摘要:

Several homologues of two structurally different types of mesogenic difluoroterphenyls have been prepared. One set of compounds contains two inner-core fluoro-substituents and the other set possesses both an inner-core and an outer-core fluoro-substituent. The use of various types of palladium-catalysed cross-coupling reactions was invaluable in the synthesis (for example, selective couplings, and couplings to trifluoromethanesulphonate derivatives). Additionally, a novel selective lithiation is reported. The mesogenic materials are all low melting and the types of mesophase generated are considerably structure dependent. However, most of the materials support the tilted Scphase which is required of ferroelectric host materials. The mesomorphic behaviour of these novel materials is discussed in terms of the molecular structure and the structure/property relationships are compared with those for difluoroterphenyls with different arrangements of lateral fluoro-substitution.

ISSN:0267-8292    

DOI:10.1080/02678299508036584

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

X-ray investigations were performed on 4-hexylphenyl 4-cyanobenzoyloxybenzoate (DB6CN) and on 4-heptyloxyphenyl 4-cyanobenzoyloxybenzoate (DB7OCN). For both compounds, the smectic A2structure is observed. From the small angle meridional reflections, the angular distribution of the smectic units around the normal to the smectic layers and the longitudinal correlation length were studied. The orientational order parameter is calculated from the wide angle equatorial reflections. Based on the assumption of a layer structure with undulation in the liquid crystalline smectic A phase, the relationship between the dimensionless parameters χch/dL (correlation length/thickness of the smectic layer) and FWHM/2θ0(full width at the half maximum/maximum point of the scattering peak) is simulated.

ISSN:0267-8292    

DOI:10.1080/02678299508036585

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Electro-optic and dielectric studies have been performed for a thioester (with a chiral chain incorporating the lactic acid ester moiety), which exhibits a monotropic room temperature ferroelectric Sc* phase. Moreover, there are some hints that this substance also shows the recently discovered helicoidal SA* phase. The dielectric and electro-optic measurements have been carried out using ITO coated glass plates (d= 10 μm). Planar alignment was obtained by means of an AC electric field (v= 1Hz andVp-p= 70V). The tilt angle and spontaneous polarization temperature dependences show that the SA*-Sc* transition is of second order. Soft mode behaviour in the pretransitional region also confirms this result. The monotropic Sc* phase exhibits a rather strong Goldstone mode (GM) with pronounced temperature dependence of the dielectric parameters. The molecular reorientation about the long axis has been found for both principal alignments. The homeotropic effective alignment was obtained by applying a magnetic field of 1–2 T to a thick sample (c. 100μm) placed in a specially designed cell for anisotropic high frequency measurements. The reorientation about the long axis gives a broad spectrum centred atc. 100MHz. This spectrum shows minor changes at the SA*-Sc* transition.

ISSN:0267-8292    

DOI:10.1080/02678299508036586

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A thin cell method for dielectric time domain spectroscopy is described. The cell allows one to study the dielectric dispersion and absorption spectra for both components of the dielectric permittivity tensor of uniaxial systems like nematic and smectic A and B phases. Results are presented for 4-n-pentyl-4′-cyanobiphenyl in the isotropic phase and upon orientation in the nematic phase.

ISSN:0267-8292    

DOI:10.1080/02678299508036587

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

New stable structures of the liquid crystalline smectic C* phase are observed in free standing films of a material with high spontaneous polarization. A stable configuration of thick films is a stripe of state with an in-plane rotation of the director. In ultra-thin films an anisotropic state was observed. The director fields of the observed structures are modelled and compared with theoretical predictions.

ISSN:0267-8292    

DOI:10.1080/02678299508036588

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Preliminary results are presented on molecular dynamics simulations of the alignment of 8CB (4-n-octyl-4′-cyanobiphenyl) liquid crystal molecules on a graphite surface. Their stable alignment direction was found to be the <3120> direction of the basal plane of graphite. The calculated adsorption energy of an 8CB molecule was 151·5 kJ mol−1and its alkyl chain was the main adsorption portion. Molecular dynamics simulation could therefore be an effective probe to the surface-induced alignment mechanism of liquid crystal molecules.

ISSN:0267-8292    

DOI:10.1080/02678299508036589

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The high resolution phase diagram of the tetramethylammonium heptadecafluorononanoate (TMAHFN)/D2O system has been mapped out using2H and14NNMR spectroscopy. The14N quadruple splittings are more than an order of magnitude larger than corresponding2H splittings, while the line widths are only two to three times larger. Thus,14NNMR offers an order of magnitude improvement over2H NMR in the resolution of the spectra from coexisting phases. The2H spectra of samples in biphasic regions are often complicated by chemical exchange of D2O molecules between coexisting phases, particularly at low TMAHFN concentrations. Analysis of the2H line shapes of a TMAHFN/D2O sample with a weight fraction of TMAHFN of 0.230 obtained at various times following cooling of the sample into the isotropic/nematic biphasic region shows that the mean diameter for the dispersed nematic droplet grows from about 7 to about 26 μm over a period of 2 h. At a mean droplet size of 7 μm the exchange of TMA+ions between the coexisting phases is slow on the NMR time-scale and exchange effects are not observed in14N spectra. The TMAHFN/D2O phase diagram exhibits the generic form of those of the CsPFO/water and APFO/D2O systems, which are the only other systems composed of stable discotic micelles for which high resolution phase diagrams are currently available, but the nematic phase is displaced to smaller TMAHFN concentrations. Specifically, a discotic nematic phase ND+, intermediate between an isotropic micellar phase I and a lamellar phase L, exists for weight fractions of TMAHFN between 0.149 (φ = 0.105) and 0.420 (φa = 0.325) and temperatures between 277.3 and 327.6 K.

ISSN:0267-8292    

DOI:10.1080/02678299508036590

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Bicomponent mixtures of 4-cyanobiphenyl-4′-yl 4′-alkylbiphenyl-4-carboxylates (nCBB) and 4-alkoxy-4′-cyanobiphenyls (nOCB) were studied and their phase diagrams obtained by thermomicroscopy. The strong enhancement of a smectic Adphase in mixtures with the smectic members of thenOCB (n⩾ 8) series and the induction of a smectic Adphase in the shape of an island surrounded by a nematic phase in mixtures with the nematic members of thenOCB (n< 8) series were observed. The influence of aliphatic chain length on the observed phenomena is discussed.

ISSN:0267-8292    

DOI:10.1080/02678299508036591

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The texture of a copolyesteramide, Vectra® B950, has been studied by optical microscopy. At rest, the defects are typical of a biaxial nematic phase, but under the influence of a magnetic field or under shear, the nematic transforms into a uniaxial phase. The Freedericksz geometry yields aK1elastic constant of the order of 10−8N. The diffusivitiesKIII/nandKII/nare obtained by measuring the light intensity transmitted through a sample during shear start-up and relaxation, and from this we estimateKIII≅ 10−9N andKII≅ 10−10N. Since the biaxiality is small, these uniaxial constants adequately describe the phase at rest.

ISSN:0267-8292    

DOI:10.1080/02678299508036592

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Dielectric relaxation measurements of two classes of liquid crystalline polymers with different degrees of polymerization were carried out. The only difference between the samples is that in the first case the liquid crystalline (LC) part is coupled to the polyethylene chain by a -COO group, whereas in the second case, an ether group (O) connect the same parts. Increasing the degree of polymerization increases the clearing temperatures and decreases the dielectric relaxation frequenciesfRof reorientation of the LC part around both its short and long molecular axes. At higher degrees of polymerization, a saturation effect offRis observed which is related to the chemical structure of the linking group.

ISSN:0267-8292    

DOI:10.1080/02678299508036593

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor