摘要:

The nematic phase of the CsPFO/water system, when held in a cylindrical glass tube, spontaneously forms a detailed ‘cellular’ texture with an axialS= +1 disclination. This texture is the lyotropic discotic analogue of that previously found for a calamitic thermotropic system.

ISSN:0267-8292    

DOI:10.1080/02678299608032050

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Achiral ‘swallow-tailed’ liquid crystalline materials are known to give alternating-tilt smectic C phases (SCalt) which have structural similarities to the chiral antiferroelectric phases denoted as S*CA. This paper describes the synthesis and characterization of three achiral branchedalkyl 4-(4′-dodecyloxybiphenyl-4-carbonyloxy)-3-fluorobenzoates. Optical microscopy and differential scanning calorimetry confirm that these materials show SCaltand overlying SAphases. The compounds were investigated as potential hosts which could be doped with a chiral ferroelectric liquid crystal so as to provide a viable- antiferroelectric mixture. These studies (microscopy and differential scanning calorimetry), to characterize the properties of the mixtures, show that antiferroelectric phases are induced. However, switching studies show that the antiferroelectric phases are extremely stable, a property which is almost certainly a consequence of the length of the lateral branching groups (ethyl, propyl and butyl).

ISSN:0267-8292    

DOI:10.1080/02678299608032051

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A new series of ethylpyridinium saltsN-substituted with a 4′-methoxybiphenyl-4-yloxyalkyl group has been synthesized and the mesomorphic properties examined. The effect of positioning the ethyl group at each of the three available positions (2; 3- or 4-) on the pyridinium ring was examined and compounds with alkyl chains containing odd numbers of carbon atoms from C5 to C11 were investigated. It was found that the longer chain (C9-C11) 2- and 4-ethylpyridinium compounds form smectic phases, whereas the corresponding 3-substituted compounds do not. X-ray diffraction studies indicated that the smectic phases were of types A, B and in one case E, with the molecules lying in a head to tail fashion in single layers with interdigitated alkyl chains. We explain the difference in mesogenic properties of the three different ring-substituted compounds in terms of packing considerations in the ionic regions of the structures. It is suggested that the 3-derivatives pack more efficiently than the 2- and 4-substituted compounds.

ISSN:0267-8292    

DOI:10.1080/02678299608032052

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

We generalize an earlier density functional theory of liquid crystals by Mederos and Sullivan. The original theory took account of anisotropic hard core interactions, and for suitable intermolecular interactions predicted nematic and smectic A phases as well as isotropic liquid and vapour phases. In this generalization we also take into account quadrupolar or dipole induced dipole interactions. The modified theory now also predicts the existence of a smectic C phase.

ISSN:0267-8292    

DOI:10.1080/02678299608032053

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

High pressure studies for then= 12 homologues of the 3-fluoro-4((R) or (S)-methylheptyloxy)-4′-(4-alkyloxy-3-fluorobenzoyloxy) tolanes series have been performed by thermobarometric analysis. The phase sequence presents two Twist Grain Boundary phases denoted TGBAand TGBC. The pressure-temperature phase diagram has been determined; it exhibits a SC* phase, and an induced phase identified as SAwhich does not exist under atmospheric pressure. The TGBAtemperature stability domain versus pressure first increases, then rises to a maximum and finally disappears. Such phase behaviour leads to an experimental observation of two multicritical points SC*-SA-TGBAand SA-TGBA-N* for the pure compound under high pressures. This result confirms the existence of singular points previously predicted by the Renn-Lubensky theory.

ISSN:0267-8292    

DOI:10.1080/02678299608032054

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

In the present paper a discrete mean-field model for thin smectic A liquid crystal films with two boundary surfaces is offered. The model accounts for the recently observed phenomenon of layer-thinning transitions in free-standing smectic A films upon heating. In particular, the model predicts the observed multiple layer jumps, as well as, for films thinner than 13 layers, the observed power law dependence of the layer-thinning transition temperatures on film thickness.

ISSN:0267-8292    

DOI:10.1080/02678299608032055

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

In-plane alignment of nematic liquid crystals was regulated by polarized-light-irradiation of a cell assembled with a silica plate, the surface of which was modified by attaching 4-hexyl-4′-hexyloxyazobenzene at itso-position through surface silylation. The photoisomerizability of the chemisorbed azo-chromophore was affected by their surface density and the nature of photoinactive co-modifiers. The efficiency of the photoregulation of liquid crystal alignment was optimized by two-dimensional dilution of the chromophore with ethyltriethoxysilane (ETS) or 3-aminopropyltriethoxysilane (ATS). As a result, favourable procedure was to modify a silica surface with a crude azo-silylating reagent contaminated by ATS. The rate of the photoinduced reorientation of liquid crystals was followed by monitoring the alteration of the alignment direction of a dichroic dye dissolving in a mesophasic layer upon exposure to linearly polarized light. Exposure energy for the in-plane reorientation of a liquid crystal was about 100mJcm−2of 445 nm light at elevated temperatures close toTNI.The effect of positional isomerism on the photoregulation was determined with the use of 4-cyano-4′-hexyloxyazobenzene as a commander molecule. It was confirmed that the photoalignment efficiency was markedly enhanced by attaching the chromophore at theortho- ormeta-position. The exposure energy for the reorientation of a liquid crystal was reduced by linking thep-cyanoazobenzene at themeta-position so that the reorientation was complete with an exposure energy of 20mJcm−2.

ISSN:0267-8292    

DOI:10.1080/02678299608032056

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The recently published Baylis-Hillman methodology has been used to prepare a number of side chain liquid crystalline poly(allylalcohols) incorporating a ferroelectric mesogenic side chain. These poly(allylalcohols) exhibited wide range S*Cphases and, in the case of two of these materials, low glass transition temperatures. The transition temperatures and phase behaviour of the SCLC poly(allylalcohols) were compared to acrylate and methacrylate SCLC oligomers containing a similar mesogenic side chain. The response times for two poly(allylalcohols) exhibiting low glass transition temperatures were also measured over a wide temperature range. Although the poly(allylalcohols) had comparable response times to the analogous acrylate and methacrylate SCLCP, they showed the greater temperature dependence of the response time. However, at 39°C one of the SCLC poly(allylalcohols) showed a response time of 65 ms.

ISSN:0267-8292    

DOI:10.1080/02678299608032057

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

We present a homologeous series of liquid crystalline trioxadecalin compounds having a terminal alkoxy chain and a nitro group. The (1S, 3R, 6R,8R)-3-(4″-nitrophenyl)-8-(4′-octoxyphenyl)-2,4, 7-trioxabicyclo [4.4.0]decane2 cshows a temperature-dependent inversion of the cholesteric helix at lower temperatures. For higher temperatures, the reciprocal helical pitch reaches a minimum, then it increases, tending to a second inversion point just above the clearing point. An additional chiral centre in the side chain leads for nitro compounds3to non-mesomorphic behaviour. For the cyano compounds4, the change in the cholesteric helix is suppressed, for theRconfiguration, but for theSconfiguration helical inversion occurs at high temperatures and selective reflection above the transition to the TGBAphase.

ISSN:0267-8292    

DOI:10.1080/02678299608032058

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

We report upon a new method for producing a homogeneous alignment with a low pre-tilt angle for nematic liquid crystals. This method is significantly simpler to implement than many existing methods, and requires knowledge of only the optical properties of the liquid crystal used. In addition, we have developed a new technique for measuring the pre-tilt angles that is straightforward as well as intuitively appealing. The sensitivity of this method increases as the pre-tilt angle decreases. Results obtained using this method agree satisfactorily with those yielded by traditional techniques.

ISSN:0267-8292    

DOI:10.1080/02678299608032059

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor