Dielectric spectroscopy results

The results show that the temperature of the a-transition is significantly reduced for cast films compared to the bulk polymer, with a lower value for chloroform samples. The decrease of the a-transition temperature can be explained by the presence of residual solvent, inducing a plasticizing effect. On the contrary, an increase of the P-transition tempera- 

Toluene, THF and acetone are described as basic solvents. The residual solvent molecules can therefore only weakly interact with the acrylate groups. Interactions between chains are then favored and self-associations between PMMA chains appear. The rotation of the acrylate groups becomes more difficult compared to the bulk PMMA, probably due to self-aggregation of some PMMA chains (even if the studiedpolymer is not stereoregular). 

To resume, residual solvent molecules present in solid conventional PMMA films are able to significantly modify the polymer relaxation properties. The effect of residual solvent depends strongly on the nature of the solvent, specially its acid-base character. 

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Ese et al. found similar results on model emulsions stabilized with resins and asphaltenes extracted from North Sea oil (57). The dielectric spectroscopy results showed that the stability of model emulsions could be eharaeterized. Stability was found to depend mainly on the amount of asphaltenes, the degree of aging of asphaltenes and resins, and the ratio between asphaltenes and resins. 

Fig. 1.3 Relaxation map of polyisoprene results from dielectric spectroscopy (inverse of maximum loss frequency/w// symbols), rheological shift factors (solid line) [7], and neutron scattering pair correlation ((r(Q=1.44 A )) empty square) [8] and self correlation ((t(Q=0.88 A" )) empty circle) [9],methyl group rotation (empty triangle) [10]. The shadowed area indicates the time scales corresponding to the so-called fast dynamics [11]...

Fig. 4.20 Temperature dependence of the average relaxation times of PIB results from rheological measurements [34] dashed-dotted line), the structural relaxation as measured by NSE at Qmax (empty circle [125] and empty square), the collective time at 0.4 A empty triangle), the time corresponding to the self-motion at Q ax empty diamond),NMR dotted line [136]), and the application of the Allegra and Ganazzoli model to the single chain dynamic structure factor in the bulk (filled triangle) and in solution (filled diamond) [186]. Solid lines show Arrhenius fitting curves. Dashed line is the extrapolation of the Arrhenius-like dependence of the -relaxation as observed by dielectric spectroscopy [125]. (Reprinted with permission from [187]. Copyright 2003 Elsevier)...

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. 

Films of pure PPX and PPX composites with nanoparticles of various metals resulted from cryochemical solid-state synthesis were studied by the dielectric spectroscopy method [104], Dielectric spectroscopy has proven very useful for studying the structure and dynamics of polymer materials as well as the transport mechanism of charge carriers. To study features of the polymer structure dielectric test methods were used due to their high sensitivity to morphological changes. 

Similar results were obtained by Broadband Dielectric Spectroscopy (Fig. 17) no shifts in the relaxation time of the dynamic glass transition were detected, even for PS films as thin as 20 nm. 

This chapter concentrates on the results of DS study of the structure, dynamics, and macroscopic behavior of complex materials. First, we present an introduction to the basic concepts of dielectric polarization in static and time-dependent fields, before the dielectric spectroscopy technique itself is reviewed for both frequency and time domains. This part has three sections, namely, broadband dielectric spectroscopy, time-domain dielectric spectroscopy, and a section where different aspects of data treatment and fitting routines are discussed in detail. Then, some examples of dielectric responses observed in various disordered materials are presented. Finally, we will consider the experimental evidence of non-Debye dielectric responses in several complex disordered systems such as microemulsions, porous glasses, porous silicon, H-bonding liquids, aqueous solutions of polymers, and composite materials. 

The characterization of the physical and chemical changes that occur in montmorillonite/PDMS nanocomposite elastomers as they are thermally aged is reported. Broadband Dielectric Spectroscopy (BDS) was used to track changes in the physical interaction between the polymer and clay associated with increases in non-oxidative thermal stability (as determined by TGA). The evolution of volatile siloxane species from the elastomers was characterized with Thermal Volatilization Analysis (TVA). Results suggest that the improved thermal stability and the increases in polymer/clay association are a result of significant re-structuring of the polymer network. 

Another important source of information is dielectric spectroscopy, because dipolar ion pairs contribute to the static dielectric constant of the solution [42, 43], In polar solvents the dielectric spectra reflect two modes caused by the reorientation of solvent and of ion-pairs. In non-polar solvents one solely observes ion pair reorientation. For Bu4N-iodide (BU4NI) in dichloromethane (CH2CI2) an increase of the total dielectric constant e with the concentration of the salt is found as result of the ion pair formation. A decrease in the particle density of the solvent causes a minute decrease of the solvent contribution. The dielectric constant does, however, not increase linearly with the salt concentration. A decreasing slope at high salt concentrations may result from the redissociation of the ion pairs but at a quantitative level, redissociation alone is... 

We can conclude that the result from dielectric spectroscopy—that the a-dispersion is invariant to T and P at constant xa—appears to be quite general, with respect to both the material and the experimental technique. The limitation is only that sufficiently broad spectra must be obtained under different conditions of temperature and pressure. 

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