ISSN:0267-8292    

DOI:10.1080/026782998205804

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Nematodynamic equations are applied to the description of a cylindrical nematic sample, rotating around its axis with constant angular velocity, in the presence of a perpendicular magnetic field. The system is described by the director orientation n and by the velocity vector v fields in the cylinder volume. Equations are simplified by considering the director orientation n constrained in a planar section of the cylinder and by neglecting coupling with the velocity field, which is completely determined by the angular speed rate. Boundary conditions for perfect alignment of the director n perpendicularly to the walls are assumed. It is shown that a dynamical equation can be obtained which is amenable to numerical analysis for the spatial and time dependence of the director orientation. Transient distributions and stationary solutions are found and discussed.

ISSN:0267-8292    

DOI:10.1080/026782998205813

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Optical and electro-optical measurements have been performed on free-standing chiral smectic films sufficiently thick (10 000 layers) to preserve the natural smectic helix. The Goldstone mode appears at about 200Hz, showing that these films are a much better approximation of the 'ideal' smectic bulk state than a thick planar sample between glass plates (where the Goldstone mode is found at about 3 kHz). In these films the unwinding of the helix is studied, as a function of applied electric field, by monitoring Bragg reflections and their Fourier components. When the helix deforms, a reflection appears which at first sight might be taken for a subharmonic, but must be interpreted as the main 'full pitch mode' reflection relative to the 'half pitch' reflection from the undeformed helix. Our measurements further confirm that in anticlinic materials no helix unwinding takes place prior to the antiferroelectric-ferroelectric transition.

ISSN:0267-8292    

DOI:10.1080/026782998205822

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

We have investigated 'hexa- n -alcanoyloxytruxene' HATXC H which exhibits a disk-like thermotropic nematic phase between 65 and 84 C. Using a light-beating technique we have measured the orientational diffusivities D a correspond respectively to twist and bend deformations. D twist increases monotonically as temperature increases, while D remains constant. This result means that the curvature bend elastic constant K changes more rapidly than the viscosity eta 22 in the same ratio. This means also that in the discotic thermotropic nematic phase, when temperature increases the effective hydrodynamic object, in orientational fluctuation, changes shape to be disk-like rather than rod-like. 9 19 nd D for several temperatures, which twist bend, while K and eta change twist 33 bend

ISSN:0267-8292    

DOI:10.1080/026782998205831

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Interest in the naphthalene unit is increasing considerably. Recently, low and high molecular mass mesogens of naphthalene derivatives exhibiting attractive properties have been reported in the literature. In the present study, three mesogenic homologous series containing a naphthalene moiety have been synthesized. All the homologous series were characterized by elemental analysis and spectroscopic techniques. The behaviour of all three series and their mesophase thermal stabilities are compared with each other and with other structurally related series.

ISSN:0267-8292    

DOI:10.1080/026782998205840

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

By measuring the transmission through an antiferroelectric liquid crystal device (placed between crossed polarizers) as a function of both orientation and applied field in the so-called electroclinic-like, pretransitional regime the behaviour of the effective optic axis tilt angle (psi) and effective optic anisotropy (Delta n) is determined. The relatively poor alignment obtained in antiferroelectric liquid crystal devices is allowed for in the data interpretation and high quality results are presented.

ISSN:0267-8292    

DOI:10.1080/026782998205859

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

In thin films of smectic C liquid crystals, localized regions containing additional smectic layers form circular inclusions that carry a topological charge. Such inclusions nucleate a companion topological defect. These inclusion-defect pairs are modelled as topological dipoles within the context of a one-coupling constant approximation to the 2D Frank free energy. Deviations of the dipole direction from a preferred orientation cause the dipoles to acquire a logarithmic charge. Thermal fluctuations of the dipole direction are calculated and found to be large, scaling as the logarithm of the system size. In addition to dipole-dipole interactions arising from the topological charges, we also find that the thermal fluctuations of the dipole directions are coupled through a preference for global charge neutrality of the logarithmic charges.

ISSN:0267-8292    

DOI:10.1080/026782998205868

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The low frequency (LF) dielectric constant was studied as a function of temperature in the Schiff 's base liquid crystalline compounds of N (p - n -alkoxybenzylidene)- p - n -alkylanilines with the applied field parallel to the director to determine the phase transition temperatures and the underlying dipolar association. The low frequency (1 kHz to 10 MHz) complex dielectric spectrum systematically studied in the smectic polymorphs of these compounds implied multiple relaxations. The rotational degrees of freedom were found to be squeezed with the decrease of temperature as the low temperature smectic phases were approached. The ColeDavidson method was used to differentiate the fast dynamics of the rigid core-associated dipole from the slow component of the flexible end chain. A comparative study of the orientational dynamics of the rigid core dipole (type I) was carried out by designing a normalized scale in smectic phases. Arrehenius dependence was used in the estimation of smectic relaxation frequency at a common normalized smectic temperature scale point. The results imply the downward shift of frequency with increase in length of the flexible end chain. The distinct loss of rotational degrees of freedom of the rigid core dipole with increasing chain length suggests its solute particle behaviour in the solvent concentration.

ISSN:0267-8292    

DOI:10.1080/026782998205877

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Polymer coated substrates modified both by mechanical rubbing and optical buffing have been found to cause liquid crystal molecules to align. Atomic force microscopy (AFM) was used to characterize the differences and similarities in polyimide substrate coatings after being subjected to these two processes. Though the buffing processes caused similar alignment on the surfaces, it was found that the mechanical rubbing created grooves on the order of 250nm, whereas optical buffing resulted in no changes to topographic structure on the order of 100nm scale and some variations at smaller scales. From this observation it was confirmed that the interaction causing the alignment must be associated with molecular alignment rather than large scale physical grooving.

ISSN:0267-8292    

DOI:10.1080/026782998205886

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The extinction efficiency factor and the angular distribution of radiation for polymer dispersed liquid crystal (PDLC) droplets have been calculated in the new discrete-dipole approximation (DDA) and compared with the data obtained in the commonly used Rayleigh-Gans approximation (RGA) and in the anomalous-diffraction approximation (ADA). The errors of calculations by the above mentioned methods for droplets with a homogeneous and for droplets with a radial director configuration are considered. The results of calculations of the optical properties of droplets with a radial and with a bipolar director configuration in the DDA are presented. Based on the results obtained the potentialities of the above methods for describing the scattering by liquid crystal droplets are illustrated.

ISSN:0267-8292    

DOI:10.1080/026782998205895

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor