摘要:

A homologous series of liquid crystallinecis, cis-(3,5-dihydroxycyclohexyl) 3,4-bis(alkoxy)-benzoates ('two chain' diols) is described. The formation of different mesophase structures by these compounds is influenced by the length of the alkoxy side chains; changes from smectic via cubic to hexagonal columnar mesophases have been established in a certain chain length range. The aggregation to 'supramolecular' structures via hydrogen bonding seems to be necessary for the different mesophases. Not only the behaviour of the freeze dried products, but also that of samples which had absorbed water, either from a saturated atmosphere or from direct contact, is described.

ISSN:0267-8292    

DOI:10.1080/02678299508036654

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The complex permittivity of a liquid crystalline side group polymer has been measured at pressure from ambient up to 300 MPa. The samples were initially not aligned, then planar and finally homeotropically aligned by the field of a 7.2 Tesla magnet. Temperatures ranged up to 130°C, and measurement frequencies from 5 Hz to 5 MHz. The equilibrium permittivity εshas been interpreted in terms of an effective dipole moment μ. The frequency of maximum loss has been analysed in terms of enthalpy of activation and volume of activation for α and β relaxation processes.

ISSN:0267-8292    

DOI:10.1080/02678299508036655

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Liquid crystal layer characteristics have for some time been determined by analysis of the optical guided modes supported within such layers. A novel technique for guided mode analysis, relying on scattering and re-radiation of light from a liquid crystal filled Fabry-Perot, is presented here and compared with a more standard technique, where the transmissivity of light through such a structure, as a function of the angle of incidence, is measured. The new technique which we label ‘momentum space imaging’ is found to hold some advantages over traditional methods, with data acquisition on the millisecond time-scale and full liquid crystal layer characterization being achievable with minor modification to existing theoretical multilayer modelling.

ISSN:0267-8292    

DOI:10.1080/02678299508036656

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The microstructure of the rectangular phase in the system sodium decylsulphate/decanol/water is investigated by means of deuterium NMR. By analysing the lineshape from selectively deuteriated decylsulphate and decanol, we separate the effects of (i) the shape anisotropy of the aggregate cross-section and (ii) the inhomogeneous distribution of the two surfactants within the aggregate. The aspect ratio of the cross-section is determined to 1.39±0.01, substantially smaller than previous estimates. We find no evidence for anisotropic growth of the aggregate cross-section in the hexagonal phase, as previously suggested. Rather, the aggregate shape appears to change abruptly at the hexagonal-rectangular phase transition with little change (with temperature) thereafter. The distribution of decylsulphate and decanol within the aggregates of the rectangular phase is highly non-uniform; the decanol concentration is 3 times higher in the central lamellar region than in the curved edges, while the decylsulphate accumulates in the curved regions. This molecular segregation can be rationalized in terms of electrostatic interactions within and between the aggregates.

ISSN:0267-8292    

DOI:10.1080/02678299508036657

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Room temperature measurements of the stationary noise of the monochromatic light scattered by a polymer-dispersed liquid crystal (PDLC) have been performed in order to get information about the parameters affecting the fluctuation modes within the LC droplets, such as the effective viscosity constant of the anisotropic medium. The power spectra of the scattered light appear to be described in terms of lorentzian functions, whose intensity and cut-off frequency are strongly dependent on both the scattering angle and the electric field across the sample. Hints about the optical-axis configuration within the LC droplets are provided by a thorough analysis of the measured noise. The main discrepancies between the experimental results and the predictions for an infinite nematic LC are pointed out and discussed in terms of the finite size of the LC droplets.

ISSN:0267-8292    

DOI:10.1080/02678299508036658

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A homologous series ofN-cyanopropyl-N-alkyl-N,N-dimethylammonium bromides was synthesized and characterized. Its thermotropic liquid crystalline behaviour was studied by differential scanning calorimetry, polarizing optical microscopy, dilatometry, and X-ray diffraction. A smectic A mesophase was thus identified. The smectic layers were found to be formed of two ionic planes alternately separated by sub-layers of disordered alkyl chains and cyanopropyl groups. The smectic structure was compared with that of the crystal at room temperature and also that of the smectic T structure of theN,N-dialkyl-N,N-dimethylammonium bromides already described in the literature.

