Isotherms dielectric

The quantitative analysis of the dielectric spectra of the glass-transition process was carried out by fitting the isothermal dielectric loss data according to the HN law (21). It was found from the fitting that (5 1 for the glass-transition... 

Figure 41. The inset shows isothermal dielectric loss spectra of DOP at ambient pressure. The y-relaxation is the only resolved secondary relaxation. The main figure is obtained by time-temperature superposition.

As an example, this effect is shown by the isothermal dielectric loss spectra of BIBE at ambient pressure and at elevated pressures in Fig. 4. 

FIGURE 3.21 Isothermal dielectric loss curves for a thermodynamically-miscible blend of 25% 1,2-polybutadiene with natural rubber. Notwithstanding the homogenous morphology, the respective mobiUties of the components differ, whereby two peaks are observed in the spectrum. Reprinted from Alegria et al. (1994). 

The dielectric loss data of the 50% PVME/50% PS blend taken at ambient pressure by Schwartz et al. (2007) belong also to the a-relaxation, a conclusion we reached from the agreement of the relaxation times with those of Cendoya et al. (1999). The situation is different in blends jc% PVME/(1 — x)% PS with smaller x = 0.25 and 0.20, where the a-loss peak of PVME is broadened immensely and becomes difficult or impossible to resolve by dielectric spectroscopy. In the isothermal dielectric loss spectra by Schwartz et al. (2007) for the 25% PVME blend and by Lorthioir et al. for the 20% PVME blend, observed instead is the resolved JG /3-relaxation (called a relaxation by Lorthioir et al. (2003a)). We have already shown in connection with the 20% PVME blend in Eigure 5.25 that the observed JG /3-relaxation is distinctly different from the a-relaxation because the latter was found by TSDC, and it has much longer... 

To date only few dielectric relaxation studies have been reported on thermosetting nanocomposite systems. Kanapitsas et al. [109] reported isothermal dielectric relaxation studies of epoxy nanocomposite systems based upon three different clay modifications, a low viscosity epoxy resin based on the diglycidyl ether of bisphenol-A type (Araldite LY556, CIBA) and an amine hardener in a temperature range of 30-140 °C. Whilst details on the epoxy system investigated and the nanocomposite morphology were vague, it was reported that the overall mobility is reduced in the nanocomposite compared to the neat matrix resin. 

FIGURE 26 Isothermal dielectric loss data of 1,4 poly butadiene that show resolved Johari-Goldstein relaxation in the supercooled liquid state above Tg. Representative KWW fit to the a-relaxation peak are shown as line. The value of n so determined is given in the figures. Each vertical arrow pointing toward certain data taken at some temperature indicates the location of the independent relaxation frequency, Vo = l/2 rToa, where To is calculated using Eq. (38). A, O, V, , are data taken at -97.5, -95, -92.5, and -91.2 C, respectively. 

Figure 10.14 Arrhenius diagram showing WLF and Arrhenius temperature dependence. The lines are obtained from isothermal dielectric loss data as a function of log frequency.

Figure 8.2c shows isothermal dielectric loss, ", data as a function frequency for bulk PVDF-TrFE 56 44 copolymer. In isothermal plots, the P relaxation appears as... 

Fig. XVI-7. Dielectric isotherms of water vapor at 15°C adsorbed on a-FeiOa (solid points indicate desorption). A complete monolayer was present at P/P = 0.1, and by P/P = 0.8 several layers of adsorbed water were present. (From Ref. 110.)...

At large distances the curve of Fig. 8b is a plot of — (c2/ r)> where t is the macroscopic dielectric constant of the solvent at the temperature considered. For small values of r the curve deviates from this value but at every point the slope of the curve must represent the mean intensity of the mutual attraction or repulsion at the particular temperature considered. If the curve of Fig. 86 for dissociation in solution is to be useful, every point on this curve must belong to the same temperature T. That is to say, when we consider any change in the distance r between the ions, we are interested in an isothermal change in r. 

If a piece of metal, such as silver, is dipping into a solvent, and a positive atomic core is taken from the surface into the solvent, the ion is again surrounded by its electrostatic field but free energy has been lost by the dielectric, and a relatively small amount of work has had to be done. The corresponding potential-energy curve (Fig. 96) is therefore much less steep and has a much shallower minimum than that of Fig. 9a. For large distances d from a plane metal surface this curve is a plot of — c2/4td where t is the dielectric constant of the medium at the temperature considered The curve represents the work done in an isothermal removal of the positive core. 

At the same time, when we increase the separation of the charges from r to (r + Sr) isothermally, there is a change of entropy in the dielectric. For large values of r this amounts to... 

I,et the spherical conductors A and B of Fig. 15 have radii a and b and let them be situated m a solvent whose dielectric constant obeys (14). The sphere A initially hears a charge q, while B is uncharged. Find expressions for the values of AF, AS, and All for the isothermal transfer of the charge to the distant sphere B. 

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. 

Gel time values of the three systems measured as abrupt change in the slope of G (t) under isothermal curing conditions show that gelation occurs earlier in PWE system at all temperatures considered as shown in Table 11.27. ETPI behaves like a catalyst for the primary epoxy-amino reaction which dominates the cure until vitrihcation occurs. Dynamic mechanical analysis and dielectric spectroscopic analysis carried out by the authors also confirm the above conclusions. 

Figure 8. Hydration isotherm for Nation 117 (equivalent weight (EW) of 1100 g/equiv) and the distribution of the dielectric constant and protonic charge carrier concentration across the hydrated hydrophilic channels (pores) for three different water contents (top).

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