Showing papers on "Liquid crystal published in 2005"

Molecular Gels: Materials with Self-Assembled Fibrillar Networks

Abstract: Introduction.- Theory: Theory Of Molecular Association And Thermoreversible Gelation: Thermodynamic theory of network-forming liquid mixtures. Some important examples of non-gelling associating mixtures. Gelling solutions and mixtures. Summary.- Growth And Chirality Amplification In Helical Supramolecular Polymers: Introduction. Helical aggregation. Discotics. Linear self-assembly. A two-state model. Aggregate ends. Chirality amplification. Sergeants and soldiers. Conclusions and outlook.- Self Assembling Peptide Gels: Theoretical model of self-assembling chiral rod-like units. Experiments illustrating predictions of the model. Stabilization by twist. Nematic fluids and gels. Properties for new materials.- Kinetics Of Nucleation, Aggregation And Aging: Introduction. Some basic thermodynamic concepts. Basic concepts of the theory of nucleation and cluster growth. Spinodal decomposition. Secondary aggregation, coarsening and aging. Discussion.- Soft Glassy Rheology: Introduction. Rheology. The SGR model. Rheological aging: imposed strain. Rheological aging: imposed stress. Discussion and conclusion.- Rheological Chaos In Wormlike Micelles And Nematic Hydronamic: Deterministic Chaos in viscoelastic materials in shear flow. Spatiotemporal rheological oscillations and chaotic dynamics.- Wetting of Fibres: Introduction. The Rayleigh-Plateau instability. Drop shapes. Heterogeneous fibers. Invasion of a network of fibers. Conclusion.- Techniques: Gel Formation: Phase Diagrams Using Tabletop Rheology And Calorimetry: Introduction. Detecting the sol-gel transition by tabletop theology. Thermodynamics of gelation: sol-gel transition by calorimetry. Conclusions and perspectives.- Direct-Imaging And Freeze-Fracture Cryo-Transmission Electron Microscopy Of Molecular Gels: Introduction. Cryo-TEM. Cryo-Tem Investigations of Molecular Gels. Future Developments.- Molecular Gels And Small-Angle Scattering: Foreword. Introduction. Basic principles. Form-factors of rod-like scatterers. Semi-rigid fibers. Fibers with anisometric sections. Tubes. Helices. Scattering by the junction zones in the networks. Structure factor peak in poorly organized fibrillar scatterers. Oriented fibers. Real space data. Kinetic studies. Useful hints for a standard SANS investigation of molecular gels. Conclusion.- X-Ray Diffraction Of Poorly Organized Systems And Molecular Gels: Introduction Long range ordering. Single crystal diffraction. X-rays and neutrons. Applications of diffraction.- Optical Spectroscopic Methods As A Tool To Investigate Gel Structures: Introduction. Electronic absorption and emission spectroscopy. Ifrared spectroscopy.- Circular Dichroism For Studying Gel-Like Phases: Introduction. Technique. Applications to the study of gel-like phases. Perspectives.- Systems - Organogels: Low Molecular Mass Organo-Gelators: Analyses of the natures of gelators and liquids for efficient gelation: Introduction. Classification of low molecular-mass organo-gelators (lmogs). The role of liquid in gelation by lmogs. Future directions.- Design And Functions Of Low Molecular Weight Gelators Bearing Sugars And Steroids In Their Backbone: Introduction. Steroid derivatives for gelating organic liquids. Sugar derivatives for gelating liquids. Other related lmogs. Outlook.- Safin Gels With Amphiphilic Molecules: Introduction. Amphiphilic molecules. Gels with amphiphilic molecules. Gemini amphiphilic molecules. Conclusions.- Hydrogels: Advances In Molecular Hydrogels: Introduction. Historical perspective. Amino acid derivatives and oligopeptide based hydrogelators- b -Peptides as hydrogelators.Carbohydrate based hydrogelator

Liquid crystal ‘blue phases’ with a wide temperature range

Abstract: 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, Kossel diagrams and differential scanning calorimetry, and show, using a simple polarizer-free electro-optic cell, that the reflected colour is switched reversibly in applied electric fields over a wide colour range in typically 10 ms. We propose that the unusual behaviour of these blue phase materials is due to their dimeric molecular structure and their very high flexoelectric coefficients. This in turn sets out new theoretical challenges and potentially opens up new photonic applications.

Liquid crystal–carbon nanotube dispersions

Abstract: Parallel alignment of nanotubes can be obtained by dispersion in a self-organizing anisotropic fluid such as a nematic liquid crystal. Exploiting the cooperative reorientation of liquid crystals, the overall direction of the nanotube alignment can be controlled both statically and dynamically by the application of external fields. These can be electric, magnetic, mechanic, or even optic in nature. Employing multiwall as well as single-wall carbon nanotubes, we show their parallel alignment along a uniform liquid crystal director field and electrically verify their reorientation behavior for two complementary geometries. These demonstrate electrically controlled carbon nanotube OFF–ON and ON–OFF switches. Further applicational potential will be outlined.

Nematic and Cholesteric Liquid Crystals: Concepts and Physical Properties Illustrated by Experiments

Abstract: Foreword Preface to the English edition Nematic and Cholesteric Liquid Crystals PART A: OVERVIEW Some History Georges Friedel and Liquid Crystals The Discovery of Birefringence in Fluid Biological Substances by Buffon, Virchow, and Mettenheimer: Lyotropic Liquid Crystals Observation of the Surprising Behavior of Cholesteryl Esters by Planer and Reinitzer: Thermotropic Liquid Crystals Fliessende Kristalle or "The Flowing Crystals" of Otto Lehmann Modern Classification of Liquid Crystals The Terminology Introduced by Georges Friedel Modern Definition of Mesophases Broken Symmetries Short- and Long-Distance Order Classification of Smectic Phases Classification of Columnar Phases Chiral Smectic Phases Mesogenic anatomy Thermotropic Liquid Crystals Lyotropic Liquid Crystals Liquid Crystal Diblock Copolymers Colloidal Liquid Crystals PART B: MESOPHASES WITH AN ORIENTATIONAL ORDER Structure and Dielectric Properties of the Nematic Phase Quadrupolar Order Parameter The Uniaxial Nematic-Isotropic Liquid Phase Transition The Uniaxial Nematic-Biaxial Nematic Phase Transition Low-Frequency Dielectric Properties Optical Properties Nematoelasticity: Frederiks Transition and Light Scattering Grupp Experiment Frank-Oseen Free Energy Free Energy Minimization: Molecular Field and Elastic Torques Interpretation of the Grupp Experiment Magnetic Field Action Action of an Electric Field: Displays Elastic Light Scattering and the Determination of the Frank Constants Nonlinear Optics Appendix 1: Calculating the Scattering Cross-Section Appendix 2: Free Energy Expression in the Fourier Space Nematodynamics and Flow Instabilities Preliminary Observations Illustrating Some Fundamental Differences Between a Nematic and an Ordinary Liquid Equations of the Linear Nematodynamics Laminary Couette and Poiseuille Flows Laminary Flows and Their Stability Convective Instabilities of Electrohydrodynamic Origin Thermal Instabilities Appendix 1: "Derivation Under the Integral" Theorem Appendix 2: Rotational Identity Appendix 3: Calculation of the Irreversible Entropy Production Appendix 4: Energy Dissipation and Constitutive Laws in the Formalism of Leslie-Ericksen-Parodi Appendix 5: The Rayleigh-Benard Instability in Isotropic Fluids Defects and Textures in Nematics Polarizing Microscope Observations The Volterra-de Gennes-Friedel process Energy of a Wedge Planar Line in Isotropic Elasticity Continuous Core Model: Landau-Ginzburg-de Gennes Free Energy Interaction Energy Between Two Parallel Wedge Lines Dynamics of a Planar Wedge Line: Calculating the Friction Force Wedge line Stability: Escape in the Third Dimension Bloch and Ising Walls Induced by the Frederiks Instability Anchoring and Anchoring Transitions of Nematics on Solid Surfaces Precursors On the Notion of Interface Interface Symmetry and Classification of the Different Types of Anchoring Wetting and Anchoring Selection Anchoring Transitions Measuring the Anchoring Energy in the Homeotropic Case The nematic-isotropic liquid interface:static properties and directional growth instabilities. Anchoring Angle, Surface Tension, and Width of the Nematic-Isotropic Interface Landau-Ginzburg-de Gennes Theory Instabilities in Confined Geometry Elastic Correction to the Gibbs-Thomson Relation Directional Growth of the Nematic-Isotropic Liquid Front Cholesterics: the First Example of a Frustrated Mesophase Cholesteric Frustration Cholesteric Order Parameter and the Cholesteric-Isotropic Liquid Phase Transition Optical Properties of the Cholesteric Phase Defects and Textures of the Cholesteric Phase Unwinding Transition Cholesteric Hydrodynamics Blue Phases: a Second Example of a Frustrated Mesophase Experimental Evidence for the Cubic Symmetry of Blue Phases I and II Uniaxial Models for the Blue Phases: Disclination Lattices Biaxial Model of the Blue Phases by Grebel, Hornreich, and Shtrikman Landau Theory of the Blue Phases by Grebel, Hornreich, and Shtrikman Experiments BPIII or Blue Fog Overview of Growth Phenomena and _the Mullins-Sekerka Instability Gibbs-Thomson Relation and the Phase Diagram of a Diluted Binary Mixture The Minimal Model Stationary Plane Front Plane Front in the Diffusive Regime (GBPG

Shear rheology of lyotropic liquid crystals: a case study.

Abstract: We have investigated the rheological properties of lyotropic liquid crystals (LCs) formed by self-assembled neutral lipids and water, their relationship with the topology of the structure, and their dependence on temperature and water content. The phase diagram of a representative monoglyceride−water system, determined by combining cross-polarized optical microscopy and small-angle X-ray scattering (SAXS), included four structures: lamellar, hexagonal, gyroid bicontinuous cubic (Ia3d), and double diamond bicontinuous cubic (Pn3m), as well as several regions of two-phase coexistence of some of the above structures. Rheology in the linear viscoelastic regime revealed a specific signature that was characteristic of the topology of each structure considered. The order−order transitions lamellar-to-cubic and cubic-to-hexagonal, as well as the order−disorder transitions from each LC to an isotropic fluid, were easily identified by following the development of the storage and loss moduli, G‘ and G‘ ‘, respectiv...

Electro-tunable optical diode based on photonic bandgap liquid-crystal heterojunctions.

Abstract: Manipulation of light is in strong demand in information technologies. Among the wide range of linear and nonlinear optical devices that have been used, growing attention has been paid to photonic crystals that possess a periodic modulation of dielectric function. Among many photonic bandgap (PBG) structures, liquid crystals with periodic structures are very attractive as self-assembled photonic crystals, leading to optical devices such as dye lasers. Here we report a new hetero-PBG structure consisting of an anisotropic nematic layer sandwiched between two cholesteric liquid-crystal layers with different helical pitches. We optically visualized the dispersion relation of this structure, displaying the optical diode performance: that is, the non-reciprocal transmission of circular polarized light at the photonic-bandgap regions. Transmittance spectra with circularly polarized light also reveal the diode performance, which is well simulated in calculations that include an electro-tunable diode effect. Lasing action was also confirmed to show the diode effect with a particular directionality.

Electric Field Tuning of Plasmonic Response of Nanodot Array in Liquid Crystal Matrix

Abstract: In this work we demonstrate the feasibility of electric-field tuning of the plasmonic spectrum of a novel gold nanodot array in a liquid crystal matrix. As opposed to previously reported microscopically observed near-field spectral tuning of individual gold nanoparticles, this system exhibits macroscopic far-field spectral tuning. The nanodot-liquid crystal matrix also displays strong anisotropic absorption characteristics, which can be effectively described as a collective ensemble within a composite matrix in the lateral dimension and a group of noninteracting individual particles in the normal direction. The effective medium model and the Mie theory are employed to describe the experimental results.

Electrically tunable photonic bandgap guidance in a liquid-crystal-filled photonic crystal fiber

Abstract: Tunable bandgap guidance is obtained by filling the holes of a solid core photonic crystal fiber with a nematic liquid crystal and applying an electric field. The response times are measured and found to be in the millisecond range.

Triphenylene‐based discotic liquid crystal dimers, oligomers and polymers

Abstract: Discotic liquid crystals are unique nanostructures with remarkable electronic and optoelectronic properties. Triphenylene derivatives play a major role in the research on discotic liquid crystals. Following recent reviews of the chemistry of triphenylene‐based monomeric liquid crystals, this article now reviews the chemistry and physical properties of triphenylene‐based discotic dimeric, oligomeric and polymeric liquid crystals.

Elasticity-mediated self-organization and colloidal interactions of solid spheres with tangential anchoring in a nematic liquid crystal.

Abstract: Using laser tweezers, we study colloidal interactions of solid microspheres in the nematic bulk caused by elastic distortions around the particles with tangential surface anchoring. The interactions overcome the Brownian motion when the interparticle separation r-->p is less than 3 particle diameters. The particles attract when the angle theta between r-->p and the uniform far-field director n0 is between 0 degrees and approximately 70 degrees and repel when 75 degrees

Electrical-field effect on carbon nanotubes in a twisted nematic liquid crystal cell

Abstract: We have fabricated twisted nematic cells doped by carbon nanotubes (CNTs). The CNTs with a minute amount of doping did not perturb the liquid crystal orientation in the off and on state. The hysteresis studies of voltage-dependent transmittance and capacitance under ac and dc electric field showed that the residual dc, which is related to an image sticking problem in liquid crystal displays, was greatly reduced due to the ion trapping by CNTs.

Self-assembly of polymer-tethered nanorods.

Abstract: We present results of molecular simulations that predict the phases formed by self-assembly of nanorods functionalized by a polymer ``tether.'' Microphase separation of the immiscible tethers and rods coupled with the liquid crystal ordering of the rods induces the formation of a cubic phase, a smectic C phase, a tetragonally perforated lamellar phase, and a honeycomb phase; the latter two have been observed experimentally but have not been predicted. We also predict a new phase---a racemic mixture of hexagonally ordered chiral cylinders that self-assemble from these achiral building blocks.

Frank–Kasper, quasicrystalline and related phases in liquid crystals

Abstract: The review covers 3-dimensional and some 2-dimensional self-assembly patterns of supramolecular liquid crystals possessing either quasi-periodic or closely related truly periodic order. Compounds showing such structures are amphiphilic, and most often wedge-shaped, with dendrons being the most common examples. The topology is described in terms of 3D and 2D tiling of a variety of polyhedra or polygons, respectively. Analogy is made with structures of metallic alloys and soap froth. The recently discovered dodecagonal liquid quasicrystal is compared with the different tetrahedrally close packed Frank–Kasper phases in thermotropic and lyotropic liquid crystals. Parallels are also drawn with honeycomb columnar phases with related plane tilings, including that of distorted pentagons. The potential for the creation of nearly isotropic photonic bandgap materials is mentioned.

Magnetic alignment of the chiral nematic phase of a cellulose microfibril suspension.

Abstract: Stable suspensions of tunicate cellulose microfibrils were prepared by acid hydrolysis of the cellulosic mantles of tunicin. They formed a chiral nematic phase above a critical concentration. External magnetic fields were applied to the chiral nematic phase in two different manners to control its phase structure. (i) Static magnetic fields ranging 1-28 T were used to align the chiral nematic axis (helical axis) in the field direction. (ii) A rotating magnetic field (5 T, 10 rpm) was applied to unwind the helices and to form a nematic phase. These phenomena were interpreted in terms of the anisotropic diamagnetic susceptibility of the cellulose microfibril. The diamagnetic susceptibility of the microfibril is smaller in the direction parallel (chi( parallel)) to the fiber axis than in the direction perpendicular (chi( perpendicular)) to the fiber axis, that is, chi( parallel) < chi( perpendicular) < 0. Because the helical axis coincides with the direction normal ( perpendicular) to the fiber axis, the helical axis aligned parallel to the applied field. On the other hand, the rotating magnetic field induced the uniaxial alignment of the smallest susceptibility axis, that is, chi( parallel) in the present case, and brought about unwinding of the helices.

Liquid crystal elastomers, networks and gels: advanced smart materials

Abstract: The principal concepts, preparation methods, important features of liquid crystal elastomers, anisotropic networks and gels, as well as recent exciting progress in this field are reviewed. Discussions are focused on their potential applications as smart materials for converting electrical or optical energy into mechanical energy and vice versa – such applications include artificial muscles, light scattering electro-optical switches and display materials, electro- or photo-controllable micro- or nano-machinery, electrically switchable color-tunable reflectors (mirrors) and full-color reflective display, fine-tunable and low-threshold mirror-less lasing, etc. In addition, some innovative anisotropic networks consisting of ferroelectric liquid crystals, discotic mesogens and mesomorphic ladder-like polysiloxanes are also highlighted.

Alignment Technology and Applications of Liquid Crystal Devices

Abstract: Contents Preface Overview, Kohki Takatoh Development of the LCD Market Improvement of Viewing Angle Dependence of the Contrast Ratio Ferroelectric and Antiferroelectric Liquid Crystals Development of Novel Alignment Method The Characteristics of this Book RubbingTechnologies: Mechanisms and Applications, Masaki Hasegawa Introduction Rubbing Mechanisms Applications Non-rubbing Methods Introduction, Masaki Hasegawa Photoalignment, Masaki Hasegawa Oblique Evaporation Method, Masanori Sakamoto Liquid Crystal Alignment on Microgroove Surfaces, Kohki Takatoh LB Membranes for the Alignment Layer, Kohki Takatoh PTFE Drawn Films for Alignment Layers, Kohki Takatoh Liquid Crystalline Alignment on Chemically Treated Surfaces, Kohki Takatoh Applications of Nematic Liquid Crystals Summary of Molecular Alignment and Device Applications, Ray Hasegawa Twisted Nematic (TN), Ray Hasegawa Super Twisted Nematic (STN), Nobuyuki Itoh The IPS (In-Plane Switching) Mode, Mitsuhiro Koden Vertical Alignment (VA) Mode and Multi-domain Vertical Alignment (MVA) Mode, Kohki Takatoh Pi Cell, Masaki Hasegawa Multi-domain Mode, Kohki Takatoh Polymer Dispersed Liquid Crystals (PDLC), Ray Hasegawa Alignment Phenomena of Smectic Liquid Crystals Layer Structure and Molecular Orientation of Ferroelectric Liquid Crystals, Nobuyuki Itoh Alignment and Bistability of Ferroelectric Liquid Crystals, Kohki Takatoh Applications of Ferroelectric and Antiferroelectric Liquid Crystals Molecular Orientations and Display Performance in FLC Displays, Mitsuhiro Koden Alignment and Performance of AFLCD, Kohki Takatoh Application of FLC/AFLC Materials to Active-matrix Devices, Kohki Takatoh Index

Smectic and columnar liquid crystals : concepts and physical properties illustrated by experiments

Abstract: Dedication Preface to the English edition SMECTIC AND COLUMNAR LIQUID CRYSTALS Structure of the Smectic A Phase and the Transition Toward the Nematic Phase Lamellar Structure of the Smectic A Phase The Smectic A-Nematic Transition: a Simplified Approach De Gennes' Theory of the Smectic A-Nematic Transition: Analogy With Superconductivity Pretransitional Effects in the Nematic Phase Influence of the Fluctuations in Director Orientation on the Transition Order: the Theory of Halperin, Lubensky, and Ma An Alternative Theory Appendix 1: Superconducting Vortices and Screw Dislocations Appendix 2: X-ray Scattering and the Determination of the Structure Factor Continuum Theory of Smectics A Hydrodynamics Static Description Layer Undulation Instability Equations of Smectic Hydrodynamics Sound Propagation in Smectics Continuous flow Dislocations, Focal Conics, and Rheology of Smectics A Focal conics Dislocations Rheological Behavior at Low Shear Rate and Lubrication Theory Rheological Behavior at High Shear Rates Microplasticity Ferroelectric and Antiferroelectric Mesophases Do Ferroelectric Mesophases Exist? Genesis of the Smectic C* Phase Experimental Evidence for the Spontaneous Polarization Measurement and Use of the Spontaneous Polarization Hydroelectric Effect Electromechanical Effect The SmA -> SmC* Transition Anticlinic, Mesophases, Antiferroelectric and Ferrielectric Mesophases Formed by Banana-Shaped Molecules The Twist-Grain Boundary Smectics The Renn-Lubensky Model Discovery of the TGBA Phase Other TGB Phases Elasticity of the TGB Phases Smectic Blue Phases Hexatic Smectics Theory of Two-Dimensional Melting: the Hexatic Phase Hexatic Smectics Quasi-Elastic Light Scattering Rheology Appendix 1: Peierls Disorder The Smectic B Plastic Crystal Structure of Smectic B Phases Elastic and Plastic Properties Equilibrium Shape of a Smectic B Germ Herring instability Instabilities of the SmA-SmB Front in Directional Growth Appendix 1: Creep by Crossing of Localized Obstacles Smectic Free Films Making Smectic Films Some Crucial Experiments Thermodynamics of Thermotropic Films Steps in Smectic Films Film Structure and Phase Transitions at a Fixed Number of Layers Smectic Films as Model Systems Columnar Phases Structure and Optical Properties Elasticity Developable Domains Dislocations Measurement of the Elastic Constants Mechanical Instabilities Dynamics of the Buckling Instabilities Light Scattering Threads of Columnar Mesophases Growth of a Columnar Hexagonal Phase Phase Diagram and Physical Constants of the Material Growth in the Diffusive Regime (0

High charge-carrier mobility in pi-deficient discotic mesogens: design and structure-property relationship.

Abstract: Hexaazatrinaphthylene (HATNA) derivatives with six alkylsulfanyl chains of different length (hexyl, octyl, decyl and dodecyl) have been designed to obtain new potential electron-carrier materials. The electron-deficient nature of these compounds has been demonstrated by cyclic voltammetry. Their thermotropic behaviour has been studied by means of differential scanning calorimetry and polarised optical microscopy. The supramolecular organisation of these discotic molecules has been explored by temperature-dependent X-ray diffraction on powders and oriented samples. In addition to various liquid crystalline columnar phases (Col(hd), Col(rd)), an anisotropic plastic crystal phase is demonstrated to exist. The charge-carrier mobilities have been measured with the pulse-radiolysis time-resolved microwave-conductivity technique. They are found to be higher in the crystalline than in the liquid crystalline phases, with maximum values of approximately 0.9 and 0.3 cm(2) V(-1) s(-1), respectively, for the decylsulfanyl derivative. Mobilities strongly depend on the nature of the side chains.

Aggregation Behavior And Chromonic Liquid Crystal Properties Of An Anionic Monoazo Dye

Abstract: X-ray scattering and various optical techniques are utilized to study the aggregation process and chromonic liquid crystal phase of the anionic monoazo dye Sunset Yellow FCF. The x-ray results demonstrate that aggregation involves - stacking of the molecules into columns, with the columns undergoing a phase transition to an orientationally ordered chromonic liquid crystal phase at high dye concentration. Optical absorption measurements on dilute solutions reveal that the aggregation takes place at all concentrations, with the average aggregation number increasing with concentration. A simple theory based on the law of mass action and an isodesmic aggregation process is in excellent agreement with the experimental data and yields a value for the “bond” energy between molecules in an aggregate. Measurements of the birefringence and order parameter are also performed as a function of temperature in the chromonic liquid crystal phase. The agreement between these results and a more complicated theory of aggregation is quite reasonable. Overall, these results both confirm that the aggregation process for some dyes is isodesmic and provide a second example of a well-characterized chromonic system.

Optical characterization of the nematic lyotropic chromonic liquid crystals: light absorption, birefringence, and scalar order parameter.

Abstract: We report on the optical properties of the nematic (N) phase formed by lyotropic chromonic liquid crystals (LCLCs) in well aligned planar samples. LCLCs belong to a broad class of materials formed by one-dimensional molecular self-assembly and are similar to other systems such as "living polymers" and "wormlike micelles." We study three water soluble LCLC forming materials: disodium chromoglycate, a derivative of indanthrone called Blue 27, and a derivative of perylene called Violet 20. The individual molecules have a planklike shape and assemble into rodlike aggregates that form the phase once the concentration exceeds about 0.1 M. The uniform surface alignment of the N phase is achieved by buffed polyimide layers. According to the light absorption anisotropy data, the molecular planes are on average perpendicular to the aggregate axes and thus to the nematic director. We determined the birefringence of these materials in the N and biphasic N-isotropic (I) regions and found it to be negative and significantly lower in the absolute value as compared to the birefringence of typical thermotropic low-molecular-weight nematic materials. In the absorbing materials Blue 27 and Violet 20, the wavelength dependence of birefringence is nonmonotonic because of the effect of anomalous dispersion near the absorption bands. We describe positive and negative tactoids formed as the nuclei of the new phase in the biphasic N-I region (which is wide in all three materials studied). Finally, we determined the scalar order parameter of the phase of Blue 27 and found it to be relatively high, in the range 0.72-0.79, which puts the finding into the domain of general validity of the Onsager model. However, the observed temperature dependence of the scalar order parameter points to the importance of factors not accounted for in the athermal Onsager model, such as interaggregate interactions and the temperature dependence of the aggregate length.

Structure And Thermodynamics of Weakly Segregated Rod-Coil Block Copolymers

Abstract: The self-assembly of a series of monodisperse rod−coil block copolymers is studied in the weak segregation limit. This unusual weakly segregated system consists of polyisoprene (PI) coil blocks and poly(alkoxyphenylene vinylene) (PPV) rod blocks solubilized with alkoxy side groups. The order to microphase disorder transition (ODT) and nematic isotropic (NI) transition are experimentally investigated to produce a rod−coil block copolymer phase diagram in a system that follows polymeric scaling relationships. Small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), polarized optical microscopy, depolarized light scattering, and wide-angle X-ray scattering (WAXS) are used to map the phase diagram. As the symmetric diblock copolymer is heated, a series of transitions from lamellar to nematic to isotropic phases are observed. The NI transition temperature decreases with increasing coil fraction, and at high coil fractions only an isotropic phase is observable. The phase behavior is in quali...

Liquid crystal lens element and optical head device

Abstract: There is provided a liquid crystal lens element having a lens function capable of performing spherical aberration correction including a power component equivalent to a focal point change of the stable incident light in accordance with the voltage applied. One (12) of the two transparent substrates (11, 12) has a transparent electrode (15) and a Fresnel lens surface (17) while the other (11) of the two transparent substrates has a phase correction surface (18) and a transparent electrode (16). Thus, by arranging the Fresnel lens surface (17) and the liquid crystal layer (13) between the two transparent electrodes (15, 16), the substantial refraction factor distribution of the liquid crystal layer (13) is changed according to the voltage applied, so that positive/negative power is given to the wavefront transmitting through the liquid crystal layer (13), the Fresnel lens surface (17), and the phase correction surface (18).

Self-assembly of polymer-tethered nanorods

Abstract: We present results of molecular simulations that predict the phases formed by self-assembly of nanorods functionalized by a polymer ``tether.'' Microphase separation of the immiscible tethers and rods coupled with the liquid crystal ordering of the rods induces the formation of a cubic phase, a smectic C phase, a tetragonally perforated lamellar phase, and a honeycomb phase; the latter two have been observed experimentally but have not been predicted. We also predict a new phase---a racemic mixture of hexagonally ordered chiral cylinders that self-assemble from these achiral building blocks.

Continuously tunable devices based on electrical control of dual-frequency liquid crystal filled photonic bandgap fibers.

Abstract: We present an electrically controlled photonic bandgap fiber device obtained by infiltrating the air holes of a photonic crystal fiber (PCF) with a dual-frequency liquid crystal (LC) with pre-tilted molecules. Compared to previously demonstrated devices of this kind, the main new feature of this one is its continuous tunability due to the fact that the used LC does not exhibit reverse tilt domain defects and threshold effects. Furthermore, the dual-frequency features of the LC enables electrical control of the spectral position of the bandgaps towards both shorter and longer wavelengths in the same device. We investigate the dynamics of this device and demonstrate a birefringence controller based on this principle.

Noncrystalline phases in poly(9,9-di-n-octyl-2,7-fluorene).

Abstract: Selective formation of amorphous, nematic (N), and β phases in poly(9,9-di-n-octyl-2,7-fluorene) (PFO) films was achieved via judicious choice of process parameters. Phase structure and film morphology were carefully examined by means of X-ray diffraction as well as electron microscopy. “Amorphous” thin films were obtained by quick evaporation of solvent. Slow solvent removal during film formation or extended treatment of the amorphous film with solvent vapor resulted in predominantly the β phase, which corresponds to a frozen (due to decreased segmental mobility upon solvent removal) and intrinsically metastable state of transformation midway between a solvent-induced clathrate phase and the equilibrium crystalline order in the undiluted state. The frozen transformation process is reactivated upon an increase in temperature beyond 100 °C. Compared to the amorphous film, extended backbone conjugation in the β phase is evidenced from the emergence of a characteristic absorption peak around 430 nm near the ...

Very high time-of-flight mobility in the columnar phases of a discotic liquid crystal

Abstract: 1,4,8,11,15,18,22,25-Octaoctylphthalocyanine shows the highest time-of-flight (“long-range”) hole mobility so far reported for the columnar phase of a discotic liquid crystal. Unlike most other high long-range mobility columnar discotics, there is no clear evidence from x-ray diffraction of high order.