Page 40 - Handbook of Thermal Analysis of Construction Materials

24 Chapter 1 - Thermoanalytical Techniques

temperature, and frequency. The capacitive nature of a material is the
ability to store electric charge whereas the conductive nature is the ability
to transfer electric charge. The parameters measured in dielectric analysis
are permitivity (ε´) and the loss factor (ε´´). [60] The former is the alignment
of the molecular dipoles in the material and the latter represents the energy
required to align the dipoles or move trace ions.
DEA is used in the characterization of thermoplastics, thermosets,
composites, adhesives, and coatings, and it is complementary to other
thermoanalytical techniques such DSC, DMA, TG, and TMA. DEA is an
important technique because it has high inherent sensitivity, wide fre-
quency range, and the ability to easily detect rheological changes that occur
during heating of uncured materials. [61]
The mobility of ions and dipoles is measured by applying a
sinusoidal voltage to the sample and measuring the current (Fig. 9). [62]
Process behavior, the physical and chemical structure of polymers, and
other organic materials can be investigated through the measurement of
their electric properties. The charged sites found in organic and inorganic
polymers are typically ions and dipoles. Dipoles in the material will attempt
to orient themselves with the applied electric field, while charged ions,
usually present as impurities, will move towards the electrodes of opposite
polarity. Changes in the degree of alignment of dipoles and in the ion
mobility provide information about physical transitions in the material and
about material properties such as viscosity, rigidity, reaction rate, and
degree of cure. [62]

Figure 9. DEA excitation and response. The mobility of ions and dipoles is
measured applying a sinusoidal voltage to the sample and measuring the current.
(Reprinted with permission.) [62]