摘要:

A series of new dimeric liquid crystalline compounds were synthesized and their thermotropic properties studied by differential scanning calorimetry and observation of their melts by polarizing microscopy. These compounds consist of two bis (p-ethoxyphenoxy)terephthalate units interconnected through an oxypolymethyleneoxy spacer on the central terephthaloyl units resulting in a structure of 'H-shaped' dimeric twin compounds. The length of spacers was varied from oxytetramethyleneoxy (n4) to oxyoctamethyleneoxy (n8). The compound containing the oxydodecamethyleneoxy (n12) was also included in this study. With the exception only of the dodecamethylene compound, all of the present compounds are monotropically nematic. In contrast, the compound with the oxydodecamethyleneoxy spacer is enantiotropically nematic. It was found that the heats (DeltaHm) of melting of the monotropic compounds are exceptionally high, 70 100kJ mol1.The DeltaHm value of the compound having the oxydodecamethyleneoxy spacer, however, is much less, about 17 kJ mol1. It is concluded that 'H-shaped' compounds possess less tendency to be thermotropic and also that they tend to vitrify when the spacer length exceeds 6.

ISSN:0267-8292    

DOI:10.1080/026782998206074

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

A new liquid crystalline phase, induced by the addition of small amounts of a non-mesogenic solute (such as dimethyl sulphoxide or methyl iodide) to a quaternary ammonium salt, N -methyl- N, N, N-trioctadecylammonium iodide (MTAI), has been detected by NMR and optical microscopic studies. In some cases, there is a coexistence of nematic and smectic phases. Information on the ordering of the phases in the magnetic field of the spectrometer has been derived from NMR spectra of a dissolved molecule, 13C-enriched methyl iodide. The low order parameter of the pure thermotropic nematic phase of the salt provides firstorder spectra of the dissolved oriented molecules. Analyses of spectra of cis, cis -mucononitrile exemplifies the utility of the MTAI nematic phase in the determination of structural parameters of the solute.

ISSN:0267-8292    

DOI:10.1080/026782998206083

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The present paper is a study of the ferroelectric and antiferroelectric behaviour in low molar mass organosiloxane liquid crystal materials classed as mono-mesogens (AB type) and bi-mesogens (ABA type). A systematic study of series of materials all based on the same chiral mesogenic moiety is presented. The mesogen is a biphenylyl benzoate with a halogen X attached laterally to the benzoate ring which is closest to the chiral centre. Three homologous series, where the halogen is either fluorine, chlorine or bromine, are investigated. The number of silicon atoms in the linkage or end group is varied between two and nine. It is found that the materials have a broad (40 C) ferroelectric or antiferroelectric phase with a high P s (90nC cm2). The siloxane moiety forces the tilt angle to a temperature independent value close to 45. The antiferroelectric order is observed only in bi-mesogens with an odd number of silicon atoms in the siloxane link. The antiferroelectric order is attributed to the conformation of the molecule rather than to antiferroelectric interactions between layers.

ISSN:0267-8292    

DOI:10.1080/026782998206092

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Two series of ferroelectric liquid crystals containing a phenyl biphenyl carboxylate mesogenic group and oligomethylene spacers (n 3, 4, 6, 8) were synthesized and characterized. The chiral moieties are the (S)-2-methylbutoxycarbonyl and (S)-1-methylheptoxycarbonyl groups. The mesomorphic behaviours of the compounds were characterized using differential scanning calorimetry and optical polarized microscopy. Spontaneous polarization (P) of the liquid crystals was measured by an automated polarization tester over a wide temperature range. The mesomorphic properties are discussed as a function of spacer length and position of the chiral centre. It is found that the phase transition temperature decreases with increasing number of oligomethylene units. The phase behaviours of the LC with (S)-1-methylheptoxycarbonyl are more complex than those with (S)-2-methylbutoxycarbonyl, due to the flexible tail in the former. Moreover, the spontaneous polarization depends mainly on the position of the chiral centre, with Ps 15 nC cm 2 and Ps 60 123 nC cm 2 for the series s with (S)-2-methylbutoxycarbonyl and (S)-1-methylheptoxycarbonyl groups, respectively.

ISSN:0267-8292    

DOI:10.1080/026782998206100

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

This paper discusses the influence upon a smectic C liquid crystal cell of backflow induced by the relaxation of alignment following the removal of a strong electric or magnetic field. Our study, based upon a recently proposed continuum theory, concentrates upon the homeotropic configuration in which the smectic layers are parallel to the boundary plates, but some consideration is also given to the bookshelf geometry. Although the governing equations prove to be rather complex, some progress is possible analytically by repeating an approximation made in the corresponding problem for a nematic.

ISSN:0267-8292    

DOI:10.1080/026782998206119

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

A theory of dielectric relaxation in nematics is developed for a molecular dipole moment directed at an arbitrary angle to the molecular long axis. Both exact and simple approximate analytical formulae for the longitudinal and transverse components of the complex dielectric permittivity tensor are obtained for the non-inertial rotational Brownian motion of a molecule in the mean field potential of Maier and Saupe. It appears that both longitudinal and transverse relaxation processes are effectively described by two Debye type mechanisms with corresponding relaxation times and dielectric strengths expressed in terms of the order parameter. The generalization of the theory for an arbitrary axially symmetric mean field potential is given.

ISSN:0267-8292    

DOI:10.1080/026782998206128

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

New liquid crystals consisting of non-chiral, banana shaped molecules are investigated by the generation of the second harmonic (SHG) of an optical wave. The temperature and electric field dependence of the SH activity of the apparent thermodynamic phases of several homologues are studied and first quantitative values of the effective non-linear optical coefficients are given.

ISSN:0267-8292    

DOI:10.1080/026782998206137

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The phase ordering kinetics of various liquid crystalline materials from well known series (PCH, CCH, nO. m, BCH and CB) when turning from a disordered to an ordered phase induced by a rapid temperature quench have been investigated. The process of phase ordering can generally be described by the growth of a characteristic length L. The exponents n of the theoretically predicted universal growth law L (t) tn have been determined. In this paper we present in particular the results for the kinetics of phase formation which takes place under the mechanism of nucleus growth. As characteristic length we chose the diameter of the nuclei. We obtained values for n between 0.5 and 1 dependent on the depth of the temperature quench. These results for mainly spherical shaped nematic and smectic germs are discussed in terms of the Allen-Cahn theory for the movement of an antiphase boundary (APB). As a lower limiting value, 0.5 confirms the classical theory, while 1 as an upper limiting value can be explained by the introduction of a volume driving force.

ISSN:0267-8292    

DOI:10.1080/026782998206146

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

When an s-polarized laser beam impinges on a homeotropically aligned film of nematic liquid crystal at small incident angle, undamped oscillations of the molecular director may be produced. At high beam intensities the oscillations break up into deterministic chaos. Although this effect has been known for a long time and the route to chaos has been qualitatively analysed, no detailed study of the actual molecular director motion has been carried out. We have designed an experimental apparatus to monitor the dynamics of the molecular director, as described by its two polar angles. We observed different dynamical regimes, depending on the laser intensity: steady states, ocillations, rotation and, at the highest laser intensities, deterministic chaos. Moreover, the transitions between the oscillation and rotation regimes are characterized by intermittency.

ISSN:0267-8292    

DOI:10.1080/026782998206155

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

We investigate the influence of geometric confinement on the dynamics in a ferroelectric smectic C* material. Molecular and collective relaxation processes are studied by means of broadband dielectric spectroscopy. In microporous material (pore size 200nm) the Goldstone mode (GM) is still observed, but it is shifted to lower frequencies compared with bulk samples. In nanoporous material (pore size 7.5 nm), the collective reorientation modes are completely suppressed. In all samples, the molecular processes seem to be uninfluenced by the confinement.

ISSN:0267-8292    

DOI:10.1080/026782998206164

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor