Dielectric Polarization Mechanisms

The dielectric behaviour of proteins in aqueous solutions was first extensively studied by Ondey and co-workers. They interpreted the data in terms of rotational polarization of permanent dipole moments. The latter were found to be in the range of 100—1000 D (1 D = 10- e.s.u.) while the relaxation times came out at ca. 10" s. Despite some ta-itidsm, the preferential-orientation effect must still be considered the prindpal dielectric-polarization mechanism of proteins. - This view is also supported by dieledric dispersion studies of various proteins in solvents of different viscosity. The measured relaxation times were indeed proportional to rj as predided by (29) and (30). Nevertheless, for very large molecules (M, > 10 ) indications of other mechanisms, whose relaxation does not depend on the bulk viscosity of the solvent, have been observed. ... 

Principles in Processing Materials. In most practical apphcations of microwave power, the material to be processed is adequately specified in terms of its dielectric permittivity and conductivity. The permittivity is generally taken as complex to reflect loss mechanisms of the dielectric polarization process the conductivity may be specified separately to designate free carriers. Eor simplicity, it is common to lump ah. loss or absorption processes under one constitutive parameter (20) which can be alternatively labeled a conductivity, <7, or an imaginary part of the complex dielectric constant, S, as expressed in the foUowing equations for complex permittivity ... 

The observed dielectric constant M and the dielectric loss factor k = k tan S are defined by the charge displacement characteristics of the ceramic ie, the movement of charged species within the material in response to the appHed electric field. Discussion of polarization mechanisms is available (1). 

Because of very high dielectric constants k > 20, 000), lead-based relaxor ferroelectrics, Pb(B, B2)02, where B is typically a low valence cation and B2 is a high valence cation, have been iavestigated for multilayer capacitor appHcations. Relaxor ferroelectrics are dielectric materials that display frequency dependent dielectric constant versus temperature behavior near the Curie transition. Dielectric properties result from the compositional disorder ia the B and B2 cation distribution and the associated dipolar and ferroelectric polarization mechanisms. Close control of the processiag conditions is requited for property optimization. Capacitor compositions are often based on lead magnesium niobate (PMN), Pb(Mg2 3Nb2 3)02, and lead ziac niobate (PZN), Pb(Zn 3Nb2 3)03. 

When a voltage is applied to a dielectric (insulator), a current passes that decays with time owing to various polarization mechanisms [ 133]. Conductivity is always time-dependent. This general time dependency affects conductivity measurement for nonconductive liquids, where the peak initial current is used to calculate conductivity. Test methods are given in 3-5.5 and... 

Aryl and, more so, chlorine substituents on silicon enhance thermal stability of silacyclobutanes. The rate of the first-order thermal decomposition of silacyclobutanes varies inversely with the dielectric constant of the solvent used. Radical initiators have no effect on the thermal decomposition and a polar mechanism was suggested. Thermal polymerization of cyclo-[Ph2SiCH212 has been reported to occur at 180-200°C. The product was a crystalline white powder which was insoluble in benzene and other common organic solvents [19]. 

Dielectric relaxation study is a powerful technique for obtaining molecular dipolar relaxation as a function of temperature and frequency. By studying the relaxation spectra, the intermolecular cooperative motion and hindered dipolar rotation can be deduced. Due to the presence of an electric field, the composites undergo ionic, interfacial, and dipole polarization, and this polarization mechanism largely depends on the time scales and length scales. As a result, this technique allowed us to shed light on the dynamics of the macromolecular chains of the rubber matrix. The temperature as well as the frequency window can also be varied over a wide... 

The structure of the ferroelectric, tri-glycine fluoberyllate [8, 9], has been determined, and participation of H-bonds in the polarization mechanism at the Curie point is described. Glycine silver nitrate is also ferroelectric [10], and a polarization mechanism, also involving H-bonds, is suggested. It is evident, from dielectric and structural... 

The Marcus treatment uses a classical statistical mechanical approach to calculate the activation energy required to surmount the barrier. It assumes a weakly adiabatic electron transfer process and non-equilibrium dielectric polarization of the solvent (continuum) as the source of activation. This model also considers the vibrational contributions of the inner solvation sphere. The Hush treatment considers ion-dipole and ligand field concepts in the treatment of inner coordination sphere contributions to the energy of activation [55, 56]. 

Microwave radiation, as all radiation of an electromagnetic nature, consists of two components, i.e. magnetic and electric field components (Fig. 1.3). The electric field component is responsible for dielectric heating mechanism since it can cause molecular motion either by migration of ionic species (conduction mechanism) or rotation of dipolar species (dipolar polarization mechanism). In a microwave field, the electric field component oscillates very quickly (at 2.45 GHz the field oscillates 4.9 x 109 times per second), and the strong agitation, provided by cyclic reorientation of molecules, can result in an... 

Typical concentrations of dopants (0.05-5 at.%) must result in the formation of dipolar pairs between an appreciable fraction of the dopant ions and the vacancies, e.g. 2La A-VA or 2Fel i+ -V( ). Donor-cation vacancy combinations can be assumed to have a stable orientation so that their initially random state is unaffected by spontaneous polarization or applied fields. Acceptor-oxygen vacancy combinations are likely to be less stable and thermally activated reorientation may take place in the presence of local or applied fields. The dipoles, once oriented in a common direction, will provide a field stabilizing the domain structure. A reduction in permittivity, dielectric and mechanical loss and an increase in the coercive field will result from the inhibition of wall movement. Since the compliance is affected by the elastic movement of 90° walls under stress, it will also be reduced by domain stabilization. 

In this section, a simple description of the dielectric polarization process is provided, and later to describe dielectric relaxation processes, the polarization mechanisms of materials produced by macroscopic static electric fields are analyzed. The relation between the macroscopic electric response and microscopic properties such as electronic, ionic, orientational, and hopping charge polarizabilities is very complex and is out of the scope of this book. This problem was successfully treated by Lorentz. He established that a remarkable improvement of the obtained results can be obtained at all frequencies by proposing the existence of a local field, which diverges from the macroscopic electric field by a correction factor, the Lorentz local-field factor [27],... 

Let us assume now, for example, that a step-like constant electric field of magnitude E0 is applied within a dielectric at any time t0, and remains constant for t>t0 (see Figure 1.27). Then, Pao = Pe + P, is almost instantaneously established. Thereafter, the acting relaxation processes (i.e., dipolar and/or charge-hopping and/or space charge polarization mechanisms) provoke that the polarization is not instantaneously established. 

With an alternating current (AC) field, the dielectric constant is virtually independent of frequency, so long as one of the multiple polarization mechanisms usually present is active (see Section 8.8.1). When the dominating polarization mechanism ceases as the frequency of the applied field increases, there is an abmpt drop in the dielectric constant of the material before another mechanism begins to dominate. This gives rise to a characteristic stepwise appearance in the dielectric constant versus frequency curve. For each of the different polarization mechanisms, some minimum dipole reorientation time is required for reahgnment as the AC held reverses polarity. The reciprocal of this time is referred to as the relaxation frequency. If this frequency is exceeded, that mechanism wUl not contribute to the dielectric constant. This absorption of electrical energy by materials subjected to an AC electric held is called dielectric loss. 

Instead of reviewing the extensive experimental work on the dielectric behaviour of biomolecuiar systems, this diapter will be mainly concerned with a discussion of the basic aspects of various mechanisms which may yield relevant dielectric polarization. These result from the following effects of an applied external electric field ... 

The same mechanism must in prindple apply to globular proteins. However, because of the small dimensions of these biopolymers the dielectric increments are of comparatively small magnitude. Furthermore, their dispersion falls in the same frequency range as that of the rotational polarization mechanism of permanent dipole moments. Since the latter is apparently predominant, it would be difficult to distinguish the coimterion effect. 

Both processes (86) and (87) apparently involve formation or destruction of the zwitterion dipole moment Therefore, application of an electric field must di lace the respective chemical equilibrium to some extent depending on the an e 0 between the directions of the zwitterion dipole and the field E. For 0 < 90° an increase of the number of zwitterions is favoured whereas for 0 > 90° this number will be decreased. Therefore, the whole system tends to develop preferential orientation of dipoles parallel to the field. Proton transfer of the kind involved here has generally been proved to be practically difiiision controlled so that the reaction rates could be extremely high. The chemical mechanism of dielectric polarization may thus be fast enough in comparison with the rotational difiusion of the zwitterion, especially if the latter is a macromolecule (e.g . a protein). 

Owing to their definite structures, most biomolecules have an appreciable permanent dipole moment which must lead to dielectric polarization via the rotational mechanism of preferential orientation. Thus pertinent experimental investigation permits a direct determination of the molecular dipole moments and rotational relaxation times (or rotational diffusion coefficients, respectively). These are characteristic factors for many macro-molecules and give valuable information regarding structural properties such as length, shape, and mass. 

The counterion mechanism of dielectric polarization is also in reasonable agreement with the data collected for DNA in the double-helical state as well as in the coiled denatured form. For the latter, much smaller electric increments and relaxation times are observed, indicating contraction of the polymer upon denaturation. The rotational relaxation time for helices as measured by means of flow birefringence was generally found... 

However, this mechanism of motion does not provide any great contribution to the Kerr effect since the dispersion curves of EB fall to virtually zero (Fig. 60). This difference may be interpreted by the proportionality of the orientational EB of a ripd-chain polymer to the square of the number of monomer units in segment whereas increment Ae/c related to the orientational mechanism is proportional to S (see Sects. 5.8 and 5.9). Hence, in the case of dielectric polarization the part played by the deformational mechanism as cmnpared to the orientational mechanian can be more important than in the case of EB. 

FIGURE 22.2 Frequency dispersion behavior of dielectric materials with various polarization mechanisms. 

Fundamental properties, such as the van der Waals volume, cohesive energy, heat capacity, molar refraction and molar dielectric polarization, are directly related to some very basic physical factors. Specifically, materials are constructed from assemblies of atoms with certain sizes and electronic structures. These atoms are subject to the laws of quantum mechanics. They interact with each other via electrical forces arising from their electronic structures. The sizes, electronic stmctures and interactions of atoms determine their spatial arrangement. Finally, the interatomic interactions and the resulting spatial arrangements determine the quantity and the modes of absorption of thermal energy. 

It would appear from the foregoing discussion that a correspondence between dielectric and mechanical relaxation can be expected when the molecular motions responsible for a mechanical relaxation involve reorientation of a polar group. Since there is a formal analogy between the complex mechanical... 

---