摘要:

There have been disagreements about the assignment of the quadrupolar splittings in the deuteriatedn-hexyl chain in 4-n-hexyloxy-4′-cyanobiphenyl (6OCB); these are resolved by comparing the splittings of the deuteron resonances in the fully deuteriated chain compound with those in the material with a selectively deuteriated chain. The methods available for assigning the quadrupolar splittings in fully deuteriated chains are then applied to 6OCB and it is concluded that measurement of spin-lattice relaxation times,T1, is the best method.

ISSN:0267-8292    

DOI:10.1080/02678299408036571

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Alignment control at the molecular level is crucial for realizing high-performance LCDs. In particular, the structure of the interface between the liquid crystal and the alignment layer must be clarified. By utilizing RAS (reflection-absorption spectroscopy), a highly sensitive FT-IR spectroscopic technique, we have obtained the following information about the orientation of liquid crystal molecules; (1) the cyano group in a liquid crystal molecule behaves like an electron donor with respect to an SiO layer, but like an electron acceptor with respect to a polyimide layer; (2) with the use of polarized IR spectroscopy, it was discovered that the liquid crystal 8CB is aligned with a slant away from the aligning direction, regardless of the type of alignment layer. The angle between the molecular short axis within the plane of the core and the surface normal was found to be smaller on polyimide layers than on SiO layers.

ISSN:0267-8292    

DOI:10.1080/02678299408036572

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Optical activity has been measured for cholesteric (Ch), twisted nematic (N*) and ferroelectric (S*c) substances confined in cylindrical channels of anodic oxidized aluminium (Anoporemembranes).1H and13CNMR studies of the adsorbed N* phase in combination with measurements of the rotary power suggest a conic helical director field in the pores, as in a common S*cstructure, with the helical axes oriented along the cylindrical channels and the helicoidally ordered local directors on average being inclined at an angleθ< 90° to the cylinder axes. Supposing the validity of the de Vries formula, the sign and magnitude of the optical activity in Anopore indicate a change in birefringence and pitch with respect to the bulk sample.

ISSN:0267-8292    

DOI:10.1080/02678299408036573

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

We report new photomechanical effects in the ferroelectric liquid crystal SCE13 doped with a photoisomerizing guest azo dye. Low concentrations of dye (∼5 per cent wt:wt) are shown to cause an isothermal, reversible disruption of smectic phases when the system is illuminated with low power density (∼ 1 mW cm−2) UV light. In the case of a sample initially in the S*cphase, this results in a fall in the magnitude of spontaneous electrical polarization (Ps) and changes in electro-optic switching characteristics. If the sample is illuminated in the SAphase, the electroclinic switching decreases. In contrast to this, when systems containing higher concentrations of dye (≥ 10 per cent wt: wt) are UV illuminated in the SAphase, a reversible, isothermal transition to a biphasic S*c/isotropic state occurs. In this case, thePsis seen to rise from zero in the SAphase to a finite value(∼2 nC cm−2) in the biphasic mixture and hysteresis occurs in the electro-optic switching. When these higher dye concentration mixtures are held initially in the S*cphase and UV illuminated, a more complicated variation ofPsoccurs with the sample again undergoing a transition to a biphasic S*c/isotropic state. Possible mechanisms for the transition are discussed.

ISSN:0267-8292    

DOI:10.1080/02678299408036574

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The existence of the induced helical smectic A (S*A) phase was proved by textural, optical and calorimetric studies and by small angle X-ray scattering using binary systems which consisted of achiral 4-n-hexyloxyphenyl 4-n-octyloxybenzoate (C ↔ Sc↔ N ↔ I) and chiralN-(4-n-nonylphenylbenzoyloxy)benzylidene-S-1-benzylethylamine (C ↔ SA↔ I). The induced S*Aand cholesteric (N*) phases appeared even at small concentrations of the chiral compound (≈ 1 mol per cent) together with increases in the S*C↔ S*Atransition temperatures, compared those for the Sc↔ N transitions, and decreases in the θctilt angle. Structural factors are discussed as possible causes of the formation of the induced S*Ablue phases I, II and III, and another polymorphic state in the systems studied.

ISSN:0267-8292    

DOI:10.1080/02678299408036575

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Electron diffraction has been used to study the freezing in free-standing hexatic B films of several members of then-alkyl 4′-n-alkyloxybiphenyl-4-carboxylate (nmOBC) homologous series. It was found that 65OBC exhibits the surface crystal E phase and the accompanying orientational epitaxy previously observed in 75OBC. In 46OBC, the surfaces also appear to attain a higher order ahead of the interior, but retain a six-fold symmetry before the entire film abruptly transforms to the crystal E phase. In 3(10)OBC, the behaviour is similar to that of 46OBC, but the film breaks prior to the observation of any crystal E transition.

ISSN:0267-8292    

DOI:10.1080/02678299408036576

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

By using dielectric relaxation spectroscopy it has been observed that while the molecular reorientation around the long axis in the smectic B-crystal phase is active, the rotation around the short axis isfrozen. Thus we show that the molecular dynamics of the liquid crystal phase designated as smectic B-crystal is intermediate between that of crystals and that of conventional smectic phases (like smectic A, B, C etc.). The temperature dependence of the complex dielectric permittivity ε* (ε* = ε′ - jε″) has been studied in the frequency regime 10Hz to 1 GHz on a binary mixture of a chiral epoxy compound and a non-chiral compound. The measurements were carried out on the isotropic phase (110–102°C), the narrow range cholesteric phase, the chiral smectic A phase (98–59°C), a chiral smectic B phase, so far classified as smectic B-crystal (59–33°C), and a crystal phase (below 33°C). The dielectric measurements were made for planar and homeotropic orientations. From the analysis of the dielectric absorption curves in the planar and homeotropic orientations, it is found that while the molecular reorientation around the long axis is characterized by a distribution of relaxation times, the rotation times, the rotation around the short axis is described by a single relaxation mechanism.

ISSN:0267-8292    

DOI:10.1080/02678299408036577

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Using a combination of solid state2H NMR spectroscopy on selectively deuteriated samples and dielectric spectroscopy, the molecular dynamics of discotic charge-transfer (CT) complexes were investigated. These complexes show particular thermodynamic and flow properties. Considered were mixtures of low molar mass donors and acceptors, low molar mass donors with main chain acceptor polymers and covalently linked donor-acceptor twins with different lengths of the spacer. A main result is that correlated rotational motions of discotic molecules or groups about the columnar axis are observed in all systems except for the twin with the short spacer. This type of motion seems to be a general feature of columnar phases. The non-discotic acceptor which is incorporated in the columns participates in this motion. The twin possessing a long spacer displays at high temperatures an additional process: it performs a diffusion process between the columns. A further result is that broad biphasic regions exist in CT mixtures at the transition from the discotic to the isotropic state.

ISSN:0267-8292    

DOI:10.1080/02678299408036578

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Formulae are calculated for the main contributions to the energy in FLC-cells in the one-dimensional case. Special emphasis is given to the modelling of the smectic layers and to the influence of the electric field. For the study of structural questions in FLCDs, an efficient numeric simulation method is proposed. The use of it is illustrated with an example.

ISSN:0267-8292    

DOI:10.1080/02678299408036579

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Novel nematic and chiral nematic liquid crystals capable of vitrification have been synthesized using 1,3,5-cyclohexanetricarboxylic and (1R,3S)-(+)-camphoric acids as the base structures to which cyanotolan, cyanobiphenyl, methoxybiphenylbenzoate nematogenic groups, (S)-(−)-1-phenylethylamine and (S)-(+)-1,3-butanediol chiral moieties are attached. Once the glassy state is induced by quenching or controlled cooling from the isotropic state, these liquid crystals showed no detectable tendency towards crystallization upon heating through the glass transition temperature. In all cases, the ΔCpand ΔHcvalues resulting from the DSC heating scans are comparable to those previously reported for polymer analogues and other low molar mass glassy liquid crystals. In the nematic series with varying spacer lengths, bothTgandTcare consistently lower than the linear polymer counterparts, in contrast to the siloxane-based systems. It was also demonstrated that cholesteric mesomorphism can be induced following one of the three approaches: chiral nematic mixture, chiral nematic cyclic cooligomer, and pendant nematogenic groups attached to a chiral ring.

ISSN:0267-8292    

DOI:10.1080/02678299408036580

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor