Dielectric relaxational behavior

The dielectric behavior of PMCHI was studied by Diaz Calleja et al. [210] at variable frequency in the audio zone and second, by thermal stimulated depolarization. Because of the high conductivity of the samples, there is a hidden dielectric relaxation that can be detected by using the macroscopic dynamic polarizability a defined in terms of the dielectric complex permittivity e by means of the equation  

The transformation performed by this equation is a good way for the analysis of the dielectric relaxations in the zone of high temperatures and low frequencies of the spectrum. 

At the same time, the activation energy calculated from the maxima in tan 8 increases from 28 to approximately 56 kcal mol-1. This effect together with the asymmetric shape of the curves would indicate the presence of two sub relaxations, Pi and p2 in the increasing temperature sense. 

The high conductivity observed at low frequencies would overlap the existence of another new relaxation (Fig. 2.78) what is very similar to that found for transfer complexes [243, 244] in which some of them have a pronounced semiconductor character. In this kind of compounds, the observation of dielectric relaxations over room temperature is inhibited by the high conductivity observed. To avoid this problem and to detect the conductivity effect, it is possible to use the complex polarizability a defined by equation [182], The transformation defined by equation (2.45), has been applied with good results in the case of dielectric relaxation peaks in terms of a or tan 8 . 

The variation of tan 8a (referred to the polarizability a) as function of temperature is shown in Fig. 2.79. The activation energy value for this relaxation, calculated according to an Arrhenius equation for the five maxima, is 16kcal mol 1. This is a small value for a glass transition temperature, but not too much considering that in this case a small part of the macromolecule is activated from the dielectric point of view at higher temperatures than that of the p relaxation. 

---

Optical and electro-optical behavior of side-chain liquid crystalline polymers are described 350-351>. The effect of flexible siloxane spacers on the phase properties and electric field effects were determined. Rheological properties of siloxane containing liquid crystalline side-chain polymers were studied as a function of shear rate and temperature 352). The effect of cooling rate on the alignment of a siloxane based side-chain liquid crystalline copolymer was investigated 353). It was shown that the dielectric relaxation behavior of the polymers varied in a systematic manner with the rate at which the material was cooled from its isotropic phase. 

The accessibility of chitin, mono-O-acetylchitin, and di-O-acetylchitin to lysozyme, as determined by the weight loss as a function of time, has been found to increase in the order chitin < mono-O-acetylchitin < di-O-acetylchitin [120]. The molecular motion and dielectric relaxation behavior of chitin and 0-acetyl-, 0-butyryl-, 0-hexanoyl and 0-decanoylchitin have been studied [121,122]. Chitin and 0-acetylchitin showed only one peak in the plot of the temperature dependence of the loss permittivity, whereas those derivatives having longer 0-acyl groups showed two peaks. 

Poly(cyclobuty methacrylate)s Dielectric relaxational behavior... 

The dielectric relaxational behavior of several poly(diitaconate)s containing cyclic rings in the side chain (see Scheme 2.18) show different behaviors at low temperatures depending on the chemical structure of the polymers. 

Important contributions to the effect of the substituents in the carbon atom between both phenyl rings i.e. the interphenylic carbon atom was reported by Sundararajan [258-260], Bicerano and Clark [261,262], as well as Hutnik and Suter [263,264], Diaz Calleja et al. [265] have reported the dielectric relaxational behavior of a family of poly(thiocarbonate)s with the basic chemical structure shown in Scheme 2.19. 

Williams, G., Watts, D. C. Non-symmetrical dielectric relaxation behavior arising from a simple empirical decay function, Trans. Farad. Soc., 66, 80 (1970)... 

The situation is more complex in the case of the so-called non-Debye liquids -the protic solvents. Due to their internal structure, these liquids exhibit a complicated dielectric relaxation behavior. This group of solvents comprises alcohols, formamide, propylene carbonate, and some other liquids. One should remember that in the In vs. In Tl analysis (Sec. 3.1.3), the rate constants measured in these solvents deviated from the values measured in aprotic solvents. 

This short discussion shows that in the case of the non-Debye solvents further work is necessary on both the electrode kinetics and the dielectric relaxation behavior of the solvents in the presence of various electrolytes. There are also significant discrepancies between the results on the relaxation dynamics of these solvents reported by various authors (see [169]). 

As far as comparison with experimental data is concerned, the fractional Klein-Kramers model under discussion may be suitable for the explanation of dielectric relaxation of dilute solution of polar molecules (such as CHCI3, CH3CI, etc.) in nonpolar glassy solvents (such as decalin at low temperatures see, e.g., Ref. 93). Here, in contrast to the normal diffusion, the model can explain qualitatively the inertia-corrected anomalous (Cole-Cole-like) dielectric relaxation behavior of such solutions at low frequencies. However, one would expect that the model is not applicable at high frequencies (in the far-infrared region), where the librational character of the rotational motion must be taken... 

Table 6. Dielectric relaxation behavior of epoxy resin systems before crosslinking ...

Noel, T.R., Parker, R., and Ring, S.G. A comparative study of the dielectric relaxation behavior of glucose, maltose, and their mixtures with water in the liquid and glassy states, Carbohydr. Res., 282, 193, 1996. 

Hynes [43] has also discussed the dynamic solvent effect for solvents with more complex dielectric relaxation behavior. In the following it is assumed that the... 

Molecular theories describing the dielectric relaxation behavior of polymers have been developed and are summarized in references 5, 12 and 13. Again, if the dipoles are rigidly attached along the chain contour, normal mode theories such as those of Rouse and Bueche described in Chapter 3 for the mechanical case might be expected to be applicable. In addition, the time-temperature superposition principle also generally applies. 

Cheng ZY, Zhang QM, Bateman FB (2002) Dielectric relaxation behavior and its relation to microstructure in relaxor ferroelectric polymers high-energy electron irradiated poly(vinylidene fluoridetrifluoroethylene) copolymers. J Appl Phys 92 6749... 

The WLF equation can be used to describe the temperature dependmee of dynamic mechanical and dielectric relaxation behavior of polymCTS near the glass transition where the response is no longer described by an Arrhraiius relation. This will be dealt with in Chapter 13. 

Analysis of the Dielectric Relaxation Behavior Far Below Percolation... 

---