Cooling dielectric spectra

We have seen in Chapter 2 that the frequency of an EPR spectrum is not a choice for the operator (once the spectrometer has been built or bought) as it is determined by the combined fixed dimensions of the resonator, the dewar cooling system, and the sample. Even if standardized sample tubes are used and all the samples have the same dielectric constant (e.g., frozen dilute aqueous solutions of metalloproteins), the frequency will still slightly vary over time over a series of consecutive measurements, due to thermal instabilities of the setup. By consequence, two spectra generally do not have the same frequency value, which means that we have to renormalize before we can compare them. This also applies to difference spectra and to spectra... 

Figure 35. Time evolution of the secondary dielectric relaxation loss spectrum of DPGDB on isothermal annealing at 173.15 K after rapid cooling from 300 K. From top to bottom, the data were obtained after the sample has been annealed isothermally at 173.15 K for times, ta, equal to 93 s, 745 s, 1353 s, 3752 s, and 7272 s elapsed after the thermal stabilization. Solid circles represent the spectrum obtained by slowly cooling the sample at 0.05 K/min. Vertical arrows show the frequencies of the maximum loss for the JG P- and the y-processes.

As shown in Figure 11.1, to obtain a complex TSC spectrum, a static electric field E is applied to the sample at a polarization temperature labeled Tp for a time tp, which is necessary to obtain polarization saturation, i.e., the equilibrium polarization. Afterward, the sample is cooled down to a temperature T0 in such a way that the dielectric relaxation proceeds extremely slowly, so that after removal of the field the sample retains a frozen-in polarization. The depolarization current, Id, caused by the return to equilibrium of dipolar units, is then recorded by increasing the temperature at a constant rate from T0 up to the final temperature Tf, where Tf > Tp. The plot of Id as a function of temperature is a complex TSC spectrum. 

The dielectric rotational relaxation spectrum of two glass forming small molecules both in the bulk and in 4 nm porous Vycor glass have been measured. Theoretical studies have been conducted using Monte Carlo simulation techniques on short polymer chains to study the effect of confinement on glass formation as a polymer system is cooled fix>m the melt using different rates of coding. 

Fig. 4.17. Temperature dependence of dielectric loss spectrum as a function of frequencies of (a) 5 Hz, (b) 50 Hz, (c) 500 Hz, (d) 5 kHz, (e) 50 kHz, for slowly cooled PET4oHBAgo- (Taken with permission from Macromolecules, 20, 988, 1987, American Chemical Society, Ref. 104.)...

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