Relaxation mechanisms, dielectric strength

The study of the aP coupling is advantageous in poly( -alkyl methacrylate)s due to the accessibility to the crossover region by both dielectric and mechanical spectroscopies and to the fact that both secondary and main processes are associated with high dielectric strength values, in contrast to a variety of other materials where the P relaxation is much less intense,. 

In this paper, we analyze the effect of fluorine substitution in the polymers listed above by dielectric analysis (DEA), dynamic mechanical analysis (DMA) and stress relaxation measurements. The effect of fluorination on the a relaxation was characterized by fitting dielectric data and stress data to the Williams, Landel and Ferry (WLF) equation. Secondary relaxations were characterized by Arrhenius analysis of DEA and DMA data. The "quasi-equilibrium" approach to dielectric strength analysis was used to interpret the effect of fluorination on "complete" dipole... 

Figure 6.6. Schematic presentation of the frequency dependences of e and e" for typical dielectric relaxation modes in polymers. The traces of relaxation mechanisms with higher molecular mobility (shorter relaxation time) are recorded at higher frequencies. Compare with the isochronal dielectric spectrum of Fig. 6.4. Considering the temperature dependence of the dielectric strength (Ae), it becomes clear that the Ae values measured from isothermal or isochronal isothermal spectra alone are not necessarily equal.

The dielectric strength of a particular relaxation mechanism can be determined by the depolarization charge <2si, obtained from the area under the related TSC peak, by... 

The y Relaxation. In common with many other polymers (8) both PPO and PS display significant loss maxima below room temperature at the frequencies under consideration. Whereas the process responsible for the a loss is at least qualitatively understood in terms of a main chain relaxation associated with the glass transition, y losses can often only tentatively be attributed to specific mechanisms. In PPO, for example, it does not seem unreasonable to propose that the y loss is associated with librations in the two pendant methyl groups this view is somewhat reinforced by the observation that in the dielectric measurements the relaxational strengths of the y and a loss processes are comparable. As the latter can be well interpreted (6) in terms of a dipolar relaxation of the main chain in which the entire dipolar contributions arise from the methyl groupings, it seems plausible to assume that the same dipoles are responsible for the y loss mechanism. In polystyrene there is a similar... 

The dielectric (e ) and loss (e") constants are important properties of interest because these two parameters, among others, determine the suitability of a material for a given application. Dielectric relaxations are studied to reduce energy losses in materials used in practically important areas of insulation and mechanical strength. 

When studying a polymer on a large frequency/time scale, the response of a given material under a dynamic stimulus usually exhibits several relaxations. Moreover, the peaks are usually broad and sometimes and are associated with superposed processes. The relaxation rate, shape of the loss peak, and relaxation strength depend on the motion associated with a given relaxation process [41]. In general, the same relaxation/retardation processes are responsible for the mechanical and dielectric dispersion observed in polar materials [40]. In materials with low polarity, the dielectric relaxations are very weak and cannot be easily detected. The main relaxation processes detected in polymeric systems are analyzed next. 

It is not easy in mechanical measurements to vary the frequency over a wide range, as is possible for measurements of dielectric relaxation, so that direct determination of relaxation strengths as defined in section 7.6.1 is not usually possible. A further complication is that the amorphous and crystalline regions are coupled together mechanically, so that they do not contribute independently to the spectrum of relaxations... 

Figure 9.4 shows a comparison of the dielectric and mechanical relaxation spectra of various forms of polyethylene. The most obvious feature is that the main relaxations, here the a, (3 and y relaxations, occur at approximately the same temperatures in both spectra, although their relative relaxation strengths in the two spectra are diiferent. This is a feature common to the spectra of many polymers. The peaks are not, however, in exactly the same positions in the two spectra for the same type of polyethylene. In addition to the possible reasons for this described above, the frequencies of measurement are diiferent. The dielectric measurements were made at a much higher frequency than that used for the mechanical measurements, as is usual. Molecular motions are faster at higher temperatures, so this factor alone would lead to the expectation that the dielectric peaks would occur at a higher temperature than the mechanical peaks. The y peak, which is assigned to a localised motion in the amorphous material and is in approximately the same place for all samples, behaves in accord with this expectation. 

Similar to polyethylenes the morphology of these polymers is also described as a lamellar stack of crystalline and non-crystalline layers. This so-called two phase model is applied for the interpretation of X-ray diffraction data as well as for heat of fusion or density measurements. However, it is well known that several mechanical properties, as well as the relaxation strength at the glass transition temperature, cannot be described by such a simplistic two-phase approach, as discussed by Gupta [59]. Prom standard DSC measurements [60], dielectric spectroscopy, shear spectroscopy [61], NMR [62], and other techniques probing molecular dynamics at the glass transition (a-relaxation) temperature, the measured relaxation strength is always smaller than expected... 

In order to analyse the mechanism of the 3 process, the dielectric relaxation strength (Ae = o... 

The viscoelastic behaviour of the majority amorphous phase is important in terms of the mechanical properties of the material. Dynamic mechanical (S) and dielectric studies (8) reveal three relaxations, labelled a, p and 7 in order of decreasing temperature. Variations in the strength of these relaxations with systematic changes in the mole fraction of each component have led to the association of the 7 relaxation with the HBA component and the p relaxation with the HNA component, with the a relaxation displaying features typical of a glass transition process. Support for these assignments has been obtained from analysis of the proton NMR second moments (9,10),... 

---