摘要:

The synthesis of three new series of chiral Schiff's bases containing benzilideneaniline and 2-hydroxybenzilideneaniline moieties as mesogenic cores is presented. Differential scanning calorimetry, optical polarizing microscopy and X-ray diffraction measurements were used to study the phase transition temperatures and behaviour. The results reveal that most of these materials show chiral smectic mesomorphism.

ISSN:0267-8292    

DOI:10.1080/02678299508031982

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Reaction of pyridine with 1-bromoalkanes leads toN-alkylpyridinium bromides which can be further reacted with silver dodecylsulphate to giveN-alkylpyridinium dodecylsulphates in good yield. These salts are mesomorphic and their mesophases have been characterized by optical microscopy, DSC and X-ray scattering techniques.

ISSN:0267-8292    

DOI:10.1080/02678299508031983

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

We report the observation of a liquid crystal tilt transition from homeotropic to planar orientation induced by photopolymerization of the alignment layer in the absence of liquid crystal. The alignment agent is a unique, polymerizable lecithin (DC23PC), which induces homeotropic alignment before UV exposure. After non-polarized UV exposure, a tilted orientation is obtained. Moreover, further buffing of the UV treated substrate yields a homogeneous alignment. We believe that the conformation change in the lecithin array caused by solid state polymerization is primarily responsible for the transition. These results help to explain the mechanism of liquid crystal alignment and will lead to several potential applications.

ISSN:0267-8292    

DOI:10.1080/02678299508031984

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Textures and defects in ethyl-cyanoethyl cellulose [(E-CE)C]/dichloroacetic acid (DCA) cholesteric liquid crystalline solutions and in (E-CE)C/polyacrylic acid (PAA) composites were observed and studied by polarizing microscopy and electron microscopy. The existence of χ, λ and τ disclinations were observed in the mesophase with disk-like and band-like textures. Pairs of disclinations with different signs were also found in the mesophase with the band-like texture. Domain walls were observed in (E-CE)C/PAA composite films with cholesteric order by TEM. The orientation of polymer chains in the vicinity of the core of the disclinations is discussed.

ISSN:0267-8292    

DOI:10.1080/02678299508031985

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A new ionic liquid crystal system, based on compounds with the viologen group (for example,1–2:Cr 190 SA280 dec.) has been synthesized, and is found to exhibit the smectic A phase.

ISSN:0267-8292    

DOI:10.1080/02678299508031986

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Amorphous twisted nematic liquid crystal display cells have been prepared without rubbing. The textures consisted of twisted nematic multi-domains. Theoretical calculations of the optical properties of such cells have been carried out by the 4 × 4 matrix method. The optical anisotropy of the liquid crystal is optimized to suppress/optimize the background colour. The electro-optical characteristics of the a-TN have been measured. It is shown that the a-TN devices exhibit wide and uniform viewing angle characteristics and possess good gray scale capability. Viewing angles of − 47° to + 45° (up-down) and − 51° to + 46° (left-right) have been achieved. A rise time of 36.5 ms and decay time of 35.5 ms have been measured in an a-TN sample.

ISSN:0267-8292    

DOI:10.1080/02678299508031987

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A bifurcational analysis is performed on a version of Doi's equation of nematodynamics that describes the non-equilibrium isotropic-discotic nematic phase transition in the presence of steady uniaxial extensional flow. The disc-like molecular geometry and the degenerate extensional flow-induced orientation are shown to be the source of a complex bifurcation and multistability behaviour involving two physically equivalent biaxial nematic phases, one uniaxial nematic phase and one uniaxial paranematic phase. Depending on the temperature and the extension rate, the isotropic-discotic nematic transition, involving the two biaxial nematic phases and the uniaxial paranematic phase, may be continuous (2nd order), discontinuous (1st order), or it may exhibit a tricritical non-equilibrium phase transition point. A validation procedure on the validity of the predictions is implemented. The predictions presented here find practical applications in the industrial spinning of mesophase carbon fibres, and also provide new results that increase the present fundamental understanding of the rheology of discotic nematic liquid crystals.

ISSN:0267-8292    

DOI:10.1080/02678299508031988

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The orientational ordering of several liquid crystals containing a difluorosubstituted phenyl ring has been studied through the use of C-13 NMR. The fluorinated phenyl ring of these liquid crystals have Cssymmetry, so three order parameters are required to completely describe the ordering of this ring. All three of these order parameters have been calculated from carbon-fluorine dipolar coupling constants obtained from the carbon-fluorine splittings in the C-13 NMR spectra. Because of the complexity of the fluorine-coupled spectra, variable angle spinning (VAS) was used to resolve the carbon-fluorine splittings. In order to study the orientational ordering over wide ranges of temperature, we have developed an empirical correlation between the order parameter and the value of a carbon-fluorine dipolar coupling constant. This enabled us to study the change in the order parameter with temperature. The results of applying this method to several structurally similar liquid crystals containing the same type of difluorinated phenyl ring are presented. A comparison is made to a similar mono-fluorinated liquid crystal.

ISSN:0267-8292    

DOI:10.1080/02678299508031989

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Liquid crystalline elastomers derived from siloxane polymers, PMHS and PEHS, were prepared following a two-stage reaction scheme to investigate the effects of processing conditions and thermal treatment on mesomorphic characteristics. The LC elastomer samples were analysed by FTIR, DSC, stress-strain relationship, polarized optical microscopy, and X-ray diffraction techniques. Key findings include: (1) Chain length of the precursor siloxane polymer is critical to achieving a bulk alignment via stretching during the second-stage reaction; (2) the imposed strain is also critical to achieving a monodomain nematic character, which is completely recoverable upon heating beyond the clearing temperature followed by cooling back to room temperature (i.e. thermal treatment); (3) enthalpy is stored in an elastomer freshly prepared without an imposed strain, but the imposed strain helps to release the stored enthalpy, which can be almost completely released upon thermal treatment; and (4) while the stored enthalpy does not increase order parameter (HOFA), its release through molecular relaxation contributes to an increased HOFAvalue.

ISSN:0267-8292    

DOI:10.1080/02678299508031990

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A new homologous series of 4-(4′-ethoxybenzoyloxy)-2-alkoxy-4′-(4-butoxysalicylaldimine)-azobenzenes has been synthesized. These compounds contain four rings in the main core and a lateral alkoxy branch on one of the inner rings and present enantiotropic nematic properties. The X-ray crystal structure (MoKαradiation, λ = 0.7071 Å, graphite monochromator) of 4-(4′-ethoxybenzoyloxy)-2-butoxy-4′-(4-butoxysalicylaldimine)azobenzene was investigated (T= 293K,R= 0.059 for the 2855 observed reflections). This compound (C36H39N3O6) crystallizes in the monoclinic system with C2/c space group; (Z= 8);a= 28.052(7) A,b= 7.545(4) Å,c= 33.823(7) Å and β = 112.70(1)°. There is a strong chelated hydrogen bond between the OH group and the neighbouring nitrogen atom engaged in the imine bridge. In the solid state, the molecule is roughly linear with a 31 Å length. The lateral butoxy chain is quasi-perpendicular to the long axis of the main core. The crystal cohesion is due to numerous Van der Waals interactions. At the solid to nematic transition, a change in the conformation of the lateral chain occurs from an alltrans-conformation to one involving acis-conformation in the first part of the lateral chain. This change can be monitored by the evolution of the13C chemical shift of the OCH2belonging to the lateral alkoxy chain.

ISSN:0267-8292    

DOI:10.1080/02678299508031991

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor