Dielectric spectroscopy dynamic susceptibility

Other parameters which have been used to provide a measure of a include physical dimensions (thermomechanical analysis, TMA) [126], magnetic susceptibility [178,179], light emission [180,181], reflectance spectra (dynamic reflectance spectroscopy, DRS) [182] and dielectric properties (dynamic scanning dielectrometry, DSD) [183,184], For completeness, we may make passing reference here to the extreme instances of non-isothermal behaviour which occur during self-sustained burning (studied from responses [185] of a thermocouple within the reactant) and detonation. Such behaviour is, however, beyond the scope of the present review. 

It is noteworthy that the neutron work in the merging region, which demonstrated the statistical independence of a- and j8-relaxations, also opened a new approach for a better understanding of results from dielectric spectroscopy on polymers. For the dielectric response such an approach was in fact proposed by G. Wilhams a long time ago [200] and only recently has been quantitatively tested [133,201-203]. As for the density fluctuations that are seen by the neutrons, it is assumed that the polarization is partially relaxed via local motions, which conform to the jS-relaxation. While the dipoles are participating in these motions, they are surrounded by temporary local environments. The decaying from these local environments is what we call the a-process. This causes the subsequent total relaxation of the polarization. Note that as the atoms in the density fluctuations, all dipoles participate at the same time in both relaxation processes. An important success of this attempt was its application to PB dielectric results [133] allowing the isolation of the a-relaxation contribution from that of the j0-processes in the dielectric response. Only in this way could the universality of the a-process be proven for dielectric results - the deduced temperature dependence of the timescale for the a-relaxation follows that observed for the structural relaxation (dynamic structure factor at Q ax) and also for the timescale associated with the viscosity (see Fig. 4.8). This feature remains masked if one identifies the main peak of the dielectric susceptibility with the a-relaxation. 

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