Keizou Itoh

Frequency-dependent switching behavior under triangular waves in antiferroelectric and ferrielectric chiral smectic phases

A realistic model structure of the ferrielectric phase is proposed by taking into account the apparent tilt angle measurements. On the basis of the model, the electric and optical responses are investigated in the ferrielectric SmCγ* as well as in the antiferroelectric SmCA* phases of MHPOBC (C8H17O---COO--COOC*H(CH3)C6H13) under triangular waves of various frequencies. At low frequencies, D-E curves show double and triple hysteresis loops characteristic of the antiferroelectric and the ferrielectric phases, respectively. T (transmittance)-E curves also reveal similar hysteresis loops. With increasing frequency, the direct switching between the two ferroelectric states occurs without passing through the antiferroelectric state or the ferrielectric states. The complicated frequency dependence is interpreted based on the spatially inhomogeneous domain switching.

Temporal and Spatial Behavior of the Field-Induced Transition between Antiferroelectric and Ferroelectric Phases in Chiral Smectics

Optical transmittance change and stroboscopic micrographs have been obtained in homogeneously aligned thin cells of S-MHPOBC, C8H17O–--COO–COO-*CH(CH3)C6H13, and R-TFMNPOBC, C8H17O–--COO–-COO-*CH(CF3)C8H17, by varying the initial applied DC voltage stepwise into the final one across the threshold. There exist two components, fast and slow, in the transmittance change due to the phase transition from antiferroelectric SmCA* to ferroelectric SmC*. The fast component is due to the pretransitional effect. The movement of the domain boundaries is responsible for the slow component. In the transmittance change due to the transition from SmC* to SmCA*, only the slow component is observed. The movement speed is mainly determined by the difference between the final and threshold voltages.

Self-Recovery from Alignment Damage under AC Fields in Antiferroelectric and Ferroelectric Liquid Crystal Cells

Alignment recovery from damage has been confirmed to occur under an AC electric field, e.g., 30 Vppµm-1, in antiferroelectric chiral smectic liquid crystal cells of MHPOBC and a related fluorinated compound. The recovery partly results from internal shearing produced by reversible layer switching between bookshelf and quasi-bookshelf. Contrary to the widely accepted notion, even in a ferroelectric liquid crystal cell of a compound similar to MHPOBC, reversible layer deformation due to an electric field has been observed from X-ray diffraction and a similar alignment recovery from damage has also been confirmed to occur.

On the appearance of the antiferroelectric phase

Abstract As the origin of the appearance of the antiferroelectric phase, the importance of dimerization of molecules in adjacent layers is pointed out on the bases of some experimental results. In thermotropic main chain polymer liquid crystals, the appearance of the antiferroelectric molecular orientation shows the odd-even effect concerning the number of spacer methylene unit. The same kind of odd-even effect was observed in low molecular weight liquid crystals, when the number of carbon atom, n, in chiral end is changed; the antiferroelectric phase tends to appear when n is even. The phase diagram in enamtiomeric mixtures of an antiferroelectric liquid crystal and the layer thicknesses in these mixtures also suggest the pairng of the molecules. The image of a scanning tunnelling microscope is also shown; the core parts of molecules align in a zigzag fashion.

Dielectric Studies on Antiferroelectric Liquid Crystals

Abstract The dielectric behavior in the first and the second order phase transitions between paraelectric SmA and antiferroelectric SmC*A has been explored using three substances having trifluoromethyl group. The soft mode contribution to the dielectric constants was easily obtained in both phases having no spontaneous polarization. The temperature dependences of the inverse dielectric strength 1/Δe, determined by the Cole-Cole plot clearly reveal the variety of the transition order such as the first-order and the second-order phase transition. The temperature dependences of the relaxation frequency also show the characteristics of these transition order.

Trifluoromethylated pyranose derivatives as the chiral dopant for ferroelectric liquid crystals

Abstract The novel pyranose derivatives with a trifluoromethyl group were found to be excellent chiral dopants for ferroelectric liquid crystals, inducing large spontaneous polarization by their addition even in a small quantity. These materials showed some characteristic physical properties depending on their stereochemical relationship : thus, the corresponding cis isomers exhibited high ability in inducing large spontaneous polarization and quick response but lowered the upper limit of the temperature of the Sc phase. However, the same trend in the spontaneous polarization was observed for the compounds with trans stereochemistry, without lowering the transition temperature of SA to SC, of the host liquid crystal. MoFover, it was also proved that the adjustment of the helical pitch of the N* phase was readily carried out. Based on these results, we have succeeded in the development of the practical ferroelectric liquid crystalline compositions with good alignment and quick response.

Ferroelectric Liquid Crystal Mixtures Containing Chiral Pyranose Compounds for Response-Voltage Minimum Mode

Ferroelectric liquid crystal (FLC) materials for the response-voltage minimum (τ–Vmin ) mode were developed by doping novel chiral pyranose compounds into achiral host materials with negative dielectric anisotropy. The chiral pyranose derivatives, wherein three chiral centers and a trifluoromethyl group were fixed in the 6-membered ring, induced short τmin in the τ–Vmin characteristic. The developed FLC materials were driven by drive schemes for the τ–Vmin mode, giving short line address times.

ENZYMATIC RESOLUTION IN ORGANIC MEDIA : SYNTHESIS OF OPTICALLY ACTIVE FURANOLS POSSESSING A FLUOROALKYL GROUP

Abstract Furanols possessing a fluoroalkyl group were resolved into the corresponding optically active forms via enzymatic esterification in organic media.

Novel Antiferroelectric Liquid Crystals Derived from Trifluoromethylated Pyranoses

Abstract A new series of liquid crystals possessing trifluoromethylated pyranoses was synthesized and their mesomorphic properties were investigated. Electro-optical studies showed that some of these compounds exhibited an antiferroelectric chiral smectic C (SmCA*) phase. Further, it was found that the appearance of the SmCA* phase was strongly dependent on the conformation of the chiral part; cisand transconfigurations of the pyranose ring. Relationship between molecular structures and a propensity to the formation of the SmCA* phase was discussed.

Critical heat capacity of antiferroelectric liquid crystal 12BIMF10

Abstract Heat capacity of the antiferroelectric liquid crystal 12BIMF10 has been measured with both an ac method and a relaxation method. A very sharp heat capacity peak was observed at the Sm-Cα*-Sm-A transition. The excess heat capacity for this transition showed a non-Landau critical behavior. The exponent α determined by a fit with a preasymptotic power-law equation lied 0.10-0.21. The data were also fitted acceptably well with 3D XY exponents when the correction-to-scaling terms up to the second order have been included. These results strongly suggest that the present data reflect a crossover from the 3D XY behavior to the tricritical behavior.