Showing papers on "Liquid crystal published in 2009"

39.1: Invited Paper: Optically Isotropic Nano‐Structured Liquid Crystal Composites for Display Applications

Abstract: In this paper, we present the phase behavior and electro-optical Kerr effect of the optically isotropic liquid crystal composites, which require no surface treatment for device fabrication. Anomalously large Kerr constant, more than 10−8 mV−2 and fast response, less than sub-milli-second were observed at a room temperature.

Liquid crystalline ordering and charge transport in semiconducting materials.

Abstract: Organic semiconducting materials offer the advantage of solution processability into flexible films. In most cases, their drawback is based on their low charge carrier mobility, which is directly related to the packing of the molecules both on local (amorphous versus crystalline) and on macroscopic (grain boundaries) length scales. Liquid crystalline ordering offers the possibility of circumventing this problem. An advanced concept comprises: i) the application of materials with different liquid crystalline phases, ii) the orientation of a low viscosity high temperature phase, and, iii) the transfer of the macroscopic orientation during cooling to a highly ordered (at best, crystalline-like) phase at room temperature. At the same time, the desired orientation for the application (OLED or field-effect transistor) can be obtained. This review presents the use of molecules with discotic, calamitic and sanidic phases and discusses the sensitivity of the phases with regard to defects depending on the dimensionality of the ordered structure (columns: 1D, smectic layers and sanidic phases: 2D). It presents ways to systematically improve charge carrier mobility by proper variation of the electronic and steric (packing) structure of the constituting molecules and to reach charge carrier mobilities that are close to and comparable to amorphous silicon, with values of 0.1 to 0. 7 cm 2 . V -1 . s -1 . In this context, the significance of crosslinking to stabilize the orientation and liquid crystalline behavior of inorganic/organic hybrids is also discussed.

Electrically tunable liquid crystal optical microresonators

Abstract: Whispering-gallery-mode resonators made of nematic liquid-crystal droplets offer a wavelength tunability approximately two orders of magnitude larger than that of conventional solid-state microresonators.

Electro-optics of polymer-stabilized blue phase liquid crystal displays

Abstract: Electro-optics of polymer-stabilized blue phase liquid crystal displays (BP LCDs) is analyzed and validated experimentally. A numerical model for characterizing and optimizing the electro-optical and display properties of BP LCDs in in-plane switching and fringe field switching cells is developed. The simulated voltage-dependent transmittance curves agree well with the measured results. To lower the operating voltage while keeping a high transmittance, both electrode width and gap, and large Kerr constant make important contributions. A wide-view BP LCD using a single biaxial compensation film is simulated.

Helical Polyacetylene: Asymmetric Polymerization in a Chiral Liquid-Crystal Field

Abstract: Because the synthesis of polyacetylene (PA) thin film and the discovery of chemical doping disclosed the uncultivated field of conductive polymers,1-3 conductive polymers have been extensively investigated and widely used in such products as electrolytic capacitors and secondary batteries.4-6 Today, polymers have become essential for lightweight, highperformance batteries used in notebook computers, cellular phones, and other portable equipment. Much research and development has also been conducted on polymer lightemitting diodes, organic solar cells that are anticipated for * E-mail: akagi@star.polym.kyoto-u.ac.jp. Chem. Rev. 2009, 109, 5354–5401 5354

Shape-Controlled Colloidal Interactions in Nematic Liquid Crystals

Abstract: Robust control over the positions, orientations, and assembly of nonspherical colloids may aid in the creation of new types of structured composite materials that are important from both technological and fundamental standpoints. With the use of lithographically fabricated equilateral polygonal platelets, we demonstrate that colloidal interactions and self-assembly in anisotropic nematic fluids can be effectively tailored via control over the particles' shapes. The particles disturb the uniform alignment of the surrounding nematic host, resulting in both a distinct equilibrium alignment and highly directional pair interactions. Interparticle forces between polygonal platelets exhibit either dipolar or quadrupolar symmetries, depending on whether their number of sides is odd or even, and drive the assembly of a number of ensuing self-assembled colloidal structures.

Liquid crystal oligomers: going beyond dimers

Abstract: This review focuses on structure-property relationships in liquid crystal oligomers, which consist of molecules containing two or more mesogenic units linked via flexible spacers essentially in a linear fashion and so does not consider, for example, liquid crystal dendrimers and tetrapodes. Previous reviews have tended to focus mainly on liquid crystal dimers in which just two mesogenic units are interconnected by a single spacer. By contrast, this review is largely devoted to higher oligomers such as liquid crystal trimers and tetramers containing three or four mesogenic units connected by two or three spacers, respectively.

Chiral isotropic liquids from achiral molecules.

Abstract: A variety of simple bent-core molecules exhibit smectic liquid crystal phases of planar fluid layers that are spontaneously both polar and chiral in the absence of crystalline order. We found that because of intralayer structural mismatch, such layers are also only marginally stable against spontaneous saddle splay deformation, which is incompatible with long-range order. This results in macroscopically isotropic fluids that possess only short-range orientational and positional order, in which the only macroscopically broken symmetry is chirality--even though the phases are formed from achiral molecules. Their conglomerate domains exhibit optical rotatory powers comparable to the highest ever found for isotropic fluids of chiral molecules.

Landau–de Gennes modelling of nematic liquid crystal colloids

Abstract: Phenomenological Landau–de Gennes modelling based on the free energy of nematic liquid crystal colloids is reviewed. Nematic phase, gradient of order, and surface anchoring contributions to the total free energy are used. The numerical finite difference relaxation technique is explained as an efficient tool for the minimisation of the free energy. Effects of the mesh and mesh allocation are discussed. Various conceptually different colloidal structures are calculated to show the universality of the model. Single particles, dipolar–quadrupolar dimers, entangled dimers, dimers bound by escaped hyperbolic rings, and hierarchically patterned Saturn-ring colloidal superstructures are presented.

Phase separation behavior in aqueous suspensions of bacterial cellulose nanocrystals prepared by sulfuric acid treatment.

Abstract: Phase separation phenomena of aqueous suspensions of cellulose nanocrystals have been studied for bacterial cellulose (BC) prepared by sulfuric acid hydrolysis. Suspensions at concentrations above 0.42 wt % separated into the isotropic and chiral nematic phases with a clear phase boundary. The shape and size distribution of BC nanocrystals in both the phases were determined by transmission electron microscopy (TEM) and atomic force microscopy (AFM). The surface charge density was determined by conductometric titration. The effects of added NaCl (0-5.0 mM) on the phase separation behavior of the aqueous suspensions were investigated for a fixed total cellulose concentration. The volume fraction of the chiral nematic phase had a minimum value at a NaCl concentration of ca. 1.0 mM. At NaCl concentrations ranging from 2.0 to 5.0 mM, the suspensions did not separate into two phases, but became entirely liquid crystalline. The size of the ordered domains in the anisotropic phase decreased with an increase in the NaCl concentration from 0 to 2.75 mM. At 2.75 mM, only tactoids were observed in the entire region. At 5.0 mM, chiral nematic domains were no longer observed. The chiral nematic pitch decreased with increasing concentration of added NaCl, reached a minimum value at approximately 0.75 mM, and then increased sharply with the NaCl concentration up to 2.0 mM.

Micron-sized main-chain liquid crystalline elastomer actuators with ultralarge amplitude contractions.

Abstract: Responsive surfaces composed of cylindrical or square micrometer-sized thermoresponsive pillars made of thiol-ene nematic main-chain liquid crystalline elastomers (LCEs) are produced by replica molding. The individual pillars behave as microactuators, showing ultralarge and reversible contractions of around 300-400% at the nematic to isotropic phase transition. The nematic main-chain LCE microactuators described here present contractions as large as the best macroscopic systems reported in the literature. Moreover, the contraction observed for this new system outperforms the best values already reported for other LCE microsystems.

Optical vortices from liquid crystal droplets.

Abstract: We report on the generation of mono- and polychromatic optical phase singularities from micron-sized birefringent droplets. This is done experimentally by using liquid crystal droplets whose three dimensional architecture of the optical axis is controlled within the bulk by surfactant agents. Because of its microscopic size these optical vortex generators are optically trapped and manipulated at will, thus realizing a robust self-aligned micro-optical device for orbital angular momentum conversion. Experimental observations are supported by a simple model of optical spin-orbit coupling in uniaxial dielectrics that emphasizes the prominent role of the transverse optical anisotropy with respect to the beam propagation direction.

A Stimuli‐Responsive, Photoluminescent, Anthracene‐Based Liquid Crystal: Emission Color Determined by Thermal and Mechanical Processes

Abstract: Here, a photoluminescent liquid crystal that exhibits a change of emission color on the metastable–stable phase transition induced by external stimuli is prepared. A 2,6-diethynylanthracene derivative with amide groups and dendritic side chains exhibits a columnar phase on slow cooling from the isotropic phase and shows blue emission in this columnar phase. In contrast, a cubic phase is obtained by rapid cooling from the isotropic phase. In the cubic phase, the 2,6-diethynylanthracene cores form excimers, resulting in yellow emission. While the columnar phase is a stable liquid-crystalline (LC) phase, the cubic phase is a metastable LC phase. It is found that a change of the photoluminescent color from yellow to blue is observed on the cubic-columnar phase transition induced by heating or mechanical shearing for this 2,6-diethynylanthracene derivative in the cubic phase. This change of photoluminescent color is ascribed to the inhibition of excimer formation on the metastable–stable LC phase transition.

Liquid crystal display and method of manufacturing the same

Abstract: A liquid crystal display includes a first substrate, a second substrate, a liquid crystal material interposed between the first and second substrates. The liquid crystal display includes first protrusions and second protrusions that are alternately disposed with each other on the first substrate. A common electrode is provided at a side part of the first protrusions, and a pixel electrode is provided at a side part of the second protrusions to face the common electrode. A lateral electric field is generated between the common electrode and the pixel electrode facing each other for controlling an alignment of molecules of the liquid crystal material.

Optical film, polarizing plate, and liquid crystal display device

Abstract: An optical film, which is an optically compensatory film that comprises: at least two optically anisotropic layers comprising a first optically anisotropic layer and a second optically anisotropic layer, wherein the first optically anisotropic layer is formed by mixing two or more liquid crystal compounds having different structures with each other.

Nanoparticle-Stabilized Cholesteric Blue Phases

Abstract: The authors report the expansion of the temperature range of cholesteric blue phases by doping nanoparticles. When spherical gold nanoparticles with a mean diameter of 3.7 nm were doped in a blue phase-exhibiting multi-component liquid crystal mixture, the temperature range of the cholesteric blue phase increased from 0.5 to 5 °C, while the clearing temperature decreased by approximately 13 °C. We believe that the mechanism stabilizing the cholesteric blue phase is similar to that of polymer-stabilized cholesteric blue phases: the nanoparticles accumulate in the lattice disclinations, stabilizing the overall cholesteric blue structure.

π‐Conjugated Oligothiophene‐Based Polycatenar Liquid Crystals: Self‐Organization and Photoconductive, Luminescent, and Redox Properties

Abstract: A series of liquid-crystalline (LC) π--conjugated oligothiophenes bearing three or two alkoxy chains at their extremities has been designed and synthesized. These polycatenar oligothiophenes form various LC nanostructures including smectic, columnar, and micellar cubic phases. These properties depend on the number and length of the terminal alkoxy chains. The hole mobilities for the oligothiophenes have been measured. The layered smectic and columnar structures are capable of transporting holes, leading to mobilities of up to 0.01 cm2 V−1 s−1. The columnar LC assemblies have also been explored to produce linearly polarized light-emission. Fine red polarized fluorescence is observed from a uniaxially aligned film of the oligothiophenes. The redox properties of the oligothiophenes both in solutions and in films have been examined. The oligothiophenes exhibit electrochromism upon applying an oxidative potential. The present design strategy is useful for fabricating a variety of functional electro-active molecular assemblies.

Ferroelectric Response and Induced Biaxiality in the Nematic Phase of a Bent-Core Mesogen

Abstract: The still undiscoveredfluid ferroelectric nematic phase isexpected to exhibit a much faster and easier response to an external electric field compared to conventional ferroelectric smectic liquid crystals; therefore, the discovery of such a phase could open new avenues in electro-optic device technology. Here, experimental evidence of a ferroelectric response to a switching electric field inalowmolarmassnematicliquid crystalisreported andconnected with field-induced biaxiality. The fluid is made of bent-core polar molecules and is nematic over a range of 120-C. Combining repolarization current measurements, electro-optical characterizations, X-ray diffraction and computer simulations, ferroelectric switching is demonstrated and it is concluded that the response is due to field-induced reorganization of polar cybotactic groups within the nematic phase. This work represents significant progress toward the realization of ferroelectric fluids that can be aligned at command with a simple electric field.

Asymmetrically Substituted Benzene-1,3,5-tricarboxamides: Self-Assembly and Odd–Even Effects in the Solid State and in Dilute Solution

Abstract: Molecular organization: Chiral benzene tricarboxamides with methyl substituents at defined positions self-assemble into supramolecular polymers of preferred helicity by three-fold alpha-helical-type hydrogen bonding. The odd-even effect is operative and all derivatives are liquid crystalline showing a Col(ho) phase (see figure).Asymmetric benzene-1,3,5-tricarboxamides (aBTAs) comprising two n-octyl and one chiral methyl-alkyl side chain were synthesised and characterised. The influence of the position and the configuration of the chiral methyl group (methyl at the alpha, beta or gamma position) in the aliphatic side chains on the liquid-crystalline properties and the aggregation behaviour of the aBTAs was systematically studied and compared to symmetrical benzene-1,3,5-tricarboxamides (sBTAs). Solid-state characterisation (polarised optical microscopy, IR spectroscopy, X-ray diffraction and differential scanning calorimetry) revealed that all aBTAs show threefold, alpha-helical-type intermolecular hydrogen bonding between neighbouring molecules and exhibit a columnar hexagonal organisation from room temperature to well above 200 degrees C. Moving the chiral methyl group closer to the amide group stabilises the liquid-crystalline state, as evidenced by a higher clearing temperature and corresponding enthalpy. The self-assembly of dilute solutions of the aBTAs in methylcyclohexane ( approximately 10(-5) mol L(-1)) was investigated with circular dichroism (CD) spectroscopy. The sign of the Cotton effect demonstrated a pronounced odd-even effect, whereas the value of the molar ellipticity, Deltaepsilon, in the aBTAs was independent of the position of the methyl group. Subsequent temperature-dependent CD measurements showed that the aggregation of all aBTAs can quantitatively be described by the nucleation-growth model and that the stability of the aggregates increases when the chiral methyl group is closer to the amide moiety. The results presented herein illustrate that even small changes in the molecular structure of substituted benzene-1,3,5-tri-carboxamides affect their solid-state properties and their self-assembly behaviour in dilute solutions.

Towards in silico liquid crystals. Realistic transition temperatures and physical properties for n-cyanobiphenyls via molecular dynamics simulations.

Abstract: We have studied the important n-cyano biphenyl series of mesogens, n=4-8 using modelling and molecular dynamics simulations. We have been able to obtain spontaneously ordered nematics upon cooling isotropic samples of 250 molecules. We show that, using the united atom force field developed here, the experimental isotropic–nematic transition temperatures are reproduced within 4 K, allowing a molecular level interpretation of the odd–even effect along the series. Other properties, like densities, orientational order parameters and NMR residual dipolar couplings are also well reproduced, demonstrating the feasibility of predictive in silico modelling of nematics from the molecular structure.

Liquid-crystal electro-optical apparatus and method of manufacturing the same

Abstract: A liquid crystal device comprising: a pair of substrates having an electrode arrangement thereon; an orientation control means provided on at least one of said substrates; and a ferroelectric or antiferroelectric liquid crystal layer interposed between said substrates, said liquid crystal layer being uniaxially oriented by virtue of said orientation control means, wherein means for suppressing an orientation control effect of said orientation control means with respect to said liquid crystal layer is provided between said liquid crystal layer and said orientation control means.

Bioinspired Electrochemically Tunable Block Copolymer Full Color Pixels

Abstract: By controlling absorption, reflection, and body surface texture cephalopods display an incredible ability to manage and manipulate light – a goal of modern display technology. With these and several other aquatic species, tunable, structural color plays a fundamental role in creating vivid and eye-catching color, which is produced in part by using and modulating 1D reflectors known as iridophores. The color change of the light reflected from these tunable Bragg reflectors is accomplished by the local introduction of chemical neurotransmitters that either change the spacing between high refractive index platelets or change the thickness (and possibly the refractive index) of the platelets themselves. Efficient, refined mechanisms of visual communication and disruptive coloration – such as iridophores are key to cephalopod survival. The next generation of display materials could benefit from the lessons learned over millions of years of evolution in order to produce inexpensive, robust, low-energy, high-contrast displays. Here we present a bioinspired, electrochemically tunable, full-color pixel based on a self-assembling block copolymer (BCP). Imitating cephalopod reflectin proteins, which form layered, highly reflective structures, the lamellar structure of these BCPs exhibits a periodic refractive index. Such structures are commonly known as 1D photonic crystals and yield photonic stop bands which reflect incident electromagnetic radiation of a wavelength range dependent on the angle of incidence of light, the index of refraction of the respective layers, and their periodicity. Partial color tunability has been demonstrated in numerous photonic crystal systems which have been intensely investigated due to their inherent passive structural color which negates the need for light-absorbing color filters and backlights common in emissive technologies. A number of types of photonic crystals based on cholesteric liquid crystals, synthetic opals, and diffraction gratings have attained full color tunability using direct or indirect electrical control. Tailored absorption using electrochromic materials has also achieved full color tunability in recent years. Electrical control is highly sought after for display applications because of the ease with which it can supply a local stimulus. Previously, BCPs have been used as photonic crystals but with limited tunability arising from changes in temperature and homopolymer or solvent concentration. Recently we reported a highly tunable, structural-color reflector based on a hydrophilic/hydrophobic block copolymer (polystyreneb-poly(2-vinylpyridine)PS-P2VP) that achieved substantial tunability (over 575%) of the primary stop band from ultraviolet (350 nm) to near infrared wavelengths (1600 nm) by changing the concentration of salt in the solvent. Here we employ the same block copolymer to create our biomimetic, full-color pixel. The lamellar structure of this block copolymer and the behavior of the electrically stimulated hydrophilic gel block mimic the type of optical response of the reflectin proteins found in certain cephalopod species. These cephalopods have the ability to change the iridescent colors reflected from their skin in real time by the secretion of the common neuromodulator acetylcholine, which alters the periodic spacing of the lamellar iridophore structure. In our work, we couple the structure-forming ability of a block copolymer with an electrically induced chemical tunability of the spacing and refractive index of the gel layers to obtain a tunable, reflectin-like structure. Polyelectrolyte polymer gels are well known for their stimuli responsiveness and have been shown to react to electric fields and changes in pH caused by electric fields. By selecting a diblock copolymer that incorporates a glassy domain (polystyrene) to limit the gel forming domain (poly 2-vinylpyridine) to expansion only in the direction normal to the layers, we control the swelling behavior of the gel andmagnify the response of the periodic dielectric stack to external stimuli. We use a modest molecular weight (102 kg/mol–97 kg/mol), PS-P2VP block copolymer in combination with a simple electrochemical cell to produce red, green, and blue color through electrochemical stimulation. Our biomimetic color pixel is based on a simple electrochemical cell in which the local chemical environment at the electrode/electrolyte interface can be changed by applying sufficient voltage to induce electrochemical oxidation or reduction. Because polymeric gels can undergo significant shape change in response to changes in their chemical environments, this chemical stimulus at the electrode can have major effects on the swelling behavior of the gel, as is the case in color changing iridophores in nature. Fish such as the blue damselfish, neon tetra, and paradise whiptail all employ 1D photonic crystals

Tunable magnetic response of metamaterials

Abstract: We demonstrate a thermally tunable optical metamaterial with negative permeability working in the visible range. By covering coupled metallic nanostrips with aligned nematic liquid crystals (NLCs), the magnetic response wavelength of the metamaterial is effectively tuned through control of the ambient temperature, changing the refractive index of LC via phase transitions. By increasing the ambient temperature from 20 degree to 50 degree, the magnetic response wavelength shifts from 650nm to 632nm. Numerical simulations confirm our tests and match the experimental observations well.

Tunable magnetic response of metamaterials

Abstract: We demonstrate a thermally tunable optical metamaterial with negative permeability working in the visible range. By covering coupled metallic nanostrips with aligned nematic liquid crystals (NLCs), the magnetic response wavelength of the metamaterial is effectively tuned through control of the ambient temperature, changing the refractive index of LC via phase transitions. By increasing the ambient temperature from 20 to 50 °C, the magnetic response wavelength shifts from 650 to 632 nm. Numerical simulations confirm our tests and match the experimental observations well.

Luminescence of metallomesogens in the liquid crystal state

Abstract: Recent progress in the design of low-melting liquid-crystalline metal complexes (metallomesogens) has facilitated the study of the photophysical properties of these compounds in the mesophase. Luminescence in the liquid crystal state has been observed for metallomesogens incorporating lanthanide(III), gold(I), silver(I), copper(I) or zinc(II) ions. An alternative approach to liquid-crystalline metal-containing systems is doping a metal complex in a liquid-crystal host. A fascinating property of these materials is the ability to observe linearly polarized emission.

New type of Li ion conductor with 3D interconnected nanopores via polymerization of a liquid organic electrolyte-filled lyotropic liquid-crystal assembly.

Abstract: A new type of polymer electrolyte material for Li ion transport has been developed. This material is based on a polymerizable lyotropic (i.e., amphiphilic) liquid crystal (1) that forms a type-II bicontinuous cubic (Q(II)) phase with the common liquid electrolyte, propylene carbonate (PC), and its Li salt solutions. The resulting cross-linked, solid-liquid nanocomposite has an ordered, three-dimensional interconnected network of phase-separated liquid PC nanochannels and exhibits a room-temperature ion conductivity of 10(-4) to 10(-3) S cm(-1) when formed with 15 wt % 0.245 M LiClO(4)-PC solution. This value approaches that of conventional gelled poly(ethylene oxide)-based electrolytes blended with larger amounts of higher-concentration Li salt solutions. It is also similar to that of a bulk 0.245 M LiClO(4)-PC solution measured using the same AC impedance methods. Preliminary variable-temperature ion conductivity and NMR DOSY studies showed that liquidlike diffusion is present in the Q(II) nanochannels and that good ion conductivity ( approximately 10(-4) S cm(-1)) and PC mobility are retained down to -35 degrees C (and lower). This type of stable, liquidlike ion conductivity over a broad temperature range is typically not exhibited by conventional gelled-polymer- or liquid-crystal-based electrolytes, making this new material potentially valuable for enabling Li ion batteries that can operate more efficiently over a wider temperature range.

Phototunable Azobenzene Cholesteric Liquid Crystals with 2000 nm Range

Abstract: Phototuning of more than 2000 nm is demonstrated in an azobenzene-based cholesteric liquid crystal (azo-CLC) consisting of a high-helical-twisting-power, axially chiral bis(azo) molecule (QL76). Phototuning range and rate are compared as a function of chiral dopant concentration, light intensity, and thickness. CLCs composed of QL76 maintain the CLC phase regardless of intensity or duration of exposure. The time necessary for the complete restoration of the original spectral properties (position, bandwidth, baseline transmission, and reflectivity) of QL76-based CLC is dramatically reduced from days to a few minutes by polymer stabilization of the CLC helix.