X-ray diffraction analysis of the layer structure in a ferroelectric liquid crystal with a chiral nematic–chiral smectic C phase transition

A half-V-shaped switching ferroelectric liquid crystal (FLC) is a promising candidate for fast response displays. In the half-V FLC display, a liquid crystal with a chiral nematic–chiral smectic C phase transition is used, and the smectic layer is formed by cooling from N* to SmC* with an applied d.c. field. We studied the layer structure by means of X-ray measurements for two axes (ω and χ). By using a point-focused X-ray tube and optimizing the slit width, we succeeded in the two-axis measurement with a commercial X-ray system. The ω–χ profile of the half-V FLC showed two broad peaks in an arc-shaped high-intensity area. Our interpretation of this result is that the major part of the layer consists of a tilted-bookshelf structure and that the minor part consists of a near-bookshelf structure. Since optical microscopy observations on the half-V FLC cells showed a stripe-shaped texture, we consider that the coexistence of the tilted-bookshelf and the near-bookshelf structures forms the stripe-shaped patterns. The radius of the arc-shaped high-intensity area was nearly equal to the molecular tilt angle. This result can explain why the half-V FLC showed a desirable black appearance in spite of the stripe-shaped texture.     

Twin‐like domains in chiral smectic C liquid crystals

In the chiral smectic C phase of liquid crystals with the phase transition N*–SmC*, texture development depending on the sample thickness is reported. In very thin samples, domains of rectangular‐like shape are observed. As two possible tilts of smectic layers are possible for one anchoring direction, smectic layers inside a domain, called twin‐like domains, are tilted with respect to layers in outer regions, similarly to crystalline planes in solid crystalline twins. An elastic model of such a twin domain is proposed and its energy determined.     

First observations of a spiral instability at the smectic A—cholesteric transition under an electric field

Abstract

Archimedian spiral wave instabilities have been discovered in chemical [1] and biological systems [2]. We present here the first example of such an instability pattern, encountered in a physical medium [3]. This instability is directly observed by polarized optical microscopy, on a positive dielectric anisotropy smectic A sample with homeotropic organization between parallel glass slides. The arm of the spiral is probably constituted of a 180° Bloch wall, separating indistinguishable smectic domains, and incorporating progressively the helicity of the cholesteric phase, excluded from the smectic.     

Nonlinear optical properties of dye-doped E7 liquid crystals at the nematic–isotropic transition

This work shows the influence of a 2,1,3-benzothiadiazole-based dye in the nonlinear optical refraction and nonlinear optical absorption of the thermotropic liquid crystal E7 at the nematic–isotropic transition in the ms time-scale using the Z-scan technique. The addition of dye does not modify the critical exponent of the nonlinear birefringence observed for the undoped sample at the transition, confirming the tricritical character. Also, the order parameter based in the nonlinear absorption shows, for the samples with higher dopings, a critical exponent that deviates from the expected value in the tricritical hypothesis.     

Constant pressure molecular dynamics simulations of the crystal–smectic transition in systems of soft parallel spherocylinders as a model for liquid crystals

Abstract

We present the results of constant pressure molecular dynamics simulation under hydrostatic pressure for soft parallel spherocylinders. A clear first order transition from crystal to smectic phase is observed. The anisotropy of the molecular volume plays an important role in this transition. We study the anisotropy of the diffusion process in the smectic phase by the mean-square displacement in each direction. Also the structures before and after the transition are analysed by means of the pair distribution functions.     

Electroclinic effect in the chiral smectic A and cholesteric phases at the proximity of a N*–SmA–SmC* multicritical point

We studied the electro-optic and dielectric properties of three pure ferroelectric liquid crystal materials (C10, C11 and C12) of the same series exhibiting cholesteric (N*), smectic A (SmA) and chiral smectic C (SmC*) phases. From electro-optic investigations, the tilt angle and spontaneous polarisation were determined as a function of temperature. In the dielectric measurements carried out without a dc bias field, we studied the soft-mode relaxation in the SmA phase. From experimental data and using the results of a Landau model, we evaluated the soft-mode rotational viscosity and the electroclinic coefficient in the SmA phase. A soft-mode like mechanism was also observed in the N* phase for compounds with shorter chains (C10 and C11). This relaxation process is not detected for the homologue with a longer chain (C12). The observation of this mechanism is related to smectic order fluctuations within N* phase whose amplitude is increased when approaching the SmC*–SmA–N* multicritical point.     

Electro‐optical cells using a cellulose derivative and cholesteric liquid crystals

A cellulose derivative/liquid crystal composite‐type electro‐optical cell using a commercial cholesteric liquid crystal (CLC) was investigated. The electro‐optical properties of the system were examined, i.e. the dependence on applied voltage of the reflected wavelength and the minimum and maximum transmissions. A thin film of the CLC was dispersed with a cross‐linked cellulose film of 25 µm thickness. In the voltage dependence of the reflected wavelength it was verified that there is a hysteresis in the reflected wavelength. The variation of the reflected wavelength with temperature was also determined. The results are analysed in the framework of similar systems described in the literature for CLC dispersed in a polymer matrix.     

Synthesis and properties of (−)-menthol-derived chiral liquid crystals by introducing adipoyloxy spacer between mesogenic core and chiral menthyl

A homologous series of new chiral liquid crystal compounds, M6BnB (n = 2, 4, 6, 8, 10, 12, 14, 16, 18), with varying length of n-alkoxy chains at one end, was formed by covalently linking a chiral (?)-menthyl with biphenyl-benzoate via adipoyloxy spacer group. A combination of analysis methods such as Fourier transform infrared, ¹H NMR spectra, differential scanning calorimetry, polarised optical microscopy and X-ray diffraction was carried out to systematically investigate their phase structures and phase transition behaviours. The length of the flexible terminal alkoxy group has a profound influence on the clearing points (T_iso) and T_iso decrease with the increase of alkoxy chain (n). In addition, increasing the length of the terminal alkoxy group tends to narrow the temperature range of the N* phase and favour the development of chiral smectic C phase.     

Reflection symmetry breaking in achiral rod-shaped smectic liquid crystals?

The SmC phase of 4'-octyloxyphenyl-4-octyloxybenzoate has been examined in light of recent reports that this phase is chiral. The results suggest that two varieties of chiral domains in LC cells of the phenylbenzoate are indeed formed, driven by interactions with surfaces. Application of sensitive probes for chirality and polarity in the absence of such interfacial influences failed to find any. Currently, there is no evidence that the subject SmC phase is chiral.     

Synthesis and mesomorphic properties of new azine-type liquid crystals

A series of symmetrical azine-type liquid crystals were synthesized. The characteristic of these liquid crystals is that they had high clearing point ( ～320 ℃) and broad thermal range of nematic phase ( ～154 ℃). It was also found that the end groups of the liquid crystals had effect on the mesomorphic properties.     

A new synthetic approach based on (−)-menthone for chiral liquid crystals

Starting from (?)-menthone, a new chiral building block useful for liquid crystal preparation was synthesized. This chiral moiety was attached to selected phenols under mild conditions by esterification. Rigid cores of tolanebenzoates and phenylbenzoates were prepared using the palladium cross-coupling reaction or by traditional liquid crystal synthesis methods. This convergent approach ended with a second esterification or palladium cross-coupling reaction to furnish new liquid crystal materials with smectic A, smectic C* and N* phases, as well as blue phases (BP). Thermal behavior, and the effect of chiral moiety branches and molecular packing in the smectic phases, have been investigated using differential scanning calorimetry, optical microscopy and X-ray diffraction.     

Synthesis and mesomorphic properties of new chiral banana‐shaped liquid crystals with chiral 3‐(alkoxy)propoxy terminal groups

New chiral banana‐shaped liquid crystals with chiral 3‐(alkoxy)propoxy terminal groups (Pn‐O‐PIMB5*‐4O, n = 7, 8, 9 and 10) were synthesized and their mesomorphic properties and phase structures investigated by means of electro‐optic measurements, polarizing optical microscopy, differential scanning calorimetry, and second harmonic generation measurements. Most of these chiral bent‐core mesogens (n = 7–9) showed the antiferroelectric B2 phase, whereas P10‐O‐PIMB5*‐4O exhibited the B7 phase. Comparing with the previously reported homologue Pn‐O‐PIMB(n‐2)*, we conclude that the terminal chain structure, particularly the position of chiral centres, plays an important role in the emergence of particular phase structures.     

New steroid α,β-unsaturated ketones as chiral components of induced cholesteric liquid crystal systems

New (E)-16-arylidene derivatives of 3β-hydroxyandrost-5-en-17-one and their acetates containing different substituents in the arylidene fragment were synthesized. The ability of the synthesized chiral compounds to induce helical supramolecular ordering (their helical twisting power) upon the introduction into the nematic liquid crystal (LC) 4-cyano-4′-pentylbiphenyl (5CB) and into multicomponent mixtures E63 and LC-1289 characterized by a wide mesophase interval. The dependence of the helical twisting power of the studied chiral additives (CAd) on their molecular structure was analyzed. The highest helical twisting power (44.6–67.1 μm⁻¹) was revealed for the synthesized acetates. It was found that the composites based on LC-1289 and E63 containing the studied CAd in very low concentrations (≤10–11 mol.%) have selective light reflection in the visible spectral region. The helical twisting power of the studied α,β-unsaturated ketones is determined by the combined influence of the anisotropy of polarizability of CAd molecules and specific features of their molecular shape.     

Untwisting of the helical superstructure in the cholesteric and chiral smectic C∗phases of cross-linked liquid-crystalline polymers by strain

Abstract

It is possible to untwist reversibly the helical superstructure of elastomers with cholesteric and chiral smectic C?phases by using strain. In that way a cholesteric structure can be transformed into a nematic structure and a chiral smectic C?into a smectic C structure. The latter case is especially interesting because a structure without a macroscopic polarization (chiral smectic C?) is transformed into one with a macroscopic polarization (smectic C like arrangement).     

Improving the electro‐optical properties of bistable reflective cholesteric liquid crystal displays

We have improved the electro‐optical properties of a bistable cholesteric liquid crystal display (Ch‐LCD) that is driven by a 3+2 dynamic driving scheme (3+2 DDS). The best contrast ratio is achieved at the selection period of 1.2–1.5 ms/line and the temperature range 26–30°C. The suitable preparation period and evolution period for contrast ratio are 40 times and 20 times the selection time, respectively. In the 3+2 DDS, rubbing the homeotropic alignment layer increases reflectivity of the ON state and keeps the reflectivity of the OFF state at the same level, so reflectivity and contrast ratio alike increase as a result. However, in a delayed homeotropic reset driving method, when the homeotropic alignment layer is rubbed, the reflectivity of both the ON and OFF states increases, thus the contrast ratio decreases. The combination of driving method and aligned surface morphology influences the relaxation mechanism in the cholesteric texture. By optimizing panel condition, we have demonstrated an 8.4″ foldable VGA Ch‐LCD that exhibits high reflectivity and contrast ratio with an addressing speed of around 1.2 ms/line.     

Synthesis and mesomorphic properties of novel tetrafluorinated biphenyl acetylene liquid crystals with a blue phase

Five novel tetrafluorinated biphenyl acetylene molecules containing two chiral centres in the side chain were synthesized. The mesomorphic behaviour of these compounds was characterized by differiential scanning calorimetry (DSC) and optical polarizing microscopy. All these compounds exhibit cholesteric and blue phases.     

Synthesis and ferroelectric properties of new chiral liquid crystals derived from (S)‐lactic acid with alkoxyethanols

A homologous series of chiral materials derived from (S)‐lactic acid with alkoxyethanols has been synthesized and their mesomorphic properties investigated. The mesophases and their corresponding transition temperatures were identified by differential scanning calorimetry and polarizing optical microscopy. The spontaneous polarization and electro‐optical response of ferroelectric SmC* phase are measured and reported.