摘要:

Orientational modes at a solid-nematic interface with Rapini-Papoular boundary conditions are analysed in the framework of continuum nematodynamics, including the coupling of the director and fluid velocity. In the case of planar orientation, a surface mode appears in addition to the ordinary bulk modes, if the coupling between the director and fluid velocity, governed by the Leslie coefficient μ2, is large enough. The dispersion relation for the surface mode exhibits two distinctive regimes as μ2increases. In a realistic situation, the relaxation rate of the surface mode is several times slower than that of the corresponding bulk mode.

ISSN:0267-8292    

DOI:10.1080/02678299408036556

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Ribbon phases consist of long cylindrical aggregates that have non-circular normal sections. We have recently pointed out that scattering data for a large number of different intermediate ribbon phases of lower than hexagonal symmetry found in ionic surfactant systems indicate that these phases have a structure possessing a centred rectangular symmetry. In this communication, we have investigated the aggregate dimensions for the phases with cylindrical aggregates, i.e., the hexagonal phases and the centred rectangular ribbon phases. Previously published phase diagrams, small angle X-ray and neutron scattering data and2HNMR data for these phases in different systems have been used for this purpose. The results are that the axial ratios of the aggregates increase when the temperature decreases, when the surfactant concentration increases, and when the average surfactant charge decreases. Models that semi-quantitatively describe the thermodynamics of the micellar, hexagonal and lamellar phases, which are based on the Poisson–Boltzmann cell model approach, have previously been presented in the literature. We have extended these models to treat also the ribbon phases. The results from the calculations show the same trends with respect to changes in the dimensions of the non-circular aggregates upon changes in temperature, surfactant concentration and average surfactant charge, as those obtained experimentally. Theoretically calculated phase diagrams with ribbon phases are also presented. Based on the predictions of the model and some previously published experimental data for hexagonal phases, it is proposed that the formation of non-circular, cylindrical aggregates is a general property of single-chain, ionic surfactant/water systems, and that these aggregates in general pack on hexagonal lattices. The normal sections of these aggregates are circular on average, on account of the fact that the degree of deformation and the orientation of deformation changes along the axis of the aggregates and with time. Only for some systems, temperatures and surfactant concentrations do the asymmetric aggregates line up and ribbon phases with centred rectangular symmetry are obtained. The driving mechanisms for the transition from the hexagonal phase with asymmetric (fluctuating) cylinders and further to the centred rectangular phase with asymmetric (stiff) cylinders is also discussed. It is argued that this phase transition is of the first order.

ISSN:0267-8292    

DOI:10.1080/02678299408036557

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The orientation of a cyclic side-chain thermotropic liquid crystalline material in an AC field was monitored in real-time using synchrotron radiation. Monitoring the realignment processes in the millisecond-to-minute time-scale was made possible by the high X-ray flux. Orientation parameters and response times were calculated as a function of temperature and frequency. Response times decreased exponentially with temperature due to a decrease in the viscosity. Very little dependence of the response time on frequency was observed, except at low temperatures, where a switch from homeotropic to planar alignment of the molecules was detected. This reorientation of the director was studied in real-time and the resulting complex diffraction patterns were due to equal but opposite director rotations from an alignment parallel to the applied electric field to an alignment perpendicular to the applied electric field. The orientation parameters were highest in the central portion of the mesophase temperature range. At temperatures near clearing, the net degree of orientation diminished. Cooling through the mesophase with an applied electric field resulted in much larger orientation parameters than could be obtained by aligning at a fixed temperature in the mesophase.

ISSN:0267-8292    

DOI:10.1080/02678299408036558

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Induced smectic A phases in binary mixtures of a bi-swallow-tailed component and a second component are described as ‘filled smectic A phases’, since the second rod-like component is considered to occupy spaces between the terminal branches of the bi-swallow-tailed molecules. A new system of this kind is now described where the second rod-like component has a terminal dipole, thereby enabling dielectric studies of the mixtures. These measurements coupled with calorimetric and X-ray measurements give information about the packing and dynamics of the mixed phases.

ISSN:0267-8292    

DOI:10.1080/02678299408036559

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The dielectric relaxation study on a set of polyvinylether based cyanobiphenyl side chain liquid crystal polymers with three different alkyl spacer lengths of 7, 9, and 11 shows that the spacer decouples the motion of the mesogen from the polymer backbone so that both the low-frequency δ process and the high-frequency α process of the side group have been detected in smectic A phase. At a given temperature the relaxation frequency increases with spacer length for the α process. This trend is also observed for the λ process except for the longest spacer length.

ISSN:0267-8292    

DOI:10.1080/02678299408036560

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The effect on the liquid crystal transition temperatures of introducing various groups (for example incorporating C˭C, O, CO2and CO) into the terminal alkyl chain of a weakly polar model compound 1-[trans-4-(trans-4-propylcyclohexyl)cyclohexyl]pentane has been investigated systematically. Only the compound containing both an ester function and atrans-carbon-carbon double bond exhibited a wide-range nematic mesophase at elevated temperatures. Therefore, a wide variety oftrans-4-substituted-cyclohexyl (E)-alk-2-enoates incorporating a carbon double bond with atrans-configuration (E) in the terminal alkyl chain has been synthesized. Nearly all the two-ring esters prepared exhibit a nematic phase over a wide temperature range (≤ 100°C) at elevated temperatures (≤ 200°C). The tendency to form smectic mesophases is often low. Comparisons with the corresponding derivatives incorporating either just a carboxy group (COO) or just a carbon-carbon double bond (C˭C) in the same positions indicate that synergetic effects lead to broader nematic phases than would otherwise have been expected. The new compounds are easily prepared from known starting materials.

ISSN:0267-8292    

DOI:10.1080/02678299408036561

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

We present the first small angle neutron scattering studies on liquid crystal polymer composite systems, where small amounts of polymers are dispersed in liquid crystals. A recent study (A. Jákli, D. R. Kim, L. C. Chien and A. Saupe, 1992,Appl. Phys.72, 3161) showed that even 1 wt% of polymer can induce phase separation in the form of fibres. We have found that the cross-sectional radius of such fibres is typically 300 A and they have rough surfaces. The spatial distribution of the fibres does not change during the phase transitions of the liquid crystal. We have found that there is a correlation between the size of the separated polymer particles and the roughness of their surfaces: larger objects have less smooth surfaces.

ISSN:0267-8292    

DOI:10.1080/02678299408036562

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The structures of the highly ordered liquid crystalline smectic phases ofN-pentyl-N'-(p-pentyloxyphenyl)piperazine are identified using X-ray diffraction methods. For this compound a phase sequence hexagonal smectic B (SB)-orthofrhombic crystalline smectic H (CH)-monoclinic crystalline smectic H (CH) is observed for the first time. The changes in structural symmetry at the phase transitions are discussed.

ISSN:0267-8292    

DOI:10.1080/02678299408036563

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Four members of a homologous series of diarylethane α-chloroester ferroelectric liquid crystals [1] have been synthesized and characterized by polarizing optical microscopy and DSC measurements. Two of the compounds exhibit a twist grain boundary structure (TGB A* phase) or twisted smectic A* phase, shortly before reaching the smectic to cholesteric phase transition. The temperature dependence of the optical tilt angle θ, the spontaneous polarizationPsand the effective rotational viscosity γøwere investigated for the ferroelectric S*C, S*Iand S*Fphases. It was found that especially the spontaneous polarization and the rotational viscosity increased with decreasing length of the alkyl chain for all three ferroelectric phases. At the phase transition from S*Cto S*I, all material parameters determined showed a discontinuous change, giving evidence for a first order transition. At the transition from the S*1to the S*Fphase, the material parameters show only small, rather continuous changes.

ISSN:0267-8292    

DOI:10.1080/02678299408036564

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Small angle neutron scattering experiments have been performed on liquid crystalline 163 basepair NaDNA fragments in aqueous solution in the concentration range 190–285 mg ml−1. To induce a macroscopic alignment, a magnetic field was applied either parallel (B∥configuration) or perpendicular (B⊊configuration) to the incoming neutron beam. The isotropic scattering pattern in theB∥configuration and the anisotropic scattering in theB⊊configuration agree with the cholesteric structure of the liquid crystalline solutions. From the anisotropic scattering in theB⊊configuration, information on the orientation ordering can be derived. For this purpose, the experimental data are compared to the form function of a uniform rod including a gaussian orientation distribution. The standard deviation of this distribution is approximately 20° for all concentrations investigated here.

ISSN:0267-8292    

DOI:10.1080/02678299408036565

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor