ISSN:0267-8292    

DOI:10.1080/02678299308027300

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Progress in the field of discotic liquid crystals is summarized, with emphasis on experimental results rather than theoretical developments. Examples are given of discotic mesogens (including metallo-mesogens) and discotic polymers, and the structures of the mesophases identified to date are described.

ISSN:0267-8292    

DOI:10.1080/02678299308027301

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The relevance of scientific and technical research on liquid crystal polymers is examined and compared to the overall evolution of the general field of liquid crystals. It is shown that polymers have played a progressively increasing role during the last 25 years, and although they only became a quantitatively significant subject not much more than 10 years ago, their contribution to problems and materials now represents a considerable share. Three specific topics concerning rigid rod polymers, liquid crystalline networks and polymeric metallo-mesogens are examined and some related problems highlighted. In particular, some examples showing tractable thermotropic mesomorphic systems can be obtained with linear rigid homopolymers are examined; the phase behaviour of networks derived from segmented chain mesogenic polymers is discussed; inclusion of metallo-organic groups into polymeric mesogens is reviewed.

ISSN:0267-8292    

DOI:10.1080/02678299308027302

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The biphenylyl esters of the 4-n-alkoxyphenylpropiolic acids are a unique family of liquid-crystalline materials. In particular, when the biphenyl moiety of the compounds carries a chiral end-group, many optically active mesophases are created which exhibit unusual structures and physical properties. For instance, when the chiral group attached to the biphenyl moiety is 1-methylheptyl then Abrikosov, twist grain boundary smectic A* and antiferroelectric smectic C* phases are observed. The wide variety of chiral phases and electrochiral properties exhibited by this family of materials makes them ideal candidates for exploring chirality in the liquid-crystalline state. These investigations allow us to contrast and compare chirality dependent phenomena in liquid crystals, thereby producing a broader view of the concept of chirality in organized fluids than is traditionally presented.

ISSN:0267-8292    

DOI:10.1080/02678299308027303

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

New chiral, as well as racemic 1-methylalkyl terephthalidene-bis-aminocinnamates have been synthesized and their liquid-crystalline properties investigated. Generally, the chiral compounds possess the antiferroelectric S*ophase, whereas in the homologous series of racemates, the appearance of the Sophase depends on the length of the alkyl chain. For racemic compounds derived from 2-alkanols with an odd number of carbon atoms, a Sophase is generally observed. On the other hand when the number of carbon atoms is even, a Scphase occurs. Moreover, in racemic 1-methylbutyl terephthalidene-bis-aminocinnamate, a Sc–So–Screentrant phase sequence has been established.

ISSN:0267-8292    

DOI:10.1080/02678299308027304

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The structural, material and electro-optical properties of novel, halogenated nematic liquid crystals which contain quite different functional groups are correlated. Synergisms which lead to broad mesophases, low viscosities and large dielectric anisotropies further improve the performance of actively and passively addressed, high information content liquid crystal displays. Some recent developments, such as operation of supertwisted nematic displays with not only linear, but also circularly polarized light, are included. A recently presented, efficient liquid crystal colour projection concept, whose functional elements, i.e. polarizers, filters and modulators, consist entirely of liquid crystal devices, is reviewed. Its circular polarizers and filters are made up of novel, negative dielectric anisotropic cholesteric liquid crystals designed such that, dislocation-free, optically uniform, planar textures result from electric field alignment. Novel, non-linear optical ferroelectric liquid crystals which exhibit very large and stable second order harmonic coefficientsd22= 5 pm V−1have the potential to be used in integrated optical devices, such as frequency converters and Pockels modulators. Photopolymerization of polymer-coated substrates with linearly polarized light is shown to induce anisotropic, uniaxial orientation of the polymer side chains without mechanical treatment. The resulting anisotropic dispersive surface interaction forces align adjacent liquid crystal molecules parallel. This new, photoinduced liquid crystal aligning technique renders the generation of azimuthal director patterns possible. It opens up interesting possibilities for realizing new optical and electro-optical devices, including hybrid and stereo liquid crystal displays.

ISSN:0267-8292    

DOI:10.1080/02678299308027305

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

DSC and X-ray studies on a series of 1,2-phenylene bis[4-(4-alkoxyphenyliminomethyl)benzoates] are presented. The phases were shown to be classical nematic and/or smectic by their continuous miscibilities with reference mesophases. The layer spacings in the smectic A phase measured as a function of the alkyl chain length revealed that the mean tilt angle of the linear three-ring part of the molecule to the layer normal is about 37° and that the molecular core is V-shaped with an angle of about 70° between the two out-stretched benzoate legs. This molecular model accounts for the effects of a methyl group introduced into the 3- or 4-position of the 1,2-phenylene moiety on the mesomorphic properties. Both the nematic and smectic A properties are markedly promoted by replacement of the 1,2-phenylene group by the 2,3-naphthylene group.

ISSN:0267-8292    

DOI:10.1080/02678299308027306

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

This paper presents a constrained theory for smectic C liquid crystals that may be useful for the analysis of some effects in these materials. The theory is based on two simplifying assumptions, namely that the layers although deformed remain of constant thickness, and also that the tilt with respect to the layer normal remains fixed. The equilibrium version of the theory proves to be a non-linear generalization of the earlier Orsay theory, and promises to model a number of static effects satisfactorily. Here our aim is to examine preliminary predictions based on the corresponding dynamic theory, where some progress proves possible for shear flow, and also for a shear wave reflection-refraction experiment useful for the measurement of some viscous coefficients.

ISSN:0267-8292    

DOI:10.1080/02678299308027307

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

We review recent experimental studies of smectic A layering of asymmetric mesogens with emphasis on their relationship to phase transition physics. The formation of one and two dimensional lattices with long range order in the positions of the heads (tails) of molecules is considered as having either a polar or a steric origin. The various molecular models and phenomenological descriptions reproducing the phase behaviour of particular types of layering are evaluated. The influence of molecular factors on the delicate stability of different smectic A(C) phases is discussed in detail.

ISSN:0267-8292    

DOI:10.1080/02678299308027308

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

New anchoring properties of liquid crystals on amorphous solid surfaces are presented. In nematics (N), angular anchoring is usually described in terms of the Rapini-Papoular form, assuming constant surface order parameter. We generalize this expression, predicting a decrease of surface order for strong surface disorientation. Recent experiments on anchorings of varying strength confirm these predictions. Conjectures for the angular anchoring of smectic A on a solid amorphous surface explain the two easy layer orientations, normal to the surface or parallel, faceting inside a small critical angle. Roughness-induced surface transitions are discussed. For antagonistic nematic and smectic anchorings, we expect, below the N–SAtransition, a bent nematic surface boundary layer, recently observed by smectization under an electric field. Finally, the positional anchoring strength of smectics is introduced in terms of shear induced surface melting, and confirmed by a recent observation of oscillating shear stresses at the layer period.

ISSN:0267-8292    

DOI:10.1080/02678299308027309

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor