摘要:

On the basis of a mean-field approach, a microscopic model is presented to explain some ferroelectric properties of induced smectic C* phases of chiral dipolar guest molecules in an achiral smectic C host phase. The molecular biaxiality of the chiral dopants has been taken into account, resulting in a polar and quadrupolar ordering of the molecular short axes in the rotational distribution function of the guest molecules. This model explains the dependence of the magnitude and sign of the spontaneous polarization on the molecular structure of a series of cyclo-hexanone derivatives used as chiral dopants, as well as the effect of a local field at higher dopant concentrations.

ISSN:0267-8292    

DOI:10.1080/02678299408029145

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The thermodynamic properties and conformational structures in the crystalline and liquid crystalline phases of 4-(1-methylheptyloxycarbonyl)phenyl 4′-octyloxybiphenyl-4-carboxylate (MHPOBC) have been investigated by Raman scattering, X-ray diffraction and differential scanning calorimetry (DSC). MHPOBC can assume two different crystalline states at room temperature depending on crystallization conditions, and heating transforms the metastable crystal to the stable one before melting. In the liquid crystalline phases, Raman scattering spectra have revealed not only the flip-flop twisting vibration of the biphenyl group but also the internal rotation around C-C bonds between the carbonyl groups and the corresponding benzene rings.

ISSN:0267-8292    

DOI:10.1080/02678299408029146

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

In a previous article, we proposed a model to explain the unwinding transition in an electric field of a frustrated cholesteric liquid crystal sandwiched between two glass plates imposing a homeotropic anchoring. We found that three distinct solutions exist in materials of negative dielectric anisotropy: first, the homeotropic nematic at small thickness and small voltage, second, a translationally invariant configuration (TIC) at large voltage and, third, the cholesteric fingers. In this article, we study some optical properties of these solutions. We show first that the TIC rotates the polarization of light. Its ‘apparent’ rotatory power is calculated exactly and is compared with the experimental data when the TIC-nematic phase transition is second order. The agreement between theory and experiment is excellent. We show in particular that there exist discrete values of the voltage for which the TIC has a pure rotatory power. We then calculated the optical contrast of the fingers when they are observed between crossed polarizers. The agreement with experiment is still satisfactory, in spite of the approximate form of the director field chosen to describe the topology of the finger.

ISSN:0267-8292    

DOI:10.1080/02678299408029147

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

A series of liquid crystalline polymers has been synthesized in which fluorinated or semi-fluorinated mesogenic moieties are used as side groups linked to a polysiloxane backbone. Clear-cut differences in the structure of the smectic A phase are evidenced between fluorinated polymers and their classical non-fluorinated homologues. In a comparative analysis, we discuss the relevance of the steric hindrance of perfluorinated sequences and of the microsegregation of the backbone in response to these modifications as related to the fluorine content in the side group and to the proportion of mesogenic groups.

ISSN:0267-8292    

DOI:10.1080/02678299408029148

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Intrinsic structural defects in the lamellar (Lα) phase of the system sodium dodecyl sulphate (SDS)/decanol/water are studied by a combination of NMR2H quadrupole splittings of α-deuteriatedSDSand small angle X-ray scattering (SAXS). The focus is on the variation of the density and size of the defects with the bilayer composition (decanol/SDSmol ratio 0.46–2.5), the bilayer volume fraction (0.27–0.64), and the temperature (20–40°C). It is found that bilayer defects are promoted by a small decanol/SDSratio in the bilayers and by a low bilayer volume fraction, i.e. the same factors that drive the progression of phases towards microstructures with more highly curved interfaces. Variations of the extent of defects in the Lαphase reflect variations of the number of defects rather than changes of their size. The data are consistent with either slit defects (slit width ∼ 10 Å) or pore defects (diameter ∼ 25 Å). In either case, the interface separation across the defect is considerably smaller than the inter-bilayer separation. There is no evidence of transitions between Lαphases with different defect patterns. The defect variation with composition is analysed in terms of a simple thermodynamic model, showing that, apart from the electrostatic bilayer repulsion (which opposes defects), it is necessary to allow for considerable variations of the defect self-energy (which promotes defects).

ISSN:0267-8292    

DOI:10.1080/02678299408029149

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

We report the results of the computer simulation of a collection of particles interacting via an anisotropic potential proposed by Luckhurst and Romano and modified by scaling with part of the anisotropic well depth formalism employed by Gay–Berne. Using the molecular dynamics technique for 256 particles in the NVE ensemble, the system is shown to exhibit a variety of mesophases, as the temperature is lowered, and these are provisionally identified as isotropic, discotic nematic, a highly ordered fluid phase with some columnar features and a crystal.

ISSN:0267-8292    

DOI:10.1080/02678299408029150

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Lead(II) alkanoates with even chain lengths from octanoate to octadecanoate have been investigated by Raman spectroscopy. In the low frequency region, transverse and longitudinal acoustical modes (TAM, LAM) have been assigned. It was shown that LAM-1 is the vibration of the double chain with the node of the vibration in the Pb2+layer. A fully extended conformation of the chains in the low temperature phase was confirmed. The frequencies and intensities of the LAMs as compared with those of the alkanes and the fatty acids led to an estimate of the force constant and polarizability of the Pb2+-COO−bond relative to the C-C bond. The defects at the chain ends were investigated in the ρ(CH3) andv(CC) region. For the intermediate (CM) phase, both the Pb2+layer distance reduction and the chain length independent enthalpy contributions can be attributed mainly to defects at the chain ends.

ISSN:0267-8292    

DOI:10.1080/02678299408029151

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The orientation properties of some complexes of chromium (III) or cobalt (II) with oxygen-containing ligands are presented. The orientation obtained is mostly homeotropic. The possibility of anchoring by coordination of liquid crystal (LC) molecules to the transition metal ion within the alignment layer is discussed on the basis of spectroscopic arguments.

ISSN:0267-8292    

DOI:10.1080/02678299408029152

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Using a Rayleigh scattering light beating technique, we have made new measurements on the slow polarized fluctuations, previously identified as being of micellar origin, for a lyotropic system consisting of K-laurate, decanol, D2O and presenting only the uniaxial calamitic nematic (Nc) phase (from among the three known nematic types: uniaxial disk-like (ND), calamitic (Nc) and biaxial (NB) phases). An elaborated analysis of these data is made in order to clarify the relationship between the light scattering signals coming from the micellar fluctuations and those from (classical) orientational fluctuations. The main conclusions of this analysis are that the micellar mode is independent of the temperature and wave-vector, and that its relaxation frequency is substantially lower than that found previously for the same lyotropic components, but presenting the biaxial nematic phase. Finally, some considerations are made of the possible relationships between the micellar fluctuations and the structural properties of lyotropic nematics.

ISSN:0267-8292    

DOI:10.1080/02678299408029153

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

This article describes how the inclusion and positioning of a lateral hydroxy group influences the thermal stability of a chiral smectic C phase. An off-central position of the hydroxy group in the aromatic core has the effect of enhancing the thermal stability of the chiral smectic C phase, whereas a central position of the hydroxy group destabilizes it to the extent that there is no evidence for the phase being present. The results indicate that a hydroxy grouportho- to an ester function gives intra- rather than inter-molecular hydrogen bonding. The effects seen with lateral and terminal hydroxy groups are compared with those for analogous fluoro-substituted systems.

ISSN:0267-8292    

DOI:10.1080/02678299408029154

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor