摘要:

A theory was formulated for the description of circularly polarized fluorescence (CPF) from a chiral nematic film in the spectral region outside the selective reflection band. The CPF theory incorporates: (1) the ability of a chiral nematic film to accomplish both circular dichroism and circular polarization and (2) linearly polarized fluorescence from chromophores unaxially aligned in the nematic sublayers comprising the film. Chiral nematic fluid films consisting of a nematic fluid, BDH 18523, and a chiral dopant, cholesteryl oleate, were prepared for hosting 1,6-diphenylhexatriene as a fluorescent dye. The experimentally determined dissymmetry factor using both left- and right-handed circularly polarized excitations was found to be in good agreement with the theoretical prediction with all the system parameters determined a priori. The theory was also employed to furnish insight into the effects of the concentration of the fluorescent dye and chiral nematic film thickness on the dissymmetry factor.

ISSN:0267-8292    

DOI:10.1080/026782998207334

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Two homologous series with the terminal oxime group are synthesized to evaluate the effect of the oxime group on mesogenic properties of the system. It is observed that oxime formation eliminates the smectic mesophases exhibited by parent homologous series. The nematic phase exhibited by mesogens has higher thermal stability because of intermolecular H-bonding.

ISSN:0267-8292    

DOI:10.1080/026782998207343

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Using the continuum theory in the linear perturbation limit, the formation of static periodic distortions (PD) is studied in a planar nematic sample under the action of crossed electric (E) and magnetic (H) fields. In a nematic dielectric with positive dielectric and diamagnetic anisotropies (epsilon A > 0, chi A > 0), the wave vector of H induced PD depends on the tilt of H in a plane normal to the initial director orientation n o when E is in the sample plane; change of magnetic tilt may cause discontinuous changes in the wave vector. Similar results are obtained when electrical conductivity is taken into account with hydrodynamic effects being assumed absent. Calculations are extended to a study of PD in nematics with high elastic anisotropy for different signs of epsilon A and chi A. When such materials have susceptibility anisotropies of opposite sign, change in magnetic tilt may cause 're-entrant' appearances of distortions. When anchoring is weak enough, the saddle-splay elastic constant K 24 can influence the domain of existence of PD. Possible effects of flexoelectricity as well as results for cylindrical geometry are qualitatively discussed.

ISSN:0267-8292    

DOI:10.1080/026782998207352

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The molar heat capacity of the rod-like compound 4-(trans-4-propylcyclohexyl)benzonitrile (3-CBCN), purity of 99.8mol%, has been measured with an adiabatic calorimeter at temperatures between 15 and 385K. 3-CBCN is a nematogenic mesogen, whose melting and clearing points are 316.33 and 319.09 K, respectively. The enthalpy and entropy gained at fusion are 20.4 kJmol -1 and 64.4 J K -1 mol -1, respectively; those for the nematic-to-isotropic transition are 1.1 kJmol -1 and 3.5 J K -1 mol -1 respectively. 3-CBCN exhibits a supercooled nematic phase, whose molar heat capacities have been measured from 25 K below the melting point. The molar and transition entropies of 3-CBCN are discussed in relation to those of 4-propylbiphenyl-4-carbonitrile (3-BBCN) and trans,trans-4'-propylbicyclohexyl- 4-carbonitrile (3-CCCN). There seems to exist a correlation between these values and mesophase stability. Finally, Eidenschink's theoretical model for the nematic-to-isotropic transition has been applied to 3-CBCN; the transition enthalpy estimated according to this model agrees well with the observed value.

ISSN:0267-8292    

DOI:10.1080/026782998207361

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

An apparatus is described, which allows the precise determination of the water content of amphiphilic liquid crystals by means of coulometric Karl Fischer titration. The scope and limitations of this method are described.

ISSN:0267-8292    

DOI:10.1080/026782998207370

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

An elastic model for the twisted nematic texture observed in nematic cells containing octyloxybenzoic acid is proposed. According to the model the twisted textures are due to the presence of cybotactic groups which are responsible for the chirality of the liquid crystal. We show that by assuming for the density of cybotactic groups a critical temperature behaviour close to a special temperature, the experimental data can be well interpreted by our mechanical model.

ISSN:0267-8292    

DOI:10.1080/026782998207389

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The twisting ability of a novel series of bridged binaphthol derivatives with substituents of various lengths and chemical nature in the 6,6-positions, recently synthesized and used as dopants in nematic solvents, is investigated with the help of the model based upon surface- helicity tensors. Structures of the low energy conformers of these compounds have been generated by molecular mechanics calculations. Their orientational behaviour and the coupling between anisotropy in the alignment and molecular chirality, which are at the origin of the helical twisting power, are analysed on the basis of the anisometry and the chirality of the shape.

ISSN:0267-8292    

DOI:10.1080/026782998207398

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Liquid crystal phases can be induced chemically by mixing compounds whose specific interactions are such that the transition temperature for the induced phase is higher than the melting points of the two compounds. A particularly dramatic example of such behaviour is the creation of a columnar nematic and a hexagonal columnar phase on mixing discotic multiynes with 2,4,7-trinitrofluorenone. Although the intense colour of the mixture indicates a strong charge-transfer band, it is uncertain as to whether the charge-transfer interaction between unlike molecules is enough to stabilize the induced liquid crystal phases. An alternative explanation for the formation of such phases involves an electrostatic quadrupolar interaction between the components,whose quadrupole moments differ in sign. This interaction weakens the face-to-face attraction for like particles while strengthening it for unlike particles. We have explored this possible explanation for chemically induced liquid crystal phases in discotic systems by modelling the basic interaction between discs with a Gay-Berne potential, to which is added a point quadrupolar interaction. We have determined the phase behaviour of the pure systems and their binary mixtures with constant pressure Monte Carlo simulations. It would seem that the quadrupolar interaction can account for many of the features of chemically induced liquid crystals.

ISSN:0267-8292    

DOI:10.1080/026782998207406

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Rheological property (shear stress versus shear rate) of cholesteryl isostearyl carbonate (CISC) is measured as a function of temperature, finding that, like some other cholesteryl derivatives, CISC has blue phases (BPs) between the cholesteric (Ch) and the isotropic (I) phases. In the Ch and the BP phases, measurements of the electrorheological (ER) effect, of the electric field on the rheology, are made; a slight increase of the viscosity is observed in the BPs and no ER effect in the Ch phase, which is contrasted to the distinct ER effect in the nematic (N) and the smectic A (SmA) phases.

ISSN:0267-8292    

DOI:10.1080/026782998207415

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

A series of ferroelectric liquid crystals (FLCs), 4 (3-methyl-2-halopentanoyloxy)- 4-hexyloxybiphenyls (3M2XPHOB, X=F for fluorine, C for chlorine, B for bromine) and their racemates (3M2XPHOB-R), were synthesized and characterized. The FLCs contain a chiral tail comprised of alpha -halo acids which are derived from L-isoleucine (DL-isoleucine for the racemates). The mesogens were characterized by high-resolution 1H and 13C NMR and their phase behaviour was studied by optical microscopy and differential scanning calorimetry. The chloro and bromo derivatives show both chiral smectic C (SmC*) phases and smectic A (SmA) phases, while the fluoro derivatives exhibit only a SmA phase. The spontaneous polarization of 3M2CPHOB and 3M2BPHOB were measured in the respective SmC* phases; that of the fluoro derivative was inferred by extrapolating its concentration dependent polarization in an achiral SmC solvent, a racemic mixture of 3M2BPHOB.

ISSN:0267-8292    

DOI:10.1080/026782998207424

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor