Transition probability, dielectric relaxation

The glass transition (Tg) of the amorphous PVDF regions is in the range of -40 to -30°C, depending upon the sample and test method. Other sub-Tg transitions have been studied recently by dielectric relaxation spectroscopy (108). These studies also indicate correlations with other techniques and identify a 50°C molecular chain transition as probably related to the amorphous region at the surfaces of crystals (109). Permeation characteristics are very sensitive to these transitions as well as the usual environmental parameters (110). Water molecules trapped in the amorphous regions are monomeric, not associated and clustered as in the liquid state (111). 

However, it has to be pointed out that it is only an approximate calculation, because we did not consider intermolecular hydrogen bonds which surely also make the structure of lactose more rigid. In such a case the activation energy of the slower secondary relaxation should be comparable to that for the y-relaxation (Ea=44 kJ/mol). This may imply that the slower secondary relaxation seen in lactose may be undetectable in the case of acethyl derivative of this disaccharide, because maxima of both secondary relaxations can be too close to each other. In fact, the inspection of the dielectric loss spectra obtained for octa-0-acetyI-Iactose below its glass transition temperature (see Fig. 2) showed that there is only one secondary relaxation peak. However, a detailed analysis of the y- loss peak revealed that probably two secondary processes contribute to it. 

These results are confirmed by the TSC study. The solid dispersion relaxations associated with the dielectric manifestation of the glass transition appear to be similar to those of PEG (Figure 11.5 and Figure 11.7). However, differences in peak intensity can be noted in particular, we observe an increase of both Tgl and Tgu intensities. These evolutions denote a slight modification of the amorphous phase the oleic acid probably penetrates in the PEG and swells the polymer chains. As the DSC and TSC... 

Figure 21 shows the temperature dependence of the dielectric constant for N-pben-ylated polyurea, abbreviated NPUR. IWo relaxation processes were observed first with a small relaxation strength in the temperature range I0(>-12(FC, probably due to processes such as a local motion, and sec in the range 160-17(fC, where the dielectric constant increases to about 12 due to the glass transition. The DC conduction above IfifTC increased drastically. The NPUR has a dielectric constant of 3.4 at room temper-ature. The X-ray diffraction pattern shows a typical amotpbous structure for NPUR. 

By simply quenching bom melt, the o form (crystal form II) is formed, which is not ferroelectric. In this polymer three dielectric transitions may be delected, the p transition at about —(KTC. Ok o. transition near —10 and the a, transition near 1S0 C The p transition exhibits an Arrhenius-like relaxation behavior with an activatioo enthalpy of SO kJAnol it is attributed to local motions in the amorphous as well as in the crystalline state. The a. tiansitioo has a WLF-type relaxatkm beluvior it is attributed to the glass Iransitioa of the amorphous part The a, tiansitioo is attributed to motioos in the crystalline phase, but most probably not within the lamellae but in the partially... 

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