scispace - formally typeset
Search or ask a question

Showing papers on "Liquid crystal published in 2001"


Journal ArticleDOI
TL;DR: In this paper, it was shown that at low pump intensities, the fluorescence spectrum of the dye is modified, showing suppression of emission in the reflection band, and enhanced emission near the band edge.
Abstract: Cholesteric liquid crystals are chiral nematics, where the handedness of the constituent molecules causes the orientation of the local nematic director to vary in space. In the helical cholesteric structure, the director is perpendicular to the helix axis, and its orientation varies linearly with position along the helix axis. The spatial period of the structure is the pitch, which is determined by the concentration and the helical twisting power of the chiral constituents. As a consequence of the periodicity of the helical cholesteric structure and the birefringence of the liquid crystal, for a range of wavelengths, light propagation along the helix axis is forbidden for one of the normal modes. Since propagation is forbidden, incident light with a wavelength in this band and with the same helicity as the cholesteric is strongly reflected. The edges of this reflection band are at wavelengths that are equal to the refractive indices times the pitch. [1] Because of the existence of the selective reflection band, cholesteric liquid crystals are 1D photonic bandgap materials. The bandgap structure of cholesteric liquid crystals allows for the possibility of lasing without external mirrors that usually form a laser cavity. When a fluorescent dye is dissolved in the cholesteric host so that the peak of the fluorescent emission of the dye is in the selective reflection band of the cholesteric, propagation of one normal mode of the emitted light is forbidden. As a consequence, at low pump intensities, the fluorescence spectrum of the dye is modified, [2] showing suppression of emission in the reflection band, and enhanced emission near the band edge. As the pump intensity is increased, the linewidth of the enhanced fluorescence at the band edge narrows, and, above a pump threshold, lasing occurs. [2] Thin samples, typically 15‐30 lm in thickness, of low molecular weight cholesteric liquid crystals incorporating a variety of dyes [3] have been shown to lase. The primary role of the cholesteric liquid crystal in these systems is to act as a distributed cavity. Lasing occurs at the band edges, [2‐4] as pre

612 citations


Journal ArticleDOI
TL;DR: In this article, the physics of colloidal dispersions in nematic liquid crystals is reviewed and three possible structures are identified and discussed in detail; the dipole, the Saturn-ring and the surface-ring configuration.

579 citations


Book
01 Jan 2001
TL;DR: In this paper, the authors present a light modulation mechanism for reflective displays, including homogeneous cells, pi cells, homeotropic cells, hybrid aligned nematic cells, twisted and super-nematic cells.
Abstract: Light modulation mechanisms liquid crystal materials homogeneous cells pi cells homeotropic cells hybrid aligned nematic cells twisted nematic cells super twisted nematic cells guest-house cells cholesteric reflective displays liquid crystal and polymer composites system aspects of reflective displays.

573 citations


Journal ArticleDOI
TL;DR: The high stability and bright fluorescence of the mesophase of several of the compounds make them promising for applications as polarizers or components in (opto)electronic devices.
Abstract: A series of perylene tetracarboxylic acid bisimides 3 a–e bearing 3,4,5-tridodecyloxyphenyl substituents on the imide N atoms and zero, two, or four phenoxy-type substituents in the bay positions of the perylene core were synthesized. From investigations of their spectroscopic properties and aggregation behavior in low-polarity solvents by absorption and fluorescence optical spectroscopy, not only were these compounds found to form fluorescent J-type aggregates, but also binding constants for aggregation could be derived which reflect the number and steric demand of the phenoxy substituents for bisimides 3 a–d. In the pristine state, 3 a–d form thermotropic hexagonal columnar mesophases which exist over a broad temperature range from below −30 °C to over 300 °C. For the tetraphenoxy-substituted compound 3 e, however, a layered crystalline structure was found. This difference in behavior can be explained by the concept of microphase segregation of the aromatic cores of the molecules and the alkyl chains at the periphery. The high stability and bright fluorescence of the mesophase of several of the compounds make them promising for applications as polarizers or components in (opto)electronic devices.

554 citations


Journal ArticleDOI
01 Oct 2001-EPL
TL;DR: In this paper, a simple Landau-like phenomenological model predicts a symmetry-breaking transition inside the nematic phase, from uniform textures toward spontaneous periodic distortion, either oscillating splaybend or conical twist-bend helix.
Abstract: We propose that the natural tendency of banana-shaped mesogen molecules to induce a local bend of the nematic director can result in pathological elasticity, with negative bend elastic constant Under this hypothesis, a simple Landau-like phenomenological model predicts a symmetry-breaking transition inside the nematic phase, from uniform textures toward spontaneous periodic distortion, either oscillating splay-bend or conical twist-bend helix The predicted lower symmetry nematic phases are expected to show interesting polar properties, similar to those already reported for banana-shaped smectogens

463 citations


Journal ArticleDOI
TL;DR: In this article, an overview of recent progress in the design of novel materials capable of forming liquid crystalline phases with non-conventional mesophase morphologies is given, which includes dendritic molecules, ternary block-copolymers, rod-coil molecules and linear as well as non-linear polyphilic low molecular weight block molecules incorporating rigid segments with a specific shape.
Abstract: This article gives an overview of recent progress in the design of novel materials, capable of forming liquid crystalline phases with non-conventional mesophase morphologies. The materials include dendritic molecules, ternary block-copolymers, rod–coil molecules and linear as well as non-linear polyphilic low molecular weight block molecules incorporating rigid segments with a specific shape. Changing the shape of the rigid segments from rod-like to disc-like or to a bent shape leads to additional possibilities for the directed design of mesophase forming materials. Many of these molecules are able to form quite unusual mesophase morphologies, distinct from the conventional lamellar (smectic) and columnar mesophases of classical rod-like and disc-like liquid crystals; they include mesophases which combine lamellar and columnar organisation, columnar mesophases incorporating three discrete sets of columns, biaxial smectic phases, such as the SmAb-phase (McMillan Phase), polar smectic phases and non-conventional layer structures in which rigid units are aligned parallel to the layers. These novel mesophase morphologies were realised by increasing the number of incompatible units combined in these molecules, by changing the volume fractions of the incompatible segments, by tailoring the shape of rigid segments and by controlling the molecular topology.

437 citations


Journal ArticleDOI
22 Mar 2001-Nature
TL;DR: A material is reported that shows a further increase in electrostriction by two orders of magnitude: ultrathin (less than 100 nanometres) ferroelectric liquid-crystalline elastomer films that exhibit 4 per cent strain at only 1.5 MV m-1.
Abstract: Mechanisms for converting electrical energy into mechanical energy are essential for the design of nanoscale transducers, sensors, actuators, motors, pumps, artificial muscles, and medical microrobots. Nanometre-scale actuation has to date been mainly achieved by using the (linear) piezoelectric effect in certain classes of crystals (for example, quartz), and 'smart' ceramics such as lead zirconate titanate. But the strains achievable in these materials are small--less than 0.1 per cent--so several alternative materials and approaches have been considered. These include grafted polyglutamates (which have a performance comparable to quartz), silicone elastomers (passive material--the constriction results from the Coulomb attraction of the capacitor electrodes between which the material is sandwiched) and carbon nanotubes (which are slow). High and fast strains of up to 4 per cent within an electric field of 150 MV x m(-1) have been achieved by electrostriction (this means that the strain is proportional to the square of the applied electric field) in an electron-irradiated poly(vinylidene fluoride-trifluoroethylene) copolymer. Here we report a material that shows a further increase in electrostriction by two orders of magnitude: ultrathin (less than 100 nanometres) ferroelectric liquid-crystalline elastomer films that exhibit 4 per cent strain at only 1.5 MV x m(-1). This giant electrostriction was obtained by combining the properties of ferroelectric liquid crystals with those of a polymer network. We expect that these results, which can be completely understood on a molecular level, will open new perspectives for applications.

398 citations


Journal ArticleDOI
TL;DR: In this paper, the molecular ordering of cholesteric liquid crystals is addressed and applications in rewritable color recording are highlighted, where a sample recorded using a laser is shown.
Abstract: Fixation of the molecular ordering of cholesteric liquid crystals is essential for use of these materials in color information technology. Methods to achieve this as well as thermal and photochemical control of the cholesteric pitch of the liquid crystals are addressed and applications in rewritable color recording are highlighted. The Figure shows a sample recorded using a laser.

388 citations


Journal ArticleDOI
23 Aug 2001-Langmuir
TL;DR: A novel process for producing cubic liquid crystalline nanoparticles (cubosomes) has been developed in this article, which involves simple mixing of two water-like solutions with a minimal input of energy.
Abstract: A novel process for producing cubic liquid crystalline nanoparticles (cubosomes) has been developed. The process entails simple mixing of two waterlike solutions with a minimal input of energy. The key to this process is the inclusion of hydrotrope. Most lipids, such as monoolein, used to form cubic liquid crystals are essentially insoluble in water. The hydrotrope dissolves the lipid to create a waterlike solution. Water is added to the hydrotrope solution, resulting in a precipitous decrease in lipid solubility. Provided that the dilution trajectory falls into a cubic phase−water miscibility gap, nanometer-scale cubic liquid crystalline particles form spontaneously, presumably from a homogeneous nucleation mechanism. The process is versatile enough to accommodate any lipid and hydrotrope combination that forms cubic liquid crystalline material upon dilution. Actives and stabilizers can be formulated into either of the two solutions, allowing the production of colloidally stabilized, controlled-release d...

375 citations


Journal ArticleDOI
03 May 2001-Nature
TL;DR: A non-contact alignment process, which uses low-energy ion beams impinging at a glancing angle on amorphous inorganic films, such as diamond-like carbon, is reported, and it is found that displays of higher quality and reliability could be made at a lower cost than the rubbing technique.
Abstract: The technique used to align liquid crystals—rubbing the surface of a substrate on which a liquid crystal is subsequently deposited1,2,3—has been perfected by the multibillion-dollar liquid-crystal display industry. However, it is widely recognized that a non-contact alignment technique would be highly desirable for future generations of large, high-resolution liquid-crystal displays. A number of alternative alignment techniques have been reported4,5,6,7, but none of these have so far been implemented in large-scale manufacturing. Here, we report a non-contact alignment process, which uses low-energy ion beams impinging at a glancing angle on amorphous inorganic films, such as diamond-like carbon. Using this approach, we have produced both laptop and desktop displays in pilot-line manufacturing, and found that displays of higher quality and reliability could be made at a lower cost than the rubbing technique. The mechanism of alignment is explained by adopting a random network model of atomic arrangement in the inorganic films. Order is induced by exposure to an ion beam because unfavourably oriented rings of atoms are selectively destroyed. The planes of the remaining rings are predominantly parallel to the direction of the ion beam.

372 citations


Journal ArticleDOI
TL;DR: The role of hydrogen-bonding interactions in the formation and/or stabilization of liquid crystalline phases has been recognized in recent years and significant work has been conducted as discussed by the authors, with the aim of establishing, in the first place, the diversity of organic compounds that can be used as building elements in the process of liquid crystal formation.
Abstract: The role of hydrogen-bonding interactions in the formation and/or stabilization of liquid crystalline phases has been recognized in recent years and significant work has been conducted. Following the first and well-established examples of liquid crystal formation through the dimerization of aromatic carboxylic acids, several classes of compounds have been prepared by the interaction of complementary molecules, the liquid crystalline behaviour of which is crucially dependent on the structure of the resulting supramolecular systems. In this review the main classes of liquid crystals prepared through hydrogen-bonding interactions are presented, with the aim of establishing, in the first place, the diversity of organic compounds that can be used as building elements in the process of liquid crystal formation. Rigid-rod anisotropic or amphiphilic-type molecules, appropriately functionalized with recognizable moieties, interact in the melt or in solution and lead to the formation of supramolecular complexes tha...

Journal ArticleDOI
22 Jun 2001-Science
TL;DR: This work uses near-edge x-ray absorption fine structure (NEXAFS) spectroscopy to link the orientational bond order at three carbonaceous surfaces with the direction of liquid crystal (LC) alignment on these surfaces, showing that LC alignment can be created on any carbonaceous substrate by inducing orientational order at its surface.
Abstract: We used near-edge x-ray absorption fine structure (NEXAFS) spectroscopy to link the orientational bond order at three carbonaceous surfaces-rubbed polyimide, ion beam-irradiated polyimide, and ion beam-irradiated diamondlike carbon films-with the direction of liquid crystal (LC) alignment on these surfaces. We show that, in general, LC alignment can be created on any carbonaceous substrate by inducing orientational order at its surface. Our results form the scientific basis for LC alignment layers consisting of amorphous carbon films in which orientational order near the surface is induced by a directional low-energy ion beam.

Journal ArticleDOI
17 Aug 2001-Science
TL;DR: Systems that provide parts-per-billion (by volume) sensitivity to either organoamine or organophosphorus compounds are demonstrated, and their use for imaging of spatial gradients in concentration is reported.
Abstract: The competitive binding of a molecule forming a liquid crystal and a targeted analyte to a common molecular receptor presented at a solid surface possessing nanometer-scale topography is used to trigger an easily visualized surface-driven change in the orientation of a micrometer-thick film of liquid crystal. Diffusion of the targeted analyte from atmosphere to surface-immobilized receptor across the micrometer-thick film of liquid crystal is fast (on the order of seconds), and the competitive interaction of the targeted analyte and liquid crystal with the receptor provides a high level of tolerance to nontargeted species (water, ethanol, acetone, and hexanes). Systems that provide parts-per-billion (by volume) sensitivity to either organoamine or organophosphorus compounds are demonstrated, and their use for imaging of spatial gradients in concentration is reported. This approach does not require complex instrumentation and could provide the basis of wearable personalized sensors for measurement of real-time and cumulative exposure to environmental agents.

Journal ArticleDOI
TL;DR: In this article, the authors describe the monolithic integration of rubber-stamped thin-film organic transistors with polymer-dispersed liquid crystals (PDLCs) to create a multipixel, flexible display with plastic substrates.
Abstract: This letter describes the monolithic integration of rubber-stamped thin-film organic transistors with polymer-dispersed liquid crystals (PDLCs) to create a multipixel, flexible display with plastic substrates. We report the electro-optic switching behavior of the PDLCs as driven by the organic transistors, and we show that our displays operate robustly under flexing and have a contrast comparable to that of newsprint.

Journal ArticleDOI
TL;DR: The picture shows the orange-red electroluminescence of a light-emitting diode containing the perylene 3,4,9,10-tetracarboxylic acid ethyl ester.
Abstract: Simple ester derivatives of polycyclic arenes offer access to light-emitting diodes of nearly any visible color by making use of the good charge-transport properties of the columnar liquid crystals of these derivatives. The picture shows the orange-red electroluminescence of a light-emitting diode containing the perylene 3,4,9,10-tetracarboxylic acid ethyl ester (structure shown, R=Et). Through use of multiple layers of different esters light-emitting diodes with almost white luminescence can be obtained.

Journal ArticleDOI
19 Jun 2001-Langmuir
TL;DR: The rodlike microcrystal suspension prepared by sulfuric acid hydrolysis of bacterial cellulose was found to undergo spontaneous nematic phase separation after complete desalination as mentioned in this paper.
Abstract: The rodlike microcrystal suspension prepared by sulfuric acid hydrolysis of bacterial cellulose was found to undergo spontaneous nematic phase separation after complete desalination. This phase separation was preceded by a birefringent glassy-like state for about 1 week. Addition of trace electrolyte (<1 mM NaCl) caused remarkable changes in phase separation behavior; i.e., the separation was complete in 2 days and the anisotropic phase became chiral nematic. This phenomenon can be explained by the change in the effective particle shape, from cylindrical to the twisted rod as a result of screening of surface charge.


Journal ArticleDOI
TL;DR: A review of the thermotropic materials for which a biaxial nematic has now been claimed can be found in this article, where it is suggested that deuterium NMR spectroscopy is a powerful method with which to determine this.

Journal ArticleDOI
TL;DR: Photonic crystals made of nematic liquid crystal intercalated into the void space of close-packed silica spheres (synthetic porous opal) exhibit significant electric-field-induced shift of the optical Bragg reflection peak when the liquid crystal has the long molecular axis oriented parallel to the sphere surfaces.
Abstract: Photonic crystals made of nematic liquid crystal intercalated into the void space of close-packed silica spheres (synthetic porous opal) exhibit significant electric-field-induced shift of the optical Bragg reflection peak when the liquid crystal has the long molecular axis oriented parallel to the sphere surfaces. No such effect is observed for comparable fields when the long-axis orientation is normal to the sphere surfaces.

Patent
25 Jun 2001
TL;DR: In this paper, the authors defined the meaning of chiral compounds of formula (I) as a mixture of liquid crystalline mixtures containing at least one chiral compound of formula.
Abstract: The invention relates to chiral compounds of formula (I) R?1-X1-(A1-Z1)?m-G-(Z2-A2)n-X2-R2, wherein R?1, R2, X1, X2, A1, A2, Z1, Z2?, G, m and n have the meaning given in claim 1, to liquid crystalline mixtures comprising at least one chiral compound of formula (I), to chiral linear or crosslinked liquid crystalline polymers obtainable by polymerizing a polymerizable mixture comprising at least one chiral compound of formual (I), to the use of chiral compounds of formula (I) and mixtures and polymers obtained thereof in liquid crystal displays, active and passive optical elements, adhesives, synthetic resins with anisotropic mechanical properties, cosmetic and pharmaceutical compositions, diagnostics, liquid crystal pigments, for decorative and security applications, nonlinear optics, optical information storage or as chiral dopants, and to a liquid crystal display comprising a mixture comprising at least one chiral compound of formula (I).

Journal ArticleDOI
04 Oct 2001-Nature
TL;DR: A lyotropic liquid-crystalline lamellar phase comprising an aqueous dispersion of planar solid-like sheets in which all the atoms involved in a layer are covalently bonded is described.
Abstract: Ordering particles at the nanometre length scale is a challenging and active research area in materials science. Several approaches have so far been developed, ranging from the manipulation of individual particles to the exploitation of self-assembly in colloids. Nanometre-scale ordering is well known to appear spontaneously when anisotropic organic moieties form liquid-crystalline phases; this behaviour is also observed for anisotropic mineral nanoparticles resulting in the formation of nematic, smectic and hexagonal mesophases. Here we describe a lyotropic liquid-crystalline lamellar phase comprising an aqueous dispersion of planar solid-like sheets in which all the atoms involved in a layer are covalently bonded. The spacing of these phosphatoantimonate single layers can be increased 100-fold, resulting in one-dimensional structures whose periodicity can be tuned from 1.5 to 225 nanometres. These highly organized materials can be mechanically or magnetically aligned over large pH and temperature ranges, and this property can be used to measure residual dipolar couplings for the structure determination of biomolecules by liquid-state NMR. We also expect that our approach will result in the discovery of other classes of mineral lyotropic lamellar phases.

Book
15 Jan 2001
TL;DR: In this paper, various experimental techniques used in the study of liquid crystals are described in detail and discussed in detail, including various experimental methods used for the analysis of liquid crystal lattice structures.
Abstract: This book describes in detail various experimental techniques used in the study of liquid crystals.[...]

Journal ArticleDOI
TL;DR: The use of hydrogen bonding for the design of functional liquid crystalline materials has been shown to be a versatile approach toward the control of simple molecularly assembled structures and the induction of dynamic function.
Abstract: Liquid crystals are molecular materials that combine anisotropy with dynamic nature. Recently, the use of hydrogen bonding for the design of functional liquid crystalline materials has been shown to be a versatile approach toward the control of simple molecularly assembled structures and the induction of dynamic function. A variety of hydrogen-bonded liquid crystals has been prepared by molecular self-assembly processes via hydrogen bond formation. Rod-like and disk-like low-molecular weight complexes and polymers with side-chain, main-chain, network, and guest-host structures have been built by the complexation of complimentary and identical hydrogen-bonded molecules. These materials consist of closed-type hydrogen bondings. Another type of hydrogen-bonded liquid crystals consists of open-type hydrogen bonding. In this case, the introduction of hydrogen bonding moieties, such as hydroxyl groups, induces microphase segregation leading to liquid crystalline molecular order. Moreover, liquid crystalline physical gels have been prepared by the molecular aggregation of hydrogen-bonded molecules in non-hydrogen-bonded liquid crystals. They show significant electrooptical properties. An anisotropic gel is a new type of anisotropic materials forming heterogeneous structures.

Journal ArticleDOI
TL;DR: In this paper, the effect of increasing the proportion of long di-functional segments of main-chain nematic polymer, acting as network crosslinking, results in dramatic changes in the uniaxial equilibrium thermal expansion on cooling from the isotropic phase.
Abstract: We study the monodomain (single-crystal) nematic elastomer materials, all side-chain siloxane polymers with the same mesogenic groups and crosslinking density, but differing in the type of crosslinking. Increasing the proportion of long di-functional segments of main-chain nematic polymer, acting as network crosslinking, results in dramatic changes in the uniaxial equilibrium thermal expansion on cooling from the isotropic phase. At higher concentration of main chains their behaviour dominates the elastomer properties. At low concentration of main-chain material, we detect two distinct transitions at different temperatures, one attributed to the main-chain, the other to the side-chain component. The effective uniaxial anisotropy of nematic rubber, r(T) = / proportional to the effective nematic order parameter Q(T), is given by an average of the two components and thus reflects the two-transition nature of thermal expansion. The experimental data is compared with the theoretical model of ideal nematic elastomers; applications in high-amplitude thermal actuators are discussed in the end.

Journal ArticleDOI
TL;DR: A lattice Boltzmann algorithm to simulate liquid crystal hydrodynamics is described, written in terms of a tensor order parameter that allows both the isotropic and the nematic phases to be considered.
Abstract: We describe a lattice Boltzmann algorithm to simulate liquid crystal hydrodynamics The equations of motion are written in terms of a tensor order parameter This allows both the isotropic and the nematic phases to be considered Backflow effects and the hydrodynamics of topological defects are naturally included in the simulations, as are non-Newtonian flow properties such as shear thinning and shear banding


Patent
06 Apr 2001
TL;DR: A liquid crystal layer is sandwiched between a first substrate (1) provided on a visible side and a second substrate (5) opposed to the first substrate; a first color filter (9) is provided on the first or on the second substrate(5); a transflective reflector (24) is providing on the opposite side to the visible side of the first color filters as discussed by the authors, which transmits a part of light and reflecting almost all remaining light.
Abstract: A liquid crystal layer ( 15 ) is sandwiched between a first substrate ( 1 ) provided on a visible side and a second substrate ( 5 ) opposed to the first substrate; a first color filter ( 9 ) is provided on the first substrate ( 1 ) or on the second substrate ( 5 ); a transflective reflector ( 24 ) is provided on the opposite side to the visible side of the first color filter ( 9 ), which transmits a part of light and reflecting almost all the remaining light; and a second color filter ( 28 ) is provided on the opposite side to the first color filter ( 9 ) with respect to the transflective reflector ( 24 ), thereby forming a liquid crystal display panel

Journal ArticleDOI
TL;DR: Semiconductor ZnS nanowires were synthesized by a direct templating route in an inverted hexagonal liquid crystal formed by oligo(ethylene oxide)oleyl ether amphiphiles, n-hexane, nhexanol/i-propa... as mentioned in this paper.
Abstract: Semiconductor ZnS nanowires were synthesized by a direct templating route in an inverted hexagonal liquid crystal formed by oligo(ethylene oxide)oleyl ether amphiphiles, n-hexane, n-hexanol/i-propa...


Journal ArticleDOI
TL;DR: The experimental observation of a crystal structure formed by glycerol droplets suspended in a nematic liquid crystal that exhibits a high density hexagonal ordering and the nature of appropriate mechanisms of the interaction is described.
Abstract: We describe the experimental observation of a crystal structure formed by glycerol droplets suspended in a nematic liquid crystal. The structure exhibits a high density hexagonal ordering. We have experimentally observed a noticeable interaction between droplets with tangential boundary conditions. Within the scope of known models we discuss the nature of appropriate mechanisms of the interaction.