Liquid-crystalline physical gels

Liquid-crystalline (LC) physical gels are a new class of dynamically functional materials consisting of liquid crystals and fibrous aggregates of molecules that are called "gelators". Liquid-crystalline physical gels, which are macroscopically soft solids, exhibit induced or enhanced electro-optical, photochemical, electronic properties due to the combination of two components that form phase-separated structures. In this tutorial review, we describe the materials design and structure-property relationships of the LC physical gels. The introduction of self-assembled fibers into nematic liquid crystals leads to faster responses in twisted nematic (TN) mode and high contrast switching in light scattering mode. Furthermore, the LC physical gels can be exploited as a new type of materials for electro-optical memory. This function is achieved by the control of reversible aggregation processes of gelators under electric fields in nematic liquid crystals. Electronic properties such as hole mobilities are improved by the introduction of fibrous aggregates into triphenylene-based columnar liquid crystals. The incorporation of photochromic azobenzenes or electroactive tetrathiafulvalenes into the chemical structures of gelators leads to the preparation of ordered functional materials.     

Biphenyl-based liquid crystals for elevated temperature processing with polymers

Due to the limited thermal stability of current commercially available liquid crystals (LCs), the incorporation into polymer composites through standard processing techniques, such as melt coextrusion, has been hindered. Motivated by this dilemma, a series of smectic B liquid crystalline structures based on the 4,4?-alkyl substituted biphenyl moiety were synthesised through conventional methodologies and probed for their thermal stability and LC properties. Degradation temperatures were found to increase with increasing aliphatic chain length – up to 295 °C for C16 substituted structures, which is well above the processing temperatures of commercial polymers. Additionally, all compounds were found to be liquid crystalline in nature with crystal-to-smectic B transition temperatures ranging from 49.8 °C to 91.4 °C. Thermal stability, phase separation, and compatibility of LC/polystyrene composites were also examined. Less than 10% of 15A15 LC by weight in polystyrene exhibited good polymer miscibility, while phase separation occurred at loads higher than 15% by weight. We foresee the use of these LCs in applications that require elevated processing conditions to produce materials with enhanced mechanical or gas barrier properties.     

Colloidal shape controlled by molecular adsorption at liquid crystal interfaces

The ability to control finely the structure of materials remains a central issue in colloidal science. Due to their elastic properties, liquid crystals (LC) are increasingly used to organize matter at the micrometer scale in soft composites. Textures and shapes of LC droplets are currently controlled by the competition between elasticity and anchoring, hydrodynamic flows, or external fields. Molecules adsorbed specifically at LC interfaces are known to orient LC molecules (anchoring effect), but other induced effects have been poorly explored. Using specifically designed amphitropic surfactants, we demonstrate that large-shape transformations can be achieved in direct LC/water emulsions. In particular, we focus on unusual nematic filaments formed from spherical droplets. These results suggest new approaches to design new soft LC composite materials through the adsorption of molecules at liquid crystal interfaces.     

Photochromic composites based on porous stretched polyethylene filled by nematic liquid crystal mixtures

A number of the novel photochromic polyethylene (PE)‐based liquid crystal composites were prepared and studied. The oriented stretched porous polyethylene films were used as the polymer matrices. Commercial liquid crystals doped with new photochromic compounds were introduced into PE films and photo‐optical properties of the obtained composites were investigated. It was shown that a director of nematic liquid crystals is highly oriented along the stretching axis of PE films resulting in noticeable linear dichroism of the PE composite films. New approaches for reversible or irreversible image recording on PE LC composites by UV irradiation were demonstrated. Copyright © 2009 John Wiley & Sons, Ltd.     

Local high-density distributions of phospholipids induced by the nucleation and growth of smectic liquid crystals at the interface

Amphiphilic molecules adsorbed at the interface could control the orientation of liquid crystals (LCs) while LCs in turn could influence the distributions of amphiphilic molecules. The studies on the interactions between liquid crystals and amphiphilic molecules at the interface are important for the development of molecular sensors. In this paper, we demonstrate that the development of smectic LC ordering from isotropic at the LC/water interface could induce local high-density distributions of amphiphilic phospholipids. Mixtures of liquid crystals and phospholipids in chloroform are first emulsified in water. By fluorescently labeling the phospholipids adsorbed at the interface, their distributions are visualized under fluorescent confocal microscope. Interestingly, local high-density distributions of phospholipids showing a high fluorescent intensity are observed on the surface of LC droplets. Investigations on the correlation between phospholipid density, surface tension and smectic LC ordering suggest that when domains of smectic LC layers nucleate and grow from isotropic at the LC/water interface as chloroform slowly evaporates at room temperature, phospholipids transition from liquid-expanded to liquid-condensed phases in response to the smectic ordering, which induces a higher surface tension at the interface. The results will provide an important insight into the interactions between liquid crystals and amphiphilic molecules at the interface.     

Electroactive supramolecular self-assembled fibers comprised of doped tetrathiafulvalene-based gelators

New electroactive supramolecular fibers have been formed by self-assembly of the derivatives of tetrathiafulvalene (TTF) in liquid crystals. These derivatives are designed and prepared by introducing the TTF moiety to the scaffold derived from amino acids such as L-isoleucine whose derivatives function as organogelators. These TTF-based gelators form stable fibrous aggregates in liquid crystals. These fibers are the first example of hydrogen-bonded one-dimensional aggregates having electroactive moieties whose electrical conductivities were measured after doping. Their electronic states have also been characterized by spectroscopic methods. Unidirectionally aligned fibers are formed in the oriented liquid crystal solvents on the rubbed polyimide surface for further functionalization of the fibers.     

Self-assembly of carbazole-containing gelators: alignment of the chromophore in fibrous aggregates

Carbazole-containing gelators derived from l-isoleucine have been developed. They form elongated self-assembled fibers in common organic solvents and in liquid crystals, leading to the efficient gelation of these solvents. Spectroscopic studies indicate that the carbazolyl moieties are one-dimensionally stacked in the fibers. The stacking of the carbazolyl moieties is reversible by the association and dissociation of the hydrogen bonding. Moreover, anisotropically aligned fibers have been obtained in a homogeneously oriented smectic state of liquid crystals. This template behavior would serve as a versatile approach to the functionalization of self-assembled fibers.     

溶致液晶的结构及应用研究进展

本文评述了溶致液晶的相态和结构以及其相态随浓度变化的特点，并着重介绍了溶致液晶在新型纳米材料全盛中的应用，在生物医学中的应用，在化学反应中的催化作用，以及在工业中的作用。     

meso-四(邻烷氧基苯基)卟啉及其铜配合物的合成、表征和性能研究

合成了meso-四(邻烷氧基苯基)卟啉及其铜配合物两个系列20个化合物, 其中未见文献报道的化合物16个, 研究了其合成、分离、纯化方法, 得到了这两个系列化合物的晶体或固体, 用¹H NMR, MS, IR, UV, 元素分析等方法确证了这些化合物的结构. 用差示扫描量热法(DSC)和偏光显微镜(PM)研究了这两个系列化合物的液晶性能, 发现19个化合物具有液晶性, 其液晶行为表现为升温单变液晶.     

Nanoparticle-doped chiral nematic liquid-crystal composite and its effect in magnetic-response and electric-response flexible display

A composite system of Fe3O4 nanoparticle-doped chiral nematic liquid crystals (N*LC) with flexible display performance was proposed. Fe3O4 nanoparticle and the nanoparticle-doped N*LC composite were detailed prepared and investigated. The influence of nanoparticle doping amount and chiral compound content on the magnetic performance as well as electrical performance of the flexible device had been studied in detail. The most suitable N*LC composites for magnetic-driven display had been found. With the characteristics of simple preparation, good stability and high resolution, the Fe3O4 nanoparticle-doped N*LCs had promising applications for power-free magnetic-driven flexible LC paper or display board.     

The effect of voltage controlled orientation order of liquid crystals in non-acrylic polymer dispersed liquid crystals films

The nematic liquid crystals (LCs) are randomly dispersed material with random orientation order in polymer dispersed liquid crystal (PDLC) films. The LCs change their orientation from random to vertical as electric field is applied. This transformation of orientation order of nematic liquid crystals in the PDLC films is controlled by many factors operating simultaneously. For instance, some factors like the internal forces of attractions among the neighboring LC molecules, anchoring with polymeric matrix, ITO glass boundaries, and chemical structures of the materials are less studied. The learning of extent of vertical orientation of liquid crystal droplets in an electric field is essential to attain optimum electro optical properties of PDLCs. In this finding, bipolar and radial LCs droplets with random orientation have been observed in non-acrylic polymeric media. It is learned that with small increase of contents of external material, the extent of vertical orientation has been varied intensely. The extent of vertical orientation of LCs molecules increases as the contents of external non-acrylic polymeric material decreased. For this study, the orientations of LCs with respect to material type/contents, external applied force, and restoration of electric filed as hysteresis have been studied in details.     

Active colloids in liquid crystals

Active colloids in liquid crystals (ACLCs) are an active matter with qualitatively new facets of behavior as compared to active matter that becomes isotropic when relaxed into an equilibrium state. We discuss two classes of ACLCs: (i) “externally driven ACLCs”, in which the motion of colloidal particles is powered by an externally applied electric field, and (ii) “internally driven ACLCs”, formed by self-propelled particles such as bacteria. The liquid crystal (LC) medium is of a thermotropic type in the first case and lyotropic (water based) in the second case. In the absence of external fields and self-propelled particles, the ACLCs are inactive, with the equilibrium LC state exhibiting long-range orientational order. The external electric field causes ACLCs of type (i) to experience translations, rotations, and orbiting, powered by mechanisms such as LC-enabled electrokinetics, Quincke rotations and entrapment at the defects of LC order. A dense system of Quincke rotators, orbiting along circularly shaped smectic defects, undergoes a transition into a collective coherent orbiting when their activity increases. An example of internally driven ACLCs of type (ii) is living liquid crystals, representing swimming bacteria placed in an otherwise passive lyotropic chromonic LC. The LC strongly affects many aspects of bacterial behavior, most notably by shaping their trajectories. As the concentration of bacteria and their activity increase, the orientational order of living liquid crystals experiences two-stage instability: first, the uniform steady equilibrium director is replaced with a periodic bend deformation, then, at higher activity, pairs of positive and negative disclinations nucleate, separate, and annihilate in dynamic patterns of topological turbulence. The ACLCs are contrasted to their isotropic counterparts.     

基于含POSS-dendrimer超强液晶凝胶的柔性可拉伸液晶光散射显示器

利用凝胶因子自组装可赋予凝胶网络形状、强度等的特性,设计制备了多面体齐聚倍半硅氧烷(POSS)核有机无机杂化dendrimer(POSS-G1-BOC)凝胶因子并将其引入到液晶客体分子中,获得了兼具力学性能与响应特性的超分子液晶凝胶.在系统研究该液晶凝胶的凝胶行为、响应特性、表面形貌、组装机理及力学性质的基础之上,制备了基于透明柔性可拉伸导电薄膜(PU/Ag NWs)的三明治结构液晶光散射显示器件.该器件在低电压(10 V,DC)驱动下即可实现较高对比度的显示效果,不仅可以在弯曲至曲率为0.14 cm-1的条件下使用,而且在拉伸至原始长度的120%时,仍可保持自身的电控响应特性,有望将其广泛应用于可穿戴设备、智能响应性材料等领域.     

On crystal versus fiber formation in dipeptide hydrogelator systems

Naphthalene dipeptides have been shown to be useful low-molecular-weight gelators. Here we have used a library to explore the relationship between the dipeptide sequence and the hydrogelation efficiency. A number of the naphthalene dipeptides are crystallizable from water, enabling us to investigate the comparison between the gel/fiber phase and the crystal phase. We succeeded in crystallizing one example directly from the gel phase. Using X-ray crystallography, molecular modeling, and X-ray fiber diffraction, we show that the molecular packing of this crystal structure differs from the structure of the gel/fiber phase. Although the crystal structures may provide important insights into stabilizing interactions, our analysis indicates a rearrangement of structural packing within the fibers. These observations are consistent with the fibrillar interactions and interatomic separations promoting 1D assembly whereas in the crystals the peptides are aligned along multiple axes, allowing 3D growth. This observation has an impact on the use of crystal structures to determine supramolecular synthons for gelators.     

Influence of low concentration silver nanoparticles on the electrical and electro-optical parameters of nematic liquid crystals

Thermodynamical, optical, dielectric and electro-optical characterisation of nematic liquid crystals (LCs) and silver nanoparticle (NP) composites have been carried out. Transition temperatures of pure and composites systems have been measured. Thermodynamical studies suggest increase of clearing temperature of the composite material as compared to the pure material. Threshold voltage for switching from bright to dark state and splay elastic constant of the pure and composite materials have been determined. From frequency dependence of dielectric measurements, permittivity, loss, relaxation frequency and dielectric strength of flip-flop mechanism of LC molecules in the nematic phase have been calculated. Dielectric properties of composites have been explained in reference of Maier and Meier theory. The effects of doping of NPs on dielectric and electro-optic properties of LC-NP composites have been discussed.     

Versatile supramolecular gelators that can harden water, organic solvents and ionic liquids

We developed novel supramolecular gelators with simple molecular structures that could harden a broad range of solvents: aqueous solutions of a wide pH range, organic solvents, edible oil, biodiesel, and ionic liquids at gelation concentrations of 0.1-2 wt %. The supramolecular gelators were composed of a long hydrophobic tail, amino acids and gluconic acid, which were prepared by liquid-phase synthesis. Among seven types of the gelators synthesized, the gelators containing L-Val, L-Leu, and L-Ile exhibited high gelation ability to various solvents. These gelators were soluble in aqueous and organic solvents, and also in ionic liquids at high temperature. The gelation of these solvents was thermally reversible. The microscopic observations (TEM, SEM, and CLSM) and small-angle X-ray scattering (SAXS) measurements suggested that the gelator molecules self-assembled to form entangled nanofibers in a large variety of solvents, resulting in the gelation of these solvents. Molecular mechanics and density functional theory (DFT) calculations indicated the possible molecular packing of the gelator in the nanofibers. Interestingly, the gelation of an ionic liquid by our gelator did not affect the ionic conductivity of the ionic liquid, which would provide an advantage to electrochemical applications.     

A new reverse mode light shutter from silica-dispersed liquid crystals

Nanoparticle dispersions in liquid crystalline materials at low concentrations allow both investigating the formation of defects in liquid crystal (LC) and enhancing the light-scattering properties of LC optical devices. Reverse mode LC dispersions are LC devices, which look like transparent in their OFF state, when no electric field is applied, and opaque in their ON state. In this paper, a new reverse mode device, formed by a dispersion of a LC mixture in a silica nanoparticle crosslinked network, is presented. The morphology and the electro-optical properties of these silica nanoparticle/LC composites were investigated for two different LC mixtures with a negative dielectric anisotropy. The observed transmittances and relaxation times were found to depend strongly on the silica amount and chemical–physical properties of LC used in the sample preparation.     

氯化meso-四(对烷氧基苯基)卟啉合铁(Ⅲ)的合成、表征和性能

设计合成了10个氯化meso-四(对烷氧基苯基)卟啉合铁(Ⅲ)配合物, 其中7个尚未见文献报道. 用1H NMR, MS, IR, UV和元素分析等技术表征了该系列配合物的结构. 用差示扫描量热法和偏光显微镜研究结果表明8个配合物具有液晶性, 其液晶行为分别表现为升温单变液晶和升温降温互变液晶; 有1~2个中介相, 相变区间Δt最宽为128 ℃, Δt最窄为42 ℃, 液晶起始相变温度最高为80 ℃, 最低为42 ℃; 清亮点tc最高为181 ℃, 最低为110 ℃; 考察了烷氧基链长、配位金属离子及配合物分子空间结构对液晶性能的影响. 通过荧光光谱分析进一步验证了氯化卟啉合铁(Ⅲ)可以转化为μ-氧-双卟啉合铁(Ⅲ).     

The fabrication of novel optical diffusers based on UV-cured polymer dispersed liquid crystals

Polymer dispersed liquid crystals (PDLCs) with different sizes of the LC droplets are prepared based on the ultraviolet (UV) light curable acrylate monomers/LCs composites to fabricate the optical diffuser films. To acquire light diffusers with high optical performance, the effects of the monomer structure and the UV light intensity on the micro-structure of the PDLC films are studied. Results show that the PDLC films could exhibit a strong light scattering at the premise of maintaining high transmittance in the visible region. As the LC droplets are spherically dispersed in the polymer networks, when the size of LC droplets is about 3.0 μm, the haze can reach 88.5% and the transmittance is nearly 90.0%, which can be used as a bottom diffuser film. While when the size of LC droplets is about 10.0 μm, the haze and transmittance are 39.2% and 90.2%, respectively; hence, it can be a good choice for a top diffuser film. With the advantages of simple preparation, roll-to-roll industrial production and tunable optical properties, it is supported that the films based on UV-cured PDLC films can be applied as outstanding optical diffuser films in the liquid crystal display industry.     

Microfluidic sensing devices employing in situ-formed liquid crystal thin film for detection of biochemical interactions

Although biochemical sensing using liquid crystals (LC) has been demonstrated, relatively little attention has been paid towards the fabrication of in situ-formed LC sensing devices. Herein, we demonstrate a highly reproducible method to create uniform LC thin film on treated substrates, as needed, for LC sensing. We use shear forces generated by the laminar flow of aqueous liquid within a microfluidic channel to create LC thin films stabilized within microfabricated structures. The orientational response of the LC thin films to targeted analytes in aqueous phases was transduced and amplified by the optical birefringence of the LC thin films. The biochemical sensing capability of our sensing devices was demonstrated through experiments employing two chemical systems: dodecyl trimethylammonium bromide (DTAB) dissolved in an aqueous solution, and the hydrolysis of phospholipids by the enzyme phospholipase A(2) (PLA(2)).