Relaxation mechanisms, dielectric

A new dielectric-relaxation mechanism, important in the THz region, is proposed, relevant to vibration of a nonrigid dipole in a direction perpendicular to that of the H-bond. 

The TV-dielectric relaxation mechanism allows us to (i) remove the THz deficit of loss e" inherent in previous (see GT2) theoretical studies, (ii) explain the THz loss and absorption spectra in supercooled (SC) water, (iii) describe, in agreement with the experiment, the low- and high-frequency tails of the two bands of ice H20 located in the range 10-300 cm-1, and (iv) describe the nonresonance loss spectrum in ice in the submillimetric region of wavelengths. Specific THz dielectric properties of SC water are ascribed to association of water molecules, revealed in our study by transverse vibration of the HB charged molecules. 

As the main purpose of this conference is to understand phenomena occuring at the interfaces in polymeric composites and because dielectric methods of investigation are relatively less utilized by composite scientists (perhaps with the exception of cure montioring in thermoset composites) it seems worthwhile to introduce briefly a dielectric relaxation mechanism specific to composite materials, interfacial polarization, and to discuss its applicability to solve practical problems. 

The influence of individual molecular motion on the dielectric relaxation was first considered by Martin, Meier and Saupe [154]. Whereas the onset of the nematic ordering will not drastically influence the rotation around the long molecular axis, the motion around the short molecular axis will be strongly hindered by the nematic order. As a result, we expect four dielectric relaxation mechanisms (see [155]). These are the relaxations of a /tj dipole moment along the long molecuar axis in a direction (1) around the nematic director and (2) perpendicular to the nematic director as well as the relaxations of a H2 dipole moment perpendicular to the long axis, in a direction (3) around... 

Polytetrafluoroethylene transitions occur at specific combinations of temperature and mechanical or electrical vibrations. Transitions, sometimes called dielectric relaxations, can cause wide fluctuations in the dissipation factor. 

Relaxations of a-PVDF have been investigated by various methods including dielectric, dynamic mechanical, nmr, dilatometric, and piezoelectric and reviewed (3). Significant relaxation ranges are seen in the loss-modulus curve of the dynamic mechanical spectmm for a-PVDF at about 100°C (a ), 50°C (a ), —38° C (P), and —70° C (y). PVDF relaxation temperatures are rather complex because the behavior of PVDF varies with thermal or mechanical history and with the testing methodology (131). 

Up to now it has been tacitly assumed that each molecular motion can be described by a single correlation time. On the other hand, it is well-known, e.g., from dielectric and mechanical relaxation studies as well as from photon correlation spectroscopy and NMR relaxation times that in polymers one often deals with a distribution of correlation times60 65), in particular in glassy systems. Although the phenomenon as such is well established, little is known about the nature of this distribution. In particular, most techniques employed in this area do not allow a distinction of a heterogeneous distribution, where spatially separed groups move with different time constants and a homogeneous distribution, where each monomer unit shows essentially the same non-exponential relaxation. Even worse, relaxation... 

There is greatly renewed interest in electron solvation, due to improved laser technology. However it is apparent that a simple theoretical description such as implied by Eq. (9.15) would be inadequate. That equation assumes a continuum dielectric with a unique relaxation mechanism, such as molecular dipole rotation. There is evidence that structural effects are important, and there could be different mechanisms of relaxation operating simultaneously (Bagchi, 1989). Despite a great deal of theoretical work, there is as yet no good understanding of the evolution of free-ion yield in polar media. 

Roudaut et al. (1999a) used low-frequency pulsed-proton NMR and dielectric dynamic mechanical spectroscopies to study molecular mobility in glassy bread (<9%) as a function of temperature. Based on NMR results, they reported that some (if not all) of the water molecules were much more mobile than the polymer matrix whose relaxation time could not be measured within the 20-p,s dead time of the RF probe. 

The jump rates obtained by the line shape simulations are plotted on the relaxation map in Fig. 22 together with values obtained by other experimental methods. The points of the mechanical and dielectric relaxations correspond to the process of the large-scale side chain motions refered to as the -process and follow the WLF equation very well above Jg,. 11 It should be noted that the present 2FI NMR results are located on the curve obtained by other relaxation experiments. This fact shows that... 

For transport in amorphous systems, the temperature dependence of a number of relaxation and transport processes in the vicinity of the glass transition temperature can be described by the Williams-Landel-Ferry (WLF) equation (Williams, Landel and Ferry, 1955). This relationship was originally derived by fitting observed data for a number of different liquid systems. It expresses a characteristic property, e.g. reciprocal dielectric relaxation time, magnetic resonance relaxation rate, in terms of shift factors, aj, which are the ratios of any mechanical relaxation process at temperature T, to its value at a reference temperature 7, and is defined by... 

Paddison et al. performed high frequency (4 dielectric relaxation studies, in the Gig ertz range, of hydrated Nafion 117 for the purpose of understanding fundamental mechanisms, for example, water molecule rotation and other possible processes that are involved in charge transport. Pure, bulk, liquid water is known to exhibit a distinct dielectric relaxation in the range 10—100 GHz in the form of an e" versus /peak and a sharp drop in the real part of the dielectric permittivity at high / A network analyzer was used for data acquisition, and measurements were taken in reflection mode. 

D. Axelson These spectra were obtained at 57.9 MHz, but that s not the problem. We can measure correlation times regardless of the frequency. The correlation time at the glass temperature is very long. From a measurement of the correlation time we should be able to tell whether it is a true glass. In all these cases the correlation times are six to nine orders of magnitude lower than can possibly exist in a glass. For this reason I think the correlation between the NMR measurement and dielectric relaxation and dynamic mechanical do not relate one to one because of the frequency effects in the other measurements. 

It was thought for some time that central peaks were due to impurities, defects and other such extrinsic or intrinsic factors. A number of models and mechanisms based on entropy fluctuations, phonon density fluctuations, dielectric relaxation, molecular... 

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