Induced blue phases in cholesteric mixed systems

The induction of blue phases (BPs) was observed in binary mixtures comprising nematogenic compounds and cholesteryl esters which do not exhibit BPs by themselves. Phase diagrams and critical pitches p_c above which no BP occurs were determined. If p <p_c two BPs were observed. In mixtures with pitches close below p_c only one BP seems to exist.     

Flat optics with cholesteric and blue phase liquid crystals

ABSTRACT

The cholesteric (Ch) phase is an ancient liquid crystal (LC) phase, with its roots dating back to the days of Friedrich Reinitzer. It is most well known for its ability to selectively reflect circularly polarised light with the same rotation handedness as the helical structure, and have found applications in polarising and display devices. Most studies to date utilising the reflective properties of ChLCs, however, have treated ChLCs as simple dielectric mirrors in which light follows the law of reflection. We have recently shown that through controlling the phase of the Ch helix, it is possible to control the phase of reflected light. Because the phase can be controlled over 0–2π, the reflected wavefront from a planar device can be designed to possess non-specular properties, such as deflection and lensing. In this paper, we aim to provide a comprehensive understanding of the phenomenon by presenting theoretical and experimental results on the dependence of the reflected light phase on the helix phase, and the effect of chiral handedness on the phenomenon. Also, we show that wavefront manipulation based on the same concept can be achieved in Ch blue phases (BPs), which are chiral LCs with three-dimensional periodicity.     

Molecular design of flexible liquid crystal oligomers stabilising the chiral frustrated phases

ABSTRACT

Chirality induces structural frustration in liquid crystal systems, producing various kinds of chiral frustrated phases, for example, twist grain boundary (TGB) phases, blue phases (BPs) and dark conglomerate (DC) phases. Almost all molecules exhibiting these frustrated phases have a rigid shape. Especially, a bent–core unit is regarded as a key structure for BPs and DC phases. This paper describes that some flexible liquid crystal oligomers being far from a rigid bent–core molecule stabilise these phases. The LC oligomers have a supermolecular structure in which mesogenic units are connected via flexible spacers. By designing intermolecular interactions, they can exhibit various molecular packing structures in the liquid-crystalline phases as follows: chiral dimers inducing TGB phases, U-shaped and T-shaped oligomers stabilising BPs and achiral liquid crystal trimers exhibiting DC phases. I discuss how the designed liquid crystal oligomers produce the chiral frustrated phases.     

Selective stabilisation of blue phase liquid crystal induced by distinctive geometric structure of additives

We successfully stabilised liquid crystalline blue phases (BPs) by introducing two suitable additives with different geometric molecular structures, linear-shaped cobalt oleate complex (Co-OL) or tetrahedral-shaped tetraoctadecylammonium bromide (TODAB), into a liquid crystal (mixture of 4?-pentyl-4-biphenylcarbonitrile, JC-1041XX and chiral dopant). The BPs temperature range and phase sequence depending on the addition amount and shape of additives were systematically investigated to determine the optimal concentration and shape dependency required to achieve a stabilising effect for BPs. From the polarising optical microscope results, the BPs temperature range for all of the samples with additives was not only broadened but also shifted to room temperature compared to that of BPs without additives. The widest BPs temperature range was increased to 15.3°C by the addition of 3 wt% Co-OL. According to the UV-vis reflection spectroscopy results, the Co-OL has a more significant stabilisation effect on BP I than on BP II, and the widest BP I range increases to 11.0°C. On the other hand, TODAB is effective for BP II stabilisation with the broadest BP II range reaching 1.8°C. These selective stabilisation effects are attributed to the specific shape of additives that closely match the structures of the disclination lines of the BPs.     

Structure parameter of electrorheological fluids in shear flow

A structure parameter, Sn = η(c)γ/τ(E), is proposed to represent the increase of effective viscosity due to the introduction of particles into a viscous liquid and to analyze the shear behavior of electrorheological (ER) fluids. Sn can divide the shear curves of ER fluids, τ/E(2) versus Sn, into three regimes, with two critical values Sn(c) of about 10(-4) and 10(-2), respectively. The two critical Sn(c) are applicable to ER fluids with different particle volume fractions φ in a wide range of shear rate γ and electric field E. When Sn < 10(-4), the shear behavior of ER fluids is mainly dominated by E and by shear rate when Sn > 10(-2). The electric current of ER fluids under E varied with shear stress in the same or the opposite trend in different shear rate ranges. Sn(c) also separates the conductivity variation of ER fluids into three regimes, corresponding to different structure evolutions. The change of Sn with particle volume fraction and E has also been discussed. The shear thickening in ER fluids can be characterized by Sn(c)(L) and Sn(c)(H) with a critical value about 10(-6). As an analogy to friction, the correspondence between τ/E(2) and friction coefficient, Sn and bearing numbers, as well as the similarity between the shear curve of ER fluids and the Stribeck curve of friction, indicate a possible friction origin in ER effect.     

Electrorheological Effect and Electro‐Optical Properties of Side‐on Liquid Crystalline Polysiloxane in a Nematic Solvent

The electrorheological (ER) effect and the electro‐optical properties of a ′′side‐on′′ liquid crystalline polysiloxane (PS) are investigated. A large ER effect is observed and the response to the shear stress of neat PS in the nematic phase is shown to be affected by the shear rate. PS is also mixed with a low‐molar nematic liquid crystal (5CB) in order to improve the response behavior to the applied electric field. The rheological properties of such mixtures are highly dependent on the concentration of 5CB. The composites respond faster to the applied electric field and have improved electro‐optical properties. This study offers a new perspective on the development of liquid crystal materials for the ER effect.     

Imidazolium‐based polymerized ionic liquid crystals containing fluorinated cholesteryl mesogens

A series of polymerized ionic liquid crystals (PILCs) bearing fluorinated cholesteryl mesogens were synthesized in this work, which include polymerized imidazolium bromides (PIBs) and polymerized imidazolium hexafluorophosphates (PIHs). The PIBs were synthesized using alkyl bromine‐containing polysiloxanes and 1‐butyl‐1H‐imidazole, and the PIHs were synthesized by anion metathesis reaction using the corresponding PIBs and KPF₆. The chemical structures, liquid crystalline (LC) properties, and electrorheological (ER) effect of these PILCs were characterized by use of various experimental techniques. All the PILCs showed smectic A mesophase on heating and cooling cycles. The smectic layer structure of these PILCs are originated from the rigid fluorinated cholesteryl mesogens and the flexible moieties in the LC phase, but the ion pairs (imidazolium cations–PF₆^?, Im⁺–PF₆^?; or imidazolium cations–Br^?, Im⁺–Br^?) can disperse in the polysiloxane matrix and expand the d‐spacing in the smectic layers. The PIHs show lower T_g and T_i than the corresponding precursor PIBs, which is due to the larger ion volume of Im⁺–PF₆^? for PIHs than that of Im⁺–Br^? for PIBs. A series of 40 V% ER fluids were prepared by mixing the PILCs with polydimethylsiloxane (PDMS), and the ER behaviors were studied. All the PILC/PDMS fluids showed ER effect, and the PIH/PDMS fluids show a little greater ER effect than the PIB/PDMS fluids. The PILC droplets in the ER fluids become deformed owing to both the orientation of fluorinated cholesteryl mesogens and the suppression of ionic migration when a DC electric field was applied, resulting in the occurrence of ER effect. Copyright © 2015 John Wiley & Sons, Ltd.     

一代树状碳硅烷液晶研究-端基含12个4-硝基偶氮苯介晶基元

用发散法合成了以四碳硅烷为核心，周边含12个4-硝基偶氮苯介晶基元（M5） 端基的新的一代树状碳硅烷（D1）液晶，并用元素分析、核磁共振、基质辅助激光 解吸离子化飞行时间质谱（MALDI-TOF-MS）、红外、紫外、偏光显微镜、差示扫描 量热（DSC）和X射线衍射法（WAXD）表征。介晶基元化合物（M5）显示向列相，树 状物D1显示胆甾相和S_E相。D1的液晶相相行为是K70Ch188I185Ch58S_E-48K。     

Distinct columnar and lamellar liquid crystalline phases formed by new bolaamphiphiles with linear and branched lateral hydrocarbon chains

A universal building block for the convergent synthesis of a wide variety of different T-shaped ternary amphiphiles was developed and used for the synthesis of a series of new liquid-crystalline materials composed of a rigid biphenyl core with polar glycerol groups at both ends and linear or branched alkyl chains in a lateral position. In addition, compounds with bulky achiral (2,4,6-trimethylphenoxy, adamantane-1-carboxylate, benzoate) or chiral (menthyl or cholesteryl) substituents attached to the end of the lateral alkyl chain were also investigated. In all cases the lateral chains were connected to the aromatic core by an ether linkage. The effect of the ether linking unit on mesophase stability and mesophase type is discussed with respect to conformational effects. The liquid-crystalline phases were investigated by polarizing microscopy, calorimetry, and X-ray diffraction of surface aligned samples. Upon enlarging the lateral chains a series of different polygonal cylinder phases was observed, which were replaced by lamellar phases and a non-cylinder hexagonal columnar phase by further increasing the size of these substituents. Remarkably, only pentagonal, hexagonal, and giant hexagonal cylinder phases could be observed, whereas mesophases composed of cylinders with a smaller number of sides are missing. No distinct chirality effects were observed for the menthyl- and cholesteryl-substituted compounds. However, the rodlike shape of the polycyclic cholesteryl core leads to a unique phase structure combining an organization of the alicyclic cholesteryl cores perpendicular to the layer planes and the aromatic biphenyl cores parallel to the layer planes.     

Electrorheological Effects in Lecithin Organogels with Water and Glycerol

The effect of an external electric field on ternary mixtures consisting of lecithin, n-decane, and small amounts of polar additives (water or glycerol) has been studied by oscillating rheology, polarizing microscopy, and electric birefringence. It is shown that an electric field that is applied induces a so-called electrorheological (ER) effect, i.e., an increase in the viscosity and dynamic shear moduli of all the examined mixtures. The ER effect is absent in solutions of nonoverlapping micelles. The electric field causes the formation of fiber-like structures in the interelectrode gap. The ER effect becomes evident at a critical field of about 40 and 100 V/mm for water- and glycerol-containing organogels, respectively. In the latter case, a region of the reproducible and stable ER effect is extended up to 1700 V/mm, which is 3-4 times greater than that observed in the jelly-like phases with water. The buildup, as followed from birefringent measurements, includes fast and slow processes. Those correspond to both the local motions of parts of micellar chains and the restructuring of the whole network under the action of an external electric field. The ER response depends on the molar ratio of the polar additives to lecithin. Diagrams describing the behavior of ternary mixtures under the electric field have been constructed. They differ for water- and glycerol-containing organogels. The dependence of the stable ER effects on the molar ratio of glycerol to lecithin has a maximum in the vicinity of the phase separation of the homogeneous organogel, whereas for water-containing systems there is a gradual increase up to and including mixtures with the solid precipitate. A new rheological regime has been first established for solutions of polymer-like micelles. This feature is the square-root scaling of the dynamic moduli with the frequency. Such a scaling is inherent in polymers. A possible mechanism is considered, basing on the ordering of cylindrical micelles under the action of an external electric field. Copyright 1999 Academic Press.     

Cerebroside Langmuir monolayers originated from the echinoderms: II. Binary systems of cerebrosides and steroids

Two-component Langmuir monolayers formed on a subphase of 0.5M sodium chloride solution were investigated for two different cerebrosides (LMC-1 and LMC-2) with steroids of cholesterol (Ch) and cholesteryl sodium sulfate (Ch-S); i.e. LMC-1/Ch, LMC-1/Ch-S, LMC-2/Ch, and LMC-2/Ch-S were examined in terms of surface pressure (pi), the surface potential (DeltaV) and the dipole moment (mu( perpendicular)) as a function of surface area (A) by employing the Langmuir method, the ionizing electrode method, and the fluorescence microscopy. Surface potentials (DeltaV) of steroids were analyzed using the three-layer model proposed by Demchak and Fort. The miscibility of cerebrosides and steroids in the insoluble monolayers was examined by plotting the variation of the molecular area and the surface potential as a function of the steroid molar fraction (X(steroid)) based upon the additivity rule. From the A-X(steroid) and DeltaV(m)-X(steroid) plots, partial molecular surface area (PMA) and apparent partial molecular surface potential (APSP) were determined at the different surface pressures. The PMA and APSP with the mole fraction were discussed for the miscible system. Judging from the two-dimensional phase diagrams, they can be classified into two types. The first is a completely immiscible type; the combination of cerebrosides with cholesterol. The second is a negative azeotropic type, where cerebrosides and cholesteryl sodium sulfate are completely miscible both in the expanded state and in the condensed state. In addition, a regular surface mixture (the Joos equation for the analysis of the collapse pressure of two-component monolayers) allowed calculation of the interaction parameter (xi) and the interaction energy (-Delta epsilon) between the cerebrosides and Ch-S. The miscibility of cerebroside and steroid components in the monolayer state was also supported by fluorescence microscopy.     

Microstructure and viscoelasticity of electrorheological suspensions with hybrid core‐shell microspheres

A novel type of electrorheological (ER) fluids with hybrid microspheres as dispersed phases was prepared and their rheological properties in dynamic and oscillatory modes in the presence of electric field were studied. Hybrid microspheres are new types of inorganic‐organic composites consisting of inorganic hollow cores covered with a thin layer of conjugated polymer poly (3‐octylthiophene)—P₃OT, followed by a polyurethane electrolyte shell of defined thicknesses and controlled (electronic and ionic) conductivities. It has been found that the rate of ER response for the applied electric field of the order of few kV/mm, as well as the recovery time after high shear loads of the novel ER fluids, was significantly improved in comparison to the typical solid electrolyte‐based materials. It has been shown that upon the application of an electric field the suspensions of hybrid microspheres form a gel‐like network structure at low strain region with reasonable rigidity characterized by the domination of G′ moduli over G″ and broad linear viscoelastic range. It was shown that at electric fields as high as 3 kV/mm, the investigated ER materials exhibited predominantly elastic behavior and were able to endure strains up to 3% without significant deformation of the material microstructure. Moreover, the novel ER materials exhibited much faster microstructure recovery after high shear loads in comparison to ER fluids comprising core‐shell composites without poly (3‐octylthiophene) interlayer, which makes them more suitable for the applications requiring immediate response to an external electric field.     

宽温域蓝相液晶材料

蓝相常在高手性液晶体系的清亮点附近温度区间出现,由于具有优异的光学特性如无双折射现象和选择性反射可见光等,近年来蓝相在光电和光子领域越来越受到人们的关注。本文综述了蓝相的发现、分子排列和光学特性等,详细介绍了宽温域蓝相液晶材料在国内外的研究进展和应用现状。最后分析了蓝相液晶用于平板显示领域在技术方面存在的主要问题和未来发展方向。     

Electrorheological fluids based on glycerol-activated titania gel particles and silicone oil with high yield strength

This article describes an electrorheological (ER) fluid based on glycerol-activated titania organic-inorganic hybrid gel particles and silicone oil with high yield strength. Based on a physical picture of a water-activated ER system, glycerol that has a high dielectric constant and boiling point is in situ prepared in the amorphous titania gel during the sol-gel processing. A small amount of ionic surfactant hexadecyltrimethylammonium bromide (CTAB) is employed to enhance charge carriers in particles. FTIR and XRD techniques are used to determine the nature and structure of the hybrid gel. Rheology test results show that a large static yield stress greater than 12.6 kPa is obtained when 3 kV/mm dc electric field is applied. This value is close to the value predicted by H. Conrad (MRS Bull. 8 (1998) 35) in theory. Furthermore, dynamic shear stress as a function of shear rate and temperature is also investigated. This ER fluid exhibits strong temperature dependence and a wide working temperature range from 0 to 120 degrees C, while its leaking current density is still low. More interesting is that the glycerol content is demonstrated to have an influence on ER effect and temperature dependence. Measurement of the dielectric properties of ER fluids shows enhancement of the dielectric constant and dielectric loss due to addition of glycerol and a regular dependence of them on temperature, which well explains the strong ER effect.     

Correlation of the Dielectric Properties of Dispersed Particles with the Electrorheological Effect

The dc field rheological properties and frequency dependent dielectric properties of a set of electrorheological (ER) fluids composed of oxidized polyacrylonitrile or aluminosilicate materials dispersed in silicone oil were examined in this paper. Our experimental investigations show that there is a complicated relationship between the dielectric properties of dispersed particles and the ER effect. The dielectric loss of dispersed particles, which has not attracted much attention in previous work, was found to play a considerable role in ER response. The large dielectric loss tangent, experimentally around 0.10 at 1000 Hz, is found to be needed for a strong ER effect. A good ER solid material should first have large dielectric loss, and then the higher the dielectric constant, the stronger the ER effect. The large dielectric loss would facilitate the turning of dispersed particles, and the high dielectric constant would maintain the fibrillation structure stable and strong. Two processes, the particle turning process and the particle polarization process, are thought to be involved in ER activity. Our findings, in connection with the Wagner model, can better explain why the strongest ER effect occurs at particle conductivity of 10⁻⁷S/m; why the shear stress of some ER fluids decreases with frequency while with others the shear stress increases with frequency; and why trace water can enhance the ER effect considerably, which would help in understanding the mechanism of the ER effect. Too large a dielectric loss is thought to be unfavorable for the ER effect, and its suitable range is worth further study. The results also present a method of designing high performance ER fluids, which would significantly promote development of electrorheology and its application in industrial areas.     

ELECTRORHEOLOGICAL PROPERTIES OF POLYANILINE/PUMICE COMPOSITE SUSPENSIONS

Electrorheological （ER） properties of polyaniline （PAni）, pumice and polyaniline/pumice composites （PAPC） were investigated. Polyaniline and PAni/pumice composite were prepared by oxidative polymerization. PAni/pumice particlesbased ER suspensions were prepared in silicone oil （SO）, and their ER behavior was investigated as a function of shear rate, electric field strength, concentration and temperature. Sedimentation stabilities of suspensions were determined. It has been found that ER activity of all the suspensions increases with increasing electric field strength, concentration and decreasing shear rate. It has shown that the suspensions have a typical shear thinning non-Newtonian viscoelastic behavior. Yield stress of composite suspensions increased linearly with increasing applied electric field strength and with concentrations of the particles. The effect of high temperature on ER activity of purrfice/silicone oil systems was also investigated.     

用含苯甲酸甲氧基苯酯的聚硅氧烷组成均相电流变液及其智能阻尼特性的研究

Based on the synthesis and characterization of side-chain liquid crystal polysiloxanes containing p-methoxyl phenyl benzolate, the homogeneous electrorheological (ER) liquid and its smart damper were fabricated. The properties of the homogeneous ER liquid and its smart damper were tested. The results indicated that the homogeneous ER liquid with significant ER effect at room temperature is obtained by mixing the side-chain liquid crystal polysiloxanes containing p-methoxyl phenyl benzolate and silicone oil. Its shear stress reached 1550Pa at electrical field strength (E) of 2.2 kV/mm and shear rate of 300 s-1. The damping forces of the smart damper based on the homogeneous ER liquid enhanced if E and oscillation frequency increased. Consequently, an effective way for semi-active control using the smart damper based on the homogeneous ER liquid was put forward to reduce the seismic responses of the structures.     

Electrorheological property and microstructure of acetamide-modified TiO<Subscript>2</Subscript> nanoparticles

A sol–gel method has been proposed to prepare uniform TiO₂ nanoparticles whose average size is about 30 nm. The prepared nanometer TiO₂ particles are modified by acetamide via different self-assembled processes. X-ray diffraction analyses, scanning electron microscopy, and Fourier transform infrared spectrometry are used to determine the structure of the nanoparticles. The results indicate that the different synthesis processes do not change the morphology of the TiO₂/acetamide nanoparticles; nevertheless, they affect the interaction between amide and acetamide. The electrorheological (ER) activity is studied by shear stress under DC electric field. The acetamide-modified TiO₂ ER fluid shows notable ER activity with shear stress about 45 kPa (at 5 kV/mm), which outclasses the shear stress (2 kPa) of unmodified TiO₂ ER fluid. It is also found that the ER effect is very sensitive to the interaction of molecules on TiO₂ particles. The chemical bond between core and shell can enhance the ER activity of the sample.     

Rheological properties of chiral liquid crystals possessing a cholesteric-smectic A transition

We have measured the rheological properties of two cholesterol derivatives (cholesteryl myristate and cholesteryl nonanoate) in the vicinity of their cholesteric-smecticA transitions. The results for the two compounds differ qualitatively, and are in agreement with results based on optical observations of new defects in cholesteryl nonanoate showing that this material, traditionally considered as a typical cholesteric, in fact exhibits a TGBA phase between the cholesteric and smectic A phases.