摘要:

Two types of side group polysiloxanes which differ in their spacer between backbone and mesogenic part were synthesized via polymer analogous reaction. The impact of an oxygen atom localized in the γ position to the siloxane main chain on the ferroelectric properties was investigated. In addition, the link between the terminal chiral moiety and the mesogenic part in each series was varied. The phase transition behaviour, spontaneous polarization and the electro-optical response time for the two series of polymers are compared. The mesophase structures were determined by X-ray measurements on magnetically oriented samples at different temperatures. The additional oxygen atom in the spacer near to the main chain leads to broader range smectic C* phases and shorter electro-optical response times, which suggests that the ether linkage promotes a better decoupling between main chain and mesogenic side group.

ISSN:0267-8292    

DOI:10.1080/02678299608033159

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A number of tailor-made side chain liquid crystalline terpolysiloxanes, containing chiral cyanohydrin ester, phenyl pyrimidine and fluorinated phenylbenzoate as the side groups, have been synthesized for potential applications as ferroelectric liquid crystal polymers. All the polysiloxanes exhibit the mesomorphic sequence of crystal-chiral smectic C-smectic A-isotropic phases.

ISSN:0267-8292    

DOI:10.1080/02678299608033160

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A side-on fixed liquid crystal polyacrylate has been investigated in Langmuir and Langmuir-Blodgett (LB) films. High in-plane orientation of the mesogenic groups has been observed within the LB multilayers, showing the ability of the LB technique to align a liquid crystal. The analysis of the in-plane order versus the dipping speed suggests some self-aggregation of the polymer in the monolayer. Within these aggregates, the molecular orientation (in-plane and out-plane) has been deduced from infrared dichroism experiments; the mesogenic group lies mainly parallel to the dipping direction and relatively flat on the substrate. This alignment of the polymer is however partially lost with time, leading to materials with less in-plane anisotropy.

ISSN:0267-8292    

DOI:10.1080/02678299608033161

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

This paper presents a theoretical study of the behaviour of smectic A liquid crystals subject to shear flow. The liquid crystal is assumed to consist of uniform, planar smectic layers, on which no external moments and no external boundary conditions have been imposed. It is shown that of the five viscosity coefficients needed for a full description of dynamical behaviour of the SAphase, only two, λ1and λ4, enter the expression of the shearing torque. A stability analysis establishes the possible flow alignment configurations of the smectic layers, and the geometrical arrangements of the flow aligned smectic layers are depicted, parametrized by the crucial parameter λ1/λ4. Finally, the effective viscosities of the system are derived for some fundamental orientations of the smectic layers with respect to the shear plane, and these allow the derivation of a few inequalities that the viscous coefficients should satisfy.

ISSN:0267-8292    

DOI:10.1080/02678299608033162

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A series of new alkene monomers [MS3BDBEn, n=1–3] containing 4-oligo (ethylene oxide) monomethyl ether 4-biphenyl ether carboxyl benzoate as terminal groups were synthesized. These polymers were prepared by grafting these monomers onto the poly (methylhydrosilox-ane) (PMHS) backbone. The transition temperatures, liquid crystalline textures, and thermal stability of the polysiloxane polymers have been determined by thermal data, by optical texture, and by X-ray diffraction patterns. Polymers PS3BDBEnshowed smectic or smectic and nematic phases which were not analogous to their precursor nematic monomers. The terminal length of the polymers affects not only the mesophase transition temperatures but also the layer-spacing length(d1)and the side-chain distance(d2). The long- and short-range orders can remain to some extent above the isotropization temperature and below the melting point. The polymer PS3BDBE3decomposed in air 20°C above the isotropization temperature and lost its short range orders as detected by the X-ray diffraction analysis.

ISSN:0267-8292    

DOI:10.1080/02678299608033163

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Dielectric measurements on laterally substituted molecules were carried out in the frequency range 10 Hz to 10 MHz. In these systems, the main part of the molecules exhibits only a very small dipole moment in the direction of thepara-axis, whereas the lateral substituent is strongly polar. Depending on the position of the dipole (o,m,p), a more or less strong dipolar correlation in the parallel direction was detected. The absorption data at higher frequencies were fitted to two Cole-Cole mechanisms. The low frequency relaxation was interpreted as angular vibration of the dipole moment of the lateral group, and the high frequency one as the reorientation about thepara-axis of the main part of the molecule and of the lateral group. The appearance of two high frequency mechanisms is unexpected and demonstrates that in complicated molecules the dynamics also become differentiated.

ISSN:0267-8292    

DOI:10.1080/02678299608033164

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

We report experimental and theoretical investigations of the dynamic behaviour of a π/2 twisted NLC layer in a magnetic field. When a magnetic fieldHis applied in the layer plane at a suitable angle β with respect to the easy axis on the first surface, the relaxation towards the equilibrium texture occurs through a slow decay of unstable textures. Depending on the values ofHand β, the relaxation of the system can be nearly homogeneous or strongly inhomogeneous. In this paper we restrict our attention to the case where the relaxation occurs in a nearly homogeneous way. The theoretical relaxation time τw of the unstable textures is found to depend strongly on the angle β and on the amplitude of the magnetic field. The experimental dependence of τw onHand β is found to be in good agreement with the theoretical predictions. The relaxation process is extremely sensitive to small dishomogeneities of the director easy alignment on the surfaces. From the measured relaxation we are able to estimate a spread of 0.3° on the surface easy axes at a planar anchored SiO surface.

ISSN:0267-8292    

DOI:10.1080/02678299608033165

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The three sheet-shaped palladium organyls1a-cexhibit lyotropic nematic properties in apolar organic solvents. The occurrence of two nematic lyomesophases was verified by the investigation of the orientational properties of these liquid crystalline systems. In cells with obliquely deposited SiOxlayers, the director of the high temperature phase N2is aligned in a homogeneous, planar manner. In the low temperature phase N1, a different orientational pattern is observed. The specific pattern type depends on the deposition conditions of the SiOxlayers. In samples homeotropically aligned in the N2phase, the director is tilted away from the cell normal after the transition into the N1phase. These changes of orientation are reversible. The experimental observations presented here support our proposed model for the structural change at the nematic-nematic phase transition.

ISSN:0267-8292    

DOI:10.1080/02678299608033166

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Doping the novel chiral photochromic dye (R,R)-6,6′-bis(1-methylheptyloxy) thioindigo into the Scliquid crystal host MX6120 at a concentration of 7.2 mol % gives rise to a ferroelectric S*cphase with a reduced polarization (P0) of +14.4 nCcm−2. Results of time-resolved experiments demonstrate that the dye undergoestrans→cisphotoisomerization in the Scphase, although the efficiency of photoisomerization is significantly lower than in the nematic and isotropic liquid phases. A reversible modulation ofPson the order of 10 per cent is observed for the doped S*cphase upon photoisomerization of the dye from a thermally relaxedtransform (+6.2 nC cm−2) to acis-enriched photostationary state (+5.6 nC cm−2) atT – Tc= −10°C. Control experiments using mixtures of the racemic and optically active dye in a S*chost suggest thatPsphotomodulation does not arise from a photomechanical effect. Consideration of the Boulder model for the molecular origins ofPsprovides a rationale for the observedPsphotomodulation based on changes in the conformational equilibria of the 1-methylheptyloxy side-chains in the Scbinding site going from thetransform to thecisform of the thioindigo dopant.

ISSN:0267-8292    

DOI:10.1080/02678299608033167

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Dielectric, birefringence and voltage-transmission characteristics of novel cyclic enone compounds are reported. The new materials are shown to exhibit high dielectric anisotropies and modest refractive index anisotropies. These results are explained by structural and substituent effects and are in accordance with the results expected from molecular modelling. The analogy found between structural and physical properties may be used to design improved materials for TFT-LCDs. The use of cyclic enones as dopants to contribute to a reduction in the threshold voltage of liquid crystal mixtures for TFT-LCDs is foreseeable.

ISSN:0267-8292    

DOI:10.1080/02678299608033168

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor