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.     

Observation of surface and bulk phase transitions in nematic liquid crystals

The behaviour of liquid crystal (LC) molecules near a surface is of both fundamental and technological interest: it gives rise to various surface phase-transition and wetting phenomena, and surface-induced ordering of the LC molecules is integral to the operation of LC displays. Here we report the observation of a pure isotropic-nematic (IN) surface phase transition-clearly separated from the bulk IN transition-in a nematic LC on a substrate. Differences in phase behaviour between surface and bulk are expected, but have hitherto proved difficult to distinguish, owing in part to the close proximity of their transition temperatures. We have overcome these difficulties by using a mixture of nematic LCs: small, surface-induced composition variations lead to complete separation of the surface and bulk transitions, which we then study independently as a function of substrate and applied magnetic field. We find the surface IN transition to be of first order on surfaces with a weak anchoring energy and continuous on surfaces with a strong anchoring. We show that the presence of high magnetic fields does not change the surface IN transition temperature, whereas the bulk IN transition temperature increases with field. We attribute this to the interaction energy between the surface and bulk phases, which is tuned by magnetic-field-induced order in the surface-wetting layer.     

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.     

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.     

液晶纤维素及其衍生物材料的结构与性能

基于国内外最新研究文献，系统论述了液晶纤维素及其衍生物薄膜、纤维、功能分离膜、交联凝胶等材料的成型、结构与性能。指出这类液晶聚合物具有胆甾相或向列相液晶态。与各向同性态的同种材料相比，各向异性态的液晶纤维素及其衍生物材料具有更优异性能。双轴取向液晶三乙酸纤维素薄膜的拉伸强度和弹性模量分别可310MPa和6．3GPa；由液晶态干湿法纺丝所获得的高分子量纤维素纤维的抗张强度和弹性模量分别可高达2．2GPa和65GPa。液晶纤维素及其衍生物分离膜也表现出高得多的气体选择透过性能。其液晶交联凝胶中，凝胶所占质量分数高达98％，以及具有显著不同于各向同性态凝胶的液胀行为。液晶纤维素及其衍生物可望作为生产高级包装膜、功能纤维、气体分离膜、各向异性功能凝胶等的重要材料。     

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.     

Solitons and production of defects in flow-aligning nematic liquid crystals under simple shear flow

The production of defects in flow-aligning nematic liquid crystals under simple shear flow is analyzed by linear stability analysis based on Leslie-Ericksen theory. It is pointed out that the equation of motion of the nematic director under simple shear flow conforms to the driven over-damped sine-Gordon equation and has a soliton solution of amplitude π. It has also been shown that the stationary state with the director uniformly oriented at a Leslie angle is only a metastable state and that the potential, which governs the motion of the director, has infinite numbers of stable stationary states. Therefore, the defects, appearing as a stable solitary solution, can be nucleated from a uniformly aligned flow-aligning type of nematic liquid crystal by shear flow. On the other hand, the bands with long axis parallel to the vorticity axis, appearing as an unstable solution, can be observed as transient patterns at low shear rate and low shear strain value. The theoretical predictions are compared with previous experimental observations.     

Archimedean-like tiling on decagonal quasicrystalline surfaces

Monolayers on crystalline surfaces often form complex structures with physical and chemical properties that differ strongly from those of their bulk phases. Such hetero-epitactic overlayers are currently used in nanotechnology and understanding their growth mechanism is important for the development of new materials and devices. In comparison with crystals, quasicrystalline surfaces exhibit much larger structural and chemical complexity leading, for example, to unusual frictional, catalytical or optical properties. Deposition of thin films on such substrates can lead to structures that may have typical quasicrystalline properties. Recent experiments have indeed showed 5-fold symmetries in the diffraction pattern of metallic layers adsorbed on quasicrystals. Here we report a real-space investigation of the phase behaviour of a colloidal monolayer interacting with a quasicrystalline decagonal substrate created by interfering five laser beams. We find a pseudomorphic phase that shows both crystalline and quasicrystalline structural properties. It can be described by an archimedean-like tiling consisting of alternating rows of square and triangular tiles. The calculated diffraction pattern of this phase is in agreement with recent observations of copper adsorbed on icosahedral Al(70)Pd(21)Mn(9) surfaces. In addition to establishing a link between archimedean tilings and quasicrystals, our experiments allow us to investigate in real space how single-element monolayers can form commensurate structures on quasicrystalline surfaces.     

向列相液晶中相位光栅光束耦合特性

为了研究向列相液晶中相位光栅光束耦合特性,从理论上分析了相位光栅的光束耦合和衍射规律,采用掺杂C60垂直排列的向列相液晶(5CB),厚度为20μm的样品,进行二波耦合(TBC)实验,研究动态衍射和光束放大特性。结果表明:相位光栅的衍射强度分布呈现明显不对称。根据取向光折变效应表面电荷调制机制和电场作用下的Carr-Helfrich效应对此现象给出定性解释,认为非对称光强分布来自于样品中产生的非对称相位光栅。液晶中非正弦调制的空间电荷场产生了非对称相位光栅。     

甲亚胺液晶共聚醚的合成及研究（Ⅱ）

利用刚性较强的介晶基团与较柔软的非介晶基团共聚的方法合成了一系列具有较低相变温度的向列型液晶高分子─甲亚胺液晶共聚醚，并利用热台偏光显微镜、差示扫描量热计（ＤＳＣ），Ｘ─光衍射对其相变温度和液晶态进行了测定和表征，研究了非介晶基团对液晶共聚醚相变温度、液晶态的影响，以及液晶聚合物的相变温度与其共聚组成之间的关系。     

梳型高分子聚合物液晶的相变

利用Freiser关于液晶分子相互作用模型,建立了梳型高聚物分子的哈密顿量,适用变分程序导出高聚物液晶结合相序参数自洽方程,得到在给定分子结构参数时的各向同性相至单轴向列相以及单轴至双轴向列相的相变。结果表明,当分子主、侧链间隔基团的耦合项中排斥作用以及结合相中较强向列相场者强度增大时,单轴至双轴向列相相变温度升高,并从一级相变逐渐变为二级相变。     

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.     

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．     

取向向列型液晶的VV—VH Raman散射

把只适用于各向同性系统的振动和转动相关函数分离理论推广到各向异性的取向向列型液晶中,并用MBBA在液晶相进行VV-VH实验,分离出振动和转动相关函数,分别研究其振动和转动弛豫。     

添加微量Y、La对Zr基非晶合金耐蚀性的影响

通过计算机编程建立Zr2Cu晶体相中以Cu原子为中心的原子团簇模拟Zr基非晶中二十面体原子团簇模型,应用实空间的递推方法计算了Zr2Cu晶体相中的费米能级及团簇中Cu与近邻合金元素Y、La、Zr的键级积分.结论分析表明:Y、La易于占据Zr-Cu晶体相中Zr原子,使非晶形成能力上升,同时可提高耐腐蚀性,因此,通过加入少量的Y、La元素可制备出具有较高耐腐蚀性能的Zr基大块非晶材料.     

芳香-脂肪族共聚酰胺的液晶性能

采用低温溶液缩聚法，以对苯二胺、对苯二甲酰氯和己二酰氯为单体合成了一系列不同己二酰单元含量的芳－脂族共聚酞胺．并对其硫酸溶液的液晶性能进行了研究。结果表明，该类共聚酰胺在己二酰单元含量低于９０％（ｍｏｌ）情况下，其硫酸溶液仍保持溶致液 晶特性。经偏光显微镜、小角激光光散射和宽角Ｘ光衍射实验证明，这类芳－脂族共聚酰胺硫酸液晶溶液属向列型液晶，其临界浓度Ｃ＊随己二酰单元含量的增加而增加。     

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.     

2,3,5-三甲基氢醌、对苯二甲酸、2,6-萘二甲酸类聚芳酯液晶的研究

本文合成了一类以2,3,5-三甲基氢醌(TMHQ)、对苯二甲酸(TPA)、2,6-萘二甲酸(2,6-NDA)为基本共聚单体并加有少量聚对苯二甲酸乙二酯(PET)的芳香共聚酯;研究了共聚酯的组成对溶解度、热性质、液晶性质、流变性质及晶体结构的影响。实验表明,共聚酯在某个温度范围内呈现向列型液晶的特征,其熔体粘度的切变依赖特性也证实了这一点。DSC 热谱和广角 X-射线衍射图分析证明,对某些组成,在共聚酯的分子链中存在着相应于 TMHQ-TPA 和 TMHQ-2,6-NDA 两种短嵌段结构。这是与非均相缩聚条件有关的。     

Fluidity of water confined to subnanometre films.

The fluidity of water in confined geometries is relevant to processes ranging from tribology to protein folding, and its molecular mobility in pores and slits has been extensively studied using a variety of approaches. Studies in which liquid flow is measured directly suggest that the viscosity of aqueous electrolytes confined to films of thickness greater than about 2-3 nm remains close to that in the bulk; this behaviour is similar to that of non-associative organic liquids confined to films thicker than about 7-8 molecular layers. Here we observe that the effective viscosity of water remains within a factor of three of its bulk value, even when it is confined to films in the thickness range 3.5 +/- 1 to 0.0 +/- 0.4 nm. This contrasts markedly with the behaviour of organic solvents, whose viscosity diverges when confined to films thinner than about 5-8 molecular layers. We attribute this to the fundamentally different mechanisms of solidification in the two cases. For non-associative liquids, confinement promotes solidification by suppressing translational freedom of the molecules; however, in the case of water, confinement seems primarily to suppress the formation of the highly directional hydrogen-bonded networks associated with freezing.     

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·