摘要:

The optic tensor configuration in a surface stabilized ferroelectric liquid crystal cell is investigated using optical excitation of half leaky guided modes. A thin ferroelectric liquid crystal layer is confined between a high index pyramid, with an index greater than the maximum of the liquid crystal, and a glass substrate having an index less than the minimum of the liquid crystal. Using standard attenuated total reflection experimental procedures, over a small angle range a series of sharp resonant peaks are recorded in the s-polarized reflectivity using p-polarized incident light. These peaks are extraordinarily sensitive to details of the optical tensor configuration within the cell. Fitting theoretically modelled reflectivities from multilayer Fresnel theory to the data allows determination of near surface alignment, bending of the chevron, surface tilt angle and biaxiality. To give a clear physical explanation for the great sensitivity of the technique, the electromagnetic field component distributions in the cell are also presented and analysed. The results confirm that the half leaky guided mode method has enormous potential for the study of the optic tensor configuration in liquid crystal layers.

ISSN:0267-8292    

DOI:10.1080/02678299308029048

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Several derivatives with thetrans-stilbene oxide moiety as the central chiral core were synthesized. With the appropriate substituents, nematic and smectic C phases were observed. One derivative was obtained with aneeof 47 per cent by asymmetric synthesis. The chemical stability of these epoxides is low and they rearrange to give substituted phenyl benzylketones which are also liquid-crystalline. The structure of the two solid phases present in all the derivatives and of the smectic C phases was investigated by X-ray diffraction.

ISSN:0267-8292    

DOI:10.1080/02678299308029049

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A simple method for modelling the textures of liquid-crystalline phases, recently proposed by Bedfordet al., is here rewritten in matrix form. A formal similarity is found between their method and the continuum theory of liquid crystals. Disclination patterns are simulated by solving a modified matrix equation.

ISSN:0267-8292    

DOI:10.1080/02678299308029050

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The behaviour of polymer dispersed cholesteric droplets in electric fields has been investigated for materials with both negative and positive dielectric anisotropy. Microscopic observations parallel and perpendicular to the electric field were performed. In materials with εa< 0, the cholesteric pitch remains constant but a disclination ring occurs due to reorientation of the local helix directions. In systems with εa> 0, however, helical unwinding was observed.

ISSN:0267-8292    

DOI:10.1080/02678299308029051

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Investigations of 4-[(S, S)-2, 3 epoxyhexyloxy]-phenyl-4-(decyloxy)-benzoate by polarizing microscopy, the Cano-Grandjean method, optical rotation dispersion and UV-VIS spectroscopy reveal a cholesteric phase with temperature induced reversal of the helical twist. Switching time experiments in the Sc* phase show that the intrinsic helix can be unwound reversibly and irreveribly by application of electric fields of different strengths.

ISSN:0267-8292    

DOI:10.1080/02678299308029052

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The cholesterol derivative tetraethoxycholesteryl semisuccinate is both a mesogen and an amphiphile. This combination of both molecular prerequisites permits two types of supramolecular self-organization: the formation of a thermotropic liquid-crystalline phase and of various forms of aggregates in contact with water or other solvents. Depending on the pH of the aqueous medium the compound self organizes in micelles or liposomes. At high concentrations lyotropic liquid-crystalline phases are obtained. The formation of liposomes and lyotropic phases is not restricted to water as a solvent but can also be induced in pure organic media such as water-free diethyleneglycol. Due to the broad range of supramolecular structures that depend both on molecular shape and on amphiphilic properties we propose to call the title compound a model for amphotropic phase behaviour.

ISSN:0267-8292    

DOI:10.1080/02678299308029053

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

2-Hydroxy-5,5-dialkyl-δ-valerolactone derivatives have shown interesting properties as a chiral dopant for ferroelectric liquid crystals. In small amounts these compounds induce high magnitudes of spontaneous polarization and long helical pitches in the chiral nematic phase (N*). Alkyl chains attached at the C-5 position play an important role in these properties. Here we discuss the stereochemistry and ferroelectric properties of these lactones and compare them with those of 2-hydroxy-5-alkyl-δ-valerolactone derivatives reported previously. The ring structure of 5,5-dialkyl-δ-valerolactone is found to be a rigid ‘pseudo-chair’ conformation by *H NMR studies.

ISSN:0267-8292    

DOI:10.1080/02678299308029054

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The synthesis and characterization of cyclic main chain liquid-crystalline oligomers based on 1-(4-hydroxy-4′-biphenyl)-2-(4-hydroxyphenyl)butane (TPB) with 1,7-dibromoheptane (TPB-7(c)] are described. These oligomers were synthesized by the phase transfer catalysed polyetherification of TPB with 1,7-dibromoheptane under high dilution conditions and separated by column chromatography. Their cyclic structure was confirmed by 200 MHz1H NMR spectroscopy. The mesomorphic behaviour of TPB-7(c) was characterized by differential scanning calorimetry and polarized optical microscopy. The cyclic dimer is only crystalline, while the cyclic trimer, tetramer and pentamer exhibit an enantiotropic nematic mesophase. The high molecular weight linear homologue TPB-7(1) exhibits a nematic mesophase which has an isotropization transition temperature located in the very close proximity of its glass transition temperature. Therefore, this nematic phase is kinetically controlled. Due to the higher rigidity of cyclics versus linear structures the cyclic trimer, tetramer and pentamer exhibit higher isotropization transition temperatures than their linear homologue. Subsequently, the kinetically controlled nematic mesophase of the linear homologue is transformed into a thermodynamically controlled phase via cyclization.

ISSN:0267-8292    

DOI:10.1080/02678299308029055

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Transient disclinations of strength ± 3/2 were observed in the nematic phase of a single-component, thermotropic, rigid rod polytolane. These disclinations were characterized by 6 extinction brushes in textures observed by hot stage transmitted polarized light microscopy. The small number and short lifetime of these disclinations is discussed in terms of their energy per unit line length, and in terms of the force that causes them to be attracted to and annihilated at other disclinations.

ISSN:0267-8292    

DOI:10.1080/02678299308029056

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

We propose a simple model for the chevron structure observed in recent experiments by cooling a smectic A liquid crystal. We discuss the influence of the cell thickness and of the anchoring conditions on the temperature dependence of the layer tilt angle, and the formation of this structure in the vicinity of a smectic A—nematic transition. Below this critical point, a transition between a bookshelf structure and a chevron one appears. This transition is second order, with continuity of the tilt angle, the threshold being a function of the cell thickness. In addition to a classical layer thinning mechanism, we discuss another possibly based on the temperature dependence of the elastic moduli. We also propose an explanation for the existence of a critical thickness below which the chevrons do not appear.

ISSN:0267-8292    

DOI:10.1080/02678299308029057

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor