Theory of a helicoidal cholesteric phase induced by an external field

We present a mean field theory to describe a helicoidal cholesteric phase for mixtures of a chiral nematic liquid crystal (LC) and a polymer chain as well as for pure chiral nematic LC molecules in the presence of a longitudinal external field parallel to the pitch axis of a cholesteric (Ch) phase. The free energy of the helicoidal Ch phase (Ch_H) is derived as a function of a usual orientational order parameter and an order parameter of the Ch_H phase. On increasing the strength of the external field, we find that the Ch phase changes to the nematic (N) phase through the Ch_H phase. Depending on the temperature and the strength of the external field, we find the second-order N–Ch_H and Ch_H–Ch phase transitions and the first-order paranematic (pN)–N, pN–Ch_H and pN–Ch phase transitions. We also predict phase diagrams in mixtures of a flexible polymer and a Ch LC molecule under the external field.     

Optical and thermal properties of Fe3O4 nanoparticle-doped cholesteric liquid crystals

A composite system of Fe₃O₄ nanoparticle-doped cholesteric liquid crystals with properties of broadband reflection and controllable temperature under high-frequency electric field is proposed. The broadband reflection can shield the near-infrared light in summer and the electromagnetic-thermal effect by the Fe₃O₄ nanoparticles can deice or defrost in winter for high transmittance and good safety. Furthermore, the thermal effect may be precisely controlled and significantly enhanced by adjusting the factors of the doped concentrations and the applied electric field parameters (duration time, magnitude and frequency). This composite system may have potential applications for multifunctional windows of architectures and vehicles.     

基于胆甾相液晶的可调制光子晶体

将胆甾相液晶填充进胶体晶体内部空隙, 通过胆甾相液晶与胶体晶体的耦合, 构建了一种新型可调制液晶光子晶体. 填充于胶体晶体内部的胆甾相液晶织构呈现典型的手性近晶相(S)特征. 由于胆甾相液晶具有特定的选择性反射, 当胶体晶体的带隙处于胆甾相液晶的反射波长范围之内, 则随着温度的改变, 胶体晶体的带隙与胆甾相液晶的带隙同时发生蓝移. 在一定温度条件下, 胆甾相液晶的带隙将与胶体晶体的带隙发生耦合, 实现了光子晶体带隙在单峰与双峰之间的可逆切换.     

Liquid-crystal-droplet optical microcavities

ABSTRACT

The use of liquid-crystal droplets as optical microcavities and lasers is reviewed and possible applications are discussed. Liquid-crystal droplets are prepared by simple methods that enable scalable production since their internal structure is formed by self-assembly. Light is trapped in droplets due to total internal reflection on the surface due to refractive index mismatch or because of a photonic bandgap structure in cholesteric liquid crystals (CLCs). Light confinement gives rise to a variety of optical modes and by employing a fluorescent dye end external optical pumping, lasing can be achieved. Liquid-crystal-droplet cavities are largely tunable by applying an electric field or a temperature change. Such cavities can be used as temperature and chemical sensors, and tunable light sources and filters in future integrated soft photonic circuits.     

Thermal tuning band gap in cholesteric liquid crystals

The phase of a liquid crystal (LC) changing from a nematic phase to a cholesteric (Ch) mesophase is achieved by adding different ratios of chiral dopants S811. By studying the transmission spectrum, we are able to measure the helical pitch in cholesteric phase. The pitch in the mixtures of nematic E7 and chiral dopants S811 as a function of the concentration of the dopant and temperature is investigated. The sensitivity of the selective reflection notch of the cholesteric phase to the thermal tuning depends strongly on the ratios of the chiral dopants. It reveals that the influence of temperature is more profound for those cholesteric liquid crystals (CLCs) which exhibit smectic A (SmA) at lower temperatures. When fitted using Keating's formula, the helical pitch calculated from our experimental results lies on the predicted curve. Optimised ratios of the mixture CLCs for the optimised reflection band with the specified wavelength ranging from 467 nm to 2123 nm are suggested.     

Potassium-ion-selective sensing based on selective reflection of cholesteric liquid crystal membranes

It is well-known that cholesteric liquid crystals have an optical property, selective reflection, due to changes in the pitch of their helical structure. This unique property of cholesteric liquid crystals can be used to attain a visual sensing system showing color changes as the detection signal. In this paper, we report a visual sensing membrane comprising cholesteric liquid crystals, in which a 15-crown-5 derivative was incorporated as ion recognizing sites, for K⁺ in aqueous solution. The resulting CLC membrane showed a shift of the reflection peak sensitive to K⁺ in water. We have also designed polymer-dispersed liquid crystal membranes that showed ion-selective reflection peak shifts with improved response time.     

Temperature dependence of pitch and twist elastic constant in a cholesteric to smectic A phase transition

To understand chirality in cholesteric (Ch) liquid crystals, we performed an experimental study on the Ch-smetic A (SmA) transition of a cholesteric liquid crystal. By studying the reflection spectrum at zero field and at the critical electric field used to unwind the helical structure, we were able to measure the helical pitch P and the twist elastic constant K ²² in the Ch phase. As the temperature was lowered toward the Ch-TGB phase transition, the helical pitch and twist elastic constant diverged. The results support the model that short range SmA forms in the Ch phase. When the results were fitted by power-law temperature dependence, the exponent for P was 0.78 and the exponent for K ²² was 1.36.     

Chiral,Thermally Irreversible and Quasi-Stealth Photochromic Dopant to Control Selective Reflection Wavelength of Cholesteric Liquid Crystal

A chiral and thermally irreversible photochromic fulgide derivative incorporating an (R)-binaphthol unit in its acid anhydride moiety was used for the photoswitching of the pitch length of cholesteric liquid crystals. Since the absorption maximum wavelengths of both thermally stable photoisomers are nearly in the UV region (quasi-stealth photochromism), it can be exposed to visible light without inducing photochromic reactions. Therefore, when the photoswitching molecule is added to a permanent cholesteric liquid crystal whose reflection light wavelength is in the visible region, the UV light-induced photochromic reaction of the photoswitching molecule changes the wavelength of the reflection light in the visible light region. We have succeeded in regulating the color of cholesteric liquid crystalline cells between red and blue upon UV light irradiation. Attempts to introduce this system in polymer dispersed cholesteric liquid crystals are also described.     

New side chain ferroelectric liquid crystalline polymers based on (2S, 3S)-2-chloro-3-methylpentanoic acid: synthesis and phase behaviour

The synthesis of four new chiral mesogenic monomers (M₁–M₄) and side chain ferroelectric liquid crystalline polymers containing (2S, 3S)-2-chloro-3-methylpentanoate is described. The chemical structures and phase behaviour of the monomers and polymers obtained in this study were characterised by Fourier transform infrared, proton nuclear magnetic resonance, polarising optical microscopy, differential scanning calorimetry, thermogravimetric analysis and X-ray diffraction. The selective reflection of light was investigated with ultraviolet/visible (UV/Vis). Their structure–mesomorphism relationships were discussed. M₁ and P₁ all showed a chiral smectic C (SmC*) phase. M₂ and M₃ revealed a SmC* phase and cholesteric phase, while their corresponding polymers P₂ and P₃ revealed a SmC* phase and smectic A (SmA) phase. M₄ only exhibited a cholesteric phase, whereas the corresponding polymers P₄ showed a SmA phase. Moreover, the selective reflection of light shifted to the long-wavelength region at the SmC* phase range and to the short-wavelength region at the cholesteric range with increasing temperature, respectively. The results seemed to demonstrate that the tendency towards melting temperature (T_m), glass transition temperature (T_g), isotropic temperature (T_i) and mesophase range for the monomers and polymers increased by increasing the mesogenic core rigidity or the number of phenyl ring. The polymerisation effect could lead to higher liquid crystalline to isotropic phase transition temperature, wider mesophase range and more ordered smectic phase formed. In addition, all the obtained polymers had a very good thermal stability and the corresponding T_d increased by increasing the number of phenyl ring.     

Synthesis and properties of side-chain liquid crystalline polymers grafted with chiral dimers containing cholesteryl groups

ABSTRACT

Two series of novel side-chain liquid crystal (LC) polysiloxanes grafted with chiral liquid crystalline dimers containing cholesteryl mesogens were synthesised. The chemical structure and LC properties of comonomers and polymers were characterised by FTIR, ¹H-NMR, DSC, TGA, POM and XRD. M₁ and M₂ were chiral nematic (N*) dimers, and M₃ was an achiral LC monomer displaying nematic mesophase in a narrow mesomorphic temperature range, while the copolymers exhibited N* mesophase whose mesomorphic temperature ranges were much wider than those of the comonomers. Moreover, the glass transition temperatures and isotropization temperatures of the polymers all decreased with decreasing the dimer components. Reflection spectra showed that P_a series tend to attain wide-band selective reflection at long wavelengths, while P_b series were more potential at short wavelengths with narrow bandwidths. Decreasing the dimer components led the wavelength of the selective reflection to blue shift, which was an abnormal phenomenon in chiral mixture system.     

Temperature dependence of pitch and twist elastic constant in a cholesteric to smectic A phase transition

To understand chirality in cholesteric (Ch) liquid crystals, we performed an experimental study on the Ch-smetic A (SmA) transition of a cholesteric liquid crystal. By studying the reflection spectrum at zero field and at the critical electric field used to unwind the helical structure, we were able to measure the helical pitch P and the twist elastic constant K22 in the Ch phase. As the temperature was lowered toward the Ch-TGB phase transition, the helical pitch and twist elastic constant diverged. The results support the model that short range SmA forms in the Ch phase. When the results were fitted by power-law temperature dependence, the exponent for P was 0.78 and the exponent for K22 was 1.36.     

A temperature and pH double sensitive cholesteric polymer film from a photopolymerizable chiral hydrogen-bonded assembly

In this study, a novel H-bonded cholesteric polymer film responding to temperature and pH by changing the reflection color was fabricated. The H-bonded cholesteric polymer film was achieved by UV-photopolymerizing a cholesteric liquid crystal (Ch-LC) monomers mixture containing a photopolymerizable chiral H-bonded assembly (PCHA). The cholesteric polymer film based on PCHA can be thermally switched to reflect red color from the initial green/yellow color as temperature is increased, which is due to a change in helical pitch induced by the weakening of H-bonded interaction in the polymer film. Additionally, the selective reflection band (SRB) of the cholesteric polymer film in solution with pH > 7 showed an obvious red shift with increasing pH values. While the SRB of the cholesteric polymer film in solutions with pH = 7 and pH < 7 hardly changed. This pH sensitivity in solutions with pH > 7 could be explained by the breakage of H-bonds in the cholesteric polymer film and the structure changes induced by―OH and―K + ions in the alkaline solution. In contrast, it couldn’t happen in the neutral and acidic solutions. The cholesteric polymer film in this study can be used as optical/photonic papers, optical sensors and LCs displays, etc.     

Phase diagrams of cholesteric liquid crystals obtained with a generalized Landau-de Gennes theory

Abstract

Phase diagrams of chiral nematic liquid crystals are studied within the framework of a generalized Landau-Ginzburg-de Gennes theory. Using the parametrization of Grebel, Hornreich, and Shtrikman for the tensor order parameter Q, all relevant elastic terms are included for the helicoidal phase and the blue phases of chiral nematic liquid crystals up to fourth order in Q and its gradient ?Q. The influence of the additional elastic terms on the phase diagrams of the chiral nematic phases is then investigated. The theory correctly describes the variation of the pitch with temperature and the induced biaxiality of the cholesteric phase. The results resolve the discrepancies encountered by Hornreich and Shtrikman in the comparison of experiment and theory. New features in the topology of the phase diagrams of blue phases, like re-entrant phase transitions, are predicted.     

Electro-optic effects in blue phases

Abstract

A theory of electro-optic and elasto-optic effects in the blue phases of cholesteric liquid crystals is developed. The case of small structure deformations and a weak field is considered: |E| ? E _c, where E _e is the critical field for the cholesteric-nematic transition. The theory explains all of the main experimental facts: the field-induced birefringence and biaxiality, the distortion of the cubic structure (electrostriction) and the orientation of the blue phase monocrystals in an electric field.     

The effect of different arms on the properties of chiral branched-arm liquid crystals based on isosorbide as the chiral core

A new series of two-arm ( MA₁ , MA₂ ), four-arm ( MA₃ ) and six-arm ( MA₄ ) chiral liquid crystalline compounds containing isosorbide as the chiral core were synthesised. Four precursors of branched-arm A₁–A₄ were first obtained, and then were esterised separately with isosorbide to obtain four chiral branched-arm liquid crystals ( MA₁–MA₄ ). The structure and properties have been characterised. The results show that MA₁ was smectic–cholesteric phase with a fan-like and oily streak texture; MA₂–MA₄ showed a cholesteric phase with oily streak texture, or lined texture and finger print texture. Isosorbide successfully induced a cholesteric phase. The melting point and clearing point values for MA₁–MA₄ first increased and then decreased. The branched-arm and chiral core quantity affected the liquid crystalline properties. At the same time, the difference in side-arms resulted in different directions of rotation. MA₁–MA₂ showed right-handed rotation and had selective reflection; MA₃–MA₄ demonstrated left-handed rotation and did not have selective reflection.     

The NTB phase in an achiral asymmetrical bent-core liquid crystal terminated with symmetric alkyl chains

ABSTRACT

The characteristics of the twist-bend nematic (N_TB) phase of an achiral asymmetrical rigid bent-core liquid crystal (LC), the ends of which are terminated by symmetric alkyl chains, are reported. The nematic–nematic phase transition and its properties are studied by differential scanning calorimetry (DSC), polarising microscopy and the electro-optic techniques. Large domains of opposite handedness are observed in the absence of the external field in the N_TB phase. Another set of periodic striped pattern consisting of domains with sharp boundaries is formed when a high-frequency electric field with a magnitude above its threshold is applied across a planarly aligned cell. The neighbouring domains are of opposite chirality. The temperature dependence of the heliconical angle θ₀ is determined from the birefringence measurements using Haller’s extrapolation technique. This material shows lower values of the heliconical angle (~9.3° at a temperature of 155°C within the N_TB phase) when compared with the previously reported dimer-based twist-bend nematic LCs (31°±3°).     

Synthesis and characterization of side chain cholesteric liquid crystalline elastomers derived from chiral bisolefinic crosslinking units

In this work we prepared a nematic monomer (4′‐allyloxybiphenyl 4′‐ethoxybenzoate, M₁ ), a chiral crosslinking agent (isosorbide 4‐allyloxybenzoyl bisate, M₂ ) and a series of new side chain cholesteric liquid crystalline elastomers derived from M₁ and M₂ . The chemical structures of the monomers and polymers were confirmed by FTIR and ¹H NMR spectroscopy. The mesomorphic properties were investigated by differential scanning calorimetry, thermogravimetric analysis, polarizing optical microscopy and X‐ray diffraction. The effect of the content of the crosslinking unit on phase behaviour of the elastomers is discussed. Polymer P₁ showed a nematic phase, P₂ –P₇ showed a cholesteric phase; P₆ formed a blue Grandjean texture over a broad temperature range 145–209.6°C, with no changed on the cooling. Polymers P₄ –P₇ , with more than 6?mol?% of chiral crosslinking agent, gave rise to selective reflection. Elastomers containing less than 15?mol?% of the crosslinking units displayed elasticity, reversible phase transition with wide mesophase temperature ranges, and high thermal stability. Experimental results demonstrated that, with increasing content of crosslinking agent, the glass transition temperatures first fell and then increased; the isotropization temperatures and mesophase temperature ranges decreased.     

Amphiphilic cholesteric liquid crystals prepared from the quaternary ammonium surfactant S-(−)-1-hexadecyl-1-methyl-2-pyrrolidinemethanol bromide

Abstract

We describe the first successful attempt to produce amphiphilic cholesteric liquid crystals using a chiral quaternary ammonium surfactant, namely S-(?)-1-hexadecyl-1-methyl-2-pyrrolidinemethanol bromide. Only amphiphilic cholesteric liquid crystal samples were made where the micelle structure is related to disc shaped micelles in the achiral N_D phase. The surfactant concentration dependence of the twist and the temperature dependence of the twist were made using laser diffraction. The twist in the amphiphilic cholesteric liquid crystal samples was too small to produce total iridescence.     

Thermal and optical properties of newly synthesized dicholesteryl esters with a phenylene oxide link in the normal and solidified cholesteric phases

A series of liquid crystalline compounds with a central phenylene oxide group to which two cholesteryl groups are attached via two alkanoate spacers has been synthesized and investigated. All the compounds except the one with ethanoate spacers showed a cholesteric phase as a single mesophase. The phase transition temperatures, the corresponding enthalpy changes and the wavelengths of selective reflection associated with the cholesteric phases showed a strong odd-even effect as a function of the spacer length. By rapid cooling of the compounds from the cholesteric phase to 0°C, solid films maintaining a cholesteric molecular order were obtained. At room temperature, the solid film showed stable cholesteric colours controlled by changing the temperature at which the rapid cooling was begun. Heating the cholesteric solid film of the compound with hexanoate spacers gave two forms of crystals above 80°C, whose ratio changed depending on the colour of the starting solid films. This result suggests the existence of two conformational isomers in the liquid crystalline state. Since it is possible repeatedly to fix stable red, green and blue colours by thermal treatment of this compound, we may apply it to a rewritable full colour recording in the thermal mode.     

Hydrostatic pressure effects on polymer cholesteric liquid-crystalline structure. I. Equilibrium structure of hydroxypropyl cellulose aqueous solution

Abstract

The effect of pressure on the polymer cholesteric liquid-crystalline structure of hydroxypropyl cellulose aqueous solutions was studied using reflection spectra measurements. Pressures applied to the polymer liquid crystals ranged from 1 to 2000 bar. The equilibrium reflection spectrum of the cholesteric structure shifted to longer wavelengths, showing that the cholesteric pitch of the liquid-crystalline structure increases as the applied pressure increases. At pressures higher than 200 bar the maximum wavelength of reflection shifted linearly with the increase in applied pressure. At lower pressures, the cholesteric structure was influenced by the surface plane of the quartz window.