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Unformatted text preview: EXPERIMENTAL DATA (FREQUENCY DOMAIN) W e have acquired sufficient theoretical foundation to understand and interpret the results of experimental measurements obtained in various materials. Both the dielectric constant and dielectric relaxation will be considered and results presented will follow, as far as possible, the sequence of treatment in the previous chapters. Anyone familiar with the enormous volume of data available will appreciate the fact that it is impossible to present all of the data due to limitations of space. Moreover, several alternative schemes are possible for the classification of materials for presentation of data. Phase classification as solids, liquids and gases is considered to be too broad to provide a meaningful insight into the complexities of dielectric behavior. A possible classification is, to deal with polar and non-polar materials as two distinct groups, which is not preferred here because in such an approach we need to go back and forth in theoretical terms. However, considering the condensed phase only has the advantage that we can concentrate on theories of dielectric constant and dielectric loss factor with reference to polymers. In this sense this approach fits well into the scope of the book. So we adopt the scheme of choosing specific materials that permit discussion of dielectric properties in the same order that we have adopted for presenting dielectric relaxation theories. As background information a brief description of polymer materials and their morphology is provided because of the large number of polymer materials cited. We restrict ourselves to experimental data obtained mainly in the frequency domain with temperature as the parameter, though limited studies at various temperatures using constant frequency have been reported in the literature. Measuring the real part of the dielectric constant centers around the idea that the theories can be verified using molecular properties, particularly the electronic polarizability, and the dipole moment in the case of polar molecules. A review of studies of dielectric loss is published by Jonscher 1 which has been referred to previously. The absorption phenomena in gases and liquids in the microwave region has been has been treated by Illinger 2 and we restrict ourselves to the condensed phases. The experimental techniques used to measure dispersion and relate it to the morphology, using electrical methods include some of the following: 1. Measurement of s'and z"at various frequencies; each set of frequency measurement is carried out at a constant temperature and the procedure repeated isothermally at other selected temperatures (See fig 5.36 for an example). Plots of s"- log/ exhibit a more or less sharp peak at the relaxation frequency. In addition the loss factor due to conductivity may exhibit a low frequency peak. The conductivity may be inherent to the polymer, or it may be due to absorbed moisture or deliberately increased in preparing the sample to study the variation of conductivity with temperature or frequency. Fig. 5.1the sample to study the variation of conductivity with temperature or frequency....
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- Spring '10
- Polymer, glass transition temperature, Marcel Dekker