ISSN:0267-8292    

DOI:10.1080/02678299408027845

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Acetylenic and diacetylenic liquid crystalline monomers have been prepared in order to obtain conjugated polymers with an orientational character. Unlike the polydiacetylene derivatives obtained and which do not exhibit any mesomorphic behaviour, a smectic phase, stable over a large temperature range, occurs in the case of all the polyacetylenes. This mesophase appears to be stabilized by anin situpolymerization of the monomers oriented in the nematic state. After iodine doping, a nematic phase appears in the polymer in addition to the smectic phase. AC complex conductivity measurements, realized over a large frequency range. are reported for some doped and undoped polyacetylenes. Preliminary results concern both non-oriented polymers and an undoped polymer in which the mesogenic groups have been oriented after polymerization.

ISSN:0267-8292    

DOI:10.1080/02678299408027846

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

In liquid crystal dot-matrix displays light may leak through the display area between the pixels. To obtain sufficient contrast this non-pixel area has to be made non-transmissive. For ferroelectric liquid crystal (FLC) displays this may be done by switching the material in the gaps between the picture elements to a non-transmissive state by the stray electric fields that occur during application of voltages to the pixel electrodes. This is experimentally studied for test cells with an electrically modified smectic layer structure. The gap region considered is an asymmetric environment of the FLC material, as the transparent conductive coating has been removed on one substrate, whereas on the other substrate a conductor covers the glass. The FLC molecules in the non-pixel area prefer to direct their dipoles towards the covered substrate. To switch the FLC material with the stray electric fields, it is a prerequisite to outweigh this preference. We made spatially resolved observations for various gap widths and various applied voltages on 2 μm thick FLC layers. With bipolar voltage pulses of 64 μs width each, amplitudes of about 25 V are needed to switch the FLC in 3·2 or 4·0 μm wide gaps. It was found to be more difficult to switch gaps that, are 7 μm wide than was anticipated on the basis of the results for 4 μm gaps. This is attributed to the surface polarization charge due to the FLC permanent dipoles built up at the FLC-glass interface. Experimental results supporting this explanation are presented.

ISSN:0267-8292    

DOI:10.1080/02678299408027847

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

A series of new ferroelectric copolysiloxanes with systematically varied comonomer content (‘dilution’) has been synthesized. Good planar alignment could be achieved for all copolysiloxanes and they were studied with respect to their mesomorphic and ferroelectric properties. Broad enantiotropic S*Cphases and spontaneous polarizations up to 286 nC cm−2are found. X-ray diffraction experiments show a linear increase of the smectic layer spacing by ‘dilution’. This points to a microphase separated structure of mesogenic groups and siloxane chains. It is found that the fixation of a mesogen to homopolysiloxane leads to an increase ofPs, whereas the ‘dilution’ of the mesogens with dimethylsiloxane units decreasesPsagain. Based on the microphase separated model, it can be shown that the decrease ofPsis not only due to the decrease of the vol % of mesogenic groups. The coupling between different mesogens mediated by the polymer chain, has additionally to be taken into consideration. A remarkable drop in the response times τ with decreasing mesogen content is confirmed and switching times less than 1 ms were measured.

ISSN:0267-8292    

DOI:10.1080/02678299408027848

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Bright-field image contrast of thermotropic cholesteric liquid crystalline materials in the transmission electron microscope (TEM) is investigated. Possible sources of contrast for these systems are discussed in terms of their molecular anisotropy. A cholesteric side-chain liquid crystalline compound was investigated with TEM, low voltage high resolution scanning electron microscopy (LVHRSEM), and atomic force microscopy (AFM) to determine the origin of the strong contrast observed in these systems using bright-field transmission electron microscopy. Initial contrast of thin microtomed sections, as viewed with TEM low dose techniques and an image intensifier, was much weaker than observed under normal viewing conditions. The periodic steady state contrast typically observed for these materials is the result of beam damage. Furthermore, the surface of microtomed samples (parallel to the cholesteric helical axis) is corrugated with a periodicity of 1/2 the pitch due to a preferred fracture path in the glassy cholesteric state. AFM profile analysis shows an average peak to valley height of approximately 20–25 nm. AFM of free surfaces from aligned films also indicates a corrugation with a periodicity equal to 1/2 the pitch with substantially smaller average corrugation depths. TEM indicates a series of +1/2 and −1/2 disclination lines at the surface due to a rotation of the preferred helix direction parallel to the surface, consistent with previously reported models.

ISSN:0267-8292    

DOI:10.1080/02678299408027849

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The temperature dependence of the infrared dichroism of a side chain polymeric liquid crystal was investigated for two homogeneously oriented samples, one aligned with a PVA coating and the other aligned by a magnetic field. From the apparent order parameters, it was found that the rotation of the mesogenic side group was restricted. Not only the rigid part of the side group, but also the alkyl chains were found to be partly ordered. Realignment of the side groups was observed in the case of the magnetically aligned sample at the smectic-nematic transition temperature. Unlike the other absorption bands, the C˭O bond vibration band shifted to higher wavenumbers with increasing temperature. This behaviour was explained by the influence of the neighbouring mesogenic groups on the conjugation existing between the π-electron cloud of the benzene rings and the π-electrons of the C˭O bond.

ISSN:0267-8292    

DOI:10.1080/02678299408027850

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The phase diagram of a mixture of the discogens triphenylene hexa-n-heptanoate (HET7) and triphenylene hexa-(2-methyl-4-n-decyloxy)benzoate is presented. This mixture provides a more accessible nematic temperature range than was previously possible. The refractive indices of one mixture composition are measured and a rather small, negative birefringence found. The two-parameter Haller extrapolation method is used to calculate the molecular polarizability ratio Δα/α. This has a rather small magnitude compared with calamitic mesogens, although the individual polarizabilities are much larger, due to the greater molecular mass of these compounds. The calculated value of Δα/αis used to give the uniaxial orientational order parameterS, whose value is comparable to those of calamitic examples. Tabulated bond polarizability data are used to predict a value for Δα/α, which is found to be similar to the experimental result.

ISSN:0267-8292    

DOI:10.1080/02678299408027851

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

We have recently reported the introduction of a carbon-carbon double bond into a wide variety of 5-n-alkyl-2-(4-n-alkoxyphenyl)pyrimidines to produce the corresponding alkenyloxy derivatives. The position and nature (E/Z) of the double bond were varied systematically and the effect on the liquid crystal transition temperatures studied. The position and nature (E/Z) of the double bond changed the conformation of the alkenyloxy chain substantially. This resulted in higher smectic C and nematic transition temperatures for compounds with atrans-double bond (E) at an even number of carbon atoms from the molecular core. Significantly lower transition temperatures (including the melting point) were observed for materials with acis-double bond (Z) at an odd number of carbon atoms from the molecular core. We have now performed the same operation on the related 2-(4-n-alkylphenyl)-5-n-alkoxypyrimidines to produce the corresponding alkenyloxy derivatives. An interesting feature of the new results is thehighmelting points of thetrans-substituted materials and thelowmelting points of the terminally substituted compounds. The smectic C transition temperatures of both series are high. No nematic phases could be observed. However, in admixture with other smectic C components, the new compounds lead to surprisingly fast switching times, high smectic C transition temperatures and low melting points/crystallization temperatures in experimental mixtures designed for electro-optic display devices based on ferroelectric effects.

ISSN:0267-8292    

DOI:10.1080/02678299408027852

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The thermal motion of a substrate is shown to have strong effects on the orientation of liquid crystal molecules in contact with it. Using an invertedpendulum model, we find that the orientation of the liquid crystal molecules can have a sequence of transitions between planar and homeotropic orientations. Analytical expressions for stability conditions for the homeotropic orientations are found for both monochromatic and some multiple-mode thermal motions, and, in both homeotropic and planar anchoring cases. Numerical simulations confirm the analytical model calculations and show that strong interactions between molecules favour processes of dynamic stabilization and destabilization of the homeotropic orientation.

ISSN:0267-8292    

DOI:10.1080/02678299408027853

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

We compare the ratios of the Frank elasticity coefficients calculated within a recently developed molecular theory, based on the approximation of perfect local orientational order, with the results of computer simulations presented by Frenkel, Allen, Tjipto-Margo and Evans for fluids of hard prolate and oblate ellipsoids. Good agreement is found for high densities, which correspond to those of thermotropic nematics, and for realistic values of the axial ratio. By constrast, at lower densities the approximation of perfect local order appears to be inadequate and the results of computer simulations follow the predictions of mean-field-like theories.

ISSN:0267-8292    

DOI:10.1080/02678299408027854

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor