摘要:

A waveguide consisting of a thin (< 1 μm) layer of nematic liquid crystal sandwiched between two parallel aligned gratings is described. Grating coupling to guided modes is demonstrated by monitoring the reflection and transmission from the cell as a function of incident angle. A reduced birefringence is seen compared to the bulk case which is explained in terms of disordered regions of liquid crystal close to the grating surfaces. The movement of guided modes under electrical bias is shown and time resolved experiments demonstrate high contrast switching with rise times as fast as 18μs.

ISSN:0267-8292    

DOI:10.1080/02678299308026334

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The conditions for initiation of EHD instability at frequencies of 50–150 Hz and for DC for eight nematic materials having a positive dielectric anisotropy (λϵ > 3) have been experimentally studied. The dependences of the threshold voltage for the EHD instability on electrical conductivity, as well as on the translational and rotational viscosities of the LC materials have been found. For the measurements substances with strongly differing (from 10−9to 5 × 10−7Ω−1m−1) electrical conductivities and profound temperature dependence of the viscosities were chosen. Analysis of the results based on the isotropic model of EHD instability shows good agreement between theory and experiment.

ISSN:0267-8292    

DOI:10.1080/02678299308026335

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Measurements of the orientational and translational order parameters for the nematic, smectic A and smectic C phases of a commercial liquid crystal material are reported. The order parameters have been obtained by analysis of the angular distribution of the intensity of X-rays scattered by a sample aligned by a magnetic field. Results are presented as a function of temperature, and it is found that the apparent orientational order parameter in the smectic C phase decreases with decrease in temperature. This is explained using a model of random tilt. The experimental order parameters are compared with those calculated from a mean field model.

ISSN:0267-8292    

DOI:10.1080/02678299308026336

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The excitation of optical modes is used to study the optical tensor configuration in a thin ferroelectric liquid crystal layer, cooled from the initially homeotropically aligned nematic phase. By monitoring the angular dependent reflectivity for plane polarized radiation coupled into the guided modes in the smectic C* layer and subsequently fitting the recorded data to predictions from multilayer optics theory, the optical tensor configuration in the layer is fully evaluated. Iteratively modelling the full tilt/twist profile in the cell, progressively converging the predicted reflectivity to experimental data, gives a complete and very well specified picture of the optical tensor throughout the cell. By studying the cell at various temperatures, the temperature dependence of the tilt of the major axis of the optic tensor (which may be related to the cone angle if the smectic layers are parallel to the cell surface) has been established. The temperature dependent optical dielectric constants have also been obtained.

ISSN:0267-8292    

DOI:10.1080/02678299308026337

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The model proposed in [7] to calculate the broadening effect produced by elastic modes on NMR line shapes of nematic liquid crystals, in which all modes with relaxation times longer than a NMR time scale Δτ are static, and infinitely fast in the opposite case, is put on a more rigorous theoretical basis, by considering all modes with their actual relaxation times. The correlation function of the transverse magnetization is calculated and expressed in terms of the self-correlation function of the components nxandnyof the local director, assumed to be equivalent and independent gaussian random variables. Formal expressions are given for the general case, and in the one constant, cylindrical and spherical cases, approximations. The general procedure describing how to use this formalism for a NMR spectrum composed of many lines, is given. This formalism is then used to analyse the same data as in [7] concerning a main chain nematic polymer, in the spherical approximation. It is shown that fits with the same quality are obtained. These results provide (i) theoretical support for the model of [7], (ii) an operational way to define Δτ and (iii) a practical example for discussion of the controversial problem of the ‘cut-off wavevector(s)’ of the modes, which define the size of the elementary uniaxial object in the nematic medium. It is shown that, for this polymer, the smallest size corresponds to a volume between one and four repeat units. The analysis of line shapes provides the viscoelastic parametern½K3/2, wherenandKare the average viscosity and average elastic constant. Values ofnandKcan be deduced from the theory. The limitations of the model are discussed. It is shown that the present model and the one of [7] are complementary. It is argued that the present formalism may be useful to analyse NMR line shapes in conventional polymers.

ISSN:0267-8292    

DOI:10.1080/02678299308026338

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The smectic A (SA)-chiral smectic C (S*C) transition in binary mixtures of methylchloropentanoyloxy heptyloxybiphenyl (C7) and butyloxyphenyl decyloxybenzoate (10O.4) changes from first order to second order as a function ofX, the mole per cent of 10O.4. High resolution heat capacity measurements show that the Cpbehaviour is well-described by Landau mean field theory. Classical tricritical behaviour is observed nearXtc≃ 10·6 and the coefficientbof the quadratic term in the free energy varies like (X - Xtc) in good agreement with results from polarization data.

ISSN:0267-8292    

DOI:10.1080/02678299308026339

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The bond selectivity of the Raman technique is exploited with a time resolved Raman microprobe to determine the effective splay viscosity coefficients and elastic constants in the nematic phase of the 4-n-alkyl-4-cyanobiphenyl homologues 5,6,7 and 8CB. The Raman effect, using the signal associated with the core and the flexible alkyl chains, was used to probe the Fréedericksz transition in the limit of small deformations. The data for the cores show good agreement with data published elsewhere. Odd-even effects were found and, in the case of 8CB, pretransitional effects due to the lower temperature SAphase were observed. The viscosity data for the flexible alkyl chains, at a reduced temperature of 3°C, yield values for the apparent viscosity coefficients that are approximately half that of their respective rigid cores. This is considered to be indicative of the influence of the molecular flexibility on the effective director field. The data illustrate the usefulness of the technique for studying microscopic volumes of multicomponent systems.

ISSN:0267-8292    

DOI:10.1080/02678299308026340

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A homologous series of branched, non-ionic surfactants with the general formula CnG(EmM)2, where Cndenotes an alkyl chain, G = glycerol and EmM = oligo-oxyethylene mono-methyl ether, has been prepared from alkyl bromides (n= 10–16) and several monodisperse 1,3-di(methoxyoligo-oxyethylene) ethers of glycerol (m= 3–5). The branched hydrophilic chain is introduced to modify the interfacial area compared to corresponding linear oligo-oxyethylene surfactants (I-amphiphiles) without essentially changing the hydrophilic-lipophilic balance. The phase behaviour of these Y-surfactants in aqueous solution reveals that according to established packing models the branched hydrophilic group strongly stabilizes the cubic and hexagonal mesophases, while a lamellar phase is not observed.

ISSN:0267-8292    

DOI:10.1080/02678299308026341

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

To continue the search for novel series of ferroelectric liquid crystals, an additional two series of compounds, CS(+)-4-[(2-methylbutyl)phenyl]4′-trans-n-alkylcyclohexyl) benzoates and DS(+)-4-[(4-methylhexanoyl)phenyl]4′(4″-trans-n-alkylcyclohexyl) benzoates, where alkyl = C5-C6are reported in this paper. Their mesomorphic properties have been characterized by optical textural observations and DSC. Some of these compounds show a C-S*C-SA-N*-I phase transition sequence. The effects of the alkyl chains on transition temperatures and optically active terminal groups on phase behaviour are discussed.

ISSN:0267-8292    

DOI:10.1080/02678299308026342

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

T(p) phase diagrams for the mesomorphic compoundstrans-4-n-butylcyclohexane-1-carboxylic acid 4-cyanophenyl ester (D4N),trans4-n-pentylcyclohexane-1-carboxylic acid 4-n-pentylphenyl ester (D55),trans-4,4′-di-n-propyl-1,1-bicyclohexyl-cis-4-carbonitrile (33CCN),trans-4-methoxy-4′-propyl-1,1′-bicyclohexane (3O1CCH) andtrans-4-methoxy-4′-n-butyl-1,1′-bicyclohexane (4O1CCH) are presented. The experiments were performed using high pressure microcomputer-assisted differential thermal analysis equipment in the temperature range 300 to 500 K up to a maximum pressure of 8 kbar. Some special high pressure effects for liquid crystals, such as pressure-induced or pressure-limited phases and changes from monotropic to enantiotropic polymorphism were observed. For the correlation of the experimental results, an extension of the Simon equation was used; one of the regression parameters of the clearing curve equation was found to be correlated with the molecular structure of the liquid crystal substances under test.

ISSN:0267-8292    

DOI:10.1080/02678299308026343

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor