Dielectric properties, of condensed

The purpose of this Chapter is to describe the dielectric properties of liquid crystals, and relate them to the relevant molecular properties. In order to do this, account must be taken of the orientational order of liquid crystal molecules, their number density and any interactions between molecules which influence molecular properties. Dielectric properties measure the response of a charge-free system to an applied electric field, and are a probe of molecular polarizability and dipole moment. Interactions between dipoles are of long range, and cannot be discounted in the molecular interpretation of the dielectric properties of condensed fluids, and so the theories for these properties are more complicated than for magnetic or optical properties. The dielectric behavior of liquid crystals reflects the collective response of mesogens as well as their molecular properties, and there is a coupling between the macroscopic polarization and the molecular response through the internal electric field. Consequently, the molecular description of the dielectric properties of liquid crystals phases requires the specification of the internal electric field in anisotropic media which is difficult. 

Potential energy surfaces of weakly bound dimers and trimers are the key quantities needed to compute transition frequencies in the high resolution spectra, (differential and integral) scattering cross sections or rate coefficients describing collisional processes between the molecules, or some thermodynamic properties needed to derive equations of state for condensed phases. However, some other quantities governed by weak intermolecular forces are needed to describe intensities in the spectra or, more generally, infrared and Raman spectra of unbound (collisional complexes) of two molecules, and dielectric and refractive properties of condensed phases. These are the interaction-induced (or collision-induced) dipole moments and polarizabilities. 

Microwave irradiation has been used for promoting the reaction of isatin with malononitrile to give 3-dicyanomethyleneoxindole and gives better results in comparison to the usual method422. The dielectric properties of this condensation product have been studied423. 

It is well-known that the simplest approach to the study of the optical properties of condensed media is the macroscopic electrodynamics approach, making use of the concept of the dielectric constant tensor f..tJ (oj, k) where w and k are the frequency and the wavevector of the light wave. Calculation of this tensor for a specific medium is, however, a problem of microscopic theory. For instance, the procedures for calculating the tensor k) for the excitation region of the... 

The dielectric properties of a material are determined by the polarizability of its molecules. There are three primary contributions to the electric polarization of a dielectrics electronic, ionic and dipole reorientation - related (Uchino, 2000). The intensity with which each mechanism occurs depends on the frequency of applied electric field. The electronic polarization causes a displacement of the electrons with respect to the atomic nuclei and can follow alternating field with the frequencies up to - lOi Hz. The ionic polarization relies on a displacement of the atomic nuclei relative to one another and responds up to lO - lO Hz. Both mentioned polarization mechanisms are related to the non-polar molecules. The third mechanism associated with the dipole reorientation is valid only in the case of polar molecules. It can follow with the frequency of alternating electric field up to 10 - lO Hz. The dielectric permittivity of a material represents the ratio of the capacitance of a plane condenser filled with the dielectric to that of the same condenser under vacuum and is to calculate from the expression ... 

The structural features and properties of condensed matter in nanophases open new possibilities of the application of nanocrystals not only in microelectronics and electrotechnics (e.g. for the development of supercapacitors [106] where large e is required) but also for the solution of fundamentals question of structural chemistry, e.g. how the electronic structure of ionic compounds changes in an environment with colossal dielectric permittivity. This change can result in weakening the electrolytic dissociation even to the point of transition into the molecular state (because the Coulomb interaction is inversely proportional to e) and for the same reason, to make salts soluble in organic media. Here, we have concentrated on the experimental results, the theoretical explanation of which is still required. 

Recently. Ohishi el al. [26] measured dielectric properties of aliphatic polylhiourcas which were synthesized by thermal condensation of diamine with carbon disulfide as shown below. Polymerization of carbon disulfide and ali 4iatic diamine polymer Is called polythiourea-Af. where AT is the number of Of, groiqis. 

In experiments, the percolation transition of hydration water can be detected by conductivity and dielectric measurements. Formation of a condensed hydrogen-bonded network of water should provide a media for the charge transport (proton or ions) and should change qualitatively the dielectric properties of the system. Sharp stepwise increase of the conductivity of the system with increasing water content at some threshold hydration level may directly indicate the appearance of an infinite hydrogen-bonded water network via a percolation transition. The dielectric response is also expected to increase drastically at the percolation threshold. Note, however, that the strongly attractive sites on the surface, which immobilize water molecules, may complicate interpretation of the results. As these effects occur on the surfaces, their experimental observation is possible first in the system with high surface/volume ratio (in various porous media). 

Bill Baker rose from a bright graduate of Princeton to one of the most revered men in American science and industry. He retired as Chairman of the Board of AT T Bell Laboratories in 1980. He worked with Charles P. Smyth at Princeton on the dielectric properties of organic crystals. He joined Bell Laboratories in 1939, after he received his Ph.D. in physical chemistry, and quickly applied his deep knowledge of dielectrics to problems in polymer science. His solid state perspective on polymers helped Bell Labs to become the pre-eminent research center in the world in condensed matter materials science. 

The presence of phenol hydroxx gi-oups in Xox olaks is indicated by Koebner s finding that these products are soluble in alcoholic sodium 1 "-droxide. The dielectric properties of thermo-plastic phenolics is repoi-ted to be due to the rotation of these polar hydroxy gi oups . Meyer s studies on phenol-resin structures led him to the conclusion that the Xovolaks obtained by acid condensation of phenol and formaldehyde have terminal P-H0CbH4- groups. 

Two parallel plates of conducting material separated by an insulation material, called the dielectric, constitutes an electrical condenser. The two plates may be electrically charged by connecting them to a source of direct current potential. The amount of electrical energy that can be stored in this manner is called the capacitance of the condenser, and is a function of the voltage, area of the plates, thickness of the dielectric, and the characteristic property of the dielectric material called dielectric constant. 

These properties are sometimes grouped as the dielectric properties but this is not entirely logical as dielectric simply means insulating. Relative permittivity of a material can, for practical purposes, be defined as the ratio of the capacitance of a condenser having the material as the dielectric to the capacitance of a similar condenser having air, or more precisely, a vacuum as the dielectric. The word relative is usually dropped and the property simply called permittivity and is the same thing as used to be called dielectric constant (this term is apparently still used in the USA). 

The dielectric constant varies with coal rank (Chatterjee and Misra, 1989). The theorem that the dielectric constant is equal to the square of the refractive index (which is valid for nonconducting, nonpolar substances) holds only for coal at the minimum dielectric constant. The decreasing value of dielectric constant with rank may be due to the loss of polar functional groups (such as hydroxyl or carboxylic acid functions), but the role of the presence of polarizable electrons (associated with condensed aromatic systems) is not fully known. It also appears that the presence of intrinsic water in coal has a strong influence on the dielectric properties (Chatterjee and Misra, 1989). 

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