摘要:

Experimental and conceptual problems associated with the identification of thermotropic mesophases are reviewed and discussed. Theories linking the observed optical properties of mesophases with their molecular building principle have considerable shortcomings. Therefore the identification of mesophases is mainly based on empirical rules pertaining to the optical behavior of characteristic textures. Certain ambiguities of a strictly optical approach may only challenge the ingenuity of the investigator, while other difficulties can prevent the determination of mesomesophase and liquid-mesophase transition temperatures. Since phase transitions are defined by variations of more general thermodynamic quantities, thermal analysis is suggested as a remedy. The requirements on temperature scanning thermal analysis necessary to insure detection of all mesophases of a given sample are specified. The significance and the determination of sample impurities by themal analysis are discussed. Comments are made on the correlation between characteristics textures and mesophases. The relevancy of uniaxial optical properties to phase identification and the homeotropic conditions of cholestreic mesophases are considered briefly.

ISSN:0369-1152    

DOI:10.1080/15421406808082889

出版商:Taylor & Francis Group    

年代:1968

数据来源: Taylor

摘要:

A spectroscopic and photoelectric study is made of matrix isolated crystallites of phenanthrene in rigid 3-methylpentane (3-MP) at 77°K. The crystallites appear to be optically thin and exhibit persistent internal polarizaton (PIP) following photosensitization in the phenanthrene absorption region. A variety of studies including lux-ampere experiments strongly support the picture of a bi-excitonic primary charge generation. The charges exhibit lengthy storage properties. They may be directly excited in the near infrared or they may also be excited in the phenanthrene absorption region, the latter probably by an exciton collision. In either case typical polarization and polarization release curves are generated. A small steady photocurrent is also seen which corresponds to electrons passing through the 3-MP matrix. Changes in viscosity of the matrix alters the mobility of the carriers associated with the crystallites. Lux-ampere studies of the peak of the polarization curve show good agreement with the predictions of Rose for an exponential trap distribution. A simple electrical analogue model is developed which fits the polarization decay curve with one parameter and predicts correclty the total accumulated charge during the polarization. The model is based on a specific distribution of charge carrier mobilities suggested by a random orientation of anisotropic crystallites in the applied field. It is suggested that this system may serve as a prototype study for photoelectric examination of photosensitive units of small dimensions.

ISSN:0369-1152    

DOI:10.1080/15421406808082890

出版商:Taylor & Francis Group    

年代:1968

数据来源: Taylor

摘要:

The problem of recombination in molecular crystals and liquids is considered from the point of view of continuity of current. The probability for escape of an electron from its parent molecule depends exponentially upon how far it travels before thermalization. When account is taken of the temperature dependence of this distance, good agreement with experiment is obtained. Comparison between β particle and photo excitation suggests that initial back recombination between the emitted electron and the parent molecule may not be important in photoconductivity.

ISSN:0369-1152    

DOI:10.1080/15421406808082891

出版商:Taylor & Francis Group    

年代:1968

数据来源: Taylor

摘要:

The principal molecular polarizabilities of some molecules of the 4, 4′-di-n-alkyloxy-azobenzenes and of the 4, 4′-di-n-alkyloxy-azoxy-benzenes are calculated of the high frequency dielectric constants ϵ1∞and ϵ2∞of the magnetically oriented nematic liquid crystals. The increases of the principal polarizabilities of such molecules in complete trans-forms by adding CH2groups to the alkyl chains are known from an estimation. By comparing these values to the differences between the principal polarizabilities of the investigated molecules it follows that the molecules in the liquid crystals are in a stretched form.

ISSN:0369-1152    

DOI:10.1080/15421406808082892

出版商:Taylor & Francis Group    

年代:1968

数据来源: Taylor

摘要:

The rotational diffusion rates of molecules and of mobile molecule segments in crystalline solids are expressed as relaxation rates relative to the corresponding torsional oscillation rate of the molecule or molecule segment. The resulting dimensionless relaxation time τi(ΔHs/Ii)1/2turns out to be a function of packing density (*) of the crystal and of the characteristic temperaturefRT/ΔHs, wherefis the number of external degrees of freedom per molecule. The dimensionless activation energy ΔE/ΔHsis a uniform function of the packing density for crystals composed of rigid molecules and another function of * for crystals composed of flexible molecules. The rate determining factor for molecular rotation in crystals containing hydrogen bonds appears to be the breaking energy of the hydrogen bond.

ISSN:0369-1152    

DOI:10.1080/15421406808082893

出版商:Taylor & Francis Group    

年代:1968

数据来源: Taylor

摘要:

The nemotogenic compoundp-methoxybenzylidene-p'-cyanoaniline in the nematic and isotropic liquid states has been studied by X-ray diffraction. A structure is proposed by comparing calculated intensities of a model with experimental diffraction intensities. The structure is comparable to the “herringbone” packing observed in many organic crystals. The procedure presented in this paper illustrates the practicability of using the method of comparison of experimental intensities with those calculated for a particular structural model. Comparisons of the experimental X-ray diffraction intensities and the radial distribution functions calculated from the intensities at different temperatures indicate an essentially unchanged structure throughout the nematic range but a gross structural change upon passing to the isotropic liquid state.

ISSN:0369-1152    

DOI:10.1080/15421406808082894

出版商:Taylor & Francis Group    

年代:1968

数据来源: Taylor

摘要:

s-Triazine has been counted an exception to the fairly general rule taht aromatic compounds with threefold molecular symmetry occupy general positions in the crystal. It is shown that a low temperature phase ofs-triazine exists which conforms with this rule. The transition occurs over a large temperature range below −60° C.

ISSN:0369-1152    

DOI:10.1080/15421406808082895

出版商:Taylor & Francis Group    

年代:1968

数据来源: Taylor