Dichroism Of Absorption Bands Research Articles

Spectral features of a nematic liquid crystal consisting of biaxial molecules with internal rotation

Expressions for the components of the optical permittivity tensor of a nematic liquid crystal consisting of biaxial π-conjugated molecules with a conformational degree of freedom (internal rotation) are obtained. A relationship between the intensity and dichroism of absorption bands in isotropic and nematic phases and the parameters of conformational, orientational, and mixed conformational and orientational orders of the molecules is established. A distribution function for the molecules is obtained that takes into account the mutual correlation of their conformational and orientational degrees of freedom. The effect of this correlation on the above-noted order parameters, the dichroism of the absorption bands, and the dependence of the measured oscillator strengths of molecular transitions on the character and degree of orientational order of biaxial molecules is studied. On the basis of comparison with experimental data, the relative role of different terms in the distribution function that are responsible for the correlation between the conformational and orientational degrees of freedom of the molecules is ascertained.

Dichroic Dyes for Liquid Crystal Devices

Abstract Using data on geometry of molecules and direction of electronic oscillator a large series of dichroic dyes have been synthesized exhibiting positive, negative or zero dichroism, as well as two colour dichroic dyes with opposite signs of dichroism of absorption bands. Ways of obtaining dichroic dyes with a higher solubility have been proposed.

The Structure of oriented films of polyamidoimides and polyester amidoimides

Methods of X-ray analysis, IR spectroscopy and thermomechanical analysis have been used to investigate the structure of oriented polyamidoimides (PAI) and polyester amidoimides (PEAI) differing as to the structure of the diamine fragments. It is shown that heat treatment of polyamidoacid films with 150–250% elongation is accompanied by a marked change in the dichroism of absorption bands in the temperature interval 270–300° for polymers having phenylene in the meta-position. In this case it appears from the X-ray data that a highly oriented crystalline or mesomorphic structure is formed. Crystallographic parameters were determined for the ordered structures and conformational structures for the PAI and PEAI are proposed. For PAI containing the p-phenylene unit in the diamine the energetically most favourable packing of chains in the crystal lattice was calculated on the basis of atom-atom potentials.

Dichroism of absorption bands of planar molecules oriented in films of transparent polymers. Decomposition of absorption spectra of molecules of high symmetry into molecular components

Dichroism of absorption bands of planar molecules oriented in films of transparent polymers. Decomposition of absorption spectra of molecules of high symmetry into molecular components

Investigation of physical state and phase behavior of polyester‐imides

AbstractChanges in properties of linear aromatic heterocyclic polymers of the polyimide class after introduction of ester linkages into their dianhydride component and of the oxygen and sulfur atoms into their diamine component have been studied. Thermomechanical, dilatometric, thermographic characteristics and x‐ray diffraction patterns of poly(ester amic acids) and polyester‐imides have been investigated. On heating, poly(ester amic acids) and amorphous polyesterimides exhibited a peculiar effect of self‐orientation of macromolecules. The x‐ray diffraction patterns of unoriented films indicate that they exhibit axial‐planar texture with the c axis of macromolecules in the film plane. The formation of the texture is preceded by mutual arrangement of macromolecules, i.e., by a precrystallization process beginning in the amorphous state. Because of this, on heating at temperatures ranging from 200 to 380°C, polyester‐imides yield highly crystalline products. Crystallographic parameters of the polymer lattices on x‐ray diffraction patterns and dichroism of absorption bands in polarized infrared spectra, the most probable conformations of macromolecules of some polyesterimides are proposed.

Chain‐backbone motion in glassy polycarbonate studied by polarized infrared spectroscopy

AbstractChain‐backbone motion in glassy polycarbonate has been investigated both under isothermal stress, and also under zero stress during isothermal annealing of freely contracting film specimens. In both types of experiment, backbone motion was detected by measuring the change in infrared dichroism. The dichroism of absorption bands at 1364 and 2971 cm−1, which have transition moment vectors directly related to the chain‐backbone orientation, was studied. Under tensile stress in the homogeneous region of deformation, changes of up to 2.2° in the mean chain‐backbone orientation angle were measured at 23°C. With the onset of cold drawing a total orientation change of some 8° was observed. For the isothermal annealing experiments, a film specimen holder employing conductive heating with radiative losses was employed. It enables infrared measurements to be made while the temperature of the contracting specimen is maintained constant to ± 0.5°C. Oriented specimens were prepared by isothermal stretching of polycarbonate films to strains of the order of 100%. Changes in the mean chain‐backbone orientation angle were observed during annealing of these oriented films at temperatures between 80°C and the glass transition (149°C). Chain motion proceeded during annealing, and chain segments were observed to move cooperatively. The temperature at which the polymer is prestretched has a pronounced effect on its subsequent relaxation during annealing: when the sample was stretched at 23°C. motions were detected during annealing at temperatures as low as 81°C, while, if it was stretched at 154°C, no motion was detected at annealing temperatures below 127°C. The data are discussed in comparison with theories of the glassy state that predict the absence of chain‐backbone motion at temperatures significantly below the glass transition. A shift in frequency of the νa (CH3) absorption peak in stretched polycarbonate was measured by using polarized radiation. The effect was interpreted in terms of changes in the intermolecular bonding structure of the oriented polymer.