摘要:

This work continues our study of the mesomorphic properties of the p-n-alkoxybenzylidenep-fluoroanilines. Previous studies have shown that the straight chain homologues, with the number of carbon atoms in the chain ranging from 4 to 8, form smectic phases. In this paper we describe H NMR studies of the homologues with n 4, 5 and 8 and the synthesis and properties of a chiral branched chain derivative, (S)-p-2-methylbutyloxybenzylidene-p-fluoroaniline. This compound is not mesomorphic, but does show solid state polymorphism. From an optical, electro-optical, DSC and X-ray diffraction study we infer that the higher temperature crystalline form is a CrE phase.

ISSN:0267-8292    

DOI:10.1080/026782997209450

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The synthesis and properties of laterally 4-nitrobenzyloxycarbonyl substituted phenyl benzoates are described; these compounds represent the first two-ring mesogens having a phenyl ring in the lateral branch. The lengths of the terminal chains of the core carrying the branch have a great influence, as shown by the phase behaviour of two homologous series. Compounds having long alkyloxy groups exhibit enantiotropic smectic A phases. It should be emphasized that the mesophase thermal stability of these strongly branched derivatives can be higher than that of the laterally unsubstituted parent compound. Connecting two of the new mesogens by means of an aliphatic spacer results in a novel type of twin molecule.

ISSN:0267-8292    

DOI:10.1080/026782997209469

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

It is shown that chlorination and even bromination of the mesogenic hexahexyloxytriphenylene 1 can take place at the sterically congested tetracyclic core delivering the respective monohalogenated, sevenfold substituted enantiotropic mesogens 3 and 4 in 32 and 17% yields, respectively, giving mesophases over greater temperature ranges. Miscibility tests among all mesogenic compounds of this study prove formation of the columnar hexagonal type of phase. The steric and electronic effects of those substituents, different from the six equal alkoxy groups, on the mesophase are discussed in detail. In our hands and to our surprise, the sterically hindered acceptor (nitro or halogeno) substituents of 2, 3, or 4 could not be replaced by an alkylthio group in nucleophilic aromatic substitution reactions using the aprotic polar solvent DMEU. Semi-empirical calculations on numerous hexaethers with one or two substituents, e.g. halogens or others, in one of the three bay regions of the triphenylene core reveal that these molecules are helically deformed due to the crowding in the molecule. In this new case of atropisomerism, the 1,12-disubstituted derivatives should offer the possibility of resolution into enantiomers and, therefore, of studying disc-like mesomorphic compounds possessing for the first time the origin of chirality within their molecular centre.

ISSN:0267-8292    

DOI:10.1080/026782997209478

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

We consider Poiseuille flow of polymeric liquid crystals corresponding to large values of the velocity gradient. The model employed [Ericksen, 1991] proposes governing equations for the velocity field, v, the pressure p, the director n, and the order parameter s. The constitutive functions for the Leslie coefficients alphai derived from the molecular theory of Doi [1981] play a crucial role in the modelling. In addition to the Ericksen number, E, the present model exhibits a new non-dimensional parameter I, that represents the contribution of the elastic free energy of non-gradient type with respect to Frank-Oseen's elasticity. One of the goals of the analysis was to examine the role of s in describing singularities as well as in obtaining regimes which are not predicted by the previous Leslie-Ericksen model. In particular, solutions are obtained that correspond to domain structures parallel to the flow. Such domains are separated by singular lines across which the director experiences jumps of, approximately, 45 degrees with respect to the flow direction. A condition on the size of I is required in order to support such layered structures. The contribution of the energy associated with I turns out to play the role of an elastic surface energy which is, otherwise, neglected in the present model.

ISSN:0267-8292    

DOI:10.1080/026782997209487

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

‘Oligomeric compounds’ with three, four or six mesogenic groups in the chain were synthesized by the Wittig reaction from different bis Wittig salts and various heteroaromatic aldehydes. The mesogenic groups are connected via hexamethylene moieties. All derivatives containing at each terminal position of the chain E-1,2-di-(2-thienyl)ethylene units and one, two or four E,E-2,5-bis(2-thienylvinyl)thiophene units in the chain show nematic phases while heating and/or cooling the samples (2a, 4 and 5). Moreover, the oligomeric compound having the chain from 2a and one terminal formyl group (5) also has a nematic phase. The derivative with four E-2-(2-thienylvinyl)thiophene units in the chain (3a) melts without forming a mesophase. Also two ‘oligomeric compounds’ with benzene and thiophene systems in the mesogenic groups were synthesized. The compounds all-E-1,4-bis[5-{6-[5-(2-thienylvinyl)2-thienyl]hexamethylene}-2-thienylvinyl]b (2b) with three mesogenic groups and allE-1,6-bis{4-[5-{6-[5-(2-thienylvinyl)-2-thienyl]hexamethylene}-2-thienylviny (3b) with four mesogenic groups in the chain do not show liquid crystalline phases.

ISSN:0267-8292    

DOI:10.1080/026782997209496

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Polymerization induced phase separation in mixtures of liquid crystals (LCs) and acrylates (Merck TL205/PN393) proceeds by liquid-gel demixing, in most cases of practical interest. At high LC content or low temperature of polymerization liquid-liquid separation cannot be excluded. Depending on the elasticity and homogeneity of the polymer network at the onset of phase separation, spherical or non-spherical LC domains are observed; non-spherical domains reflect an inhomogeneous gel structure. The change from spherical to non-spherical occurs in a very narrow range of LC concentrations and curing temperatures. The transition between these two morphologies can be explained using conversion phase diagrams obtained from the Flory-Huggins-Dusek theory. The contrast ratio of PDLCs made from the Merck mixture passes through a maximum when the droplet shape at the onset of phase separation changes from spherical to non-spherical. Lowering the LC content or increasing the temperature leads to smaller LC domains which scatter less efficiently. The reverse changes lead to early phase separation and large LC domains which also scatter inefficiently. It is speculated that the maximum of the contrast ratio is related to secondary phase separation, leading to subdomains of an appropriate size.

ISSN:0267-8292    

DOI:10.1080/026782997209504

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

We report on 13C NMR measurements above and below the clearing temperature of the liquid crystal 4-n-hexyloxyphenyl 4-methoxybenzoate constrained to an oriented, low concentration polymer network. The network is obtained by UV-irradiation of the reactive monomer 1,4-di-[4-(6-acryloyloxyhexyloxy)benzoyloxy]-2-methylbenzene which is in admixture with the liquid crystal. The characterization of orientational order of the mixtures before UV-radiation, and hence before polymerization, reveals the high order of the components at the polymerization temperature. The chosen geometry explains the LC director orientation only by the aligned network. Above the nematic-isotropic transition a strong pretransitional order is detected. Fast molecular translational diffusion averages the order over dimensions smaller than 1 mum. The Landau-de Gennes theory predicts a relation between pretransitional order and the lateral dimension of the LC regions. The experimental data are successfully explained by pore diameters of 35 and 98 nm for concentrations of 20 and 8 mol% of monomer, respectively. The results support the model of nearly cylindrical shaped liquid crystal domains surrounded by thin walls of crosslinked network.

ISSN:0267-8292    

DOI:10.1080/026782997209513

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The orientational ordering of nine classes of liquid crystals, namely 4-n-alkyl-413 cyanobiphenyls (nCB), 4-n-alkoxy-4-cyanobiphenyls (nOCB), 4-n-alkoxybenzilidene-4-nalkylanilines (nO.m), 4-cyanophenyl 4-alkylbenzoates (nCPB), 4-n-alkylphenyl 4-n-alkoxycinnamates (Cin-n-m), esters of alpha-chloro carboxylic acids and 4-n-alkyl-4-hydroxybiphenyls (An, Bn and Cn), 4,4-di-n-alkyldiphenyldiacetylenes (PTTP), 4-n-alkylphenyl 4-n-alkoxybenzoates (nOm), and 4-n-alkoxyphenyl 4-n-alkoxybenzoates (nOm), have been investigated by the use of 13C NMR. The order parameters of the phenyl rings were determined by using a 2D 13C NMR technique known as separated local field (SLF) spectroscopy in combination with variable angle spinning (VAS). The order parameters obtained were then correlated with their corresponding anisotropic 1D C chemical shifts in their nematic and smectic phases to give a linear relationship in each case. The parameters obtained in the linear correlations provide a convenient way to obtain the order parameters from direct measurement of the 13C chemical shifts for other homologous members of these classes of liquid crystal.

ISSN:0267-8292    

DOI:10.1080/026782997209522

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A detailed derivation of the extended Jones matrix representation for twisted nematic liquid crystal displays at oblique incidence (which was previously published by the author) is given. It is shown that this representation reduces to the well-known ordinary Jones matrix representation at normal incidence.

ISSN:0267-8292    

DOI:10.1080/026782997209531

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A series of liquid crystalline copolymers, poly{2-hydroxyethyl methacrylate}-co-{6-[4-(S2-methyl-l-butyloxycarbonylphenylazo)phenoxy]hexyl methacrylate} with an azobenzene moiety as photoreactive mesogenic unit, was prepared and investigated by using DSC, polarized optical microscopy and X-ray diffraction. The results show that these polymers exhibit smectic phases. Z-type Langmuir-Blodgett films of these copolymers were successfully deposited onto calcium fluoride and quartz. Reversible homeotropic and planar liquid crystal alignments were induced by using the photochromism of the LB films of one of the copolymers containing 20.6 mol% of the azo unit.

ISSN:0267-8292    

DOI:10.1080/026782997209540

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor