摘要:

A programmable nematic liquid crystal phase plate is formed by sandwiching a layer of liquid crystal material between two glass substrates, each coated with a transparent electrode. Regions of the liquid crystal layer are independently controlled by patterning the electrode into an array of pixels and addressing each pixel with a separate voltage source. The voltage-controlled reorientation (deformation) of the liquid crystal layer far from the pixel edges is well understood; however, very few investigations of the deformation near a pixel edge have been published. We show that near the pixel edges the deformation decreases monotonically, but a transition region located in the center of the pixel results. The transition region, called a liquid crystal inversion wall, is undesirable for most applications but can be forced to form near the edge of the pixel by adding a pretilt to the surface alignment. We present computer simulation results of the voltage-dependent pixel edge deformations and relate the deformations to the phase profile experienced by a wavefront propagating through the programmable phase plate.

ISSN:1058-725X    

DOI:10.1080/10587259208030753

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

Experimental data are presented for the unwinding of the helical cholesteric structure in the presence of an applied electric field. Homeotropic surface anchoring conditions are known to reduce the magnitude of the critical unwinding voltage which depends on helical pitch and cell thickness. The experiments are unique in that the pitch is varied continuously at constant cell thickness rather than changing thickness at constant pitch. This is accomplished by employing cholesteric mixtures in which the inverse pitch changes linearly with temperature in the nematic compensation region.

ISSN:1058-725X    

DOI:10.1080/10587259208030754

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

The crystal and molecular structure of a cybotactic nematic compound bis-(4′-n-butoxybenzal—chloro-1,4-phenylinediamine (BOCP in short) has been reported. The crystals belong to monoclinic system with space group P21/c,a= 36.693,b= 14.940,c= 6.1058 Å, β = 93.89° andZ= 4. The structure was determined by direct methods, with a finalR-value of 0.049 (Rw= 0.070) for 1979 observed reflections. The molecules are almost in their extended conformation. The molecules are inclined to theab-plane, the planar aromatic core region being almost parallel to theac-plane. Molecular packing in relation to the liquid crystal phase is discussed.

ISSN:1058-725X    

DOI:10.1080/10587259208030755

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

A comprehensive series of (R—(4-substituted-phenoxy)propanoates and (R—(4-substituted-phenoxy)propanonitriles have been prepared. A wide variety of 4-substituents and ester functions have been examined to determine how the position of the chiral centre affects the cholesteric phase formation in these classes of materials; the synthesis of these novel materials, their transition temperatures and a procedure for assessing their optical purity are described and discussed. Mesogenicity is significantly depressed if the chiral centre is placed centrally within the molecule.

ISSN:1058-725X    

DOI:10.1080/10587259208030756

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

Alcohols with perfluorinated segments, F(CF2)n—(CH2)m—OH;n= 8, 10, 12;m= 4, 6, 10; have been converted into methacrylates by a dicyclohexylcarbodiimide activated esterification reaction. The resulting methacrylates show two disordering transitions prior to isotropization. Polarization optical microscopy indicates the occurrence of smectic liquid crystal phases. Organic solutions of the amphiphilic methacrylates showed a strong decrease in the surface tension at low concentrations. Peculiar surface activity had already been observed for the compound with the shortest fluorocarbon segment. Needle shaped crystallites of the semifluorinated compounds are formed when hydrocarbon solutions of methacrylates with a perfluorododecyl substituent are cooled. The crystals can form a fine mesh network which encloses the solvent in the cavities. The size of the needle crystals and the internal dimensions of the network depend strongly on the crystallization conditions.

ISSN:1058-725X    

DOI:10.1080/10587259208030757

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

Dielectric properties of a polymer-dispersed liquid crystal (PDLC) film comprising of liquid crystal, octyl cyanobiphenyl (K-24) and polymethyl methacrylate (PMMA) are reported as a function of frequency (100 Hz-1.0 MHz) in the temperature range of 25°C to 45°C. A dielectric dispersion in the low frequency region (∼1 KHz) was observed while no such dispersion was seen in either pure PMMA or pure K-24. The dispersion is due to Maxwell-Wagner polarization mechanisms. The temperature variation studies show (i) a discontinuous jump in the dielectric permittivity at the nematic-isotropic and smecticnematic transitions and (ii) also a lowering of the transition temperatures, suggesting some contamination of liquid crystal by the polymer. Simple dielectric theories of composites have been applied to estimate the molecular alignment of liquid crystal (LC) molecules inside the micro-droplets.

ISSN:1058-725X    

DOI:10.1080/10587259208030758

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

The nematic layer growing from the isotropic liquid of a free suspended film has been investigated. The selected substances possess different dielectric anisotropy, molecular orientation of the interfaces, latent heat of the N-I phase transition, a kind of phase under the nematic. It is found that the nematic layers of the two interfaces nematic-air arise and grow at very slow cooling independently if the thickness of the free film is ≥8 μm. After the upper nematic layer of some substances reaches ∼5 μm thickness, the formation of drops begins under the layer on the interface nematic-isotropic liquid. For substances in which the molecules lie approximately in the interface N-I at the same experimental conditions the drops are not seen. It is found that the transition I-N does not depend on the kind of phase under the nematic if the nematic temperature region is not too small.

ISSN:1058-725X    

DOI:10.1080/10587259208030759

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

The equations governing the flow behavior of the biaxial nematic phase are derived and presented in such a form that it is easily seen that the fluid dynamic theory of biaxial nematics can be formulated as a natural generalization of the Leslie-Ericksen theory of uniaxial nematics. It is shown that in order to describe the flow behaviour of biaxial nematics, fifteen viscosity coefficients, related by three Onsager relations, are required. With the biaxial nematic stress tensor as a starting point we discuss the concept of viscous torques, and show how a good qualitative understanding of the flow behaviour of the system follows by studying them. We show that three rotational viscosities and nine effective shearing viscosities have to be defined in order to characterize the viscous behaviour of biaxial nematics completely. Thus it is possible to design sufficient experiments to measure the twelve independent viscosity coefficients included in the theory. We also derive the possible equilibrium orientations for a biaxial nematic when subjected to shear flow, and give a brief discussion of their stability.

ISSN:1058-725X    

DOI:10.1080/10587259208030760

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

In view of the limitations inherent to phenomenological approaches, the problem of the internal structure of defects in nematics is here afforded from a molecular viewpoint. By using an extension of the Maier-Saupe mean-field potential which accounts for distorted states of nematics, the general equation applicable inside a defect is obtained by self-consistency. The specific example of the hedgehog point defect is developed in detail. It is shown that the size of the defect core depends on the order parameter characterizing the undistorted nematic phase: the core size decreases when the order parameter increases.

ISSN:1058-725X    

DOI:10.1080/10587259208030761

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

The structure of two N-alkylribonamides both in the anhydrous state and in concentrated water solution has been studied by X-ray diffraction. A Lamellar Crystalline, a Lamellar Liquid-Crystalline and a Cylindrical Hexagonal Liquid-Crystalline Structures have been found. The influence of temperature and of water concentration on the geometrical parameters of the mesophases has been established. The organization of the molecules in the three structures is described and a step by step melting behavior is proposed for the N-alkylaldonamides.

ISSN:1058-725X    

DOI:10.1080/10587259208030762

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor