摘要:

We investigated the correlation between orientational order and microscopic organization of the molecules in smectic A and chiral (racemic) smectic C phases by means of solid-state C-13 NMR, powder X-ray diffraction, and electrooptical measurements. The compounds under investigation are 4-((S)-2-methyloctanoyl)phenyl 4′-nonylbiphenyl-4-carboxylate ((S)-MONBIC) and its corresponding racemic compound ((S, R)-MONBIC). Static C-13 NMR indicates that: (1) the orientational angle of the tail with respect to the magnetic field decreases slightly both in the SAand S*Cphases as decreasing temperature, and (2) the angle of the core with respect to the field decreases in the SAphase but increases in the S*Cphase as decreasing temperature. Analysis of C-13T1reveals that the dynamic molecular deformation for the core part can occur near the transition. We discuss the dynamic molecular deformation in comparison with the reorientation of the director at the SAto S*Ctransition. Based on the experimental results, we propose the structural model in which describes the microscopic organization of the molecules in the mesophases.

ISSN:0267-8292    

DOI:10.1080/02678299508036633

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A number of multi substituted binaphthalene compounds were synthesized in an attempt to obtain a highly chiral biaxial nematic (Nb) phase or a good NLO material. Introduction of two pairs of substituents into the binaphthalene molecule

ISSN:0267-8292    

DOI:10.1080/02678299508036634

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Self-assembled, hydrogen bonded, supramolecular complexes based on the interaction of pyridine with carboxylic acids have been synthesized. These contain different proportions of a compound with two terminal benzoic acid residues and a pentaerythritol-based tetra-pyridyl. They have been investigated for possible mesomorphic properties. Surprisingly for a tetrahedral pentaerythritol-based complex, a mesophase was observed with a grainy microscopic texture, which was fluid and could be sheared. The mesogenic unit is formed on hydrogen bonding complexation between the pyridyl residues and the carboxylic acid residues. Infrared spectra, as a function of the temperature, were recorded for the 1:1 complex; these show pyridyl complexation and changes on isotropisation. These changes are discussed in terms of the hydrogen bonding. Fibres could be drawn from the (mesophase) melt, supporting a polymeric, extended-chain structure for the complexes.

ISSN:0267-8292    

DOI:10.1080/02678299508036635

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The ferroelectric switching of columnar 1,2,5,6,8,9,12,13-octakis-((S)-2-heptyloxy-propanoyloxy)dibenzo[e,l]pyr-2-heptyloxy-propanoyloxy)dibenzo[e,1]pyrene was studied in detail between 60°C and the clearing point 115°C. The switching angle (optical tilt angle) is ±24.5° up to 10V μm−1and ±37° at higher field strengths. The electric polarization is 60nC cm−2in the low field phase and 180nC cm−2in the high field phase. The switching rate has apparent activation energies that increase with voltage from 3 × 10−19to 10−18J. It varies roughly with a power of the voltage, the exponent increasing with decreasing temperature from 2 to 5. At equal voltages, switching is faster in the low field than in the high field phase. We tentatively infer the structures of the two columnar lattices from the ratio of polarizations and other data. From the switching angles, we calculate a tilt angle of 44° for the aromatic cores of the disc-like molecules stacked in the columns. Finally, we point out possible advantages of ferroelectric columnar liquid crystals over their smectic counterparts in electro-optical displays.

ISSN:0267-8292    

DOI:10.1080/02678299508036636

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The electroclinic effect, in a material having a first order SAto SC* transition, is studied using the half leaky guided mode geometry. Using an approximately 1 μm thick, homogeneously aligned cell, the voltage induced director twist is characterized at a few temperatures in the SAphase. The mean field theory readily explains the data recorded at low fields where a linear dependence on voltage is found. However, at higher fields, an unexpected saturation occurs which is most likely caused by the influence of strong surface anchoring forces.

ISSN:0267-8292    

DOI:10.1080/02678299508036637

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

High resolution voltage dependent tilt angle studies using optical excitation of half-leaky guided modes have been conducted on a homeotropically aligned ferroelectric liquid crystal mixture (Merck SCE13) in the SAphase. Uniform homeotropic alignment is realised, with no surface aligning layer, by the application of an in-plane DC electric field when the liquid crystal is in the SC* phase. The applied field unwinds the pitch of the SC* chiral helix and gives a uniformly tilted homeotropic monodomain. On warming into the SAphase, detailed studies of the voltage induced tilt, the electroclinic effect, are then conducted at various temperatures. Because there is no influence of surface anchoring forces, the linear relationship between the induced tilt angle and the DC field is obtained even under very weak fields. Further, the relationship between induced tilt and temperature confirms the predictions of a second order Landau mean-field theory with a coupling term between the tilt angle and the DC field.

ISSN:0267-8292    

DOI:10.1080/02678299508036638

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The excitation of half leaky guided optical modes to characterize fully the optical tensor profile in a thin liquid crystal layer has been used to evaluate the effect of rubbed polyimide aligning layers on the alignment of a nematic liquid crystal. A cell fabricated with rubbed polyimide alignment surfaces was studied at a wavelength of 632.8 nm. The liquid crystalline layer is sandwiched between a high refractive index top glass plate and a low refractive index glass substrate. Angular dependent reflectivities are recorded using a coupling prism and matching fluid with the same index as the top glass plate. Careful fitting of the predictions from multilayer optics theory to the observed angle dependent polarization conversion and reflectivity data yields the director profile within the liquid crystal layer in great detail.

ISSN:0267-8292    

DOI:10.1080/02678299508036639

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

In this paper a novel method is developed for measuring the twist angle and surface torsional anchoring strength in nematic liquid crystal (NLC) cells. This method is based on a technique developed from the Jones optical propagation matrix. From the measurement, the actual twist angle in NLC cell and the deviation of the LC director on the boundary of the cell from the rubbing direction of the substrate are obtained. A theoretical discussion shows that the surface torsional anchoring strength has a great influence on the value of the deviation, and hence so it can be calculated.

ISSN:0267-8292    

DOI:10.1080/02678299508036640

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

We propose a mechanism, through which the low membrane bending rigidity changes the steric polymer repulsion between polymer-grafted bilayers. We calculate the energy of a polymer chain trapped between two thermally undulating surfaces. The proposed treatment is in terms of de Gennes' theory of fluctuations in liquid crystals, later developed by Helfrich for membranes. We show that the confinement energy of the polymer increases due to membrane fluctuations. Also, the repulsive forces between soft polymer-grafted bilayers can extend further from the surface than the height of the polymer layer.

ISSN:0267-8292    

DOI:10.1080/02678299508036641

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Thirteen members of the lead(II)n-alkanoate series (from then-hexanoate to then-octadecanoate) have been synthesized, purified, and studied by differential scanning calorimetry, polarizing light microscopy, and FTIR and Raman spectroscopy. A solid-solid transition was found for all members of the series. A mesomorphic state, implying a fusion and a clearing point, was identified as smectic C, and was present only in the lower members of the series below then-tridecanoate. The higher members form the isotropic liquid through a regular fusion. Temperatures and enthalpies for the phase transitions have been measured and the corresponding entropy changes calculated. Heat capacity measurements between 280 K and 400 K were also carried out. A study of these magnitudes versus number of carbons was made. The solid phase previous to the liquid crystal in the lower members and to the isotropic liquid in the higher members could be identified as a ‘condis’ crystal (conformationally disordered phase) by an FTIR versus temperature study.

ISSN:0267-8292    

DOI:10.1080/02678299508036642

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor