Reorientation mechanism dielectric relaxation

In this discussion of dielectric constant the calculation was an equilibrium one and no account was taken of the way in which the equilibrium configuration is achieved. It was only necessary that some mechanism exist for the reorientation of molecular dipoles. In the next section we shall consider the mechanism of dielectric relaxation in detail and from this consideration will come an alternative method for calculating Cg. 

The fact that the dielectric relaxation of ice is accurately described by a simple Debye curve implies that only a single mechanism is involved. Bjerrum (1951) discussed the possible reorientation mechanisms in ice—either ion-state motion or orientational defect motion—and concluded that the latter was the relevant process. As we shall see later, his conclusion is correct for pure ice, where the greater concentration of L- and D-defects more than makes up for their lower mobility, compared with ion... 

In order to understand this complex relaxation behavior of the microemulsions, it is necessary to analyze dielectric information obtained from the various sources of the polarization. For a system containing more than two different phases the interfacial polarization mechanism has to be taken into account. Since the microemulsion is ionic, the dielectric relaxation contributions are related to the movement of surfactant counterions relative to the negatively charged droplet interface. A reorientation of AOT molecules, and of free and bound water molecules, should also be mentioned in the list of polarization mechanisms. In order to ascertain which mechanism can provide the experimental increase in dielectric permittivity, let us discuss the different contributions. 

Relevant dielectric results of type B glasses were already discussed in Section IV.C.2. The spectra below Tg exhibit a broad symmetric secondary relaxation peak that can be interpolated assuming a Gaussian distribution of activation enthalpies. Only recently it became clear that also NMR is able to identify secondary relaxation processes in glasses, moreover providing information on the mechanisms of molecular reorientation that is not easily accessible to most of the other methods. For detailed reports the reader is referred to the reviews by Bohmer et al. [11] and Vogel et al. [15]. Here, we summarize the major results. 

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. 

The greater degree of freedom enjoyed by the 1-naphthyl label in P/VN is most likely ascribable to motion independent of the polymer chain about the bond of attachment. Since the vectors for absorption and emission are located within different planes within the molecular framework for triplet emission, such motions constitute a mechanism of enhanced depolarization for the P/VN system. In the case of PMMA it has been shown (H) that independent motion of a 1-VN label occurs in the vincinity of the relaxation of the polymer but is characterized by an apparent activation energy inferior to that sensed by an ester label or as afforded by dielectric and dynamic relaxation data for the g-process. Consequently whilst the onset of motion might be consistent with an increased degree of free volume released by the g-mechanism, the activation energy for naphthyl group reorientation within the lifetime of the excited states should not be equated with that of the g-process itself in PBA. 

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... 

When a dielectric is placed in an alternating electric field the dipoles attempt to maintain alignment with the field. This process requires a finite time that is different for each polarization mechanism. At the relaxation frequency the dipoles will only just be able to reorient themselves in time with the applied field. At this frequency the dielectic is lossy and energy is lost in the form of heat. The dielectric loss is at a maximum when the frequency of the external field coincides with the relaxation frequency of a given polarization mechanism. This is the principle behind the microwave oven. It operates at the relaxation frequency of water molecules and the heat generated warms the food. 

The dielectric (e" and M") spectra and the relaxation frequencies of the p relaxation are similar to the mechanical spectra (J" and E", respectively) and the corresponding relaxation rate over a wide temperatures range (Muzeau et al. 1991 Perez et al. 1999). These observations suggest that the underlying mechanisms for the local electrical and mechanical relaxation processes in PMMA are similar. Clearly, this is not always the case for polymers, since all modes of motion of a polymer chain are not dielectrically active. When rotational diffusion occurs about a variety of different axes among which only a few reorient a dipole, the shape of the relaxation and the average rates of relaxation in a dielectric measurement may and will differ from those in a mechanical test. Dielectric, dynamic mechanical, and DSC glass transition... 

The top panel of Figme 10 shows the reorientational relaxation time Ti as a function of the solute s electric dipole moment, when the solute s center of mass is constrained to a slab of width 4 A centered at the Gibbs surface (labeled G), 3.5 A above the Gibbs smface (S), and in bulk water (B). As expected, in every location the relaxation time increases with the increase of the solute dipole moment, reflecting the increase in the dielectric friction. For relatively small solute dipole moments fi < 6D), the friction is dominated by the mechanical density-dependent contribution, and the relaxation in the higher density bulk region is much slower than the relaxation at the interface. However, as the dipole moment is increased, the bulk and the surface reorientation relaxation times become similar. This behavior, which mirrors that of the... 

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