Showing papers on "Liquid crystal published in 1975"

Ferroelectric liquid crystals

Abstract: A general symmetry argument is presented, and experiments on newly synthesized p-decyloxybenzylidene p'-amino 2-methyl butyl cinnamate are described, demonstrating that chiral smectic C and H liquid crystals are ferroelectric. Some of the properties of this new class of ferro-electrics are discussed.

Phase transitions and fluidity characteristics of lipids and cell membranes.

Abstract: The concept that the liquid crystalline or mesomorphic condition was of importance to biological systems is a relatively old idea. Thus Bernal (1933) when discussing the different types of arrangements of molecules in liquid crystals commented ‘Such structures belong to the liquid crystal as a unit and not to its molecules which may be replaced by others without destroying them and they persist in spite of the complete fluidity of the substance. These are just the properties to be required for a degree of organization between that of the continuous substance, liquid or crystalline solid and even the simplest living cell.’ Stewart (1961) some thirty years later also stated that ‘It is this property – the combination of flow and lability with a preferred and relatively stable molecular orientation – that makes the mesomorphic (i.e. liquid crystal) phase uniquely appropriate to the structure of protoplasm and living tissue.’

The optical properties of twisted nematic liquid crystal structures with twist angles ⩽90 degrees

Abstract: Liquid-crystal twisted nematic structures can be made with twist angles other than the conventional 90 degrees . It is shown that theoretically the transmission of such a structure when placed between two polarizers aligned parallel and perpendicular to the respective surface molecular directors is given by T=(1+u2)-1 sin2( theta (1+u2)12/) where u= pi d Delta n/ theta lambda , and theta is the twist angle, d being the cell thickness, Delta n the refractive index anisotropy of the nematic material, and lambda the wavelength of the transmitted radiation. In the idealized case T is taken to be zero. The departures from this, predicted by the above expression, are due to the fact that plane-polarized light becomes elliptically polarized as it passes through the nematic structure. Experiments confirm the validity of the theoretical model.

New Liquid-Crystal Phase Diagram

Abstract: The nematic-smectic-$A$ transition temperature of mixtures of HBAB {$p$-[($p$-hexyloxybenzylidene)-amino]benzonitrile} and CBOOA [N-$p$-cyanobenzylidene-$p\ensuremath{-}n$-octyloxyaniline] becomes multivalued with increasing concentration of HBAB. The nematic phase occurs at both a higher and a lower temperature than the smectic phase. Measurements of the bend elastic constant as a function of concentration are presented.

A molecular field theory for uniaxial nematic liquid crystals formed by non-cylindrically symmetric molecules

Abstract: Most molecular theories of nematic liquid crystals assume that the constituent molecules are cylindrically symmetric. However, although this may be a useful approximation the molecules of real nematogens are of lower symmetry; here we develop a theory for an ensemble of such particles based on a general expansion of the pairwise intermolecular potential together with the molecular field approximation. The dependence of the orientational properties of the uniaxial mesophase on the deviation from molecular cylindrical symmetry is calculated from the series expansion of the pseudopotential. In these calculations the number of arbitrary parameters in the orientational pseudo-potential is reduced by assuming that the anisotropic intermolecular potential originates solely from dispersion forces. The theoretical predictions for the values of the ordering matrix and the entropy change at the nematic-isotropic transition are found to be in good agreement with those observed for 4,4′-dimethoxyazoxybenzene. In addit...

Dynamic behavior of twisted nematic liquid‐crystal layers in switched fields

Abstract: The Ericksen‐Leslie equations, describing the dynamic behavior of nematic liquid crystals, have been applied to a twisted nematic layer and have been numerically solved for a number of cases. The results account well for the observed dynamic behavior of displays using a twisted nematic layer. In particular, an explanation is given for the ’’bounce’’ in the optical transmission of a twisted nematic cell between polarizers after switching off the applied field. Materials having a frequency dependence of the sign of the dielectric anisotropy show a reversal of twist after the frequency of the applied field has been switched. This can also be explained. It is shown that fluid motion is essential to the occurrence of these phenomena.

A new real-time non-coherent to coherent light image converter - The hybrid field effect liquid crystal light valve

Abstract: A new, high-performance device has been developed for application to real-time coherent optical data processing The new device embodies a CdS photoconductor, a CdTe light-absorbing layer, a dielectric mirror, and a liquid crystal layer sandwiched between indium-tin-oxide transparent electrodes deposited on optical quality glass flats The noncoherent image is directed onto the photoconductor; this reduces the impedance of the photoconductor, thereby switching the ac voltage that is impressed across the electrodes onto the liquid crystal to activate the device The liquid crystal is operated in a hybrid field effect mode It utilizes the twisted nematic effect to create a dark off-state and the optical birefringence effect to create the bright on-state The liquid crystal modulates the polarization of the coherent read-out light so an analyzer must be used to create an intensity modulated output beam

The structure of a number of nematogens

Abstract: X-ray diffraction measurements have been made on five nematogens in the nematic and isotropic liquid phases. The data have been interpreted to give information about the molecular packing. Two Schiff's bases R1 [MATH] CH = N [MATH] R2 : MBBA (R1 = CH3O, R2 = nC4H9) and EBBA (R1 = C2H5O, R2 = nC4H9) have very similar structures, the higher homologue being much the more ordered. A third Schiff's base MBCA (R1 = CH3O, R2 = CN) is thought to have a more crystal-like packing resulting in a repeat distance ("d" spacing) along the texture axis 3 A shorter than the molecular length. Two cyanobiphenyls R [MATH] CN : 5 CB (R = C5H11) and 7 CB (R = C7H15) have very similar structures, with pronounced local ordering. This is revealed most strikingly in a repeat distance along the texture axis of ~ 1.4 molecular lengths, which is interpreted as arising from an overlapping head to tail arrangement of molecules resulting in a quasi-layer (smectic A) structure on a local scale. The order extends over 100-150 molecules and is more pronounced for the higher homologue. In all cases, local order very similar to that in the nematic phase persists in the isotropic liquids.

Irreversible Thermodynamics of Nonequilibrium Alignment Phenomena in Molecular Liquids and in Liquid Crystals

Abstract: Abstract To treat nonequilibrium alignment phenomena in molecular liquids and in liquids crystals two points of the usual irreversible thermodynamics have to be modified. Firstly, in the specific energy a term quadratic in the alignment tensor is included. Secondly, terms up to 4th order in the alignment are taken into account in the entropy for a nonequilibrium situation. Then the entropy production is calculated. Constitutive laws are set up for the friction pressure tensor and for the tensor which characterizes the decay and the production of the alignment in the balance equation for the alignment tensor. As a first application, the nonlinear relaxation equation for the alignment is considered. For a uni-axial alignment zero and nonzero stable stationary values of the order parameter are found for temperatures above and below the temperature TK at which the transition from the isotropic to the nematic phase takes place. Small deviations from the equilibrium alignment decay exponentially. For temperatures below TK the relaxation time turns out to be anisotropic. In the appendix, the entropy associated with the alignment is calculated for a special case.

Liquid crystal dot color display

Abstract: A solid state dot sequential color display device is disclosed. A color image is formed in a liquid crystal display panel by breaking each picture element up into its three primary color components and presenting them in an adjacent manner. The space occupied by a trio of primary color elements is smaller than the resolving power of the human eye. Techniques of large scale integration (LSI) are employed to effect the required addressing circuitry as applied to a matrix array of triads of reflective electrodes in the dynamic scattering liquid crystal display panel. Arrangements used to form the basic color cells involved are: (a) deposition of band-pass filters in front of the display and (b) deposition of band reflection mirror in back of the liquid crystal material.

Temperature, concentration, and molecular weight dependence of the twist elastic constant of cholesteric poly‐γ‐benzyl‐L‐glutamate

Abstract: Aspects of the behavior of the twist elastic modulus K22 of a lyotropic liquid crystal formed by the α‐helical polypeptide poly‐γ‐benzyl‐L‐glutamate of three molecular weights (MW= 300 000, 310 000, and 550 000) in three solvents (dioxane, dichloromethane, and chloroform) were investigated as a function of concentration and temperature. We find that K22 in this system may be made to vary over a wide range by adjustment of solvent and polymer molecular weight. Only a slight concentration dependence of K22 throughout the liquid crystal range was, however, noted. The variations can be at least partially understood in terms of changing macromolecular interactions and recent statistical hard‐rod theoretical treatments of liquid crystal elasticity. The functional form for the temperature dependence of the order parameter was also determined for the system through the clearing point.

AC biased and resonated liquid crystal display

Abstract: A liquid crystal device biased by an alternating current having a predetermined frequency higher than the dispersion frequency of the liquid crystal material. The liquid crystal device exhibits inherent capacitance across its input terminals which is resonated by an inductor to result in a higher bias current and reduction of image smear of images displayed by such device.

Shear flow induced polarization in ferroelectric smectics C

Abstract: The existence of ferroelectricity in the chiral smectic C phase has been established from polarization measurements in the presence of an electric field [I]. In this communication we study the case where the polarization is created by applying a shear velocity parallel to the smectic planes : The helicoidal structure is distorted by the shear : The alignment effect induces the average dipoles in the smectic C planes to align perpendicular to the plane of the shear. The dynamics of the effect gives information on the elasticity and viscosity of the process. A significant increase of the polarization observed near the smectic A transition is discussed. (*) Article paru dans le Journal de Physique 36 (1975) 1005. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1976325

Growth of a smectic A from a bent nematic phase and the smectic light valve

Abstract: We show that for certain liquid crystals, characterized by very small nematic/smectic−A latent heats, the transition from a nematic phase, held firmly in a bent state, to a smectic−A phase takes place via an intermediate state which is stable in a given temperature range and which we have called ’’striped texture’’. This state consists of a corrugated layer of bent nematic which is sandwiched between two smectic−A layers whose planes are oriented in two different directions. As the temperature of the sample is further decreased, the ’’striped texture’’ becomes unstable. The corrugated layer undergoes axial buckling leading to the final smectic−A ’’honeycomb texture’’. This texture is the scattering phase of Kahn’s smectic light valve. We explain the occurrences of these phenomena on the basis of elastic and surface energy considerations.

Liquid crystal display devices

Abstract: A liquid crystal guest-host display device includes a dye dissolved in the liquid crystal material. The invention is characterized in that the dye comprises three colored dye compounds mixed together to provide gray in the absence of an applied electric field. The compounds may be Sudan Black, Sudan Red and β-carotene. The liquid crystal material preferably includes at least one 4'-alkyl- or -alkoxy-4'-cyanobiphenyl compound.

Elastic and dielectric constants in mixtures of nematic liquid crystals

Abstract: The dielectric constants and two elastic constants were measured for a series of mixtures of two nematic liquid crystals, MBBA and PEBAB. The materials covered the range from 0 to 10% PEBAB. The parameters were calculated from the measured voltage dependence of capacitance. The dielectric constants vary linearly with PEBAB concentration and the anisotropy changes from negative to positive at 1.7%. The elastic constants remain fixed over most of the range, but are reduced in mixtures of small anisotropy. The results differ significantly from recent measurements of Michel and Smith, the probable reason being that we take any nonideal alignment at the surface explicitly into account in the calculations.

Light scattering from nematic liquid crystal waveguides

Abstract: Light scattering properties of smectic‐A liquid‐crystal thin‐film waveguides are studied both theoretically and experimentally. A Green’s function analysis of the depolarized light scattering induced by thermal fluctuations in the average molecular alignment is presented. Scattered power distributions and scattering coefficients for mode conversion and for scattering out of the guide are derived. It is shown that the power distributions for scattering from nematic and smectic waveguides differ greatly and that scattering losses in smectic‐A liquid‐crystal waveguides are several orders of magnitude lower than those encountered in nematic guides. Power loss in these waveguides, due to light scattering arising from thermally induced dynamical distortions of the smectic planes, is calculated to be on the order of 2 dB/cm and is experimentally verified.

Electro-optical display device

Abstract: A non-liquid crystal organic compound having positive dielectric anisotropy is mixed with a nematic liquid crystal composition having negative dielectric anisotropy to obtain a nematic liquid crystal composition having positive dielectric anisotropy, which is used as an electro-optical element. Thus, the present invention aims at development of new use of the materials.

Non-Gaussian fluctuations in electromagnetic radiation scattered by a random phase screen. II. Application to dynamic scattering in liquid crystal

Abstract: For pt.I see ibid., vol.8, 369 (1975). The authors have studied experimentally the statistical, spatial coherence and temporal coherence properties of non-Gaussian fluctuations in light scattered by a thin layer of liquid crystal (MBBA) in its dynamic scattering state. The results are consistent with the deep phase-screen theory developed in the previous paper. With 20 V applied to the sample, experimental values of the parameters of the model are: mean square phase deviation phi 2=45.6+or-8.0 rad2, phase correlation length xi =2.63+or-0.24 mu m and phase coherence time 2.2+or-0.2s. Indications are found that, while phase fluctuations in the emergent wave-front are probably dominant, amplitude fluctuations are not entirely negligible. It is argued, however, that the effects of amplitude fluctuations on the values of the above parameters are probably quite small. Taking a broader view, the results confirm that, in many scattering experiments, detailed information concerning the scattering process can be obtained from measurements in the non-Gaussian regime.

New syntheses.the liquid crystal properties of some new mesogens

Abstract: The 4'-n-alkyl-4-cyanobiphenyls and 4'-n-alkoxy-4-cyanobiphenyls provide colourless liquid crystal phases which are chemically and photochemically stable. In many cases, the mesophases are produced at quite low temperatures, and indeed two members of the n-alkyl series are nematic at room temperature. Eutectic mixtures prepared from members of the two series give wide nematic ranges, e. g., from about 2 OC to above 60 O C . All the compounds are of high positive dielectric anisotropy and either individually or in admixture their nematic phases perform well in twisted nematic displays. The nematic ranges have been further extended, without adversely affecting the stability characteristics or the electrical properties by incorporating certain members of the series of 4\"-n-alkyl-4-cyano-p-terphenyls. For example, one eutectic mixture is nematic from 5 OC to 91 O C . Two optically active analogues of the cyanobiphenyls and cyano-p-terphenyls have been prepared by using a branched chain as the alkyl or alkoxy group. Incorporation of these compounds in mixtures with the 4'-n-akyland 4'-rr-alkoxy-4-cyanobiphenyls gives long pitch cholesteric phases which exist in the room temperature range, and which are colourless and both chemically and photochemically stable. These phases perform well in cholesteric-nematic phase change devices. These chiral solutes used in low concentration (1 %) also prevent areas of reversed twist in twisted nematic displays. 4'-n-octyl-4-cyanobiphenyl gives a room temperature smectic phase. This phase and those shown by other higher homologues of the two series have been shown by miscibility to be smectic A. However, the smectic phases have unusual X-ray characteristics. The smectic polymorphism of the 4\"-n-alkanoyl-4-bromo-p-terphenyls has been studied ; the heptanoyl compound provides only the third known example of a direct Sn-N transition. The four series of 4\"-chloro-, 4\"-methyl-, 3\"-chloroand 3\"-methyl-w-phenylalkyl 4-(4'-cyanobenzy1ideneamino)cinnamates have been studied. It is concluded that the extremely large alternations in the N-I transition temperatures as the series are ascended are explained in terms of changes in the spatial distribution of the terminal ring of the ester function which are determined by the stereochcmistry of the alkylene chain. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1975157