called “ferroelectric, anti-ferroelectric and ferrielectric.” My own answer to this is to an important part negative. At least, no liquid crystal so far is ferroelectric. I will review our current understanding of polar liquid crystals and contrast them to ferroelectric and antiferroelectric solid crystals. While some of the electric properties can be very similar, it is the optical properties that really make the liquid crystals interesting. In particular, the optical properties of antiferroelectric liquid crystals are surprisingly rich, as discovered only in the very last years.

Tilt plane orientation in antiferroelectric liquid crystal cells and the origin of the pretransitional effect.

The optic, electro-optic, and dielectric properties of antiferroelectric liquid crystals (AFLCs) are analyzed and discussed in terms of the local tilt plane orientation. We show that the so-called pretransitional effect is a combination of two different electro-optic modes: the field-induced antiphase distortion of the antiferroelectric structure and the field-induced reorientation of the tilt plane. In the presence of a helix, the latter corresponds to a field-induced distortion of the helix. Both electro-optic modes are active only when the electric field has a component along the tilt plane. Thus, by assuring a horizontal surface-stabilized condition, where the helix is unwound by surface action and the tilt plane is everywhere parallel to the cell plates, the pretransitional effect should be suppressed. We also discuss the dielectrically active modes in AFLCs and under which circumstances they contribute to the measured dielectric permittivity.

Studies of the alignment properties of antiferroelectric liquid crystals by X-ray diffraction

It is observed optically that in a parallel rubbed antiferroelectric liquid crystal device, the texture consists of domains with two distinct optic axes, which make equal and opposite angles with the rubbing direction. It is proposed that this is caused by a large electroclinic effect at the surfaces during layer formation in the SmA* phase. This hypothesis is verified by finding the layer structure in single, parallel and skew rubbed devices by using X-ray diffraction.

Dielectric properties of antiferroelectric liquid crystals composed of banana-like molecules

The dielectric properties of an antiferroelectric B2 phase representing a homolog (n=14) from a series of 4-chloro-1,3-phenylene-bis[4-(4-n-alkylphenyl)aminobenzoates] with banana-like molecules were studied. The temperature dependence of the dielectric relaxation time was measured, and the corresponding activation energy was determined. The kinetics of the dielectric permittivity was studied in the course of the polarization vector variation under the action of a low-slope triangular bias voltage in the entire temperature range of existence of the B2 phase. Dependence of the dielectric permittivity on the electric field strength (bias voltage) is determined by the presence of two polar subsystems forming the antiferroelectric B2 phase.

10.1: Invited Paper: Future Antiferroelectric Liquid Crystal Displays

AbstractIn spite of some very interesting prototypes of large‐size high‐resolution color displays, no manufacturing of antiferroelectric liquid crystal (AFLC) displays has ever started. The reason can be found in the inadequacy of the dark state. This has prompted us to investigate the optical properties of AFLCs in general. The result is not only that future AFLC displays should use materials with a tilt angle of 45 degrees, but that there is a variety of highly promising AFLC modes both for transmissive and reflective displays and both with passive and active (TFT or silicon backplane) driving. The reason for the failure of the present AFLC displays will be explained and some of the most promising alternatives discussed.

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.

Optical properties of antiferroelectric liquid crystals in free-standing films.

A theoretical explanation of the results of ellipsometric measurements, performed some time ago by Bahr and co-workers [Ch. Bahr, D. Fliegner, C. J. Booth, and J. W. Goodby, Phys. Rev. E 51, R3823 (1995)] on free-standing films of antiferroelectric liquid crystal (R)-1-methylheptyl-4-(4\ensuremath{'}-$n$-dodecyloxybiphenyl-4-$\mathrm{yl}$ carbonyloxy)-3-fluorobenzoate (in the following 12F1M7), is given within a discrete phenomenological model of antiferroelectric liquid crystals. The structure of the smecti-${C}_{\ensuremath{\alpha}}$ (Sm-${C}_{\ensuremath{\alpha}}$) phase is described as an incommensurate phase, an average dielectric tensor, in general biaxial, is determined and the ellipsometric parameter $\ensuremath{\Delta}$ is found for two orientations of the film structures, defined by external electric field. Results are in agreement with observations: we show that temperature induced changes of the structure of the Sm-${C}_{\ensuremath{\alpha}}$ phase, as derived from the model, lead to the oscillatory dependence of the ellipsometric parameters ${\ensuremath{\Delta}}_{\ensuremath{-}}$ and ${\ensuremath{\Delta}}_{+}$ on temperature.

Synthesis and Thermal Properties of Antiferroelectric Liquid Crystals Having Phenylalkanoate and Phenylalkenoate Moieties

Abstract Novel antiferroelectric liquid crystals having phenylalkanoate and phenylalkenoate moieties were synthesized and their mesomorphic properties were studied. 4-[2-{1-(Trifluoromethyl)heptyloxycarbonyl}ethyl]phenyl 4′-decyloxybiphenyl-4-carboxylate 1b exhibited the chiral smectic CA phase at a temperature below the chiral smectic C phase. Biphenyl-cinnamate derivatives of the three-ring system exhibited a stable mesomorphic phase and enantiotropic smectic CA phase, with the carbon-carbon double bond acting as a mesogenic core.

Optical and dielectric properties of antiferroelectric liquid crystals and their surface effects

Abstract A detailed study of the dielectric and optical properties of the ferroelectric liquid crystal material (R)-4′-(3-methoxycarbonyl-2-propoxycarbonyl)phenyl 4-(4-(n-octyloxy)phenyl)benzoate (3MC2PCPOPB) has been carried out. It has been found that an anomalous temperature dependence of the dielectric constant in 3MC2PCPOPB is due to the antiferroelectric and ferrielectric properties. A T(temperature)-E(electric field) phase diagram has been obtained on the basis of the apparent tilt angle measurements. In a thin cell (< 3 μm), both ferroelectric and antiferroelectric domains are simultaneously observed over a wide temperature range, and the complete antiferroelectric phase does not appear even at low temperature. A characteristic texture in which boundary focal conics are aligned parallel to a smectic layer has been observed. The movement of the zig-zag defect line caused by the application of the voltage is also observed.