摘要:

We report here a detailed study of the propagation of longitudinal ultrasound through a magnetically aligned cholesteric liquid crystal (a mixture of cholesteryl chloride and cholesteryl myristate, 1.75: 1 by weight). The observed velocity anisotropy is tentatively attributed to dispersion and an attenuation anisotropy was observed for the first time. The temperature dependence of several elastic constants and combinations of viscosity coefficients was determined. A new expression for the attenuation was derived from the hydrodynamic equations, and from a twisted nematic model. A more detailed report of the acoustic Brillouin zone effect is also presented.

ISSN:0026-8941    

DOI:10.1080/00268947808084964

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

An optical microscopic system has been employed to measure the diffusion lengths of singlet excitons in pure and doped anthracene crystals by following the quenching of the anthracene fluorescence by a dye solution. Factors such as the inhomogeniety of dye uptake across the surfaces, the decay of the dye emission with time and correction for reflectivity at the anthracenesolution interface have been accounted for. The diffusion lengths measured-normal toab(001) cleavage sections-are very dependent upon the quality of the crystal surface and the purity of the samples and range from 322 ± 80 Å in acridine doped crystals to 598 ± 51 Å in pure specimens.

ISSN:0026-8941    

DOI:10.1080/00268947808084965

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

A study of the supercooling of the isotropic to nematic transition in the homologous neniatogensp-azoxyanisole (PAA) and heptyloxyazoxybenzene (HOAOB) is reported. Differential scanning calorimeter thermograms are consistent with a small supercooling ∼0.01°K. Observations of the nematic-isotropic phase boundary by optical microscopy reveals the development of a dendritic-like growth front with increasing rate of transformation to the nematic phase. This is also consistent with a supercooling -0.01°K. The boundary surfaces induce nematogenic alignment and therefore act as heterogeneous nucleation sites. Nucleation theory is used to show that if heterogeneous nucleation could be eliminated, the observable supercooling would be no more than 0.1(To-T*), whereTois the nematic-isotropic transition temperature andT*is the pseudo-second order transition temperature.

ISSN:0026-8941    

DOI:10.1080/00268947808084966

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

dl-Camphene has been ultra-purified and the process of self-diffusion in well-annealed crystals studied as a function of added dopants and temperature using radiotracer and plastic deformation techniques.

ISSN:0026-8941    

DOI:10.1080/00268947808084967

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

The densities of cholesteryl myristate were measured in the smectic, cholesteric, blue and isotropic phase. At the transitions smectic/cholesteric and bluelisotropic within the temperature interval of 0.01 to 0.02°K discontinuities in the density curves occured. which indicate phase transitions of first order. At the transition cholesteric/blue only a change in the expansion coefficients was observed. which points to a phase transition of second order.

ISSN:0026-8941    

DOI:10.1080/00268947808084968

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

Solid state polymorphism ofp-azoxyanisole has been studied by different experimental methods. A good agreement of differential scanning calorimetry, polarizing microscope, far infrared spectroscopy and neutron diffraction results permitted three solid modifications to be identified.

ISSN:0026-8941    

DOI:10.1080/00268947808084969

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

A homologous series (I) of mesomorphic Schiff bases is synthesized by condensingp-n-alkoxybenzaldehydes withp-bromoaniline.

ISSN:0026-8941    

DOI:10.1080/00268947808084970

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

Molecular and lattice symmetry effects on the order of plastic crystal-crystal phase transitions are discussed in the context of Landau theory.

ISSN:0026-8941    

DOI:10.1080/00268947808084971

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

The dependence of helical pitch on composition and its phase diagram in many nematiccholesteric mixtures was measured. It has been known about the nematic-cholesteric mixtures that the reciprocal pitch versus composition curve deviates considerably from a straight line. In this paper, the deviation is discussed in relation to the molecular structure of components and smectic thermal stability in the mixtures. It is concluded that a lateral intermolecular interaction plays an important role in the helical pitch of nematic-cholesteric mixtures.

ISSN:0026-8941    

DOI:10.1080/00268947808084972

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

Phase transitions of pyrazine crystal have been studied by calorimetry, X-ray diffraction and low frequency vibrational spectroscopy. Three phases have been shown to exist: phase I stable up to 28.5°C. phase I1 between 28.5 and 37°C and phase III between 37 and 51°C. Phase III is characterized by the space group Cmmm and its unit cell (Z = 8) is the quadruple of that of phase I. Phase I1 is similar to phase III and the most significant spectroscopic feature of the former is the presence of two Raman bands at 23 and 10 cm-1. The lattice modes of phase 1 and the librational amplitude have been calculated. The amplitudes are very large and amount up to 10° for the motions about thex-axis. They may play a major role in the mechanism of phase transitions and are likely to be responsible for some disagreements between the calculated and observed lattice frequencies.

ISSN:0026-8941    

DOI:10.1080/00268947808084973

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor