摘要:

In contrast to nematics far less is known about the alignment of the directors in a smectic A phase by an external field. To characterise this process, which necessarily involves the flow of the molecules, we have undertaken an NMR investigation of the smectic A phase of 4-octyl-4′-cyanobiphenyl. The results of these experiments have revealed that, as expected, the alignment of the directors is a complex process strongly influenced by the nature of the containing surface for the smectic A phase. To assist in understanding the alignment process we have used computer simulations to study the field induced alignment of the directors in the smectic A phase formed by the Gay-Berne mesogen GB(4.4,20.0,1,1).

ISSN:1058-725X    

DOI:10.1080/10587250008024834

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Equilibrium shapes of smectic-A domains in an isotropic phase formed on a substrate surface have been observed using a polarizing microscope. Polarizing-microscope images of some possible structures are simulated to determine the shape of the smectic-A domains. It is found from a comparison between experimental and simulated images that the shape formed on the substrate surface is a hemisphere. The hemispherical smectic-A domains are shown to be equilibrium shapes in terms of a theory proposed earlier (H. Naito, M. Okuda and Z. Ou-Yang,Phys. Rev. Lett.70, 2912 (1993)) by taking account of the change in the transition temperature from an isotropic phase to a coexisting phase during the growth of a smectic-A phase.

ISSN:1058-725X    

DOI:10.1080/10587250008024835

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Deuterium nuclear magnetic resonance (NMR) spectroscopy has been used to investigate the electric field-induced alignment of the director of the smectic A phase of the liquid crystal, 4-α,α-d2-octyl-4′-cyanobiphenyl (8CB-d2), at 303.3K. The electric field is arranged to be orthogonal to the magnetic field. The alignment process has been investigated at different electric field strengths and the rate of director alignment was monitored by recording the deuterium NMR spectra as a function of time after the electric field was switched on. The results reveal a complex pattern of electric field-induced director alignment. At high electric field strengths a rapid process is observed in which the director switches from an orientation parallel to the magnetic field to one in which it is parallel to the electric field. An induction period is also observed in which no apparent change in director orientation occurs. This induction period becomes longer (hours in magnitude) as the electric field strength is lowered. Other, intermediate, director orientations are observed as the electric field strength is lowered further. The role of defects is invoked in trying to interpret some of the observed processes underlying the mechanism of the director alignment in the smectic A phase.

ISSN:1058-725X    

DOI:10.1080/10587250008024836

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The twist elastic constant, K2, and the rotational viscosity coefficient, γ1, are of importance when the response time for the in-plane switching mode is studied. Since adding dopants is one technique to improve the response characteristics, the effect of dopants on these physical properties is significant. The effect on K2and γ1of adding alkyl(alkoxy) phenylcyclopentenones and alkyl(alkoxy) cyanobiphenyls to the base mixture ZLI-4792 together with their temperature dependence have been investigated using different temperature scales. The reduced temperature scale showed the effect of these dopants on K2is small. On the other hand, the temperature dependence of γ1depends on both the absolute temperature scale and the reduced temperature scale. Therefore, it is clear that the choice of temperature scale with which to compare γ1for different systems raises fundamental questions which way not have a unique answer.

ISSN:1058-725X    

DOI:10.1080/10587250008024837

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Deuterium nuclear magnetic resonance (NMR) spectroscopy has been used to investigate the field-induced director dynamics in a nematic liquid crystal, 4-pentyl-d2-4′-cyanobiphenyl (5CB-d2) deuteriated in the α-position of the pentyl chain, confined between two glass plates. The NMR spectra have been measured as a function of time after turning an electric field on and off. It is demonstrated that the field-induced director dynamics in the nematic liquid crystal cells can be successfully time-resolved. In addition, it is found that the doublet NMR spectra become powder-like during the turn-on and turn-off processes. It is shown that the rotational viscosity and the diamagnetic anisotropy of 5CB-d2can be determined from the time-resolved NMR spectra by assuming uniform alignment of the director.

ISSN:1058-725X    

DOI:10.1080/10587250008024838

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

In this paper we present a theoretical analysis of shear induced melting in smectic A liquid crystals. We have performed a detailed numerical analysis and find that for a range of parameter values a first order transition, from a supersheared state to a more relaxed state, is periodically encountered as the total shear is increased. This first order transition involves the melting and reformation of the smectic layers. In this paper we present a summary of this analysis while a more detailed analysis, including analytic expressions for certain critical quantities, is presented elsewhere.

ISSN:1058-725X    

DOI:10.1080/10587250008024839

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

By constant temperature molecular dynamics simulation of the Gay-Berne system sandwiched by homogeneous anchoring walls, formation of surface stabilised order with homeotropically anchored layers (SmAII) driven by the electric field is studied. The phase SmAII is shown to be stabilised even in the anisotropic homogeneous anchoring condition where SmAII is never observed without the field, while in case dipole-dipole is not neglected the layer of SmAII is destabilised and eventually a sort of smectic C phase appears.

ISSN:1058-725X    

DOI:10.1080/10587250008024840

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

We review the evidence indicating that ion transport can be decoupled from structural relaxations of the host material in polymer electrolytes. Particular emphasis is given to side group liquid crystal polymer electrolytes consisting of poly(ethylene oxide)-based backbones and pendent mesogenic groups. In salt complexes of these materials appreciable ionic conductivities are observed below the glass transition temperature. Thus, truly solid polymer electrolytes have been obtained. Structural similarities are discussed between these materials and rigid polymers containing powerful ion solvating groups which also appear to exhibit a decoupled conductivity mechanism. The conductivity mechanism in solid polymer electrolytes is compared to that proposed for inorganic glasses.

ISSN:1058-725X    

DOI:10.1080/10587250008024841

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

We have synthesized the AB type diblock copolymer with polystyrene and poly(6-[4-(4′-methoxyphenyl)phenoxy]hexyl methacryrate) by sequential anionic living polymerization. The copolymers were prepared with quantitative yields, possessing the predictableMnvalues, compositions and narrow molecular weight distributions. The thermotropic phase behavior and structures were examined for six copolymers with the weight fraction of LC segments from 24wt% to 45wt%. All copolymers exhibit the crystal-SmA and the SmA-isotropic phase transitions. Microphase separations were examined as a function of the weight fraction of two segments and in a relation with the micro structure of LC segment.

ISSN:1058-725X    

DOI:10.1080/10587250008024842

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

There is currently great interest in the use of lyotropic liquid crystals as templates for the production of mesoporous inorganic or metallic materials having uniform pore sizes in the range 20-100 Å or greater. Such materials have potential applications as catalysts, molecular sieves or electrodes, and can accommodate molecules which are too large to fit within the pores of conventional zeolites. We have investigated the structure and alignment of mesoporous materials produced using the True Liquid Crystal Templating (TLCT) method. In TLCT, the surfactant forms a lyotropic liquid crystal phase of well defined geometry and dimensions at the start of the synthesis. We have been able to produce hexagonal mesoporous coatings within capillaries, having a specific alignment of the 2-D hexagonal lattice, as probed by X-ray diffraction. We have also followed the structural kinetics of the alignment process. These TLCT mesoporous materials may be functionalized for catalytic applications.

ISSN:1058-725X    

DOI:10.1080/10587250008024843

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor