Electro‐optic effects and phase behavior of liquid‐crystalline physical gels: self‐assembly of hydrogen‐bonded molecules for the formation of dynamically functional composites

Anisotropic physical gels of liquid crystals are obtained by the formation of non‐covalently‐bonded networks through self‐organization of low molecular weight compounds in nematic solvents. They exhibit thermoreversible transitions between isotropic liquid and isotropic gel, and between isotropic gel and liquid‐crystalline gel, whose temperatures are dependent on the components. Electro‐optic properties of liquid‐crystalline gels are examined with twisted nematic cells. A nematic liquid crystal in a gel structure can respond to electric fields twice faster than a single liquid‐crystalline component.     

Anisotropic gels and plasticized networks formed by liquid crystal molecules

Two component mixtures containing liquid crystalline diacrylates and liquid crystal molecules with no reactive groups have been made. Upon photopolymeriz-ation of the mixtures anisotropic networks containing liquid crystal molecules which were not chemically attached to the networks were created. In these systems the behaviour of the liquid crystal molecules was found to be dominated by the network molecules. Even when the liquid crystal molecules in these networks were heated above their isotropic transition temperature they remained partly oriented, thus contributing to the anisotropy of the system.     

Ferroelectric liquid crystalline elastomers: from the analysis of the molecular dynamics to the design of nanomachines

In ferroelectric liquid crystalline elastomers (FLCE) the properties of ferroelectric liquid crystals are combined with that of polymeric networks. With use of refined synthetic techniques, it is possible to align the chiral mesogenic groups in FLCE on a macroscopic scale (∼cm²). By employing that set of methods, the development of a new class of materials has been achieved having an extraordinary profile of features, among them the following: (1) FLCE are piezoelectric because of the ferroelectric order of the chiral mesogens and because of the fact that the polymeric network prevents flow. (2) Due to a photochemically or thermally induced cross‐linking reaction the viscoelastic properties of FLCE can be adjusted over a wide range. (3) The coupling between the elastomeric network and the (polar) chiral mesogens can be modified by chemical engineering, hence offering the possibility to tailor the piezoelectric as well as the viscoelastic properties of FLCE. (4) It is possible to prepare thick (∼ 100 μm), thin (∼ μm), and ultrathin (∼ nm) films of macroscopically oriented FLCE, as self‐supporting or as transferred samples. With this combination of properties FLCE have made a strong technological impact as basic materials for sensors and soft actuators in the area of microsystems technology. The present contribution is focused on the piezoelectric effect in FLCE and its molecular interpretation.     

Optical Properties of Sol-Gel Derived Vanadium Oxide Films

Vanadium oxide gels can be made from vanadate aqueous solutions or from vanadium alkoxides. The condensation of vanadic acid gives long ribbon-like oxide particles which macroscopically orient in the same direction in aqueous sols when their concentration is larger than 0.12 mol·l⁻¹. These anisotropic sols and gels should be considered as lyotropic nematic liquid crystals. Thick films in which ribbons align along the same direction can be deposited. These oriented coatings exhibit improved electrochemical properties as reversible cathodes for lithium batteries. Amorphous oxo-polymers are formed via the controlled hydrolysis of vanadium alkoxides. They allow the deposition of optically transparent thin films that exhibit interesting electrochromic properties and turn reversibly from yellow to green upon electrochemical reduction. Moreover these alkoxide derived films can be easily reduced into vanadium dioxide. These VO₂ thin films exhibit thermochromic properties and could be used as optical switches in the infrared. The transition temperature of these VO₂ films can be modified by doping the vanadium oxide with other cations.     

Smectic liquid single crystal elastomers

In the liquid crystalline (I.c.) state the backbone of l.c-polymers has to adapt an anisotropic conformation that is consistent with the I.c.-phase structure. For macroscopically uniformly aligned samples this anisotropic backbone conformation can be obtained by applying a suitable mechanical field on chemically crosslinked I.c-polymers. Introducing the network anisotropy a priori by synthesis, stable macroscopically aligned I.c.-networks can be realized. We have called these networks Liquid Single Crystal Elastomers (LSCE) because their physical properties resemble that of organic or inorganic single crystals.     

Liquid single crystal elastomers (lsce) - mechanical optical and electric properties

Liquid crystal elastomers can be macroscopically ordered with respect to the director by applying a mechanical field similar to electric/magnetic field effects of low molar mass liquid crystals. Introducing network anisotropy a priori by the synthesis, uniformly aligned nematic or smectic elastomers are available without external mechanical field. These LSCE's combine optical properties of single crystals with entropy elasticity of elastomers. Due to uniform director orientation without defects, the LSCE's exhibit excellent transparency which makes them applicable for optical elements. Non-linear optical properties can be easily optimized by attaching suitable I.c.-moieties to the polymer network. On the other hand, due to the rubber elasticity of the LSCE's, electromechanical effects can be observed, e. g. piezoelectricity of chiral smectic C-LSCE's.     

Increased alignment of electronic polymers in liquid crystals via hydrogen bonding extension

Crucial for the development of enhanced electrooptic materials is the construction of highly anisotropic materials. Nematic liquid crystals are able to control the chain conformation and alignment of poly(phenylene ethynylene)s (PPEs), producing electronic polymers with chain-extended planar conformations for improved transport properties. Here, we show that the dichroic ratio, and hence polymer alignment, increases dramatically when interpolymer interactions are introduced by end capping the PPE with hydrogen bonding groups. This increased order can be readily turned off by the introduction of a competing monofunctionalized hydrogen bonding compound. The formation of hydrogen bonds between the polymers results in the formation of gels and elastomers which may be of interest for future applications.     

柔性高分子/小分子液晶混合物的自洽场理论

发展了柔性高分子/小分子液晶混合物连续自洽场理论，将小分子液晶模型化 为取向与位置无关的单体分子，小分子液晶间存在各向异性的Maier-Saupe相互作 用，该理论可还原成高分子和各向同性小分子组成的Flory-Huggins溶液理论和纯 液晶的Maier-Squpe液晶理论，通过数值解自洽场方程组，还将理论用于研究柔性 高分子/小分子液晶混合物相分离开界面性质，得到的结果与用Helfand格子界面理 论和MOnte Carlo模拟的结果一致。     

Vinyl‐terminated liquid‐crystalline dendrimers based on dendritic polyols and their siloxane‐based elastomers

A new series of side‐chain liquid‐crystalline dendrimers (LCDs) by grafting vinyl‐terminated phenyl benzoate‐based promesogens to a novel polypropyleneimine‐derived dendritic polyols are reported. Polarized optical microscopy and X‐ray diffraction studies show that both the compounds display a smectic‐A (SmA) mesophase. The second‐generation dendrimer bearing eight‐branched promesogens exhibits a more stable SmA mesophase with a wide mesomorphic temperature range. It is demonstrated that “promoting groups” in the structure of LCD for the enhancement of mesomorphic stability are unnecessary in the case of strong anisotropic interactions. In contrast to conventional LCDs, these two compounds possess reactive vinyl terminals that endow them with the potential for the preparation of polymeric materials. For the first time, a type of thermoset elastomers is explored from LCDs via hydrosilylation crosslinking reaction of vinyl terminals and siloxane crosslinker. Two‐dimensional X‐ray diffraction study indicates that the lamellar structures of original dendrimers are reserved in the elastomer networks. Stress–strain curves reveal that these elastomers exhibit excellent elasticity under successive uniaxial compression. The combination of anisotropic structures of rigid units and elasticity of flexible networks in this novel series of elastomers makes them promising candidates for the application in artificial muscles or cartilages. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Anisotropic ionogels of sodium laurate in a room-temperature ionic liquid

Anisotropic thermally reversible ionogels of sodium laurate (SL) were prepared in the first discovered room-temperature ionic liquid (RTIL), ethylammonium nitrate (EAN). Polarized optical microscope images indicate that the gels are birefringent, illuminating the presence of anisotropic structures. Small-angle X-ray scattering results reveal that SL and lauric acid (LA) molecules are arranged to form lamellar structures, but no SL crystallites were confirmed by the X-ray diffraction measurements. With an increase of the SL concentration, the interlayer distance decreases. Rheological measurements indicate that the anisotropic ionogels are highly viscoelastic and the storage modulus (G') increases with an increase of the SL concentration in EAN. Electrochemical measurements indicate that the anisotropic ionogels may have potential applications in electrochemical fields. The intermolecular hydrogen bond as well as the solvatophobic interaction of SL and LA formed by a chemical reaction, CH(3)(CH(2))(10)COONa + CH(3)CH(2)NH(3)NO(3) --> CH(3)CH(2)NH(2) upward arrow + NaNO(3) downward arrow + CH(3)(CH(2))(10)COOH, can play a role in the formation of three-dimensional networks having lamellar structures which are responsible for the anisotropic ionogels. The formation of anisotropic ionogels by surfactants in RTILs could be a new phenomenon, but this is not a very classic case of organogels.     

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.     

光致变形型高分子材料

光致变形型高分子材料以光为激发源,在没有机械接触的情况下,能够快速改变尺寸和形状。本文介绍了光致变形的高分子凝胶、无定形高分子、液晶弹性体和光致形状记忆高分子材料,并对各种材料的光致变形机理进行解释。无定形高分子的光致变形较小,目前研究重点是具有各向异性的液晶弹性体。文中着重介绍了具有偶氮苯介晶基元的液晶弹性体的光致变形研究,在光照下这类材料只要有1%的偶氮苯介晶基元发生顺反异构,就会发生光致变形。     

Photoinduced alignment of polymerisable liquid crystals on photoreactive polymers containing 2,6-bis(benzylidene)-1-cyclohexanone units in the main chain

Photoreactive polymers containing 2,6-bis(benzylidene)-1-cyclohexanone (bisBC) units were synthesised and investigated as a photoalignment layer for polymerisable liquid crystals (PLCs) and liquid crystalline polymers (LCPs). The liquid crystalline materials were aligned homogeneously on the photoalignment layers in a wide range of irradiation dose of linearly polarised UV light (LPUVL). Specifically, for the photoalignment layer baked at 80°C, order parameters of the liquid crystalline materials were low due to the disturbance of oriented-photoreactive polymer caused by the contact with the solvent of liquid crystalline materials. However, the liquid crystalline materials were aligned homogeneously even at low irradiation doses on the thermally cured photoalignment layer baked at 180°C. In addition, the liquid crystalline materials were aligned perpendicular to the LPUVL electric field. The alignment mechanism is discussed by comparing the retardation of photoalignment layer with anisotropic polarisabilities of model molecules calculated by density functional theory (DFT). It is suggested that the liquid crystalline materials aligned along the unreacted chromophores in the photoreactive polymer.     

Self‐Assembly of Giant Spherical Liquid‐Crystalline Complexes and Formation of Nanostructured Dynamic Gels that Exhibit Self‐Healing Properties

Supramolecular self‐assembly of 24 forklike mesogenic ligands and 12 transition metal ions led to the formation of giant spherical coordination complexes that exhibit liquid‐crystalline (LC) phases. Self‐healing LC supramolecular gels were also obtained through the introduction of these LC nanostructured supramolecular giant spherical complexes into dynamic covalent networks formed by cross‐linkers and bifunctional polymers. The giant spherical structures of the Pd^II complexes with 72 rodlike moieties on the periphery were characterized by NMR, diffusion‐ordered NMR spectroscopy, and mass spectrometry. These complexes are stable and exhibit lyotropic LC behavior, while the mesogenic ligands show thermotropic LC properties. The self‐assembled LC structures of the spherical complexes can be tuned by the length of the rodlike moieties.     

Vanadium Pentoxide Sol and Gel Mesophases

Colloidal suspensions of V₂O₅ ribbon-like particles display optical textures typical of lyotropic nematic phases. Tactoids (small nematic droplets) and then isotropic phases are formed as these systems are diluted. Nematic suspensions can be oriented by applying a magnetic or an electric field. Such a liquid crystal behavior is mainly due to the highly anisotropic shape of vanadium oxide colloidal particles. Acid dissociation at the oxide/water interface gives rise to surface electrical charges and electrostatic repulsion should also be responsible for the stabilization of the nematic phases. Anisotropic xerogel layers are formed when these gels are deposited and dried onto flat substrates. X-ray diffraction patterns of such coatings exhibit a series of 00l harmonics due to the turbostratic stacking of the oxide particles. Dehydration is reversible and fluid mesophases are again obtained via a swelling process when water is added to the xerogel.     

Synthesis and properties of phenyl benzoate-based and biphenyl-based liquid crystalline thiol-ene monomers

Novel phenyl benzoate-based and biphenyl-based liquid crystalline thiol-ene monomers were synthesized and their properties investigated. By varying the bridging unit and spacer length, the type of mesophase can be tuned from the low ordered nematic and smectic A phase in the case of the phenyl benzoate-based monomers, to the highly ordered crystal E phase for the biphenyl-based monomers and their corresponding bromo precursors. We investigated the degree of order of the phenyl benzoate-based materials using the Haller method. Possible premature polymerization of these monomers was examined by size exclusion chromatography. The materials exhibit low transition temperatures and a high stability at typical handling times and temperatures. Consequently, these monomers are useful for in situ polymerization with anisotropic inert solvents, which could potentially lead to new architectures and enhanced electro-optical properties of devices. The use of the biphenyl-based monomers appears to be of limited use for polymerizations in anisotropic solution. However, as a result of their intrinsic high degree of molecular order, these monomers form a particularly interesting class of reactive materials that can be bulk polymerized to give main chain polymers with highly defined mechanical and optical or electro-optical properties.     

新型液晶聚合物的分子设计及功能

介绍了新型液晶聚合物的分子设计方法；综述了利用分子间氢键，电荷转移相互作用和防子间相互作用设计得到的新型液晶聚合物复合体系的性质和功能，概述了液晶聚合物ＬＢ膜和液晶聚合物弹性体的分子排布特征和功能，这些研究开拓了液晶聚合物研究的新领域，为液晶聚合物分子排布的控制和功能性研究提供了新方法。     

Application of electric fields in NMR

Anisotropic orientation states in materials such as liquid crystals, liquid crystalline polymers or polymeric electrets can be induced by the application of electric fields. This behaviour is exploited extensively in displays or sensors. Anisotropic optical properties induced by relatively high electric field strengths are used in the well known displays made from liquid crystalline materials. The anisotropy of electric properties in polymeric materials gives rise to properties such as the high piezoelectricity of fluorine containing electrets. Attempts have been made to investigate the orientational state induced by electric fields after poling using NMR techniques. However, there is only access to the irreversible part of the electrically induced orientation. In-situ application of (high) voltages during NMR experiments enables the investigation of the influence of the electric field on molecular orientational states or electrically-induced morphological changes. The NMR methods have been modified to investigate the dynamics of reorientation processes in electric fields and to extend the possible time range towards below1ms. The application of high voltages in NMR experiments requires experimental specifications. Some of our experiences are outlined in this paper.     

Effect of ionic aggregates of sulphonate groups on the liquid crystalline behaviours of liquid crystalline elastomers

A series of siloxane-based liquid crystalline elastomers containing biphenyl benzoate mesogenic units and ionic Brilliant Yellow moieties was synthesized. The chemical structures and liquid crystalline properties of the samples were characterized by FTIR, ¹H NMR, DSC, POM and XRD. The effective crosslink density of the ionic elastomers was determined by swelling experiments in organic/buffer mixtures. All the polymers displayed a smectic mesophase. It was shown that the glass transition and melting point temperatures of the polymers increased slightly with increasing content of ionic and mesogenic groups in the polymers, while the liquid crystal mesophase region decrease slightly.     

Coarse grained models for flexible liquid crystals: parameterization of the bond fluctuation model

We extend the bond fluctuation model, originally devised to investigate polymer systems, to contain anisotropic interactions suitable for the simulation of large flexible molecules such as liquid crystalline polymers and dendrimers. This extended model coarse grains the interaction between the flexible chains at a similar level of detail to the mesogenic units. Suitable interaction parameters are obtained by performing trial simulations on a low molar mass liquid crystalline system. The phase diagram of this system is determined as a function of the molecular stiffness. The nematic to isotropic transition temperature is found to increase with increasing stiffness.