Optical Field Induced Scattering in Polymer Dispersed Liquid Crystal Films

Abstract

Polymer dispersed liquid crystals are composite materials consisting of inclusions of liquid crystalline materials dispersed in a polymer binder. If the refractive indices of the constituent liquid crystal and polymer are appropriately matched, then films of these materials may be switched between an optically scattering state and a non–scattering transparent state^1?2 by the application of electric fields which reorient the liquid crystal in the inclusions. In this paper we discuss the response of these materials to intense laser radiation, and examine the mechanisms associated with optical field induced reorientation.     

Acid mediated tunability of stimulated laser emission from dye doped chiral microdroplets

Abstract

In the last decade, the possibility to use liquid crystal droplets as optical micro-cavities and lasers has attracted much attention since it paves the way for many applications in the field of sensors or tunable photonics. Several techniques can be used to obtain small micro-resonators as, for example, dispersing a cholesteric liquid crystal inside an immiscible isotropic fluid to create an emulsion. Since liquid crystals are extremely sensitive to external factors as temperature or external fields, laser tuning can be easily achieved. Here, we report on the possibility to tune the laser emission from dye doped cholesteric liquid crystals microdroplets dispersed in a glycerol matrix in presence of nitric acid molecules in the emulsion. Using a fluorescent dye with pH dependent optical properties, the emitted laser wavelength can be tuned in a range of 60?nm. This effect could find applications for the development of spectroscopy based sensors.     

Russian Liquid Crystal Society awards first Freederiksz Medals

Abstract

The interest in research and development of plastic liquid crystal technology originates from the stabilization of liquid crystals with a polymer matrix for manufacturing of largearea flexible electro-optical films which can be used as electrically switchable shutters, panels and privacy windows in the building, residential and transportation sectors, as well as reflective displays. In figure 1, we show typical examples of PDLC applications as electro-optical windows. The display applications of PDLC are not within the scope of the present article.     

Thermotropic Liquid-Crystalline Polymers,Re-Entrant and Discotic Liquid Crystals. A Bibliography for 1977–1984

This bibliography contains bibliographic data of books and papers concerning the thermotropic liquid-crystalline polymers, re-entrant and discotic liquid crystals for 1977–1984 years. The bibliography of the thermotropic liquid-crystalline polymers contains also bibliographic data of papers describing both the thermotropic and lyotrophic behavior of the liquid-crystalline polymers, liquid-crystalline melts, solutions of thermotropic liquid-crystalline polymers, solutions of polymer chains in a nematic solvent and some polymerization problems. In this bibliography are included also bibliographic data of papers which according to the citation in the literature are important for the study of both the thermotropic and lyotropic liquid-crystalline polymers. In the bibliography of the re-entrant liquid crystals (at atmospheric pressure) are included the papers published after the first work in this field of Cladis (Phys. Rev. Lett. 35, 48–51 (1975)) up to the end of 1984 year. Similarly, in the bibliography of the discotic liquid crystals are included the papers published after the first work of Chandrasekhar, Sadashiva and Suresh (Pramana 9, 477–480, (1977)) also up to the end of 1984 year.     

Liquid Crystals and Polymer-Based Photonic Crystal Fibers

Experimental results of polymer photonic liquid crystal fibers based on commercially available (Kiriama) PMMA and cyclo-olefin polymer (Zeonex 480R) microstructured polymer fibers infiltrated with nematic liquid crystals (2CHBT/8CHBT and PCB) are presented and thermally-tuned photonic band-gap propagation mechanism is observed. These preliminary results suggest, that polymers binding to liquid crystals much easier than silica, can offer new opportunities while using polymer-based photonic crystal fibers.     

Dielectric relaxation and electro-optical switching behavior of nematic liquid crystal dispersed in poly(methyl methacrylate)

Polymer dispersed liquid crystal composite films were prepared from poly(methyl methacrylate) and nematic liquid crystal E44 by solvent induced phase separation method. In the present investigation we report effect of liquid crystal concentration on the electro-optical and dielectric properties of the composite films. The results were interpreted in terms of phase separation of liquid crystal and polymer, shape and size of liquid crystal droplet, interfacial charge layer effect, liquid crystal loading and miscibility of liquid crystal in the polymer matrix. The miscibility between two phases at interface was investigated by employing Fourier‐Transform Infrared Spectroscopy and differential scanning calorimetry. Morphological study showed that liquid crystal phase is embedded in a spongy poly(methyl methacrylate) matrix and homogeneous distribution increased with increasing E44 content. Electro optical behavior of these composite films under the condition of an externally applied AC electric field (0–200 Vp-p, 50–1000 Hz) and wide range of temperature was determined using He–Ne laser (wave length 632.8 nm) as a light source. It was found that Poly(methyl methacrylate)/E44 (30/70) wt.% composite has more significant properties than the other concentrations. The performance of all composites showed variations with respect to applied voltage as well as temperatures. Dielectric measurement of polymer dispersed liquid crystals has been carried out in the frequency range from 20 Hz to 20 MHz and over the temperature range from 24 °C to 100 °C. The Maxwell–Wagner effect due to interfacial charge accumulation between boundaries of liquid crystal droplets and surrounding of polymer matrix has been observed.     

Material Highlights: Liquid Crystal Polymers

Abstract

A key issue in the area of side chain liquid crystal polymers concerns the conformation of the backbone within the mesophase. In order to resolve this matter, Noirez and co-workers (Liquid Crystals, 1995, 18 129) have studied a wide range of polymers exhibiting both nematic and smectic behaviour using small angle neutron scattering. Their results combined with a survey of those in the literature reveal that if the polymer is solely nematogenic, then the backbone adopts a prolate conformation in which on average it lies along the director. By contrast, in a smectic phase the backbone adopts an oblate shape and on average lies perpendicularly with respect to the director. This arises from a microphase separation in which the backbones are distributed between the mesogenic layers. However, the backbones can cross the mesogenic layers although their ability t o do so depends on, amongst other factors, the degree of polymerization. If the polymer exhibits both nematic and smectic behaviour, then in the nematic phase the backbone adopts a slightly oblate shape resulting from smectic fluctuations.     

Liquid Crystal Polymers: Conformation of Deuterated Parts by SANS

Comb like liquid crystal polymers are polymers on each monomer is grafted a hanging liquid crystal molecule. The bulk state has the classical liquid crystal phases (nematic, smectic); what is the conformation of a chain in these phases is the problem to solve with the help of small angle neutron scattering. The values of the global sizes R _|| and R _‖ of a chain (in the direction parallel and perpendicular to the director) are discussed as a function of the chemical species and the temperature. Strange results are discussed in terms of segregation of labelled parts.     

In Memoriam of Professor Pier Luigi Nordio, 1936–1998

Abstract

It is with deep regret that we bring to the attention of the liquid crystal community the untimely death, on 20 October 1998, of Pier Luigi Nordio, Professor of Theoretical Chemistry at Padova University (Italy), and author of many important contributions to the theory of liquid crystals.     

Otto Lehman Prize 1999

Abstract

The Otto Lehman Foundation was established in 1997 by Professors Mlynski and Knoll from the University of Karlsruhe, Germany. The mission of the foundation is to promote scientific contributions from the rising generation of scientists in the field of liquid crystals. The prize can be given for outstanding habilitations, doctoral theses and graduate work in fields of liquid crystal science making a significant contribution to the application of liquid crystals.     

Magnetic Field Induced Deformations in Nematic Liquid Crystals

Abstract

Unlike most organic materials, liquid crystals respond readily to magnetic fields. This response originates in the anisotropic magnetic susceptibility of nematics, and the relative ease with which they undergo orientational deformations. Because of the large optical birefringence of liquid crystals, these orientational deformations are easy to observe. An interesting demonstration is to place a small rare earth magnet on a nematic cell between crossed polarizers which is illuminated from below. As the magnetic field reorients the liquid crystal, beautiful interference colours appear, indicating director deformations.     

Liquid Crystal Light Valves with Organic Polymeric Photoconductors

Abstract

It has been suggested to use organic polymerle photoconductors in the liquid crystal light valves instead of widely used inorganic ones. Light valves with photoconductive sensibilized polyimides with various types of electro-optical effects in liquid crystals were constructed and tested. The influences of the read-on light regimes and control voltages were analysed. High resolution up to 1500 mm^?1 at diffraction efficiency 0.1%, and 40% at space frequencies up to 100 mm^?1 were obtained in pulse regimes. Thin holograms can be effectively read on such modulators in reversible mode in real time. The results obtained permit to come to a conclusion about prospects of the organic polymeric photoconductor usage in liquid crystal light valves and applications of the latter for input and output of the optical information.     

Light Scattering by Cholesteric Spherulites

Abstract

In this work, we analyze the scattering of light by the so-called spherulites or skyrmions in cholesteric liquid crystals. These are quasi-planar localized excitations of the director configuration. We compute the cross section of the polarization conversion for polarized incident light in the Born approximation, considering the anisotropic optical properties of the liquid crystal and the shape of the spherulite. We compare the results obtained by the analytical and the numerical skyrmion solutions found for several values of the model parameters. Finally, possible practical employments and theoretical extensions in the study of the phenomenon are foreseen.     

Annual Meeting of the American Chemical Society

Abstract

Ordered Fluids and Liquid Crystals was the focus of the two-day symposium on April 5 and 6 in Anaheim, California that was sponsored by the Division of Colloid and Surface Chemistry, during the Annual Meeting of the American Chemical Society. The organizers were 5. Rananavare of the Oregon Graduate Institute and R. Shashidhar of the Naval Research Laboratory who did a magnificent job in arranging a program that was faithful to the title and addressed issues on nearly all topics of liquid crystals research and technology. Both days were packed with presentations that covered lyotropics, ferroelectrics, liquid crystal polymers, molecular theory, new materials, and, of course, applications.     

Understanding protein crystallization on the basis of the phase diagram

The phase diagram of a protein–water system is described with a simple model with parameters for the interaction between the protein molecules in the crystal and in the solution. For a certain range of these parameters the phase diagram shows a metastable liquid–liquid immiscibility region. It is shown that this region corresponds to the “crystallization slot” for growing crystals, as proposed by George and Wilson [Acta Crystallogr. D 50 (1994) 361]. Nucleation in this region proceeds in two steps. First small liquid droplets with a high protein concentration are formed; then small crystalline nuclei grow inside these droplets. In the crystallization slot crystals are covered by a thin liquid film with a high protein concentration. We discuss NMR experiments on lysozyme, which show that nucleation is a transient process with an induction time.     

Bilayered Super-Structures of Antiferroelectric Mesogens

Abstract

Crystal structures of two antiferroelectric mesogens, TFMHPBC and MHPBC-10, were analysed by an X-ray diffraction method. In both crystals, mesogen molecules formed a herringbone structure which was essentially the same as that proposed for the antiferroelectric liquid crystal phase. Because of the crystallographic 2₁-symmetry along the b-axis, only the b-axis component of the dipole moment remains in a smectic layer. Since the dipole moment in the next layer has the same magnitude but the opposite direction, both crystals show no dipole moment as a whole. These structural features observed in their crystal states seem to be conserved in their antiferroelectric liquid crystal phases which are just above their crystal phases.     

Raman Scattering from Liquid Crystals

Abstract

The use of Raman scattering technique as a tool for studying local order in complex liquid crystal systems is illustrated by three examples, namely the locally anisotropic liquid L-phase, lipid membranes, and PDLC's.     

Lasing Pixels: A New Application for PDLCs

Abstract

Pixels in an intracavity polymer dispersed liquid crystal can switch lasing action on and off, yielding addressable lasing pixels for projection applications. The marriage between an efficient PDLC display element and laser illumination consolidates the positive attributes of two different approaches to projection into a very capable technology with greater application potential. This radically different approach to projection certainly meets many of the market needs where current technologies fall short. With these needs met, a new application of PDLC technology is on the horizon.     

Bowlic and Polar Liquid Crystal Polymers

Bowlic liquid crystals are made up of polar molecules. Both main-chain and side-chain bowlic polymers are possible. Exactly solvable discrete models describing the phases and conformations of these bowlic and other polar liquid crystal polymers are presented. For the ideal one-dimensional case the model is equivalent to the 1D Ising model. Susceptibility and other properties are calculated. Wave propagation and solitons in these polar polymers are discussed. Possible highly conducting and high T_c superconducting liquid crystal polymers are proposed.     

Nematic and Cholesteric PDLC Elaborated under Shear Stress

Abstract

The properties of nematic and cholesteric Polymer Dispersed Liquid Crystal (PDLC) with ellipsoidal droplets are reported. The shape of the droplets are modified by a shear stress during the preparation. Different values of the shear stress were used which permit an ellipticity ratio modification by a factor 10. The electro-optical properties are measured for different types of samples and compared with theoretical previsions: transmission or reflection under voltage, threshold voltage, response times. We also describe the preparation of the sample, the experimental apparatus used for the shear stress, and the expected theoretical quantities like response times. When the liquid crystals are nematics, we obtain an increase of the threshold voltages, as predicted. The relaxation time decreases with increasing deformation but the diminution is not as great as predicted. When the liquid crystals are cholesterics, the high deformations induce polygonal fields, which strongly modify the electro-optical properties of the micro-composite. The threshold voltages and response time at applied voltage decrease whereas the relaxation time is increasing.