摘要:

The polarizing microscopic textural changes in a thin film (12p) of a nematic liquid crystal (NLC) p-ethoxy benzylidene -p′-n-butylaniline (EBBA) as it is cooled from its isotropic to the crystal phase, have been examined. No alignment was given to the substrates; no external field was applied. Upon cooling the isotropic (1) melt, the nematic phase (N) begins to separate at the clearing point (77°C) in the form of droplets. Each drop displays a black cross the iums of which are parallel to the vibration directions of the polarizers. Interference fringes are also observed within each drop. The structure of a nematic droplet floating in an isotropic liquid medium have been studied earlier both theoretically and exprimentaly1–13.A theoretical investigation of the structure of droplets of nematic materials, floating in an isotropic liquid has been carried out by Dubois-Violette and Parodi1.1By assuming an anisotropic surface energy, these authors predict both the possible configurations depending on whether surface tension induces a normal or a tangential orientation of the molecules at the N-I interface. The surface tension of the N-I interface has been calculated in a mean field approximation for the system of rod-like molecules intracting via attraction as well as hard-core repulsion14. It is found that the excluded volume effect favours the planar orientation of molecules at the N-I interface in contrast with the case of the nematic free surface at which the effect favours the normal alignment. Thus one can expect that at the N-I interface of EBBA, the surface tension imposes tangential boundary condition on the droplet surface. As a result two point defects, classified as boojurns are obtained at the poles of the droplets. When such droplets are observed through a polarizing microscope with the droplet axis (direction connecting two poles of the droplet) parallel to the direction of incoming light, the resulting texture is an extinction cross with its arms parallel to analyzer and polarizer respectively, as can be seen in Plate 1. Certain droplets are surrounded by black ring indicating thereby that molecules on the equator of the droplets are exactly parallel to the direction of incoming light.

ISSN:1058-725X    

DOI:10.1080/10587250008024415

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The influence of lateral substitution of liquid crystals on the structure of their smectic Adphases revealed by X-ray diffraction study is discussed.

ISSN:1058-725X    

DOI:10.1080/10587250008024416

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Five series of liquid crystals incorporating a 2,3,5,6-tetrafluorophenylene moiety and a terminal fluorosubstituted phenyl group have been prepared. The mesomorphic properties have been studied by polarizing microscopic textural observation and DSC measurements. The effect of terminal fluorosubstituent on mesomorphic behaviour has been discussed.

ISSN:1058-725X    

DOI:10.1080/10587250008024417

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

We present photoalignment behaviors of low-molecular liquid crystals (LMLCs) on a photo-crosslinkable side-chain copolymer liquid crystal (CPLC) containing a 4-(2-cinnamoy-loxyethoxy)biphenyl group and a 4-cyanobiphenyl group by use of linearly polarized (LP) UV irradiation. The LMLC aligned homeotropically on the photoreacted film in the initial state of the photoreaction and the alignment direction changed homogeneously on further irradiation. The direction of the homogeneous alignment can be controlled to be parallel or perpendicular to the electric vector of the LP-UV light, by changing the irradiation temperature or the degree of the photoreaction. These alignment behaviors are explained by the interaction between the LMLCs and the photoreacted CPLC film.

ISSN:1058-725X    

DOI:10.1080/10587250008024418

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The crystal structures of hexyltrimethylammonium bromide (6TAB) adduct of (R)-(+)-1,1′-bi-2-naphthol (RBNP) and decyltrimethylammonium bromide (10TAB) adduct of rac-1,1′-bi-2-naphthol (BNP) have been determined by the X-ray diffraction method. The molecules of the 6TAB/RBNP complex are held in their aggregates by O1…Br [3.228(4)Å], O2…Br [3.225(6)Å] intermolecular hydrogen bonds and C-H…π interactions between the alkyl chain, methyl groups and aromatic rings of the naphthol moieties. It was observed in the crystal structure of 10TAB adduct of BNP that the crystal structure formation was mainly stabilized by the intermolecular hydrogen bonds between the bromide anion and OH group of BNP (01…Brl [3.241(4)Å], O2…Br2 [3.172(3)Å]). In both complexes, the packing arrangement of the monoalkylammonium bromide salts with non-planar aromatic molecules are different from those observed in complexes of monoalkylammonium bromide salts with planar aromatic guest molecules. X-ray powder diffraction studies on both complexes showed that the crystals complex can be obtained by crystallization from solution and also by mixing powdered samples in a mortar. This study has shown that a non-chiral host amphiphile can form a complex with either a chiral or racemic aromatic guest compound.

ISSN:1058-725X    

DOI:10.1080/10587250008024419

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The X-ray crystalline structure at 293 K and 150 K of the radical cation salt (C10H10S8I3) obtained from a disulfide tetrathiafulvalene (TTF) derivative, is described. The title compound crystallizes in the P1space group with the lattice constants at 293 K: a = 9.7634(12) Å, b = 9.9311(13) Å, c = 12.780(2) Å, α = 72.94(1)°, β = 74.51(1)°, γ = 63.08(3)°, V = 1043.1(6) Å3, Z = 2. The structural study at low temperature indicates a similar crystal structure without any apparent symmetry and structural modifications.

ISSN:1058-725X    

DOI:10.1080/10587250008024420

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Tetra- and octa(13, 17-dioxa nonacosane-15-sulfanyl) substituted metal free- and Ni(II) phthalocyanines have been synthesized from the corresponding phthalonitrile derivatives in the presence the anhydrous metal salt(NiCl2) or a strong organic base. The new compounds have been characterised by elemental analyses, IR, NMR and electronic spectroscopy and mass spectra. The mesogenic properties of these new materials were studied by differential scanning calorimetry(DSC), optical microscopy and X-ray investigations. All compounds show a discotic mesophase in an extremely large temperature interval including room temperature. X-ray diffraction patterns of the mesophases confirm that tetra-substituted compounds(5a, 5b) form a hexagonal ordered columnar mesophases(Dho), whereas octa-substituted compounds(7a, 7b) lead to centered rectangular disordered columnar mesophases(Drd).

ISSN:1058-725X    

DOI:10.1080/10587250008024421

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The azimuthal anchoring energy and pretilt angle of a nematic liquid crystal (E7) which was aligned on a thin layer of polyimide (PI) and poly(vinyl cinnamate) (PVCi) blend by the irradiation of polarized UV on the surface, were studied. Within a PVCi content of 10–100 wt%, the azimuthal anchoring energy remains unchanged with the decrease of the PVCi content in the blend, which is attributed to the preferential occupation of the surface by cinnamate chromophore. The blend alignment layer revealed a significantly enhanced thermal stability of azimuthal anchoring in comparison with the pure PVCi based alignment layer on account of the stiffening of the layer by the incorporated PI, which caused low thermal relaxation of the structural anisotropy developed from PVCi in the blend. The addition of PI into PVCi was also found to be effective for improving the thermal stability of the pretilt angle.

ISSN:1058-725X    

DOI:10.1080/10587250008024422

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The surface pressure-area isotherms of the monolayers of three kinds of amphiphilic derivatives of 8-hydroxyquinoline, N-hexadecyl-8-hydroxy-2-quinolinecarboxamide (1), 2-hexadecyloxycarbonyl-8-hydroxyquinoline (2) and 8-hexadecyloxy-2-quinaldinic acid (3) at an air-water interface on subphases with and without metal ion were investigated and compared with each other. Some information regarding the packing density and phase transition under different experimental conditions has been obtained. At the air-water interface1, 2and3can form metal complexes with Cu2+ion present in the aqueous phase. The formation of complexes leads to changes in the area per molecule and the collapse pressure of the monolayer. X-ray photoelectron spectroscopy analysis reveals Cu-ligand ratios in the LB films to beca.1:1 for1and2and 1:2 for3. Low-angle X-ray diffraction measurements showed the LB films of1, 2and3deposited from CuCl2subsolution had good homogeneity and were Y-type bilayer films.

ISSN:1058-725X    

DOI:10.1080/10587250008024423

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The responses of symmetric square and triangular wave pulses have been investigated in planar cells of two ferroelectric liquid crystalline mixtures FLC-6430 and SCE-13 in the SmC*phase. The samples were sandwiched between two parallel polyimide coated substrates. The cells containing SCE-13 and FLC-6430 were considered in the surface stabilised state (p>d) and volume stabilised state (p<d) respectively, (p is the helix pitch and d is the sample thickness). Dynamic processes associated with the helix unwinding and winding by applying bias -10V - 0V - + 10V has been studied. The spontaneous polarisation (Ps), response time (τR) and torsional viscosity (η) have been evaluated in these materials. Materials in surface stabilised configuration show surface switching while in DHF configuration switching is due to bulk and surface effects.

ISSN:1058-725X    

DOI:10.1080/10587250008024424

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor