Dielectric loss transition temperature

Fig. 17 Relaxation rate of the dynamic glass transition vs. inverse temperature for different film thicknesses, as indicated. Inlet, dielectric loss vs. temperature at 31 kHz showing the dynamic glass transition of thin PS films for different film thicknesses, as indicated...

As a matter of fact, the tolerance factor is a rather complex crystaUo-chemical parameter, which can reflect the structural distortion, force constants of binding, rotation and tilt of the BOg octahedrons. These in turn affect the dielectric properties, transition temperature, temperature coefficient of the dielectric constant of material, and even the dielectric loss behavior in a perovskite dielectric. 

Glass transition temperature is one of the most important parameters used to determine the application scope of a polymeric material. Properties of PVDF such as modulus, thermal expansion coefficient, dielectric constant and loss, heat capacity, refractive index, and hardness change drastically helow and above the glass transition temperature. A compatible polymer blend has properties intermediate between those of its constituents. The change of glass transition temperature has been a widely used method to study the compatibility of polymer blends. Normally, the glass transition temperatme of a compatible polymer blend can be predicted by the Gordon-Taylor relation ... 

The glass transition temperatures ofthe polyimides are 195-250 °C their 10% weight loss temperamres (dynamic thermogravimetric analysis, air, AT = 4.5 °C/min) are 390-422 °C. Of particular interest are the dielectric constants of these polyimides. At a relative humidity of 50% these constants are 2.70-2.90 and are comparable with constants of the best fluorinated polyimides [21, 50-55]. The lowest dielectric constant (2.70) was observed for polyimide based on 6F dianhydride, containing the highest amount of fluorine. Thermal treatment of this polymer film at 280-290 °C for 1 hour led to a decrease (2.45) of dielectric constant due to the possible formation of nanofoams [56]. 

For many applications low-temperature flexibility of the plasticized composition is also important. Plasticizers of low viscosity and low viscosity-temperature gradient are usually effective at low temperature. There is also a close relationship betv/een rate of oil extraction and low-temperature flexibility plasticizers effective at low temperature are usually rather readily extracted from the resin. Plasticizers containing linear alkyl chains are generally more effective at low temperature than those containing rings. Low-temperature performance is evaluated by measuremen t of stiffness in flexure or torsion or by measurement of second-order transition point, brittle point or peak dielectric loss factor. 

However, it is interesting to perform a more direct comparison of the experimental results to check whether some differences between the mechanical and dielectric behaviours could exist as a function of temperature. The appropriate quantity is E" for the mechanics and, for the dielectric response, it is the dielectric loss modulus, m" (defined as e"/ sa + s"2)). Figure 112 shows the temperature dependence of E" and m" at 1 Hz, obtained by superposing the low-temperature part of the j3 transition. 

Whereas for PMMA (Fig. 113) the two p peaks are quite well superposed, except in the p - a crossover region, in the case of the CMIM20 copolymer, the dielectric loss is weaker than the mechanical loss in the high-temperature part of the P transition. In contrast, in the p - a crossover region, the same behaviour is observed for the mechanical and dielectric responses, showing that, in CMIM20, the CMI units lead to a complete decoupling between the P and a transitions, which is not the case for PMMA. 

The temperature dependence of the dielectric loss, e", at 1 Hz for CMIM20 and MGIM21 and PMMA, is shown in Fig. 148. It appears that the two imide groups have different effects on the dielectric response of PMMA. In the low-temperature region, the MGI unit leads to e" values lower than the CMI unit. However, the largest effect arises in the high-temperature part of the f3 transition. Indeed, whereas the CMI units strongly hinder the cooperativity of the... 

The mechanical loss modulus, and the dielectric loss modulus, m", at 1 Hz, are compared for the various MGIMx copolymers in Fig. 149, using the same scales which lead in PMMA to superposition of the low-temperature parts of the j3 transition (Fig. 112). In all cases, the mechanical loss, E , is higher than the dielectric loss, m", over the whole temperature range. 

Dielectric permittivity and loss for both polymers under study can be observed on Figs. 2.17 and 2.18. In both figures a prominent peak corresponding to the dynamic glass transition temperature can be observed, which at low frequencies is overlapped by conductivity effects. Moreover, in both polymers a broad secondary peak is observed at about -50°C. This peak is more prominent in P2tBCHM which is in good... 

Figures 2.31 and 2.32 show the dielectric permittivity and loss for poly(cyclobutyl methacrylate) (PCBuM) and poly(cyclobutylmethyl methacrylate) (see Scheme 2.5). In these figures the a relaxation is associated to the glass transition temperature and the p relaxation appear as a shoulder of the a relaxation.

Because the time of electron transit over a cluster from N particles is rN = Nxt it is easy to understand that in this model intensity of losses grows with reduction of a field frequency v down to value vum 1/2t2 and then remains constant. The temperature range for these dielectric losses is determined by a relationship between ti, t2, and xt and does not depend on a frequency of the electromagnetic field that is in accordance with experimental data. 

The volumetric, elastic and dynamic properties of internally and externally plasticised PVC were studied and compared with those of unplasticised PVC. The glass transition temperature for the plasticised samples was markedly lowered and this decrease was more important for the externally plasticised ones. The positions of the loss peaks from dielectric alpha-relaxation measurements confirmed the higher efficiency of the external plasticisation. However, the shape of the dielectric alpha-relaxation function was altered only for the internally plasticised samples. The plasticisation effect was linked with a decrease in the intensity of the beta-relaxation process but no important changes in the activation energy of this process were observed. The results were discussed. 47 refs. 

Fig. 7.9 shows the temperature dependence of the dielectric constant and dielectric loss at 1 kHz for the PMN-PT ceramics obtained by sintering the calcined powders from a soft-mechanochemical route at 1200°C for 2 h. A diffuse phase transition, being typical for a relaxor, is observed for each ceramics. As x increases from 0 to 0.2, the maximum dielectric constant, K, , increases from 13000 to 27000. The temperature correspondent to K ,... 

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. 

It is important to select the components of the substrates with care and particularly to pay attention to the physical parameters they act upon, in particular the vitreous transition temperature (7g ) of the deep-frozen vaccine [20-25,30]. This temperature, also referred to as vitreous eutectic temperature, does indeed play a critical role in the deformation and collapsing of freeze-dried pellets [20,25], and possibly in the loss of infectivity titers. This temperature is dependent on the nature and concentration of the substrate molecules and may be determined in several ways [20,29,35,36]. In industrial practice, the most commonly utilized techniques are differential scanning calorimetry (DSC) as well as resistance and/or dielectric constant measurements. 

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