摘要:

An improved method for determining the bulk refractive indices of mesophases by using a polarizer at the ocular of an Abbe refractometer is presented.

ISSN:0026-8941    

DOI:10.1080/15421407408083399

出版商:Taylor & Francis Group    

年代:1974

数据来源: Taylor

摘要:

Measurements of the proton spin-lattice relaxation time in the solid phases of ethyl-[p-(p-methoxybenzilidene)-amino]-cinnamate (EMC) are reported. A model for the non-exponential relaxation observed is discussed as well as mathematical technique for treating the experimental data. This model, based on the assumption of the coexistence of two non-interacting groups of indentical spins, was tested using a model liquid-solid system. Results of the measurements on EMC indicate that the two possible solid phases coexist at low temperatures and that a slow annealing takes place near room temperature. The dominant spin-lattice relaxation mechanism at low temperatures is determined to be a rotational motion with an activation energy of ∼ 2.3 kcal/mole.

ISSN:0026-8941    

DOI:10.1080/15421407408083400

出版商:Taylor & Francis Group    

年代:1974

数据来源: Taylor

摘要:

Several time-dependent electro-optical properties of a cholesteric-nematic mixture have been investigated. In particular, the dynamics of the Grandjean focal conic texture transitions have been characterized. A cholesteric-nematic mixture was used comprising molecules having a net tendency to align with their long axes perpendicular to the applied field, allowing the study of the field induced focal conic →→ Grandjean transition. D.C. electro-hydrodynamic effects were employed in the investigation of the Grandjean →→ focal conic transition. The driven and relaxation properties of the electro-hydrodynamic induced transition are described. Also, the memory properties of intermediate textures of this transition are discussed. Thickness is found to play a crucial role in the relaxation of the textures. The time to achieve the electro-hydrodynamic induced texture transition (Grandjean →→ focal conic) is found to be controlled in a non-linear fashion by applied voltage and only weakly by current. For the field induced focal conic →→ Grandjean transition, it is determined that the transition time varies inversly with the cube of the applied voltage in the material studied. Furthermore, the transition time is found to be a strong function of sample composition, increasing rapidly with cholesteric content.

ISSN:0026-8941    

DOI:10.1080/15421407408083401

出版商:Taylor & Francis Group    

年代:1974

数据来源: Taylor

摘要:

Measurements are reported of the spectral yield and other characteristics of the photocurrent arising from photoexcitation of holes out of bulk traps into the valence band of anthracene. Vapour-grown crystals were found to contain two main sets of hole traps: an essentially discrete shallow set, about 1 eV above the valence-band edge and present with a density of ∼ 1011cm−3, and a deeper set lying 1.8–2.1 eV above the valence-band edge with a density of ∼ 5×1012cm−3. The trap density in meltgrown samples is one to two orders of magnitude lower.

ISSN:0026-8941    

DOI:10.1080/15421407408083402

出版商:Taylor & Francis Group    

年代:1974

数据来源: Taylor

摘要:

Measurements of the delayed-fluorescence excitation spectrum in anthracene could so far yield only the relative variation of the singlet-triplet absorption coefficient α with wavelength. In the present work the absolute magnitude of α is derived from studies involving the interaction of photo-generated triplet excitons with trapped electrons introduced into the sample by contact injection. Such an interaction results in the liberation of trapped electrons on the one hand and in the quenching of the triplet-exciton lifetime on the other. The former process is studied by photocurrent measurements while the latter is monitored by triplet lifetime measurements. The value of α so determined is (1.2, ± 0.2) × 10−3cm−1at the first singlet-triplet absorption peak (6780 A).

ISSN:0026-8941    

DOI:10.1080/15421407408083403

出版商:Taylor & Francis Group    

年代:1974

数据来源: Taylor

摘要:

It is found that a suspension of microcrystals of chlorophyll-a (Chl-a) in a nonpolar medium may be electrodeposited as a film on a cathode. Spectroscopic properties of the film indicate that it is quite uniform and consists of randomly oriented microcrystals. Upon heating, the electrodeposited microcrystalline film is transformed to an alternate form of aggregated Chl-a. This new film appears to resemble very closely the Chl-a films previously obtained in monolayer work. Either film may be dissolved into monomer form and it is seen how Chl-a monomer is conserved during the complete process proceeding from electrodeposition to thermal conversion and the final dissolution. Furthermore, it is found how the oscillator strength over the red-visible region is the same for the dissolved monomer and the second type of film. The simple excited state resonance model for the spectrum of the latter is thus supported. The oscillator strength of the microcrystal film, by contrast, is about 40% greater. Such electrodeposited films are quite stable at room temperature and can serve well as samples in a variety of photoelectric studies.

ISSN:0026-8941    

DOI:10.1080/15421407408083404

出版商:Taylor & Francis Group    

年代:1974

数据来源: Taylor

摘要:

Les spectres de fluorescence directe et retardée du cristal de naphtalène ont été étudiés entre 77° K et 300° K, sous différentes excitations. Le naphtalène soumis à des traitemen ts mécaniques (broyage de l'échantillon), fait apparaitre de nouvelles raies et bandes attribuées à des traces de pyrène et aux déformations du réseau qu'elles provoquent.

ISSN:0026-8941    

DOI:10.1080/15421407408083405

出版商:Taylor & Francis Group    

年代:1974

数据来源: Taylor

摘要:

The solid phases of cholesteryl acetate were studied by x-ray diffraction techniques. The data were analyzed in terms of the angular position of the reflections, their intensity, and their integral width. The unit cell is monoclinic with the parameters a, b, c, and β respectively equal to 10.00 A, 7.36 A, 37.2 A and 95°. The space group is either c2 or cm. The dimensions of the unit cell are the same for all three solid phases. Temperature dependent measurements show that the broad melting range of the transition into the isotropic phase is not a true melting range. Increasing lack of short range order occurring along the direction of the long axis of the molecule causes an increase of the expansion coefficient in that range.

ISSN:0026-8941    

DOI:10.1080/15421407408083406

出版商:Taylor & Francis Group    

年代:1974

数据来源: Taylor

摘要:

During the past few years, interest in reactions of molecules organized within a mesophase has developed considerably. In particular, an increasing number of publications describes the synthesis and polymerization of mesomorphic vinyl monomers. Vinyl oleate, for example, has been polymerized in its smectic state by Amerik and Krentzel.1Hardy and coworkers2have studied the kinetics of polymerization of cholesteryl acrylate in the smectic state and Paleos and Labes3have reported the kinetics of polymerization of N-(p-methoxy-O-hydroxybenzylidene)p-aminostyrene in its nematic state. Other reports of polymerization of cholesteryl acrylate and methacrylate4,5,6,7also been published and recently Strzelecki and Lieberts have reported the synthesis and polymerization of various monomeric Schiff bases in their liquid crystalline state.

ISSN:0026-8941    

DOI:10.1080/15421407408083407

出版商:Taylor & Francis Group    

年代:1974

数据来源: Taylor

摘要:

An experimental approach has been devised which permits the study of the effect of nonmesomorphic solutes of different sizes and shapes on nematic phase stability. The apparatus consists of a vacuum microbalance system which employs a Cahn RG Electrobalance to determine the weight of volatile solute absorbed by a known weight of nematic material at a given temperature and solute partial pressure. From such measurements, solute activity coefficients (γ2) were determined as a function of solute mole fraction (x2) at several set temperatures (T) below the nematic-isotropic transition temperature (61.2°C) of the pure nematogenp-methoxybenzylidene-p'-n-propylaniline (MBPA). The solute probes studied weren-heptane, benzene and carbon tetrachloride. For all three systems the observed behavior was: In γ2 decreased sharply with increasingx2in the nematic region, became discontinuous at a mole fraction x2corresponding to the solute induced transition point, and then decreased gradually (approaching zero) with increasingx2in the isotropic region. From studies at differentTvalues, dT/d-x2was determined for all three solutes. These experimental results are discussed in terms of the solute molecular characteristics and in light of recent theoretical results obtained from a statistical mechanical lattice model of a two component mixture of non-interacting rigid rods of different size. This study provides additional evidence of the important role of repulsive forces in governing nematic stability.

ISSN:0026-8941    

DOI:10.1080/15421407408083408

出版商:Taylor & Francis Group    

年代:1974

数据来源: Taylor