Assignment of the Dielectric Relaxations

In the copolyamides under consideration, the dipoles that are responsible for the dielectric relaxations are associated with the C = 0 groups of the amide functions. Due to the quasi-conjugated character of the CO - NH bond, the amide group takes on a rigid plane conformation in such a way that the dielectric relaxations of copolyamides should correspond to motional modes that involve amide groups and not only the carbonyls, in contrast to what happens with the ester groups encountered in polyethylene fere-phthalalc (Sect. 4.1.2). 

COahph groups, which are non-conjugated carbonyl groups situated between two aliphatic units, a flexible lactam-12 on one side and a rigid cycloaliphatic unit on the other side. In addition, the 1.8T copolyamide also possesses a C = 0 group, noted COaiiph 2 located between two lactam-12 segments. 

COarom i groups, which are located between a flexible lactam-12 or methylpentane unit and a phenyl ring and share some conjugation with the phenyl ring. 

Regarding the assignment of the transitions, the simplest compound is the MT polymer for it contains COarom 1 groups only. As its dielectric trace exhibits only one transition, this /3 transition can be unambiguously assigned to the motion of these COarom 1 dipoles. 

The dielectric /9 transition of the xTyli-y polymers (Figs. 77 and 78), whose characteristics in terms of temperature and activation energy are very close to those of MT, can also be assigned to the COarom i motions. 

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The assignment of the alpha relaxation was additionally confirmed by AC-calorimetric measurements on thin films of POHOAc [31], The effect of confinement on the overall dynamics of POHOAc is given in Fig. 24, showing the dielectric spectra for different film thicknesses, ranging from 310 nm down to 17 nm. While the beta and the gamma relaxations, as local processes, are not affected with decreasing film thickness (except a decrease of the dielectric... 

Figure 6.19 displays the temperature dependence of the relaxation rate, as derived from the maxima of the loss curves. For a comparison it also includes the temperature dependencies of the loss maxima of the mechanical a-process, as observed in measurements of either J" co) or G" lo). As we can see, the dielectric relaxation rates are located intermediately between the rates obtained in the mechanical experiments and, importantly, all three temperature dependencies are similar, the rates differing only by constant factors. The assignment of this dielectric relaxation process is therefore obvious It originates from the same group of processes as the mechanical a-process and thus is to be addressed as the dielectric a-process. 

Fast relaxation processes ( , 0) show a Williams-Landel-Ferry (WLF) type temperature dependence which is typical for the dynamics of polymer chains in the glass transition range. In accordance with NMR results, which are shown in Fig. 9, these relaxations are assigned to motions of chain units inside and outside the adsorption layer (0 and , respectively). The slowest dielectric relaxation (O) shows an Arrhenius-type behavior. It appears that the frequency of this relaxation is close to 1-10 kHz at 240 K, which was also estimated for the adsorption-desorption process by NMR (Fig. 9) [9]. Therefore, the slowest relaxation process is assigned to the dielectric losses from chain motion related to the adsorption-desorption. 

The plot shows a distribution closely around a slope of unity indicated by the solid line in Figure 2 except for the alcohols and nitrobenzene. Such anomaly in alcohols is also reported for other chemical processes and time-dependent fluorescence stokes shifts and is attributed to their non-Debye multiple relaxation behavior " the shorter relaxation components, which are assigned to local motions such as the OH group reorientation, contribute the friction for the barrier crossing rather than the slower main relaxation component, which corresponds to the longitudinal dielectric relaxation time, tl, when one regards the solvent as a Debye dielectric medium. If one takes account of the multiple relaxation of the alcohols, the theoretical ket (or v,i) values inaease and approach to the trend of the other solvents. (See open circles in Figure 2.)... 

From Figure 10 it appears that a dipolar relaxation labeled a is superimposed on the phenomenon we have just discussed. The behavior of this a peak correlates well with the behavior of the dynamic mechanical a relaxation since it increases in magnitude and decreases in temperature with increasing sulfonation. The presence of this peak in the dielectric spectra of these materials and its behavior as a function of sulfonate concentration are consistent with the assignment of the mechanical a relaxation to an ionic-phase mechanism. However, it is not possible to cite this dielectric peak as proof of the mechanical assignment the known presence of ionic impurities in these systems and the unknown origin of the large increases in tan 8 and c dictate that the dielectric results be interpreted with caution. 

Figure 9.4 shows a comparison of the dielectric and mechanical relaxation spectra of various forms of polyethylene. The most obvious feature is that the main relaxations, here the a, (3 and y relaxations, occur at approximately the same temperatures in both spectra, although their relative relaxation strengths in the two spectra are diiferent. This is a feature common to the spectra of many polymers. The peaks are not, however, in exactly the same positions in the two spectra for the same type of polyethylene. In addition to the possible reasons for this described above, the frequencies of measurement are diiferent. The dielectric measurements were made at a much higher frequency than that used for the mechanical measurements, as is usual. Molecular motions are faster at higher temperatures, so this factor alone would lead to the expectation that the dielectric peaks would occur at a higher temperature than the mechanical peaks. The y peak, which is assigned to a localised motion in the amorphous material and is in approximately the same place for all samples, behaves in accord with this expectation. 

Although the frequency dependence of the dielectric spectrum contains a (mostly orientational) response from all of the molecules (water, biomolecules, and ions) in the system, assignment to the orientational relaxation of individual species is possible when they are well separated in the frequency (or time) scales. 

Figure 2 shows the dielectric loss for the polymer PM5 versus temperature for different frequencies. Different relaxation regions indicated by peaks in e" are observed. At low temperatures a p-relaxation takes place. (For the highest frequency a further relaxation process can be seen which takes place at lower temperatures than the p-relaxation. This process has to be assigned to the Y-relaxation which is due to rotational fluctuation of the tail group.) This relaxation process is followed by the a-relaxation at higher and by the 6-process at the highest measuring temperatures. This relaxational behavior is quite the same for the other polymers (see Figure 3). However, it should to be noted that the a-relaxation is more suppressed for the even and shorter spacer lengths.

The viscoelastic behaviour of the majority amorphous phase is important in terms of the mechanical properties of the material. Dynamic mechanical (S) and dielectric studies (8) reveal three relaxations, labelled a, p and 7 in order of decreasing temperature. Variations in the strength of these relaxations with systematic changes in the mole fraction of each component have led to the association of the 7 relaxation with the HBA component and the p relaxation with the HNA component, with the a relaxation displaying features typical of a glass transition process. Support for these assignments has been obtained from analysis of the proton NMR second moments (9,10),... 

Hence the dielectric relaxation process cannot possibly be assigned to the relaxation of the transport of charges their presence either as free ions or as triplets accounts only to a minute minority of the total solute population around csQ.lM. 

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