ISSN:0267-8292    

DOI:10.1080/02678299508036659

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Achiral dyes in isotropic (acetyl)(ethyl)cellulose (AEC) solutions display no induced CD signals irrespective of acetyl content, polymer concentration or dye content. When dissolved in anisotropic AEC solutions, the dye molecules show strong induced CD bands that disappear when the phases are heated above the anisotropic/isotropic phase transition temperature. The liquid crystal induced circular dichroism (LCICD) spectra for the dyes in a series of well-characterized AEC mesophases, whose handedness depends on solvent and acetyl content, reveal that the sign of the LCICD signal is determined by the supramolecular structural features of the host matrices. Thus, acridine orange (AO) dissolved in a left-handed liquid crystalline AEC solution displays a negative LCICD whereas a positive LCICD was observed when the AO is dissolved in a right-handed AEC mesophase. The sign and intensity of the LCICD signals change with acetyl degree of substitution, solvent and temperature. In all cases, changes in sign of the CD peak correlated with changes in handedness of the chiral nematic structure, and the intensity of the CD peak increased with increasing chiral nematic pitch for a given solvent and polymer concentration. Induced circular dichroism is thus a useful probe of chiral nematic structure in these systems.

ISSN:0267-8292    

DOI:10.1080/02678299508036660

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

As part of a systematic study of the factors affecting nematic phase formation, the influence of introducing dipoles (in the form of oxygen, carbonyl and carboxy groups) and steric restrictions (in the form of carbon-carbon double bonds) in various positions, configurations and combinations in a model system (4-[trans-4-pentylcyclohexyl]benzonitrile) has been investigated. On the basis of these results, we have introduced an ester groupanda carbon-carbon double bond with atrans-configuration (E) into the terminal alkyl chain attached to the cyclohexyl ring of a variety of two- and three-ring nematic mesogens of positive dielectric anisotropy. This is a new combination of a polar ester group (dipole effect) and the added rigidity imposed by the double bond (steric effect). Most of the new (E) alk 2-enoates containing two rings in the molecular core possess high melting points. Only a few two ring esters exhibit nematic phase, although the clearing point of those esters exhibiting mesomorphic behaviour was high. The corresponding three-ring (E)-alk-2-enoates incorporating an additional phenyl or cyclohexane ring also possess high melting and clearing points, as well as wide nematic ranges. No smectic mesophases could be observed for any of the (E)-alk-2-enoates synthesized. Comparisons with the corresponnding derivatives incorporating either just an ester group, or just a carbon-carbon double bond in the same position indicate that synergetic effects lead to higher clearing points than would otherwise have been expected. The new (E)-alk-2-enoates possess a surprisingly moderate viscosity for esters. The high value of the elastic constant ratiok33/k11is of advantage for mixtures designed for supertwisted nematic LCDs.

ISSN:0267-8292    

DOI:10.1080/02678299508036661

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A mean-field theory of liquid crystalline ordering in hydrogen bonded liquids is proposed. It is shown that hydrogen bonds may cause liquid crystalline ordering. For a simple example of spherical molecules containing hydrogen bonding functional groups the existence of a nematic phase is predicted and the thermodynamics of the phase transition is discussed.

ISSN:0267-8292    

DOI:10.1080/02678299508036662

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

We report here original results characterizingin situthe interactions between a smectic liquid crystal phase and a polymer network dispersed in it. These results have been obtained by neutron scattering on smectic liquid crystal (8CB) samples containing a physical network of 1.5wt% polymer. The samples were polymerized in the isotropic, or in the smectic A phases. For the first time it is experimentally proved that the polymerization of non-mesogenic monomers in an aligned smectic A matrix induces anisotropy in the resulting network. The network becomes elongated along the liquid crystal director. When the polymerization is carried out in the isotropic phase the polymer network has an isotropic distribution even if a magnetic field, which orients the liquid crystal director, is later applied. On the other hand, studies show that after several thermal cycles, the liquid crystal orientational order still remained. Without other external constraints, the polymer network freezes the alignment of the liquid crystal. It is probably imposed by pendant reticulates on the diffuse liquid crystal-polymer interfaces.

ISSN:0267-8292    

DOI:10.1080/02678299508036663

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor