摘要:

The effect of structural variations on the mesomorphic nature of columnar liquid crystals of general structure I is studied. X-ray crystal structure analyses of compounds la and 2a reveal the columnar organization of the macrocyclic cores in the solid state. The up and down asymmetry of the cores is resolved in an alternating sequence of up-up and down-down intermolecular contacts. Specific interactions between theRsubstituents are present, which influence the stacking distance between the cores in the down-down intermolecular contacts. Statistical incorporation of different side chains on the same macrocyclic core produces, when the difference in length between the alkyl chains is appropriate, random mixtures having wide mesomorphic ranges. The increase of disorder associated with the statistical side chain distribution on the core and with the structural heterogeneity of each component of the mixture have the effect of depressing mainly the crystal-mesophase transition temperature.

ISSN:0267-8292    

DOI:10.1080/02678299308026320

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

NMR field-cycling measurements of the deuteron spin relaxation dispersionT1(v) for the fully deuteriated nematic liquid crystal 4-n-pentyl-4′-cyanobiphenyl (5CB-d19) over a broader Larmor frequency range (v≈10 kHz to 30 MHz) than reported so far in the literature basically confirm the magnetic relaxation mechanisms previously observed by frequency dependent proton spin studies of various nematogenic molecules, namely collective nematic modes of the director field in the kilohertz regime, and anisotropic reorientations of individual molecules (mainly self-diffusion for the protons and mainly rotations about the long axis for the deuterons) in the megahertz range. Within the experimental error limits such a model allows a self-consistent interpretation of the available deuteron and protonT1(v) results for deuteriated or protonated 5CB, respectively. In particular, the magnitudes of the measured order fluctuation contributions are in approximate accordance, i.e. within a factor of less than two, with theoretical estimates from NMR line splittings and the relevant material parameters. More exact and more extensive deuteron studies are needed to locate the origin of the observed minor inconsistency.

ISSN:0267-8292    

DOI:10.1080/02678299308026321

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The syntheses of new mesogenic aromatic esters containing a 1,3,5-triazine moiety by esterification of 6-substituted 2,4-bis-(4-carboxyphenoxy)-1,3,5-triazines with the appropriate phenols are described. The mesomorphic properties of these new materials are discussed in relation to their dependence on the terminal substituents of the aromatic nucleus and the substituents at the 6-position of the triazine ring.

ISSN:0267-8292    

DOI:10.1080/02678299308026322

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The phase diagram has been established for binary mixtures of nonyloxy-biphenyl cyanobenzoate (9OBCB) and pentylphenylcyanobenzoyloxy benzoate (DB5CN). These frustrated liquid crystals exhibit a rich variety of polymorphic smectic A (SA) and smectic C (SC) phases. X-ray and calorimetric studies have been made of SAd-S[Ctilde]-SC2and SAd-SA1-S[Ctilde]-SA2SC2phase sequences. Major emphasis is given to the tilted ribbon smectic (S[Ctilde])-bilayer smectic A (SA2) transition and the SA2-SC2transition. The former is first order, but there is substantial pretransitional heat capacity in the S[Ctilde]phase that can be described by an inverted Landau model. The latter transition is second order and exhibits a classical Landau step inCp, in contrast to the behaviour of SA-SCtransitions in non-polar systems where an extended Landau model with a large sixth order term is required.

ISSN:0267-8292    

DOI:10.1080/02678299308026323

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A general and versatile route to the 2-alkyl-5-(4′-cyanophenyl)thiophenes was established, involving the condensation of an aryl vinyl ketone with an aldehyde in the presence of a thiazolium catalyst (Stetter procedure) to give a 1,4-diketone which with Lawesson's reagent undergoes ring-closure to give the corresponding arylthiophene. In the final step the exchange of a bromo substituent on the phenyl ring for a cyano group is accomplished by copper(I) cyanide inN,N-dimethylforamamide at reflux. The same sequence afforded 2-alkyl-5-(4″-cyanobiphenylyl)thiophenes. Several of the 1,4-diketones were obtained by conjugate addition of nitroalkanes to aryl vinyl ketones, and treatment of the γ-nitroketones with silica gel-supported potassium permanganate. Three alkyl 5-(4-cyanophenyl)thiophene-2-carboxylates were prepared by condensing β-chlorovinylaldehydes with thioglycolates. The cyanophenylthiophenes exhibited only monotropic phases, but incorporation of an additional phenyl ring provided cyanobiphenylylthiophenes of wide nematic ranges. Transition temperatures of binary mixtures of the cyanophenylthiophenes with 4-n-pentyl-4′-cyanobiphenyl were measured, and extrapolated virtual nematic-isotropic transition temperatures were determined.

ISSN:0267-8292    

DOI:10.1080/02678299308026324

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

We report measurements of the deuterium NMR spectra of the lyotropic systemsn-octyl-,n-nonyl- andn-decyl-β-D-glucopyranosides with D2O and C6D6. From these spectra the phase diagrams are constructed and include lamellar, hexagonal and isotropic (presumably cubic) phases. The details of the phase diagrams depend upon the sugar homologue and the solvent. The results can be understood qualitatively by considering some of the factors which determine the nature of the phases in these systems. The deuterium spectra belonging to the OD groups in the polar heads ofn-octyl-α-D-glucopyranoside dissolved in C6H6were measured and compared with analogous spectra, which were reported previously, of the same compound in D2O.

ISSN:0267-8292    

DOI:10.1080/02678299308026325

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Prism coupling techniques have been used to excite optical modes in a thin nematic liquid crystal with finite surface tilt in order to study the voltage dependent director profile. The surface tilts are opposite in character and it is found that at zero applied volts the stable configuration is the substantially horizontal state. On applying the field this state is broken, probably transforming to the twisted vertical state. By modelling all the data obtained, the detailed behaviour of the director profile has been fully characterized yielding much information, including the change of surface tilt with applied voltage. For the nematic liquid crystal E7, this gives a voltage induced surface tilt of approximately 0.67° V−1for a 5.65 μm thick cell. Also using a boundary layer model, it has been possible to analyse the free energy in the cell and hence show that the observed twisted vertical state is the expected stable state under the field applied.

ISSN:0267-8292    

DOI:10.1080/02678299308026326

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Several esters and ethers of pentaerythritol have been prepared. Four of them, containing each a 1,4-trans-disubstituted cyclo-hexane unit and three hydroxyl functions, emerge as examples of a novel type of thermotropic, intermolecularly hydrogen bridge-supported multiol mesogens exhibiting the smectic A phase. A new case of a heteroatom effect preventing mesophase formation by causing stable intramolecular hydrogen bonding situations will also be discussed.

ISSN:0267-8292    

DOI:10.1080/02678299308026327

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Two-component liquid crystalline (LC) mixtures containing LC diacrylates and chiral molecules with no reactive groups were made. Upon photopolymerization of the mixtures cholestric networks containing LC molecules which were not chemically attached to the networks were created. By heating, molecules which were not chemically attached to the network could be removed leaving behind a three dimensional network with a helical structure containing no chiral molecules and changed optical properties. The changes in the optical properties of the system could also be induced locally using a laser beam, demonstrating the possibility of using the method for optical storage.

ISSN:0267-8292    

DOI:10.1080/02678299308026328

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

By endcapping mesogenic rigid rod molecules with reactive epoxy groups a novel class of liquid-crystalline thermoset has been obtained. In fact is has been shown that the nematic molecular arrangement is sustained over the crosslinking reaction of liquid-crystalline epoxy resins when the curing reaction is carried out in the thermal stability range of the liquid-crystalline phase. Calorimetric analysis was used in characterizing the isothermal cure. An unsophisticated model is proposed for evaluating the activation energies of the crosslinking reaction. For liquid-crystalline epoxy resins lower activation energies result with respect to the cure reactions for non liquid-crystalline epoxy resins.

ISSN:0267-8292    

DOI:10.1080/02678299308026329

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor