Dielectric spectroscopy temperature

At low temperature the material is in the glassy state and only small ampU-tude motions hke vibrations, short range rotations or secondary relaxations are possible. Below the glass transition temperature Tg the secondary /J-re-laxation as observed by dielectric spectroscopy and the methyl group rotations maybe observed. In addition, at high frequencies the vibrational dynamics, in particular the so called Boson peak, characterizes the dynamic behaviour of amorphous polyisoprene. The secondary relaxations cause the first small step in the dynamic modulus of such a polymer system. 

Fig. 4.5 Scaling representation of the spin-echo data at Q nax- Different symbols correspond to different temperatures. Solid line is a KWW description (Eq. 4.8) of the master curve, a Polyurethane at Qmax=l-5 A L The shift factors have been obtained from the superposition of the NSE spectra. (Reprinted with permission from [127]. Copyright 2002 Elsevier), b Poly-(vinyl chloride) at Qmax=l-2 A L The shift factors have been obtained from dielectric spectroscopy. (Reprinted with permission from [129]. Copyright 2003 Springer, Berlin Heidelberg New York)...

Fig. 4.9 Temperature dependence of the characteristic time of the a-relaxation in PIB as measured by dielectric spectroscopy (defined as (2nf ) ) (empty diamond) and of the shift factor obtained from the NSE spectra at Qmax=l-0 (filled square). The different lines show the temperature laws proposed by Tormala [135] from spectroscopic data (dashed-dotted), by Ferry [34] from compliance data (solid) and by Dejean de la Batie et al. from NMR data (dotted) [136]. (Reprinted with permission from [125]. Copyright 1998 American Chemical Society)...

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. 

Dielectric spectroscopy has been shown to be of great interest in dealing with transitions involving reorientation of permanent dipoles [93]. By monitoring the temperature and frequency dependence of the complex dielectric permittiv-... 

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. 

Dielectric spectroscopy was also used by the same group in order to study the local and global dynamics of the PI arm of the same miktoarm star samples [89]. Measurements were confined to the ordered state, where the dynamics of the PI chain tethered on PS cylinders were observed in different environments since in the SIB case the faster moving PB chains are tethered in the same point as the PI arm. The distribution of segmental relaxation times were broader for SI2 than SIB. The effect was less pronounced at higher temperatures. The PI normal mode time was found to be slower in SIB, when compared to SI2 although both arms had the same molecular weight. Additionally, the normal mode relaxation time distributions of the PI chains tethered to PS cylinders in the miktoarm samples were narrower than in P(S-h-I) systems of lamellar structure. 

Floudas and coworkers [90] employed dielectric spectroscopy to probe the interfacial width in lamellae forming non-linear block copolymers of the type (B-h-A)3B and (B-h-A)3B(A-h-B)3, where A is PI and B is PS. Their experiments were conducted at temperatures below the ODT and below the glass transition of the PS hard phase . In this temperature region the global chain dynamics of PI bridges were used to provide an estimate of the dynamic interface between... 

Broadband Dielectric Spectroscopy provides a direct experimental access to the molecular relaxations of polymers over a broad frequency and temperature range. It is also especially suitable for the investigation of thin polymer films, because it does not suffer sensitivity loses with decreasing sample amount. This technique does require a special sample preparation for thin films, because of the need to have metal electrodes and good electrical contacts at both interfaces. Spin-coating, one of the most commonly employed methods for the preparation of... 

For thin polystyrene films annealed for 12 hours at 150 °C in high vacuum (10-6 mbar) and measured in a pure nitrogen atmosphere the dynamic glass transition was characterized using two experimental techniques capacitive scanning dilatometry and Broadband Dielectric Spectroscopy. Data from the first method are presented in Fig. 15a, showing the real part of the complex capacity at 1 MHz as a function of temperature for a thin PS film of 33 nm. 

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.)...

Figure 49. Susceptibility spectra for propylene carbonate (Tg — 160 K) as measured by depolarized light scattering (top, data from Ref. 372) and dielectric spectroscopy (bottom, data from Ref. 9), each normalized by a temperature-independent static susceptibility. The full lines are fits from solutions of a two-component schematic MCT model. The dashed fines indicate a white noise spectrum. The dash—dotted line in the upper panel exhibits the asymptote of the critical spectrum. The dotted line shows the solution of the model at T — Tc with hopping terms being neglected. (From Ref. 380.)...

The important acid activity in Nafion is appropriately represented by trifluo-romethane sulfonic (triflic) acid, CF3SO3H see Fig. 1.5. Dielectric spectroscopy has suggested that a significant amount of triflic acid is not dissociated in the ionic melt at 50% mole fraction of water (Barthel et al, 1998). But the deprotonation chemistry of hydrated triflic acid hasn t been experimentally studied over the wide range of hydration and temperature that would be relevant to the function of sulfonate-based polyelectrolyte membrane materials. 

CF3SQ3H H2O probed by temperature dependent dielectric spectroscopy. J. Chem. Soc. Faraday Trans. 94, 1953-1958 (1998). 

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