Process dielectric spectroscopy

Practically, DMTA is limited to low frequencies (up to tens of hertz) and, consequently, provides information about relatively slow processes. Dielectric spectroscopy is a related approach in which an alternating electric field is applied to a sample and the complex permittivity is then obtained from phase and amplitude measurements of current and voltage again, it is possible to consider data in the frequency domain, the temperature domain, or even as frequency/temperature contour maps.2 ° 2 See Refs. 230 and 232 for a theoretical account of the underlying physics. The approach can provide information in the frequency range W -io" coupling the applied electric field... 

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. 

I. Alig, D. Fischer, D. Lellinger and B. Steinhoff, Combination of NIR, Raman, ultrasonic and dielectric spectroscopy for in-line monitoring of the extrusion process, Macromol. Symp., 230, 51-58 (2005). 

Dielectric spectroscopy, also known as impedance spectroscopy, has been used for process analysis for some time, as it offers the ability to measure bulk physical properties of materials. It is advantageous to other spectroscopic techniques in that it is not an optical spectroscopy and is a noncontact technique, allowing for measurement without disturbing a sample or process. The penetration depth of dielectric spectroscopy can be adjusted by changing the separation between the sensor electrodes, enabling measurement through other materials to reach the substrate of interest. Because it measures the dielectric properties of materials, it can provide information not attainable from vibrational spectroscopy. 

Davis RD, Bur AJ, McBearty M et al. (2004) Dielectric spectroscopy during extrusion processing of polymer nanocomposites a high-throughput processing/characterization method to measure layered silicate content and exfoliation. Polymer 45 6487-6493... 

Dielectric analysis (electrothermal analysis, dielectric spectroscopy) is the measurement of dielectric properties as a function of frequency and temperature. It is increasingly finding use in characterising polymer structure and, in particular, the curing process. Its use in this respect has been considered in Chapter 6. 

As it have been demonstrated above, dielectric spectroscopy yields a wealth of information on the different molecular motions and relaxations processes, which are... 

The dielectric spectroscopy study of conductive samples is very complicated because of the need to take into account the effect of dc-conductivity. The dc-conductivity c>o contributes, in the frequency domain, to the imaginary part of the complex dielectric permittivity in the form of additional function a0/(so ). The presence of dc-conductivity makes it difficult to analyze relaxation processes especially when the contribution of the conductivity is much greater than the amplitude of the process. The correct calculation of the dc-conductivity is important in terms of the subsequent analysis of the dielectric data. Its evaluation... 

Figure 15. Time constants of the a- and p-processes of several glass formers, as determined by dielectric spectroscopy (DS), light scattering (LS), photon correlation spectroscopy (PCS), NMR, Kerr effect (KE), neutron scattering (NS), and viscosity o-Terphenyl (OTP, type A) NMR (crosses, [177-179]), DS (filled squares [151]), KE (unfilled circles [66]), viscosity (solid line [164]). m-Tricresyl phosphate (m-TCP, type A) NMR (crosses [15]), LS (unfilled squares [181]), DS (circles [180]) and viscosity (line [182]). m-Fluoroaniline (FAN, type B) DS (stars [153]). 2-Picoline (PIC, type A) LS (unfilled circles [183]), NS (filled triangles [184]), PCS (unfilled squares [65], DS (filled diamonds, [181]). Toluene (type B) NMR (+ [11]), DS (filled squares [153]) and LS (filled circles [185]).

Figure 33. Time constants of the P-process obtained from dielectric spectroscopy as a function of the reduced reciprocal temperature Ts/T for the glasses toluene, fluoroaniline (m-FAN), ethanol, a mixture of chlorobenzene and decaline (CB/DEC), trimethyl phosphate (TMP), and polybutadiene (PB). (Compiled from Refs. 6, 137, 230, 306, 315, and 344.)...

Fig. 10. Transition map for the mixture of hydrophilic Aerosil with PDMS [27] the relaxation of chain units outside the adsorption layer is represented by symbol , anisotropic motion of chain units inside the adsorption layer is shown by symbol 0, the slowest chain motion related to adsorption-desorption processes in the adsorption layer is designated by symbol O the data of the fu t two relaxation processes are fitted by the WLF function, the tempoature dependence of the slowest relaxation shows the Arrhenius-like behavior for comparison data from previous h Ty and NMR experiments , mechanical , and dielectric spectroscopy are given...

According to the dielectric experiments [27], an activation energy for adsorption-desorption processes at PDMS-hydrophilic Aerosil interface is equal to 32 kJ moF. This value is about three times larger than that from NMR experiments [9]. This discrepancy may be due to the difference in the spatial scale of motions in the adsorption layer as detected by NMR and dielectric spectroscopy [54]. 

Cerveny investigated the development of the dynamic glass transition in styrene-butadiene copolymers by dielectric spectroscopy in the frequency range from 10 to 10 Hz. Two processes were detected and attributed to the alpha- and beta-relaxations. The alpha relaxation time has a non-Arrhenius temperature behavior that is highly dependent on styrene content... 

As in DMTA, dielectric spectroscopy can also be used to study both imposed factors such as additives,degradation.and penetrant molecules, and intrinsic molecular processes. In the latter case, a number of distinct dielectric relaxations are generally observed, which are labeled a, P, 7, etc. with decreasing temperature in the case of Nafion perfluorocar-boxylate polymers, for example, specific a, a, p, y, and 8-relaxations have... 

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