摘要:

We report measurements on the electrooptic response of thin samples (∼2-5 μm) of two antiferroelectric liquid crystals. All the phase transitions in these compounds can be very easily detected using this technique. We have been able to measure such an electrooptic effect for the first time in the antiferroelectric and smectic I* phases of a tolane compound. The response shows a relaxation at high frequencies (∼10 KHz) and is at-tributed to a soft mode which produces an asymmetry in the molecular tilt in successive layers.

ISSN:1058-725X    

DOI:10.1080/10587259508033447

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A new experimental technique of observation and quantitative characterization of interference states in polarizing light is proposed. It permits the determination of the main optical features of the heterogeneous media, namely birefringence, rotatory power, biaxiality…etc, even of the microdomains. The method consists in superposition of unknown interference state and a standard interference state which presents axial symmetry, and permits observation of extinction zones.

ISSN:1058-725X    

DOI:10.1080/10587259508033448

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The dielectric behaviour of ferroelectric liquid crystals of copper (II) Schiff bases complexes were investigated by dielectric spectroscopy. The spontaneous polarization Psand the switching timeTwere determined by means of the polarization reversal method. Temperature dependent magnetic susceptibility measurements were carried out in the temperature range of 4.2 - 450 K to examine the magnetic properties of the copper (II) complexes.

ISSN:1058-725X    

DOI:10.1080/10587259508033449

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Results are reported for dipole-dipole association of mesogenic molecules in para-xylene solution. Measurements of Kirkwood correlation factors for solutions of 4-n-pentyl-4-cyanophenyl cyclohexane (PCH5) and 4-n-heptyl-4-cyanophenyl cyclohexane (PCH7) are interpreted in terms of new madel by Dunmur et al.10for dipole-association. Results suggest that the molecular association depends on molecular shape and polarizability anisotropy of the system.

ISSN:1058-725X    

DOI:10.1080/10587259508033450

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Dielectric measurements at ambient and elevated pressure confirm the proposal1, that the two nitrophenylhydrazone compounds studied which display discotic phases, exist as dimers. The dipolar contribution to dielectric permittivity in the solid phase comes mainly from the alkoxy groups, and is augmented in the liquid phases by contributions from the hydrogen bonded o-N02group, The two tailed material with a narrow mesophase exhibits erratic behaviour suggestive of the existence of meta-stable states in the solid phase. The use of a commercial ITO coated cell for the non-discotic compound revealed two separate artefacts which limited the value of the data.

ISSN:1058-725X    

DOI:10.1080/10587259508033451

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The 6 - 10th homologues of 4-n-alkoxybenzoic acid, i.e. hexyl-, heptyl-, octyl-, nonyl- and decyloxybenzoic acids (60BA, HOBA, OOBA, NOBA and DOBA) are used for the present investigations. Far IR spectroscopy is used to study the state of hydrogen bonding at temperature variation of the nematic liquid crystal phase. The observed spectral bands are assigned to different OH deformation vibrations. From the analysis of the temperature dependence of the spectral band widths a cyclic ↔ open dimer transformation is indicated at different vibration frequencies and polarizations. The activation energy for the dimer and monomer reorientation is calculated and compared with that per hydrogen bond. The correlation lengths parallel and perpendicular to the nematic director (ξ‖ and ξ⊥) are calculated and an anisotropy is found in the temperature dependence of the critical index of the correlation length. A comparison with nonoriented nematics with hydrogen bonded molecules is presented.

ISSN:1058-725X    

DOI:10.1080/10587259508033452

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The thermal behaviour of a homologous series of cyclic siloxane liquid crystals with cyanobiphenyl terminal groups and alkoxy linkages is presented. One member of this series, with an n = 3 alkoxy spacer, exhibits a broad nematic phase and an above ambient glass transition. Excellent planar alignment was obtained in the nematic phase using low tilt rubbed polyimide, and uniform switching was seen on the application of a transverse electric field above threshold. Splay threshold voltages have been measured as a function of temperature and the splay elastic constant, K11, calculated. For equivalent reduced temperatures, the splay threshold voltage and elastic constant for the cyclic - 3 OCB are similar to those for low molar mass 5CB. Electro-optic switching times and contrast ratios have been measured in a twisted nematic geometry. The effect may be stored below the glass transition.

ISSN:1058-725X    

DOI:10.1080/10587259508033453

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Several liquid crystal dyes and fluorinated compounds with high birefringence are synthesized and their properties evaluated. These polar compounds are useful dopants for enhancing the dielectric anisotropy of the dialkyl diphenyl-diacetylene host.

ISSN:1058-725X    

DOI:10.1080/10587259508033454

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

We apply the recently developed continuum theory of liquid crystalline polymers1to interpret the behavior of the shear viscosity η(γ.) and the first normal-stress difference N1(γ.) measured for liquid crystalline solutions (c = 37%, c > c*) of HPC (Aldrich, Mw= 100,000) in acetic acid (AA) with different ages: 1 day, 1 month and 2 years. η(γ.) and N1(γ.) were measured over four decades in γ. N1(γ.) is observed to change from positive to negative and again to positive, as the shear rate γ. increases. η(γ.) shows a small newtonian plateau at low shear rates and a strong shear-thinning at higher values of γ. The rate of decrease of η(γ.) in this region is not monotonous, as usual, but shows an “hesitation” similar to one previously observed in a different system2. The aging effect promotes a “depolymerization”3of HPC. This, in turn, should have a strong influence on the behavior of Nl(γ.) and η(γ.) which is indeed observed4. All these observations can be rationalized within the framework of the theory1.

ISSN:1058-725X    

DOI:10.1080/10587259508033455

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Binary mixture of 4 octyloxy- 4′-cyano- biphenyl (80CB) and 4-n-hexyl phenyl 4-n′ pentyloxy benzoate (ME 50.6) shows enhanced smectic A phase, where 80CB has both nematic and smectic A phases and ME 50.6 has only nematic phase. In the present paper we report the phase diagram, refractive indices, densities and orientational order parameters of different binary mixtures of 80CB/ME 50.6 throughout the entire composition range. Birefringence as well as density values show minima with molr fraction of 80CB. Order of smectic to nematic phase transition has been discussed.

ISSN:1058-725X    

DOI:10.1080/10587259508033456

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor