Showing papers in "Molecular Crystals and Liquid Crystals in 1991"

Adhesion of Vesicles and Membranes

Abstract: In the presence of an attractive surface, a vesicle can undergo shape transformations between two different free states, between a free and a bound state, and between two different bound states. Adhesion can also lead to topological changes such as vesicle rupture and vesicle fusion. The interaction between the vesicle membrane and the surface is renormalized by thermally excited shape fluctuations. This renormalization leads to unbinding phenomena both for fluid and for polymerized (or solid-like) membranes.

Low-Power Optical Reorientation in Dyed Nematics

Abstract: The optical reorientation process in nematic guest-host mixtures is investigated. It is found that for certain dyes the optical Freedericksz threshold is two orders of magnitude lower as in the pure host. The observations can be interpreted in terms of an enhanced optical torque which increases with the dye concentration.

Cross-Coupling Reactions in the Synthesis of Liquid Crystals

Abstract: The synthesis of structurally complex liquid crystals by traditional linear methods is often difficult, if not impossible. When many functional groups of a specific substitution pattern are required it is very likely that the sequence of reactions will interfere with functional groups already present or the directing effect of such functional groups may lead to the wrong substitution pattern. Syntheses using small, more easily managed units of the correct substitution patterns in high-yielding coupling reactions are a much more viable approach; the possible use of the sub-units in the synthesis of many different final compounds leads to systematic and economic syntheses. Suitable methods of synthesis of liquid crystals by cross-coupling reactions of various types are discussed, examples are provided and issues of selectivity are considered. The methods based on arylboronic acids are particularly useful.

A Continuum Theory for Smectic C Liquid Crystals

Abstract: Our aim in this paper is to present a rather general non-linear continuum theory that may prove useful for the interpretation of observations and experiments upon smectic C liquid crystals. In its present formulation our theory is constrained to exclude variations in the layer spacing thickness and also changes in tilt with respect to the layer normal. De Gennes1 discusses a theory of this type for smectic A liquid crystals, but for these liquids applications of such a constrained theory are somewhat more restricted than for smectic C materials, and consequently our interest focuses on the latter. Throughout, much of our motivation and reasoning stem from experience with nematic theory, details of which are available in, for example, the reviews by Ericksen2 and Leslie3 as well as the books by de Gennes1 and Chandrasekhar.4

The effect of electric fields on blue phases

Abstract: An experimental review on electric field effects in blue phases is presented. Electric fields have a profound effect on these phases which appear in chiral liquid crystals. In addition to producing elec-trostrictive effects on blue phase lattices, they also cause new field induced phases—tetragonal BPX, hexagonal BPH3D and BPH2D, and possible other phases yet to be identified. In recent experiments on BPIII for materials with negative dielectric anisotropy, it was found that electric fields cause a dramatic increase in the otherwise weak BPIII selective reflection. These experimental results were discussed with respect to possible models for the unknown structure of BPIII. Recently, efforts were also made to investigate the dynamics of different electrooptic effects more precisely.

Cyclosiloxanes with mesogenic side groups

Abstract: Die vorliegende Erfindung betrifft cyclische Siloxane, welche mindestens eine an ein Siliciumatom gebundene Gruppe der Formel (1) enthalten The present invention relates to cyclic siloxanes, containing at least one silicon atom bonded to a group of the formula (1) worin x eine ganze Zahl im Wert von mindestens 2, vorzugsweise von 2 bis 10, wherein x is an integer having a value of at least 2, preferably from 2 to 10, R" eine chemische Bindung oder einen zweiwertigen Rest der Formel -COO-, -OOC-, -CH 2 -CH 2 -, -CH = CH-, -C=C-, -N = N-, -N = N(O)-, -CH = N-, -N = CH- oder -Si(R) 2 -, wobei die Reste R "is a chemical bond or a divalent radical of the formula -COO-, -OOC-, -CH 2 -CH 2 -, -CH = CH-, -C = C-, -N = N-, -N = N ( O) -, -CH = N-, -N = CH- or -Si (R) 2 -, where the radicals R gleiche oder verschiedene, gegebenenfalls substituierte Kohlenwasserstoffreste mit 1 bis 18 Kohlenstoffatomen sind, R are identical or different, optionally substituted hydrocarbon radicals having 1 to 18 carbon atoms, L jeweils gleiche oder verschiedenen, gegebenenfalls durch mindestens einen Rest Q in 2-, 3-, 5- und/oder 6-Stellung substituierten 1,4-Phenylen- oder 1,4-Cyclohexylenreste, L are identical or different, optionally substituted by at least one radical Q in the 2-, 3-, 5- and / or 6-position substituted 1,4-phenylene or 1,4-cyclohexylene, Q gleiche oder verschiedene Reste, namlich Wasserstoff-, Fluor- oder Chloratome, Cyano-, Methyl- oder Trifluormethylgruppen, Q identical or different radicals, namely hydrogen, fluorine or chlorine atoms, cyano, methyl or trifluoromethyl, A gleiche oder verschiedene zweiwertige Reste der Bedeutung R" oder der Formel -CH 2 -O- oder -O-CH 2 -, -CH 2 -CH 2 -, -N = N-, - N = N(O)-, -CH = N- oder -N = CH-, A identical or different divalent radicals having the meaning R "or of the formula -CH 2 -O- or -O-CH 2 -, -CH 2 -CH 2 -, -N = N-, - N = N (O) -, -CH = N- or -N = CH-, y eine ganze Zahl im Wert von 0 bis 10, vorzugsweise von 0 bis 2, y is an integer having a value from 0 to 10, preferably from 0 to 2 z eine ganze Zahl im Wert von 0 bis 10, vorzugsweise von 0 bis 2, z is an integer having a value from 0 to 10, preferably from 0 to 2 mit der Masgabe, das die Summe y + mindestens 1 betragt, und with the proviso that the sum of y + is at least 1, and T die in der Beschreibung angegebene Bedeutung hat. T has the meaning given in the description.

News on Nematic-Biaxial Liquid Crystals1

Abstract: Recent studies have established that not only some polymeric materials exhibit nematic biaxial phases, but that also low molecular weight compounds can do so if certain structural conditions are me...

Fluorinated Liquid Crystals for Active Matrix Displays

Abstract: Eight types of two- and three-ring liquid-crystalline compounds each possessing a 3,4-difluorophenyl substituent were synthesized. Their liquid-crystalline mixtures were such as to make them useful for active matrix displays, such as TFT (thin-film-transistor) and MIM (metal-insuiator-metal) systems. They have high stability and low viscosity, very important properties so far not possible with conventional substances.

The Elastic Constants of Maier-Saupe Rodlike Molecule Nematics

Abstract: The classical Maier-Saupe theory of spatially uniform nernatics is here extended to the case of distorted configurations. Under the assumption that the molecules are rodlike, the mean field potential is here defined as an average over a suitable neighborhood of the test molecule. The energy density thereby calculated automatically accounts for the distortion energy, and properly reduces to the classical expres- sion for the homogeneous situation. In the linear limit, contrary to existing predictions based on Maier- Saupe potential, the three constants of Frank elasticity are found to be different from one another, and ordered as K 1,

The Synthesis of Several Lateral Difluoro-substituted 4,4″-Dialkyl- and 4,4″-Alkoxyalkyl-Terphenyls and a Rationalisation of the Effect of Such Substitution on Mesophase Type and Transition Temperatures

Abstract: In recent years we have prepared a large number of mono-fluoro- and ortho-difluoro-substituted 4,4''-dialkyl- and 4,4''-alkoxyalkyl-terphenyls. The synthesis and transition temperatures of a variety of related difluoro compounds is presented here so that it is now possible to identify(i) broadening of the molecule, (ii) twisting about an inter-annular bond, and (iii) the presence of an inner (2-, 2'-, 3'- or 2'' substituent) or an outer (3- or 3''-substituent) fluorine, as being the major factors which influence the type and the thermal stability of the mesophases obtained

Smectic ordering in block-copolymer melts

Abstract: Microscopic theory of smectic ordering in melts of rod-coil copolymers is proposed. It is shown that a second-order phase transition nematic-smectic A or a first-order transition nematic-smectic C may occur in the system as Flory's parameter of incompatibility of the blocks, x is increased. A number of phase transitions between different smectic A and C phases are also predicted. The most stable structure for large x is always SmA2, implying head-to-head arrangement of rigid rods in double lamellar sheets.

Mesogenic Behaviour of some 1,4,8,11,15,18,22,25-Octa-alkylphthalocyanines

Abstract: Discotic columnar mesophase behaviour exhibited by ten metal-free and copper 1,4,8,11,15,18,22,25- octa-alkylphthalocyanines has been characterised by optical microscopy, differential scanning calorimetry, x-ray diffraction and electronic absorption spectroscopy. Compounds with side-chains as short as hexyl exhibit one or more mesophases, and a total of three different mesophases are apparent for the series as a whole. The highest temperature mesophase is assigned as Dhd. At lower temperatures, some compounds develop a second mesophase of hexagonal symmetry, whereas others form a mesophase of rectangular symmetry. One example shows all three mesophases.

Cure Parameters and Phase Behavior of An Ultraviolet-Cured Polymer-Dispersed Liquid Crystal

Abstract: Using calorimetry and scanning electron microscopy we have investigated the formation energetics and kinetics as well as the phase behavior of an ultraviolet-cured polymer-dispersed liquid crystal (PDLC) system. The purpose of this work was to determine those cure conditions which lead to liquid crystal microdroplet formation. We have found that a range of cure temperatures exists in which the degree of polymer cure is highest. In this range phase separation of liquid crystal from the polymer is maximized, and the best droplet microstructure occurs. Samples cured in the optimum temperature range also have the highest nematic-isotropic transition temperature. Results of calorimetric and electron microscopic studies are presented.

Computer Simulations of Mesogenic Molecules Using Realistic Atom-Atom Potentials

Abstract: Molecular dynamics simulations are reported for an atom-based model of the mesogen truns-4-(trans-4-n-pentylcyclohexyl)cyclohexylcarbonitrile (CCHS). Atom-atom potential functions take account of intermolecular contributions to the potential energy and internal molecular structure is modelled by a standard empirical force field. The calculations are carried out in the isothermal-isobaric ensemble at a range of temperatures, and the results provide predictions for phase behaviour and orientational ordering which are in good agreement with experiment. Time-averaged molecular properties are calculated from the simulation data. We discuss the possible uses of this technique in the study of liquid crystalline systems and in the prediction of the phase behaviour of real molecules.

Ferrielectric Chiral Smectic Liquid Crystalline Phase

Abstract: We summarize the experimental results observed in the ferrielectric smectic liquid crystalline phase of MHPOBC by such as DSC, dielectric, X-ray, conoscope, rotatory power and electro-optic measurements. By using the newly obtained information for the pitch in this phase, we propose a novel structure of the ferrielectric phase; the repeating unit consists of three layers, so that the molecules have ferroelectric and antiferroelectric interactions with the ratio of 1:2.

The Synthesis and Transition Temperatures of Some Lateral Monofluoro-substituted 4,4″- Dialkyl- and 4,4″-Alkoxyalkyl- 1, 1′: 4′,1″-terphenyls

Abstract: The tetrakis(triphenylphosphine)palladium(0)-catalysed cross-coupling of arylboronic acids with aryl halides has been used to prepare several lateral fluoro-substituted 4,4″-dialkyl- and 4.4″-alkoxyalkyl- 1,1′: 4′,1″-terphenyls. The melting points, mesophase types and transition temperatures of these compounds vary considerably depending on where the lateral fluoro-substituent is positioned. All the compounds have a strong tendcncy to form tilted sniectic mesophases.

Peripherally Octasubstituted Phthalocyanines with Branched Alkoxy Chains

Abstract: Phthalocyanines (H2Pc) octasubstituted with branched alkoxy chains were synthesized: 2,3,9,10,16,17,23,24- octakis(3,7-dimethyloctoxy)phthalocyanine (1a) and 2,3,9,10,16,17,23,24-octakis(3,7,11-trimethyldodecoxy)phthalocyanine (1b). Their mesomorphic properties were studied by DSC, polarization microscopy and X-ray diffraction. Compound 1a, which was initially crystalline at room temperature, maintains its mesomorphic structure on cooling back to room temperature. Compound 1b is mesomorphic at room temperature. It shows a D ho→D hd transition at 34[ddot]C and a transition of unknown nature at 173[ddot]C. Both compounds display a D→I transition.

Polymorphic Ionic Mesogens of Silver(l): Ionic Materials Exhibiting a Thermotropic Cubic Mesophase

Abstract: Reaction of tran.s-4-alkyloxy-4′-stilbazoles (n-OPhVPy) with silver dodecylsulphate (AgDOS) led to the complexes [Ag(n-OPhVPy)2][DOS] which showed thermotropic mesomorphism. All homologues from met...

Electrically and Optically Controlled Light Modulation and Color Switching Using Helix Distortion of Ferroelectric Liquid Crystals

Abstract: The electro-optic effect associated with the helix distortion of chiral smectic C (SmC*) liquid crystals and its application to light modulation are investigated. Optically and electrically addressed spatial light modulators based on this effect are demonstrated. They exhibit an analog response, response times as short as 100 μs, good contrast ratio and resolution, and color switching capability. The analog response is shown to be a result of the gradual distortion of the helix at low voltages. Optically this corresponds to a linear rotation of the average optic axis and change of the average effective birefringence. From measurements of the wavelength of the maximum transmission the birefringence has been found to change as a function of dc voltage and frequency of ac voltage and magnitude. The agreement between theory and experiment suggests that color changes are due solely to birefringence changes associated with changes in the geometry of the helix. We derive expressions for the angle of rot...

Existence of a Sm-C Grain Boundary Phase at the Chiral MAC Point

Abstract: Based on a mean field analysis of the Chen-Lubensky model we extend an earlier discussion regarding the effect of chirality on the region of the phase diagram where the Cholesteric, Smectic-A, and Smectic- C* phases meet (i.e., the N* AC* point). We show that, on the Sm-C* side of the phase diagram, a second twist grain boundary (or chiral smectic) phase occurs. This phase, the “TGB C ”, is a highly dislocated version of Smectic-C. According to the deGennes analogy, this phase corresponds to Abri- kosov's flux lattice when the Ginzburg parameter K is negative. We also show that an Ising like transition between the TGB A and TGB C occurs and that this phase boundary, together with the Sm-A/TGB A Sm-C/TGBc, and the x-y like Sm-A/Sm-C* phase boundaries, meet at a multicritical point.

An Approach to the Design of Ferroelectric Liquid Crystals with Large Second Order Electronic Nonlinear Optical Susceptibility

Abstract: Ferroelectric liquid crystal thin films in the Clark-Lagerwall surface-stabilized geometry exhibit well known spontaneous polar orientation of functional groups. The symmetry of the system thus allows the existence of bulk electronic second order nonlinear hyperpolarizability x(2) within thc context of the simple dipolar model. For all FLC materials examined to date, however, the magnitude of x(2) is small, presumably since the particular functional group arrays oriented along the polar axis possess small molecular hyperpolarizdbility β. Using the Boulder Model for the molecular origins of the polar order occurring in FLC films, it is possible to design materials with functionalized aromatic rings oriented along the polar axis of the film. Since such functional arrays may show respectable values of β, it should be possible to obtain FIX films (both low molecular weight and polymeric) with useful magnitude of x(2) using this approach. Results of initial experiments aimed at design of FLC materials...

Mesomorphic Behavior of 1,2-Phenylene Bis[4-(4-alkoxybenzylideneamino)benzoates

Abstract: A series of 1,2-phenylene bis[4-(4-alkoxybenzylideneamino)benzoates has been prepared in order to establish that the nonlinear molecular structure is not necessarily incompatible with mesophase formation. The phases appearing in the methoxy and ethoxy derivatives are classical nematic. While the propoxy to octyloxy derivatives display a smectic A phase, as well as a nematic phase, the higher homologous members are purely smectogenic. Although the introduction of a methyl group to the 3-position of the central phenylene ring eliminates the nematic phase, the thermal stability of the smectic A phase is not much affected. The 4-methylation promotes both the nematic and smectic A phases. A metastable smectic B phase is observed in many members of these three series. The above-mentioned behavior closely resembles that given by the corresponding methyl derivative of phenyl 4-(4-alkoxybenzylideneamino)benzoate.

Synthesis and Properties of Liquid Crystals with Fluorinated Terminal Substituents

Abstract: The paper reports synthesis of LC derivatives of different chemical classes containing terminal groups ‒OCHF2, ‒OCF3, ‒SCHF2, ‒OCnF2n +1 and ‒CnF2n + 1 and indicates their phase transition points and physico-chemical properties.

A Revised Model for the Molecular Arrangement in the Smectic Ad Phase of Carbohydrate Derived Amphiphiles with One Alkyl Chain

Abstract: We propose a different molecular arrangement in the bilayers of the smectic Ad phase of mesogenic Carbohydrate amphiphiles from that previously postulated. The molecular arrangement proposed in this report is consistent with all experimental data currently available. In this new model, the smectic bilayers are identical (except for the absence of water) to the fluid lamellar (Lα) lyotropic phase, i.e., the molecules within the bilayers are arranged tail-to-tail, with the carbohydrate moieties on the outside of the bilaycrs and partially interdigitized alkyl chains in the core of each layer.

Liquid Crystalline Cores for Optical Fibers

Abstract: We consider the problem of wave guiding by optical fibers with nematic liquid crystal cores. The dielectric properties of the core which govern wave guiding in the fiber are determined by the nemat...

Measurements of Third Order Optical Nonlinearities of Nematic Liquid Crystals

Abstract: In order to better understand the physical mechanisms responsible for the large observed optical nonlinearities of nematic liquid crystals, we have carried out nonlinear absorption and nonlinear refraction measurements on the pure liquid crystals 5CB and 8CB, and the liquid crystal mixture E7 using picosecond, nanosecond and millisecond pulses. We have used the recently developed Z-scan method1–2, a sensitive single-beam technique which allows the determination of the sign and magnitude of the nonlinear refraction and the magnitude of the nonlinear absorption for aligned samples3. We have performed these measurements on aligned samples at wavelengths of 514nm and 532nm. In addition, we have studied the temperature dependence of the nonlinear refractive indices. Possible mechanisms responsible for the observed nonlinearities are discussed.

From Liquid Crystalline Molecules to Anisotropic Gels

Abstract: Two component mixtures containing liquid crystalline (LC) diacrylates and LC molecules with no reactive groups were made. Upon photopolymerization of the mixtures anisotropic networks containing LC molecules which were not chemically attached to the networks were created. in these systems the behavior of the LC molecules was found to be dominated to a large extent by the network molecules. Even when the systems were heated above the isotropic transition temperature of the LC molecules they remained partly oriented, thus contributing to the anisotropy of the system. Networks containing LC molecules were clear and did not give rise to excess scattering of light. Upon application of an electric field across the system, it turned translucent making the system suitable for display applications.

Synthesis and Properties of Some Liquid Crystal Polysiloxanes with Laterally or Terminally Attached Side Chains

Abstract: A series of polysiloxanes having laterally-attached mesogenic side chains and involving variations in spacer length and the nature of the two end groups are reported. The terminally-attached analogues of some of these polymers are also reported. Optical microscopy and differential scanning calorimetry were used to establish the mesomorphic properties. An explanation is proposed for the low TN-1 values observed for the polymers with laterally attached side chains.

The Dynamical Behavior of Liquid Crystals: A Continuum Description through Generalized Brackets

Abstract: In this paper, we systematically develop a hierarchy of constitutive equations at various levels of abstraction for describing the dynamics of incompressible liquid-crystalline systems. We also introduce a corresponding hierarchy of generalized bracket formulations. The simplest brackets, in terms of the direction vector, are capable of generating the Leslie-Ericksen (LE) theory in both the standard form of the equations (inertial) as well as the form in which the inertia associated with the rotation of the director is neglected. In parallel, a second, more-complex class of brackets is introduced, in terms of a structural tensor, which generates a theory that is more general than the LE theory, in that it can describe more-diverse phenomena, such as biaxiality and phase transitions. As particular cases, the generalized equations reduce to the LE equations or the more recent theory of Ericksen under the uniaxial approximation. Thus the collection of the governing equations of the generalized theor...

Picosecond Time Resolved Electron Injection from Excited Cresyl Violet Monomers and Cd3P2 Quantum Dots into TiO2

Abstract: Picosecond time resolved luminescence was employed for monitoring the decay of electronic excited reactants on the surface of oxide semiconductors. Decay of cresyl violet monomers was determined on...