Dipolar process orientation polarization

Relaxation processes are probably the most important of the interactions between electric fields and matter. Debye [6] extended the Langevin theory of dipole orientation in a constant field to the case of a varying field. He showed that the Boltzmann factor of the Langevin theory becomes a time-dependent weighting factor. When a steady electric field is applied to a dielectric the distortion polarization, PDisior, will be established very quickly - we can say instantaneously compared with time intervals of interest. But the remaining dipolar part of the polarization (orientation polarization, Porient) takes time to reach its equilibrium value. When the polarization becomes complex, the permittivity must also become complex, as shown by Eq. (5) ... 

A dipolar process is described in terms of three quantities the orientation polarization, the vibrational polarization, and the relaxation time. The first of the three is expressed as A =85-8oc, where is as defined earlier, and is the limiting high-frequency permittivity. The vibrational polarization is expressed as A8vib=eoo- op. where op is the refractive index of the material, usually at the... 

The reaction dipole moment zfM of a dipolar equilibrium may be obtained from the measurement of continuum properties such as the dielectric permittivity as well as from direct monitoring of concentration shifts produced by an externally applied electric field. In both approaches to reaction properties it is primarily the chemical part of the total polarization that is aimed at. However, the chemical processes are intimately connected with the physical processes of polarization and dipole rotation. In the case of small molecules the orientational relaxations are usually rapid compared to the diffusion limited chemical reactions. When, however, macromolecular structures are involved, the rotational processes of the macromolecular dipoles may control a major part of the chemical relaxations. Two types of processes may be involved if a vectorial perturbation like an external electric field is applied a chemical concentration change and a change in the orientation of the reaction partners. 

For mechanical excitations, changes of molecular orientations are mainly involved in energy dissipation and storage. However, an electric field acts on the orientation of dipolar segments (OR polarization) and the deformation of the electron shell (DEF polarization). The latter has an extremely short relaxation time and contributes to dissipative processes at very high frequencies only. For low frequency experiments, "def = 0. It contributes to the permittivity e = e oR -H def and thus reduces tan Sg. 

The electrostatic polarization theory is commonly employed to describe ER response. The model assumes that ER fluids are dispersions of nonionic polarizable particles in a low dielectric medium and that free charges and charge-transfer electrochemical processes can be neglected. This model is based on the fact that, due to the permittivity mismatch between the particles 6p and the continuous phase e, the dipolar particles are polarized and aligned with the neighboring particles. When an electric field is superimposed on the point dipole interaction, the orientation of the dipoles in relation to the exter-... 

An alternative method of studying the molecular motions of a polymeric chain is to measure the complex permitivity of the sample, mounted as dielectric of a capacitor and subjected to a sinusoidal voltage, which produces polarization of the sample macromolecules. The storage and loss factor of the complex permitivity are related to the dipolar orientations and the corresponding motional processes. The application of the dielectric thermal analysis (DETA) is obviously limited to macromolecules possessing heteroatomic dipoles but, on the other hand, it allows a range of frequency measurement much wider than DMTA and its theoretical foundations are better established. 

In the spin-correlated RP the two radicals interact via electron-electron dipolar and exchange interaction which leads to line splitting. The ET process creates the RP in a strongly spin-polarized state with a characteristic intensity pattern of the lines that occur either in enhanced absorption (A) or emission (E).144 145 The spectrum is therefore very intense and can directly be observed with cw EPR (transient EPR) or by pulse methods (field-swept ESE).14 To study the RPs high field EPR with its increased Zeeman resolution proved to be very useful the first experiment on an RP was performed by Prisner et al. in 1995146. From the analysis of the RP structure detailed information about the relative orientation of the two radicals can be extracted from the interaction parameters. In addition kinetic information about the formation and decay of the RP and the polarization are available (see references 145,147). 

Theoretical formulations of reorganization in the course of electron-transfer processes have undergone a number of advances in recent years. The relative importance of various solvent contributions (including translational as well as orientational response, and inductive and dispersion as well as elecrostatic interactions) can depend strongly on the polarity (i.e., dipolar, higher multipolar, or nonpolar) as well as other molecular features of the solvent [21, 47-49]. Molecular-level perspectives on solvent response are of great utility in helping to parameterize effective cavity models (e.g., in conjunction with conventional [50] or spatially nonlocal [47] dielectric models). Additivity relationships traditionally assumed to pertain to sol-... 

Phase diagrams for strongly polar molecules in adsorbed films are still in the process of development even for the films on the basal plane of graphite [35]. These systems are made more complex because of the interplay of dipolar forces and molecular shape in determining preferred orientations relative to the surface and to neighboring molecules. A simulation of Stockmayer molecules (Lennard-Jones atoms with ideal dipoles attached) adsorbed on a featureless slit pore at low temperature [46] has shown that the dipoles tend to lie parallel to the surface in... 

Dumont and coworkers [136, 155, 156] have observed that shining doped (or functionalized) polymer thin hlms with noncentrosymmetric dipolar chromo-phores, induces a significant increase of electro-optic coefficient in the chromo-phore absorption band, corresponding to a better, polar orientation of chromo-phores. The measurements have been done by using the attenuated total reflection technique, and the optical field polarization was perpendicular to the applied low-frequency external electric field to the thin film (Fig. 33). A better stability of induced orientation was observed in the case of functionalized polymers than in guest-host system, as is usually the case with the static field poled polymers. The chromophores orient with dipolar moments perpendicular to the optical field (and parallel to the applied static (or low frequency) field. As will be discussed later, the chromophore orientation undergoes a trans-cis isomerization process (Fig. 34). 

The SoSoo term corresponds to the instantaneous response of the material to the electric field, whereas the so(s,-8oo) (t) term is related to the slower response assigned to dipolar polarization, where the dielectric function 0(0 describes the temporal development of the dipole orientation. The decay function, < )(0=1-O(0, accoimts for the decrease of polarization after removing the electric field, ( )(0)=1 and < >(oo)=0. In the model of Debye the polarization process follows a first-order kinetics, where its time variation is proportional to the equilibrium value ... 

In practice, electrical measurements do not distinguish between conduction and polarization currents, because only the total current that appears in Maxwell s equation, idPIdt), is measured. The conduction cunent Jo and the polarization current (dPIdt) need to be separated into the electrode polarization, dc conduction, and dipolar orientation contributions by using suitable procedures. This is partieularly the case for a majority of highly conducting suprainolecular structures of all types at ambient temperatures, with s and e" data that do not show features directly attributable to a molecular dielectric process. In such cases, Eqs. 16 and 17 are used to determine contributions from electrode polarization and dc conductivity. On subtraction from the measured s and... 

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