Multiple liquid crystal phases of DNA at high concentrations

DNA packaging in vivo is very tight, with volume concentrations approaching 70% w/v in sperm heads, virus capsids and bacterial nucleoids. The packaging mechanisms adopted may be related to the natural tendency of semi-rigid polymers to form liquid crystalline phases in concentrated solutions. We find that DNA forms at least three distinct liquid crystalline phases at concentrations comparable to those in vivo, with phase transitions occurring over relatively narrow ranges of DNA concentration. A weakly birefringent, dynamic, 'precholesteric' mesophase with microscopic textures intermediate between those of a nematic and a true cholesteric phase forms at the lowest concentrations required for phase separation. At slightly higher DNA concentrations, a second mesophase forms which is a strongly birefringent, well-ordered cholesteric phase with a concentration-dependent pitch varying from 2 to 10 micron. At the highest DNA concentrations, a phase forms which is two-dimensionally ordered and resembles smectic phases of thermotropic liquid crystals observed with small molecules.     

Transparent nematic phase in a liquid-crystal-based microemulsion

Complex fluids are usually produced by mixing together several distinct components, the interactions between which can give rise to unusual optical and rheological properties of the system as a whole. For example, the properties of microemulsions (composed of water, oil and surfactants) are determined by the microscopic structural organization of the fluid that occurs owing to phase separation of the component elements. Here we investigate the effect of introducing an additional organizing factor into such a fluid system, by replacing the oil component of a conventional water-in-oil microemulsion with an intrinsically anisotropic fluid--a nematic liquid crystal. As with the conventional case, the fluid phase-separates into an emulsion of water microdroplets (stabilized by the surfactant as inverse micelles) dispersed in the 'oil' phase. But the properties are further influenced by a significant directional coupling between the liquid-crystal molecules and the surfactant tails that emerge (essentially radially) from the micelles. The result is a modified bulk-liquid crystal that is an ordered nematic at the mesoscopic level, but which does not exhibit the strong light scattering generally associated with bulk nematic order: the bulk material here is essentially isotropic and thus transparent.     

Liquid crystal 'blue phases' with a wide temperature range

Liquid crystal 'blue phases' are highly fluid self-assembled three-dimensional cubic defect structures that exist over narrow temperature ranges in highly chiral liquid crystals. The characteristic period of these defects is of the order of the wavelength of visible light, and they give rise to vivid specular reflections that are controllable with external fields. Blue phases may be considered as examples of tuneable photonic crystals with many potential applications. The disadvantage of these materials, as predicted theoretically and proved experimentally, is that they have limited thermal stability: they exist over a small temperature range (0.5-2 degrees C) between isotropic and chiral nematic (N*) thermotropic phases, which limits their practical applicability. Here we report a generic family of liquid crystals that demonstrate an unusually broad body-centred cubic phase (BP I*) from 60 degrees C down to 16 degrees C. We prove this with optical texture analysis, selective reflection spectroscopy, K?ssel diagrams and differential scanning calorimetry, and show, using a simple polarizer-free electro-optic cell, that the reflected colour is switched reversibly in applied electric fields over a wide colour range in typically 10 ms. We propose that the unusual behaviour of these blue phase materials is due to their dimeric molecular structure and their very high flexoelectric coefficients. This in turn sets out new theoretical challenges and potentially opens up new photonic applications.     

Liquid-crystal-mediated self-assembly at nanodroplet interfaces

Technological applications of liquid crystals have generally relied on control of molecular orientation at a surface or an interface. Such control has been achieved through topography, chemistry and the adsorption of monolayers or surfactants. The role of the substrate or interface has been to impart order over visible length scales and to confine the liquid crystal in a device. Here, we report results from a computational study of a liquid-crystal-based system in which the opposite is true: the liquid crystal is used to impart order on the interfacial arrangement of a surfactant. Recent experiments on macroscopic interfaces have hinted that an interfacial coupling between bulk liquid crystal and surfactant can lead to a two-dimensional phase separation of the surfactant at the interface, but have not had the resolution to measure the structure of the resulting phases. To enhance that coupling, we consider the limit of nanodroplets, the interfaces of which are decorated with surfactant molecules that promote local perpendicular orientation of mesogens within the droplet. In the absence of surfactant, mesogens at the interface are all parallel to that interface. As the droplet is cooled, the mesogens undergo a transition from a disordered (isotropic) to an ordered (nematic or smectic) liquid-crystal phase. As this happens, mesogens within the droplet cause a transition of the surfactant at the interface, which forms new ordered nanophases with morphologies dependent on surfactant concentration. Such nanophases are reminiscent of those encountered in block copolymers, and include circular, striped and worm-like patterns.     

含联苯基团热致液晶聚脲的合成与表征

为合成新型热致液晶聚脲,以含液晶基元的联苯二胺（或联邻甲苯胺）、1,6-己二胺、2,4-甲苯二异氰酸酯为原料进行合成。通过改变单体加料顺序发现,仅当先加入己二胺与2,4-甲苯二异氰酸酯预聚,再加入芳香二胺聚合,所得的聚合物才有液晶性。采用这种加料顺序,通过改变芳香,脂肪两种二胺的配比,合成了两个系列的热致液晶聚脲,并确定了合成热致液晶聚脲的最佳配比范围。所得聚合物用偏光显微镜,示差扫描量热分析,广角、小角X射线衍射等方法进行表征,确定了其液晶织构属于向列型。     

Factors determining crystal--liquid coexistence under shear

The interaction between an imposed shear flow and an order--disorder transition underlies a broad range of phenomena. Under the influence of shear flow, a variety of soft matter is observed to spontaneously form bands characterized by different local order---for example, thermotropic liquid crystals subjected to shear flow exhibit rich phase behaviour. The stability of order under the influence of shear flow is also fundamental to understanding frictional wear and lubrication. Although there exists a well developed theoretical approach to the influence of shear flow on continuous transitions in fluid mixtures, little is known about the underlying principles governing non-equilibrium coexistence between phases of different symmetry. Here we show, using non-equilibrium molecular dynamics simulations of a system of spherical particles, that a stationary coexistence exists between a strained crystal and the shearing liquid, and that this coexistence cannot be accounted for by invoking a non-equilibrium analogue of the chemical potential. Instead of such thermodynamic arguments, we argue that a balancing of the crystal growth rate with the rate of surface erosion by the shearing melt can account for the observed coexistence.     

C10Zn,C12Zn及二元体系非等温固-固相变动力学

合成了具有层状钙钛矿结构的两种热致相变材料四氯合锌酸癸铵n-(C10-H21NH3)2ZnCl4和四氯合锌酸十二铵n-(C12H25NH3)2ZnC14,并在两种材料的乙醇溶液中结晶出一系列二元混合体系.对纯组分及各个二元体系利用差示扫描量热(DSC)测定了热分析曲线,采用Kissinger和Ozawa两种动力学模型研究了非等温固-固相变动力学,计算了固-固相变过程的活化能和反应级数.两种方法的计算结果相一致.     

全芳香族共聚酯类热致液晶的制备与性能

制备了两个系列的全芳香族共聚酯类热致液晶。对所得聚合物作了红外，偏光显微镜及差热分析，并测定了各聚合物的熔融指数。结果显示，所制备的聚合物为热致向列型液晶聚合物。     

Liquid crystal phase transitions in suspensions of polydisperse plate-like particles

Colloidal suspensions that form periodic self-assembling structures on sub-micrometre scales are of potential technological interest; for example, three-dimensional arrangements of spheres in colloidal crystals might serve as photonic materials, intended to manipulate light. Colloidal particles with non-spherical shapes (such as rods and plates) are of particular interest because of their ability to form liquid crystals. Nematic liquid crystals possess orientational order; smectic and columnar liquid crystals additionally exhibit positional order (in one or two dimensions respectively). However, such positional ordering may be inhibited in polydisperse colloidal suspensions. Here we describe a suspension of plate-like colloids that shows isotropic, nematic and columnar phases on increasing the particle concentration. We find that the columnar two-dimensional crystal persists for a polydispersity of up to 25%, with a cross-over to smectic-like ordering at very high particle concentrations. Our results imply that liquid crystalline order in synthetic mesoscopic materials may be easier to achieve than previously thought.     

Stacking of conical molecules with a fullerene apex into polar columns in crystals and liquid crystals

Polar liquid crystalline materials can be used in optical and electronic applications, and recent interest has turned to formation strategies that exploit the shape of polar molecules and their interactions to direct molecular alignment. For example, banana-shaped molecules align their molecular bent within smectic layers, whereas conical molecules should form polar columnar assemblies. However, the flatness of the conical molecules used until now and their ability to flip have limited the success of this approach to making polar liquid crystalline materials. Here we show that the attachment of five aromatic groups to one pentagon of a C(60) fullerene molecule yields deeply conical molecules that stack into polar columnar assemblies. The stacking is driven by attractive interactions between the spherical fullerene moiety and the hollow cone formed by the five aromatic side groups of a neighbouring molecule in the same column. This packing pattern is maintained when we extend the aromatic groups by attaching flexible aliphatic chains, which yields compounds with thermotropic and lyotropic liquid crystalline properties. In contrast, the previously reported fullerene-containing liquid crystals all exhibit thermotropic properties only, and none of them contains the fullerene moiety as a functional part of its mesogen units. Our design strategy should be applicable to other molecules and yield a range of new polar liquid crystalline materials.     

芳-脂族共聚酯酰胺热致液晶的合成和表征

利用芳－脂复合二酰氯和四种芳族二元胺合成了３个系列含有两种芳香二胺结 构单元的无规芳－脂共聚酯酰胺，其对数比浓粘度为 １．３０～２．１８ ｄＬ／ｇ。用偏光显微镜、 Ｘ光衍射和ＤＳＣ对聚合物的液晶行为进行了研究。     

Colloidal ordering from phase separation in a liquid-crystalline continuous phase

Some binary mixtures exist as a single phase at high temperatures and as two phases at lower temperatures; rapid cooling therefore induces phase separation that proceeds through the initial formation of small particles and subsequent growth and coarsening. In solid and liquid media, this process leads to growing particles with a range of sizes, which eventually separate to form a macroscopically distinct phase. Such behaviour is of particular interest in systems composed of an isotropic fluid and a liquid crystal, where the random distribution of liquid-crystal droplets in an isotropic polymer matrix may give rise to interesting electro-optical properties. Here we report that a binary mixture consisting of an isotropic fluid and a liquid crystal forming the continuous phase does not fully separate into two phases, but self-organizes into highly ordered arrays of monodisperse colloidal droplet chains. We find that the size and spatial organization of the droplets are controlled by the orientational elasticity of the liquid-crystal phase and the defects caused by droplets exceeding a critical size. We expect that our approach to forming monodisperse, spatially ordered droplets in liquid crystals will allow the controlled design of ordered composites that may have useful rheological and optical properties.     

多原子分子材料的激光感生热致折射率光栅

从理论上研究了在多原子分子材料中,由激光感生的热致折射率光栅及其驰豫效应。并利用简并四波混频实验,观察了多原子分子液晶的热致折射率光栅和它的驰豫行为,讨论了有关的物理机制     

Photoselective adaptive femtosecond quantum control in the liquid phase.

Coherent light sources can be used to manipulate the outcome of light-matter interactions by exploiting interference phenomena in the time and frequency domain. A powerful tool in this emerging field of 'quantum control' is the adaptive shaping of femtosecond laser pulses, resulting, for instance, in selective molecular excitation. The basis of this method is that the quantum system under investigation itself guides an automated search, via iteration loops, for coherent light fields best suited for achieving a control task designed by the experimenter. The method is therefore ideal for the control of complex experiments. To date, all demonstrations of this technique on molecular systems have focused on controlling the outcome of photo-induced reactions in identical molecules, and little attention has been paid to selectively controlling mixtures of different molecules. Here we report simultaneous but selective multi-photon excitation of two distinct electronically and structurally complex dye molecules in solution. Despite the failure of single parameter variations (wavelength, intensity, or linear chirp control), adaptive femtosecond pulse shaping can reveal complex laser fields to achieve chemically selective molecular excitation. Furthermore, our results prove that phase coherences of the solute molecule persist for more than 100 fs in the solvent environment.     

Observation of shear-induced nematic-isotropic transition in side-chain liquid crystal polymers

Flow-induced phase transitions are a fundamental (but poorly understood) property of non-equilibrium systems, and are also of practical importance for tuning the processing conditions for plastics, petroleum products, and other related materials. Recognition that polymers may exhibit liquid crystal properties has led to the discovery of the first tailored side-chain liquid crystal polymers (SCLCPs), which are formed by inserting a spacer between the main polymer chain and the lateral mesogen liquid-crystalline graftings. Subsequent research has sought to understand the nature of the coupling between the main polymer chain and the mesogens, with a view to obtaining better control of the properties of these tailored structures. We show here that the parallel or perpendicular orientation of the mesogens with respect to the main chain can be reversed by the application of an external field produced by a shear flow, demonstrating the existence of an isotropic nematic phase transition in SCLCPs. Such a transition, which was theoretically predicted for low-molecular-weight liquid crystals but never observed, is shown to be a general property of SCLCPs too. We expect that these SCLCPs will prove to be good candidate systems for the experimental study of these non-equilibrium phenomena.     

外电场作用下向列相液晶的光学双稳态

将Ｈｅ－Ｎｅ激光和外电场同时作用在Ｆａｂｒｙ－Ｐｅｒｏｔ腔内的向列相液晶（８ＣＢ，５ＣＢ）上，实现光学双稳态。获得双稳的最大输出光强，开启光强与外电场强度关系，同时首次获得双稳特性与电场频率的关系。     

向列相液晶由分子相互作用引起的相变

考虑一双轴性液晶分子简单相互作用模型，利用平均场理论，得到了从各向同性相至单轴向列相以及从单轴向列相至双轴向列相的相变．     

向列相液晶中热致相位光栅的研究

本文研究了向列相液晶中热致相位光栅产生的简并四波混频过程.理论上得到的半定量公式表明,混频信号上升过程由快分量和慢分量组成.向列相MBBA的实验结果与理论分析相符.     

芳香酯类三臂液晶化合物的合成与表征

以间苯三酚为中心,ω [4 (对烷氧基苯甲酰氧基)苯氧甲酰基]戊酸为液晶基元,通过酯化反应合成了芳香酯类三臂液晶化合物·这是一类不同于传统棒状及盘状液晶分子的新型结构液晶化合物·通过红外光谱(IR)、核磁共振谱(1HNMR)对其分子结构进行表征·采用差示扫描量热(DSC)、偏光显微分析(POM)、热重分析(TG)等手段对其液晶性能及热稳定性进行研究·结果表明,此类三臂液晶化合物符合分子设计,热稳定性好,5%热失重温度高于340℃,液晶区间随着分子中末端烷基链的加长而增宽,升、降温过程中均呈现丝状织构,属互变向列相液晶·     

Reconfigurable self-assembly through chiral control of interfacial tension

From determining the optical properties of simple molecular crystals to establishing the preferred handedness in highly complex vertebrates, molecular chirality profoundly influences the structural, mechanical and optical properties of both synthetic and biological matter on macroscopic length scales. In soft materials such as amphiphilic lipids and liquid crystals, the competition between local chiral interactions and global constraints imposed by the geometry of the self-assembled structures leads to frustration and the assembly of unique materials. An example of particular interest is smectic liquid crystals, where the two-dimensional layered geometry cannot support twist and chirality is consequently expelled to the edges in a manner analogous to the expulsion of a magnetic field from superconductors. Here we demonstrate a consequence of this geometric frustration that leads to a new design principle for the assembly of chiral molecules. Using a model system of colloidal membranes, we show that molecular chirality can control the interfacial tension, an important property of multi-component mixtures. This suggests an analogy between chiral twist, which is expelled to the edges of two-dimensional membranes, and amphiphilic surfactants, which are expelled to oil-water interfaces. As with surfactants, chiral control of interfacial tension drives the formation of many polymorphic assemblages such as twisted ribbons with linear and circular topologies, starfish membranes, and double and triple helices. Tuning molecular chirality in situ allows dynamical control of line tension, which powers polymorphic transitions between various chiral structures. These findings outline a general strategy for the assembly of reconfigurable chiral materials that can easily be moved, stretched, attached to one another and transformed between multiple conformational states, thus allowing precise assembly and nanosculpting of highly dynamical and designable materials with complex topologies.