摘要:

Two series of ferroelectric liquid crystals containing a phenyl biphenyl carboxylate mesogenic group and oligooxyethylene spacers were synthesized. The mesomorphic behaviours of the compounds were characterized using differential scanning calorimetry (DSC), optical polarizing microscopy (POM) and powder X-ray diffraction measurements. These series all contain a smectic A (SA), a chiral smectic C (SC) and a crystal E (CrE) phase sequence. The ordered smectic Cr E phase was observed for the short spacer chain (n= 0, 1, 2) homologues of the two series. Moreover, the mesomorphic properties are discussed as a function of the oligooxyethylene spacer length, and the position of the aromatic core rings. These results indicate that the structures with a rigid part (biphenyl group) closer to the flexible spacer chain (oxyethylene unit) tend to form the symmetrical smectic SAand Cr E phases.

ISSN:0267-8292    

DOI:10.1080/02678299608032854

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Six Schiff's base and azo liquid crystalline crown ethers were first synthesized via the intermediatescis(trans)-4,4′-diaminodibenzo-18-crown-6 and 4′-aminobenzo-15-crown-5, respectively. The structures were elucidated by elementary analysis, IR,1H NMR and MS. The mesomorphic properties were determined by DSC and optical microscopy. The relationship between the structures of the double-armed and mono-armed crown ethers, and the mesomorphic properties were studied. We realized that when theL/Dratio of the doublearmed crown ether was in the range of 4-8 and that of the mono-armed crowns was larger than 3, they would possess liquid crystallinity. This conclusion is consistent with our previous investigations.

ISSN:0267-8292    

DOI:10.1080/02678299608032855

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Two novel smectic bilayer structures have been identified in an enantiomerically enriched chiral side chain polymer containing the highly dipolar nitrile group at stereocentres. The structures were characterized by electron diffraction, electron microscopy, and X-ray diffraction. In both phases each smectic layer has a bilayer structure with backbones and spacers confined in a thin disordered region between two sublayers of mesogenic segments. One of the structures which we denote as CrE* has the unusual feature of having its side chains arranged parallel to the layer normal in spite of its enantiomeric bias and twisted nature. In the second structure side chains are tilted by 34.8° with respect to the layer normal and we denote this phase as CrH*cIn both structures each sublayer contains three different orientations of orthorhombic (CrE) or monoclinic (CrH*c) lattices which are related to one another by rotations of ± 60° about thec-axis. In both the CrH*cand the CrE* phases, lattices in each sublayer are regularly rotated about thec-axis by 5.9° relative to those in the adjacent sublayer. The observation of a chiral CrH phase is uncommon and in this specific case the structure is unique since the rotation between adjacent layers occurs about the sidechain axis (c-axis) (CrH*c) and not about the layer normal (c-axis) (CrH*c). We believe the system undergoes a change in molecular organization from CrH*cto CrE* as a result of a chemical reaction which joins a fraction of the stereocentres through covalent bonds. With increasing temperature the CrE* structure was found to transform to a special orthorhombic untwisted smectic phase in whicha= 31/2b, denoted here as CrEh. The structure then transforms to a hexatic SBphase and finally to a SAphase at yet higher temperatures.

ISSN:0267-8292    

DOI:10.1080/02678299608032856

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

We have synthesized two separate homologous series of chiral non-symmetric dimesogens: these have a two phenyl ring moiety bearing a terminal alkoxy chain linked via a methylenic spacer to a chiral three phenyl ring mesogen which itself displays antiferroelectric properties. Depending on the parity of the number of carbons constituting the spacer, two kinds of mesomorphic behaviour have been found. When the spacer is even, various Sc*phases (Sc*A, Sc*Fi, Sc*Sc*α) have been identified without doubt, whereas when the spacer is odd, a tilted modulated phase (S[ctilde]*) has been observed and characterized. In each case, we study the effect of the length of the alkoxy chain at the end of the shorter moiety. The alternation of the mesomorphic behaviour with the spacer parity has been interpreted in terms of molecular shape, but we have also discussed the influence of some chemical parameters on the stability of the Sc*Aphase.

ISSN:0267-8292    

DOI:10.1080/02678299608032857

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The microstructure of the low molecular weight thermotropic liquid crystal, bis(p-hexyloxyphenyl)terephthalate (PP6), was analysed by fluorescence. The material has an emission around 400 nm in its various liquid crystalline phases, but this emission disappears in the isotropic phase. The emission was attributed to fluorescence from an intermolecular ground state complex between two different chromophores of PP6 based on fluorescence experiments of model compounds and dilute PP6 solutions. No peak shifts were observed on changing temperature, while the fluorescence intensity decreased as the temperature was increased. The fluorescence intensity changed dramatically at the phase transitions. An abrupt change in the fluorescence intensity was observed on changing depending on the temperature especially at the smectic-nematic transition temperature. Thus, the intermolecular fluorescence of the liquid crystal is greatly affected by the phase structure and the order of liquid crystalline phase.

ISSN:0267-8292    

DOI:10.1080/02678299608032858

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

By dielectric measurements, the temperature scan method and studies of optical hysteresis loops, the phase sequences have been elucidated for two new compounds from a series of homologues containing a thiobenzoate group with a fluoro substituent in the first benzene ring (denotednFHTBBM7). Both compounds (n= 11 andn= 12) exhibit antiferroelectric (ScA), ferroelectric (Sc) and several ferrielectric (ScFI) phases, as well as the (Scα) phase. Also, in the compound 11FHTBBM7, a metastable ferrielectric phase may appear in the temperature range of the antiferroelectric phase. The dielectric relaxation study revealed contributions to the dielectric constant from several modes detected in the temperature range studied. The properties of the relaxation modes and the phase transition sequences of the two compounds have been compared with those of the homologues withn= 9 andn= 10 studied previously.

ISSN:0267-8292    

DOI:10.1080/02678299608032859

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The critical temperature at the consolute point where two smectic A phases of different compositions phase separate has been measured for binary systems which combine one compound from a newly synthesized series of fluorinated mesogens with one homologue from the commercially available cyanobiphenyl family. Taking advantage of the regular evolution of the layer spacings within a series of homologues, this systematic analysis identifies a clear connection between miscibility and difference in layer spacings of the smectic A phases of the pure compounds. However, additional data obtained on significantly different systems show the non-universal character of this analysis and suggest that factors in addition to this structural difference are relevant to this phenomenon.

ISSN:0267-8292    

DOI:10.1080/02678299608032860

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The synthesis of a new class of chiral smectic liquid crystals is described, along with transition temperatures, phase behaviour, spontaneous polarizations and related data. The general structure is shown in A. possessing a chiral unit derived from 3,4-epoxy-2-octanol. Two of these new compounds show seemingly antiferroelectric behaviour. The influence of the mesogenic core structure on mesophase behaviour and ferroelectric properties was investigated. The non-liquid crystalline representatives were examined as dopants for induced ferroelectric phases in an achiral matrix.

ISSN:0267-8292    

DOI:10.1080/02678299608032861

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A surface memory effect (SME) is found in the dimerized nematic with short range smectic C order of 4-n-octyloxybenzoic acid (OOBA) in a cell whose walls have been covered with an obliquely evaporated SiO or ITO layer. The recording and erasure processes are discussed and erasure in the nematic phase is realized. A mechanism for the surface memorization is presented which takes into account the specific character of dimerized nematics preceding cooling into a smectic C phase.

ISSN:0267-8292    

DOI:10.1080/02678299608032862

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The relationship between the helical twisting power (HTP) of cholesteric liquid crystals and the molecular structure of the chiral mesogens has been investigated. Rod-like mesogens are compared with analogues bearing a bulky lateral branch. Additionally, the HTP of induced cholesteric phases formed by chiral guest molecules in nematic host phases has been studied in terms of different molecular structures. The paper gives information on the influence of bulky lateral groups in mesogens on the HTP.

ISSN:0267-8292    

DOI:10.1080/02678299608032863

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor