摘要:

In principle, binary mixtures of rod-like and disc-like particles should exhibit a biaxial nematic phase, but in practice phase separation into two uniaxial nematic phases prevents this. Here, we report the results of a computer simulation study of an equimolar mixture of rods and discs in which phase separation is not allowed. The particles are confined to the sites of a simple cubic lattice in which each rod is surrounded by six discs and vice versa. Neighbouring particles interact such that they prefer to align with their respective symmetry axes orthogonal to each other. In contrast, the interaction between next nearest neighbours, which are either rods or discs, is such that their symmetry axes tend to be parallel. Monte Carlo simulations of this model mixture show that an orientationally ordered phase exists at low temperatures. This nematic phase has overall uniaxial symmetry and the particles have a negative second rank orientational order parameter, indicating that they tend to align at right angles to the director. The two interpenetrating sub-lattices containing either rods or discs, however, exhibit a biaxial nematic phase. The results of the simulation are found to be in reasonable agreement with the predictions of a molecular field theory for this model mixture. We have also investigated the behaviour of this mixture when the rods and discs are allowed to exchange between their lattice sites. The mixture is found to separate into two uniaxial nematic phases composed essentially of either rods or discs, as expected.

ISSN:0267-8292    

DOI:10.1080/02678299308029133

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The mesomorphic properties of 3,4,10,11,17,18,24,25-octaalkyloxy and octaalkanoyloxycyclotetraveratrylene (orthocyclophane) have been investigated by differential scanning calorimetry (DSC), polarizing optical microscopy, NMR and X-ray diffraction. Twelve homologues of the octaalkyloxy series, with equal side chains containingn= 5 to 16 carbons per side chain, were studied and found to be mesomorphic with at least one columnar mesophase. All members of the series exhibit a biaxial columnar mesophase denoted by D1. The homologues, withn= 10 to 16, also exhibit a uniaxial columnar mesophase, between the D1and the liquid phase, which is denoted by D2. Miscibility studies show that the D1phases of all the compounds, and the D2mesophases of the high members of the series are isomorphic. The D1and D2phases are completely miscible with M1and M2of the hexaalkanoyloxytriphenylene series respectively. They may, therefore, be classified as discotic B and discotic A respectively. Both phases remain stable upon addition of substantial amounts of non-mesogenic aromatic compounds. The transition between D2and D1is weakly first order (not observed by DSC), but the interconversion is spread over a wide temperature range (∼20°C for then= 13 homologue). The analysis of the X-ray results shows that D2is hexagonal and that D2is rectangular, with one and two molecules per unit cell respectively. The lattice parameters for all measured compounds in the two mesophases are reported.

ISSN:0267-8292    

DOI:10.1080/02678299308029134

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A series ofN-[4′-(dodecyloxy)resorcylidene]-4-(RO*)-anilines (HLnn= 1, R*O = (−)-cis-myrtanyl;n= 2, R*O = (−)-menthyl;n= 3, R*O = S-(−)-β-citronellyl;n= 4, R*O = R-(−)-2-octyl) has been synthesized and the mesomorphic properties investigated. These ligands are able to chelate a metal atom; accordingly three different series of palladium complexes, [(Ln)Pd(Ln)], [(Ln)Pd(Azoxy-6)] and [(L3)Pd(Ph-Pyn)] (where Azoxy-6 and PhPynare cyclopalladated 4,4′-dihexyloxyazoxybenzene and 5-substituted-2-(4-substituted-phenyl)pyrimidine, respectively) have been prepared. The mesogenicHL3andHL4ligands exhibit a S*Cphase, retained in [(L3)Pd(L3)], which changes to a cholesteric phase in [(L4)Pd(L4)] and becomes the more ordered S*Hphase in the [(Ln)Pd(Azoxy-6)] (n= 3,4) derivatives. In contrast, in the [(L3)Pd(Ph-Pyn)] compounds the mesomorphic phase is a SAphase.

ISSN:0267-8292    

DOI:10.1080/02678299308029135

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

We report the results of computer simulation studies for a bulk system composed of chiral particles interacting via the Gay-Berne potential and an additive chiral potential. Using Monte Carlo (MC) simulations in theNVTensemble, the chirality-temperature plane of the phase diagram was studied at different points by a variation of the chirality parametercdescribing the strength of the chiral potential. Additionally to the well-known isotropic, nematic and smectic phases of the Gay-Berne fluid, we localized regions of cholesteric phase. For large values of the chirality parameter we also observed blue phases. Furthermore, when starting from a cholesteric phase and decreasing the temperature at constantc, we obtained a phase region showing characteristics of the recently discovered helical smectic A* phase. All phases have been characterized by correlation functions, order parameters, and visual representations of selected configurations. All results of the simulation are limited by the small system size ofN= 256 molecules and the use of periodic boundary conditions.

ISSN:0267-8292    

DOI:10.1080/02678299308029136

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

We report an X-ray study of smectic A layering for mesogenic compounds with fluorinated substituents in terminal positions. The measurements were carried out using diffractometers with one- and two-coordinate proportional chambers. It was found that in contrast to ‒CN or ‒NO2terminated smectics, the polar ‒OCF3compounds form only a monolayer smectic A1phase. The ratio of the intensity of the second harmonic to the first in smectic A phases formed by molecules with lengthy perfluorinated chains was found to be two orders of magnitude higher than is commonly reported for low molar mass thermotropic mesogens, indicating deviations of the density distribution functionp(z) from a pure sinusoidal form. The layer periodicitydfor these mesogens exceeds the molecular lengthL:d/L≃ 1·1, which corresponds to a smectic Adphase consisting of parallel or antiparallel dimers. We have observed that lateral fluorine substitution in the benzene ring adjacent to the perfluorinated chain leads to the disturbance of uniform smectic A layering and to the formation of a defect-modulated smectic A structure of a chequer-board type. For polyphilic compounds containing both hydrogenous and perfluorinated units in the terminal chain, the in-plane structure factor displays double-peaked liquid-like profiles indicating the existence of nearest-neighbour molecular stacking at different distances. The peculiarities of smectic A layering in fluorinated mesogens are discussed in terms of steric coupling and enhanced conformational rigidity of fluorine containing moieties.

ISSN:0267-8292    

DOI:10.1080/02678299308029137

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

New differential scanning calorimetry measurements of the smectic A-cholesteric latent heat of transition for binary mixtures of cholesteryl nonanoate (C9) and cholesteryl heptanoate (C7), cholesteryl nonanoate and cholesteryl caproate (C6) are reported for cooling cycles. These measurements give evidence of a tricritical point at approximately the reduced temperatureTSACh/TChl≅ 0·92 for 63·1 mol per centC9in the mixture ofC6andC9. This tricritical point occurs at reduced temperatures and concentrations different from those already obtained for heating cycles for the same mixture.

ISSN:0267-8292    

DOI:10.1080/02678299308029138

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

New thermotropic liquid crystal compounds, with one and two mesogenic groups, have been synthesized and characterized. The structural elements of the compounds consist of thiophene ring systems, vinylene units and, for some compounds, carbaldehyde groups. Mesogenic groups involving three thiophene ring systems are normally not liquid crystalline. Only in the case ofE,E-5-[5-(5-butyl-2-thienylvinyl)-2-thienylvinyl]-2-thiophenecarbaldehyde could a monotropic nematic phase be easily observed. By replacing thiophene ring systems in the core with a benzene ring system, or by attaching one carbaldehyde group at the moieties, or by doubling the mesogenic groups via an alkylene spacer, the temperature ranges of the mesophases of the compounds with thiophene ring systems can be enlarged or liquid crystal phases induced. The small values for δSNI, which have been observed for the low molecular weight compounds, indicate a very low order in the nematic phases.

ISSN:0267-8292    

DOI:10.1080/02678299308029139

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The basic mechanisms determining the formation of optical anisotropy in stretched, thin polymer dispersed liquid crystal (PDLC) films with micron sized nematic droplets have been studied experimentally and the results analysed in terms of a proposed theoretical model. The experiments were performed on PDLC films with the bipolar nematic director configuration in the droplets, where the film transmittance, microscopic structure, and birefringence of the polymer matrix were studied. It is shown that the orientational ordering of bipolar nematic droplets, introducing the main contribution to the ability of stretched PDLC film to polarize the transmitted light, is strongly dependent upon initial droplet shape and the elastic properties of the polymer matrix. The ‘anomalous’ nematic director orientation is also observed in a portion of elongated droplets where the axes of bipolar configurations do not coincide with the major axes of the droplet cavities due to the presence of inclusions at the cavity walls. The effect of alternation of droplet size and shape upon stretching and the influence of optical anisotropy of the polymer matrix on film transmittance are analysed. On the basis of the results obtained, simple criteria for optimization of main PDLC polarizer performance are formulated.

ISSN:0267-8292    

DOI:10.1080/02678299308029140

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The dielectric spectra ofn-decane-1,2-diol andn-undecane-1,2-diol have been measured with the use of time domain spectroscopy, in the temperature range covering the isotropic and liquid crystalline phases. In the isotropic and smectic phases a relaxation process was observed which could be described by the Cole-Cole equation with the distribution parameter α ≈ 0·20. It was established that the transitions from the isotropic to smectic phases were not accompanied by a step-wise change of the relaxation times. However, the activation energy increases markedly at the transition to the S*Bphase, whereas it does not change at the isotropic-S*Atransition. A discussion of the dielectric parameters obtained for these and other low molar mass diols is presented. It was found that the static dielectric permittivity and the activation barrier change systematically with the increase of the molecular length ofn-alkyl-1,2-diols.

ISSN:0267-8292    

DOI:10.1080/02678299308029141

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A new kind of planar chevron defect, which we call the ‘mountain defect’ due to its mountain-shaped appearance under the microscope, is observed in chevron surface stabilized smectic C liquid crystal cells for both chiral (ferroelectric) and achiral materials. Polarized optical microscopy investigations reveal that this kind of defect, which can either appear spontaneously and grow slowly over days, weeks and months or can be induced by applying an electric field or mechanical distortion, mediates change in the chevron interface position, separating chevron domains of differing chevron interface position. The full three dimensional layer structure of this defect and its relation with other commonly seen line defects in such cells, like zig-zag walls and field lines, will be presented. The formation of this kind of defect indicates that chevron structure is not necessarily a stable structure in these cells.

ISSN:0267-8292    

DOI:10.1080/02678299308029142

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor