摘要:

Liquid crystal phases are typically formed by molecules having several degrees of internal freedom. These systems exhibit, therefore, complex dynamics, with internal motions superimposed on the rotational diffusion of the whole molecule. The problem of the internal transitions has been treated in terms of a master equation for jumps between configurational sites, derived by projecting the multidimensional diffusion equation for the torsional variables on a suitable set of site functions. The coupling with the overall diffusion has been taken into account explicitly, by considering the conformational dependence of both the mean field torque and the molecular diffusion tensor. A Marcelja-like potential acting on the various molecular moieties has been used, and the frictional effects have been calculated for the different chain conformations. In this way, the rates for the internal transitions are orientation dependent, and the solution of the diffusional problem requires a matrix representation in the full space of angular and site functions. The nematogen 4-n-pentyl-4′-cyanobiphenyl, for which a large amount of experimental data is available from detailed NMR relaxation measurements, is taken as a reference system. The spectral densities of the relevant correlation functions for the deuterons in the various positions of the molecule have been calculated, for different degrees of ordering and different choices of the energetic and hydrodynamic parameters.

ISSN:0267-8292    

DOI:10.1080/02678299008047375

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Further studies are reported on polar diacetylenes with conjugated side chains which show liquid-crystalline behaviour. Results obtained using differential scanning calorimetry, thermogravimetry, and hot stage microscopy are reported. One group of materials forms a smectic liquid crystal phase and all polymerize from a nematic liquid crystal phase.

ISSN:0267-8292    

DOI:10.1080/02678299008047376

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The effect of molecular structural changes on the mesomorphic nature of a new class of macrocyclic columnar liquid crystal is reported. Twenty-six new compounds with general molecular structure I were prepared, characterized and compared.

ISSN:0267-8292    

DOI:10.1080/02678299008047377

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

On the basis of the symmetry consideration of the Sc* phase, a generalized elastic free energy of ferroelectric Sc* liquid crystals is presented to account for the layer compression or dilatation and the layer distortion as well ascdirector deformation. The present elastic free energy is expressed in terms of three vectors, i.e. thecdirector and the wave vector and the spontaneous polarization vector. According to the present Sc* model withC2symmetry, it is shown that there may exist 17 non-chiral, 4 chiral terms and 14 flexoelectric terms in the Sc* phase. A few practical applications are also presented to elucidate some interesting elastic properties of Sc* (or Sc) in simplifed geometries.

ISSN:0267-8292    

DOI:10.1080/02678299008047378

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Side-chain liquid crystal polyacrylates and polysiloxanes containing different photochromic spironaphthoxazine side groups were synthesized. Thermodynamic, spectral and kinetic properties of the polymers were investigated. The structure of the mesophase is discussed.

ISSN:0267-8292    

DOI:10.1080/02678299008047379

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The orientational ordering of a series of ferroelectric thiobenzoate liquid crystals was studied by natural abundance13C NMR spectroscopy. The technique used was a combination of variable angle spinning (VAS) and separated local field spectroscopy (SLF). With rapid sample spinning about an axis forming an angle ofc.45° with respect to the magnetic field, the smectic director aligns parallel to the spinning axis, leading to narrow peaks in the13C NMR spectrum. The two-dimensional NMR spectroscopic method SLF allows the observation of decoupled13C signals in the ω2 dimension and first-order Csbnd;H splitting patterns in the ω1 dimension, from which the dipolar C‒H coupling constants for individual bonds can be obtained. The order parameters for different molecular segments of eight different compounds, all containing two phenyl rings linked by a thioester group, were obtained this way. A considerable influence of length, branching and chirality of the aliphatic chain on the order parameters was observed.

ISSN:0267-8292    

DOI:10.1080/02678299008047380

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The formation of thermotropic ionic smectic liquid crystals in binary systems of lithium and caesium acetates, propionates and butyrates has been discovered. Mesomorphism of mixed melts of non-mesogenic components results from the formation of complex anions. Correlations between anion mesogenic properties and its size, shape, anisotropy in the Coulomb charge distribution as well as the nature of the surrounding cations are discussed.

ISSN:0267-8292    

DOI:10.1080/02678299008047381

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The nature of smectic A liquid crystals and the phase transitions from smectic A to nematic phases are studied in the homologous series of 4-(4″-octyloxy-benzoyloxy)benzylidene-4′-substituted anilines, in which the substituents are H, F, Cl, Br, I, NO2and CN. Measurements have been performed for the dipole moment of the molecule, the smectic A lattice period, the temperatures and the entropies of phase transitions, and the temperature dependence of the orientational order parameter, all of which provide information on the intermolecular pair potentials. It is shown that the introduction of a terminal polar substituent brings about counteracting contributions to the stability of the smectic A state. It is suggested that the reentrant nematic phase transition can appear in the strongly polar mesogens if the extent of antiparallel association of the molecules increases on lowering the temperature.

ISSN:0267-8292    

DOI:10.1080/02678299008047382

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Broadband dielectric spectroscopy (10−1to 107Hz, 100 to 450 K) has been applied, for the first time, to investigate the molecular dynamics of recently synthesized fast switching ferroelectric side group polymers. The softmode could be studied in detail in the Sc* phase and in the S1* phase. At the transition a pronounced increase of the relaxational strength was found whereas its relaxational frequency remains constant The softmode is the molecular basis for the electroclinic effect which has high application potential for ferroelectric liquid crystal polymers.

ISSN:0267-8292    

DOI:10.1080/02678299008047383

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The molecular reorientation in anisotropic fluids is considered as twostage process—fast single molecule rotation in the volume restricted by close neighbours and slow collective relaxation of the local surrounding. The theoretical results are applied to explain the discrepancy between the rotational diffusion coefficients D⊥robtained by infrared bandshape analysis and polarized fluorescence techniques.

ISSN:0267-8292    

DOI:10.1080/02678299008047384

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor