Dielectric analysis

Dielectric measurements on PET [13], over a range of five different frequencies between 1 Hz and 10 kHz, at temperatures between - 120 and 80 °C, are shown in Figs. 12 and 13 for the dielectric constant, er the loss tangent, tan 5, respectively. 

The p peak occurs at - 70 °C at 1 Hz. As the only significant dipoles in PET that are mobile are the carboxyl groups, the dielectric p relaxation can be unambiguously attributed to the motion of these groups. Furthermore, the dielectric p peak is quite symmetrical and can be fitted by a Gaussian curve. 

The associated activation energy (Table 1) is 56=bl0kJ, a value significantly lower than the one derived from mechanical measurements (Table 1). In the same way, the activation entropy (Table 2) of 53 d= 10 J K-1 mol-1 corresponding to the centre of the relaxation peak is lower that the value obtained from mechanical measurements. The symmetrical character of the dielectric P peak is also reflected in the constant value of ASa over the temperature range (Table 2). 

From another point of view, the dielectric increment, Aer of the p relaxation process is 0.52, which is much smaller than the one calculated for 

Dielectric analysis of bulk polymer properties has been well documented. Similarly, DEA has often been employed in monitoring polymer reactions.Another, quite common application uses DEA to measure the breadth of a polymer relaxation. This analysis is directly related to the coupling parameter, which quantifies cooperativity. DEA has been used for characterizing thin polymer films. However, the polymer systems that have been studied involve free-standing films that have been sputter coated on both sides. Adding conductive substrates, such as aluminum foil, has also been accomplished, as well as relying on sputter coated, conducting surfaces. 

Capacitance, the ability of a polymer to store an electrical change, dominates at temperatures 

E (permittivity) a measure of the degree of alignment of the molecular dipoles to the applied electrical field (analogous to the elastic modulus in DMA). 

E (loss factor) it represents the energy required to align the dipoles (analogous to the viscous modulus in DMA). 

DBA is worthy of mention because, although it can be used for a very similar range of applications to DSC and DMA, it has a number of practical advantages over those techniques. 

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Dielectric analysis (DEA) measures the electrical properties of a material as it is subjected to a periodic electric field under various conditions. This technique provides quantitative information on the capacitance and conductance of the ma-... 

As applied to thermal analysis, dielectric analysis consists of the measurement of the capacitance (the ability to store electric charge) and conductance (the ability to transmit electrical charge) as functions of applied temperature. The measurements are ordinarily conducted over a range of frequencies to obtain full characterization of the system. The information deduced from such work pertains to mobility within the sample, and it has been extremely useful in the study of polymers. 

Morris et al. [1.126] proposed to use dielectric analysis (DEA) to predict the collapse temperature of two component systems. The background of DEA is explained and the take off frequency (TOF) is chosen as the best analytical method to identify the collapse temperature. Figure 1.55.5 shows the dielectric loss factor as a function of the frequency. 

Bidstrup, S.A. and Day, D.R. 1994. Assignment of the glass transition temperature using dielectric analysis A review. In Assignment of the Glass Transition (RJ. Seyler, ed.), pp. 108-119. American Society for Testing and Materials, Philadelphia, PA. 

Dielectric analysis (electrothermal analysis, dielectric spectroscopy) is the measurement of dielectric properties as a function of frequency and temperature. It is increasingly finding use in characterising polymer structure and, in particular, the curing process. Its use in this respect has been considered in Chapter 6. 

Deng and Martin (1996) also showed the necessity of including a diffusional resistance in the rate equation for the cyclotrimerization of dicyanates, well before vitrification. They observed a significant decrease in the diffusion coefficient from conversions of about 0.40, using dynamic dielectric analysis. They could fit experimental kinetic data in the whole conversion range using Eq. (5.50). Experimental values of the decrease in the diffusion coefficient with conversion were used to estimate kd for different cure temperatures. 

Morris et al. [1.126] proposed the use of dielectric analysis (DEA) to predict the collapse temperature of two-component systems. The background of DEA is explained and the >take-off frequency< (TOF) is chosen as the best analytical method to identify the collapse temperature. Figure 1.55.5 shows the dielectric loss factor as a function of the frequency. The frequency at the minimum of this curve is called TOF by the authors. TOF varies with the temperature as shown in Figure 1.55.6. The extrapolated intersection of the two linear portions identifies the collapse temperature. The predicted Tc by TOF for 10% sucrose, 10% trehalose, 10% sorbitol and 11% Azactam solution deviates from observations with a freeze-drying microscope (Table 1 in [1.126]) to slightly lower temperatures, the differences being -3, -1.4, 2.2 and 0.7 °C. 

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