摘要:

The synthesis of a number ofs-triazine-2,4,6-triones (isocyanurates) by cyclotrimerization of 4-monoalkoxy-, 3,4-dialkoxy-, or 3,4,5-trialkoxyphenyl-, benzyl-, or styryl-isocyanates is reported. The thermal and phase behaviour of the novel compounds were investigated by optical polarized light microscopy and thermal analysis. No mesophase could be observed for any of the precursors except 4-decyloxybenzoic acid. Mesophases of the isocyanurates were observed only in the series containing 3,4-dialkoxyphenyl moieties. A vinylene group between the inners-triazine ring and the outer phenylene rings gives low melting points and enantiotropic mesophases for C8and longer alkyl chains. The phenylisocyanurates may exhibit a monotropic mesophase and normally have the highest melting points of the three types, whereas a methylene spacer suppresses formation of a mesophase. According to the textures observed, the mesophases are hexagonal columnar. This will have to be confirmed by X-ray scattering techniques.

ISSN:0267-8292    

DOI:10.1080/02678299608032837

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A series of side group liquid crystalline polymethacrylates with 4′-(1H,1H-perfluoroalkoxy)-azobenzene mesogenic units was synthesized and characterized by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction methods depending on the tail length. The phase behaviour is discussed as a function of the length of the tail groups. Very high clearing temperatures up to 290°C were observed for the polymers with long tail groups.

ISSN:0267-8292    

DOI:10.1080/02678299608032838

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Five diaza-crown-ethers were synthesized and four of them shown to have smectic liquid crystalline properties. Their properties were determined by DSC and polarized microscopy. This type of smectogenic diazacrown ether has not been reported so far. A novel lyotropic crown ether liquid crystal was obtained from the thermotropic crown ether liquid crystal 7.

ISSN:0267-8292    

DOI:10.1080/02678299608032839

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The processing of an anisotropic liquid crystalline thermoset formed byp-bis(2,3-epoxypropoxy)-α-methylstilbene (DOMS) and 1-methyl-2,4-diaminobenzene (DAT) with uniformly oriented mesomorphic molecules, and consequently strongly birefringent, is described. The unidirectional alignment of the molecules was achieved during the curing process by external factors such as surface-liquid crystal interactions and/or an applied electric field. The control over the molecular orientation enabled us to produce well oriented highly anisotropic and birefringent epoxy thin films. In this paper, we discuss the formation process and the properties of the anisotropic epoxy network.

ISSN:0267-8292    

DOI:10.1080/02678299608032840

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A number of new chiral binaphthol (BN) derivatives with different substituents R,R′ in the 6,6′-positions in open (BN-diethylethers) and bridged forms (BN-acetals) have been synthesized. The syntheses of the chiral 6,6′-disubstituted-2,2′-diethoxy-1,1′-binaphthyls (R,R′ = -CH'CHCHO, -CH'CH-(p-BrPh), -CH'CH-(p-CHOPh)) and the chiral 9,14-disubstituted-dinaphtho [2,1-d:1′,2′-f][1,3] dioxepins with R,R′ = -CH'CH-(p-BrPh), -CH'CH-(p-CHOPh) are reported for the first time. The possible liquid crystalline properties and molar twisting powers (βM) in three different nematic liquid crystals (LCs) of the BN derivatives were investigated. Derivatives with spatially extended substituents in the 6,6′-positions (e.g. styryl or vinyl) show unusually high molar twisting power (up to 124.5μm−1). A direct correlation between the magnitude of βMand the length of the substituents was found. Bridged forms, in which the dihedral angle θ between the naphthyl moieties is ≊ 54°, show higher twisting power than the corresponding open forms, where θ is allowed to vary around 90° resulting in an equilibrium betweentransoidandcisoidforms. From the different temperature dependencies of βMof the open and bridged BNs, a molecular model was developed relating the molecular conformation and twisting power. Although no mesophase was found in any of the compounds synthesized, they can be considered as important precursors for the synthesis of potential chiral BN-containing LCs.

ISSN:0267-8292    

DOI:10.1080/02678299608032841

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

New polymerizable liquid crystal materials: 10-{4-[5-alkyl-1,3-dithian-2-yl]phenyloxy}decylmethacrylates (10) and 10-{4-[5-alkyl-1,3-dioxan-2-yl]phenyloxy}decylmethacrylates (11) were synthesized and converted into polymers. The mesomorphic behaviour of these compounds was measured. Though the 1,3-dithiane-type monomer did not exhibit any liquid crystal phases, the corresponding polymer exhibited the schlieren texture of a nematic liquid crystal phase at around room temperature (for example11-1: G30N76I). To exhibit a nematic liquid crystal phase at around room temperature is a principal feature of these 1,3-dithiane type polymer. The corresponding 1,3-dioxane-type polymer exhibited the focal conic fan textures of a smectic A phase (for example10-1: Cr80 SA101 I).

ISSN:0267-8292    

DOI:10.1080/02678299608032842

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Preliminary results are presented on the molecular dynamics simulations of alignment of the liquid crystal molecule, 4-n-octyl-4′-cyanobiphenyl (8CB), on a polyimide (pyromelltic dianhydride-p-phenylene diamine) oligomer monolayer. We actually simulated a three-layer system, i.e., liquid crystal molecule/polyimide oligomer/a basal plane of graphite. First, simulations of the oligomers adsorbed on graphite were done in order to obtain reasonable adsorption structures, as the pre-stage simulation of the three-layer system. Then, by placing a liquid crystal layer on top, the three-layer system was simulated. The stable liquid crystal alignment direction on the polyimide monolayer was found roughly to be the polyimide chain direction with zero pretilt in this combination of liquid crystal and polymer materials. The calculated adsorption energy of an 8CB molecule to the polyimide monolayer was 128 kJ mol−1and the carbonyl group of the polyimide was the main adsorption site.

ISSN:0267-8292    

DOI:10.1080/02678299608032843

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

We calculate using mean field theory, the cholesteric pitch in systems composed of chiral molecules, which we model as ‘corkscrews’. We consider both the rigid and semi-flexible limits. Our result for the cholesteric pitch depends on the intrinsic molecular pitch length as well as the concentration of molecules, but does not depend on the flexibility of the molecule.

ISSN:0267-8292    

DOI:10.1080/02678299608032844

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The phase behaviour of dimer liquid crystals (DLC), α, ω-bis[(4,4′-cyanobiphenylyl) oxy]alkanes (CBA-nwithn= 9,10) has been studied by differential thermal analysis (DTA) over a pressure range from 0.1 to 150 MPa. Both samples exhibit crystal (Cr)↔nematic (N)↔isotropic (I) transitions under all experimental conditions. The slopes of the phase boundary curve (dp/dt)trwere determined from thePtrvs.Ttrphase diagram, where the subscripttrdesignatesCrNorNI.Both transition temperaturesTCrNandTNIwere found to increase almost linearly as a function of pressure; CBA-9: (dp/dt)CrN= 3.92, (dp/dt)NI= 2.03; CBA-10: (dp/dt)CrN= 3.66, (dp/dt)NI= 2.17, the units being MPaK−1. As a consequence, the nematic region defined by the interval between the CrN and NI transitions becomes broader as the applied pressure increases. While the transition enthalpies δSCrNand the associated entropies δSCrNat the CrN transition decrease substantially with increasing pressure, the corresponding quantities at the NI transition remain nearly insensitive to pressure. At atmospheric pressure, the magnitude of δHNIamounts to about 10% of δHCrNfor the given samples. The transition enthalpies and entropies were also estimated from the Clapeyron relation, the volume changes required in this expression being taken from the PVT measurements previously reported. Somewhat larger values were obtained for both δHtrand δStrin the latter estimation. The odd-even character with the spacer lengthnwas clearly observed in the aforementioned thermodynamic quantities over the entire pressure range examined.

ISSN:0267-8292    

DOI:10.1080/02678299608032845

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The characteristic feature of liquid crystal dimers, in which two mesogenic groups are linked by a flexible spacer, is often thought to be the strong odd-even effect exhibited by their transitional properties. That is, the nematic-isotropic transition temperature and the entropy of transition are large for dimers with an even number of groups in the spacer in comparison with those for neighbouring dimers with an odd number of groups. However, the magnitude of the odd-even effect along a homologous series of dimers is found to depend strongly on the nature of the link between the mesogenic group and the spacer. This dependence is thought to originate in the variation of the molecular geometry with the linking group, a view which is supported by detailed molecular field theory calculations involving all of the conformational states. Here we are concerned with developing a more transparent understanding of this geometrical effect using a simple model of the dimers in which all of the conformational states are replaced by just two, a linear and a bent conformer. The model has been found to exhibit a strong odd-even effect as well as a nematic-nematic transition when the bond angle is tetrahedral. We have used this model to explore the dependence of the transitional properties of liquid crystal dimers on their geometry by varying the bond angle of the bent conformer. The behaviour predicted by the model for the nematic-isotropic transition is found to be in qualitative agreement with experiment. In addition, the nematic-nematic transition is observed to exhibit a critical behaviour as the bond angle is increased. At the other extreme, when the bond angle is reduced to its limiting value of 90° there is a very strong first order transition between a discotic and a calamitic nematic.

ISSN:0267-8292    

DOI:10.1080/02678299608032846

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor