摘要:

The crystal structures of α,ω-bis(4-n-pentylanilinebenzylidene-4′-oxy)butane and α,ω-bis(4-n-pentylanilinebenzylidene-4′-oxy)pentane have been determined by single crystal X-ray analysis. These molecules have even and odd chain length spacers linking two mesogenic groups, respectively, and are members of a family of symmetric liquid crystal dimers which possess a rich smectic phase polymorphism, as well as pronounced odd—even effects in the transitional properties as the parity of the central spacer is varied.

ISSN:0267-8292    

DOI:10.1080/02678299508036687

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Three homologues series of 4-chloro-substituted benzoate-tolanes were prepared. Thermal optical microscopy and DSC analysis show that these compounds are liquid crystalline. The effect on the mesomorphic properties of different positions of the perfluorophenyl in the molecular core unit is discussed.

ISSN:0267-8292    

DOI:10.1080/02678299508036688

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A novel chiral twin material, (R)-bis[5-octyloxy-2-(4-octyloxyphenoxycarbonyl)phenyl] 3-methyladipate, has been prepared, where two mesogenic parts are connected laterally by a spacer possessing a chiral centre. A weaker helical structure, in particular in the chiral smectic C (S*c) phase, was found to be induced by the laterally-connected twin material than by the analogous terminally-connected twin material. If laterally-connected chiral twin molecules prefer to stay in the smectic layer structure so that the two mesogenic parts exist in the same smectic layer, the twist interaction between adjacent layers cannot be produced by direct correlation of motion and directions of two mesogenic parts. Thus, the helical structure in the S*cphase induced by laterally-connected chiral twin molecules becomes weak. An analogous laterally-branched ‘monomeric’ compound, (S)-5-octyloxy-2-(4-octyloxyphenoxycarbonyl)phenyl 3-methyl-pentanoate, has also been prepared, and the induced helical structures compared.

ISSN:0267-8292    

DOI:10.1080/02678299508036689

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

In an earlier study by van Heckeet al., it was shown that the discotic mesogen hexa-octanoyloxytriphenylene (HAT8) crystallizes in four different solid forms, depending on the thermal history of the sample. The present study is an attempt to characterize three of these solids in more detail using X-ray, carbon-13 and deuterium NMR techniques. It is shown that van Hecke's modificationIVis a metastable amorphic solid, while modificationsIIIandIIare highly ordered crystalline solids. Both are apparently monoclinic, but with different unit cell dimensions. In all solid phases, the triphenylene core is static on the NMR time-scale, but the side chains are disordered to different degrees in the different modifications. Further NMR studies on the discotic mesophase of HAT8 are also reported.

ISSN:0267-8292    

DOI:10.1080/02678299508036690

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The presence of periodic diffuse meridional reflections in the X-ray diffraction pattern of the nematic phase of a pentamethylcyclosiloxane compound with pendant mesogenic groups is examined as a function of temperature and frequency of an applied electric field. These reflections are thought to be caused by a tendency of the mesogens to pack in columns of molecules that have no side-to-side registry (uncorrelated strings). Column lengths obtained from the intercepts of plots of δs2versuss2, wheresis the scattering vector and δsis the width of the reflection, indicate association of 3–6 mesogens depending on temperature and frequency. The length over which the strings are correlated are compared to values obtained from a magnetically aligned sample quenched into a glassy nematic phase. The development of a pseudo-layered packing arrangement with decreasing temperature is also discussed. The appearance of a symmetric 4-spot pattern at temperatures slightly above the crystallization temperature suggests the formation of layered domains with local ordering similar to smectic C packing. Layer correlation lengths are calculated and compared for the electrically (110 Å) and magnetically (180 Å) aligned samples.

ISSN:0267-8292    

DOI:10.1080/02678299508036691

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Hexakis(4-(4′-heptyloxy)biphenoxy)cyclotriphosphazene (HHCP) was synthesized from hexachlorocyclotriphosphazene and 4-heptyloxy-4′-hydroxybiphenyl. The mesogenicity of HHCP was studied by DSC, FTIR spectroscopy and polarizing microscopy. Enantiotropic smectic C and nematic phases were observed between 450 and 455 K and 455 and 456 K, respectively, on heating, and between 456 and 455 K (nematic) and 455 and 440 K (smectic C) on cooling from the isotropic liquid phase. The introduction of the heptyloxybiphenoxy groups as side chains into cyclotriphosphazene has generated the liquid crystalline phase. FTIR spectroscopy showed that the P=N and P-O-(C) stretching vibrations converted to lower frequencies from 1224 to 1210 cm−1and from 920 to 910cm−1, respectively, at the crystalline (C)-Scphase transition. This result suggests that the state of the cyclotriphosphazene ring dramatically changes near the C-Scphase transition.

ISSN:0267-8292    

DOI:10.1080/02678299508036692

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A novel system of non-symmetric dimers containing 4-n-alkyloxy-substituted cinnamic acid and cyanobiphenyl groups has been studied. Two series were prepared: in one the flexible spacer was varied in length while the spacer was fixed. The spacer length has a profound influence on the nematic-isotropic transition temperature of these materials and a large odd-even effect is observed for the series. The terminal chain also plays a significant role in determining the liquid crystal phase behaviour: a smectic A phase is exhibited for the ethyl and propyl homologues, in addition to a nematic phase; this smectic phase vanishes for intermediate chain lengths but then reappears for the nonyl and decyl members of the series. X-ray diffraction has revealed the structure of the smectic A phase for the ethyl homologue to be intercalated, whereas that for the decyl compound is interdigitated. The existence of the intercalated smectic A phase has previously been explained in terms of a charge-transfer interaction between unlike mesogenic groups. However, for the non-symmetric liquid crystal dimers described here this specific interaction appears unlikely and we discuss, therefore, other possible mechanisms for the formation of intercalated smectic phases.

ISSN:0267-8292    

DOI:10.1080/02678299508036693

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Two new ferroelectric oligosiloxanes, a cyclic tetramer and a twin, have been synthesized. By a comparative study with their corresponding monomer and side chain polysiloxanes, the influence of oligo- and polymerization on the liquid crystalline and ferroelectric properties have been investigated. Polymerization leads to a stabilization of LC phases through increase of the clearing temperatures and suppression of crystallization. Oligomerization also leads to mesophase broadening, but, due to the low degree of polymerization, the effect is inferior to the linear polysiloxanes. The low viscosity of the oligosiloxanes ensures response times in the microsecond region, thus being comparable with their monomer and conventional LMWFLCs. It is found that polymerization increases the spontaneous polarizationPs. This is attributed to the density increase after polymerization, enhancing the inter-mesogenic interactions. The collective and local dynamics of the OFLCs are influenced differently with respect to their molecular structures. Each oligomer is already a good model for its corresponding polymer concerning the soft mode dynamics. For the local β-relaxation a similar temperature dependence of the relaxation times τ for the cyclic tetramer and for the side chain polysiloxanes is observed. The long axial rotation of the twin, having a very efficient decoupling, is significantly faster, thus resembling the monomer.

ISSN:0267-8292    

DOI:10.1080/02678299508036694

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A way of investigating dielectric relaxation phenomena occurring in the isotropic and liquid crystalline states by the use of monotropic mesogens is suggested. The application of common methods of dielectric spectroscopy in the radio frequency range (102-108Hz) to strongly supercooled samples for the investigation of the amorphous liquid and low temperature monotropic liquid crystalline states is described. The possibility of experimental separation and investigation of the different dipole polarization mechanisms responsible for the dielectric properties of the monotropic mesogen 4-cyano-3-chlorophenyl 4-hexyl-3-chlorobiphenyl-4′-carboxylate (CChEC), whose molecules have longitudinal and normal permanent dipole moment components, is established.

ISSN:0267-8292    

DOI:10.1080/02678299508036695

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

ISSN:0267-8292    

DOI:10.1080/02678299508036696

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor