Dielectric Spectroscopy

Dielectric spectroscopy can provide information about the segmental mobility of a polymer by probing its dielectric properties. The complex dielectric properties, the loss factor(e") and the relative permittivity(e'), are determined by performing several isothermal scans as a function of frequency. An alternating current(Vrms=0.005-1.1 volts) external electric field is applied across the DUT(Device Under Test) in a capacitor plate configuration. The applied alternating electric field interacts with the electric dipole moments of the DUT. The polarization of the DUT is related to the applied electric field by (Equation 1):

(1) (2)

where Xe is the electric susceptibility(Equation 2) of the material. This global polarization is consisted upon by four different dielectric mechanisms(electronic, atomic, orientation or dipolar, and ionic polarization).

Fig. 1: Frequency response of dielectric mechanisms 1

Each dielectric mechanism effect has a characteristic relaxation frequency. As the frequency becomes larger, the slower mechanisms drop off. This in turn leaves only the faster mechanisms to contribute to the dielectric storage(k'=e'/eo). The dielectric loss factor(k"=e"/eo) will correspondingly peak at each critical frequency. 1 Electronic polarization occurs in neutral atoms when an electric field displaces the nucleus with respect to the electrons that surround it. Atomic polarization occurs when neighboring positive and negative ions "stretch" under an applied electric field. Both electronic and atomic polarization create induced moments depending on the polarizability of the atoms or molecules. 2 A permanent dipole moment is caused by unbalanced sharing of electrons by atoms of a molecule. In an absence of an external electric field, these moments are oriented in a random order such that no net polarization is present. Under an external electric field, the dipoles rotate to align with the electric field causing orientation polarization to occur. The ionic polarization is composed of ionic conductivity and interfacial or space charge polarization. At low frequencies ionic conduction is the most prevalent mechanism. Ionic conduction only introduces losses into a system. Space charge polarization occurs when more than one material component is present or when segregation occurs in a material containing incompatible chemical sequences and when translating charge carriers become trapped at the interfaces of these heterogeneous systems. 2 The electric field distortion caused by the accumulation of these charges increases the overall capacitance of a material which appears as an increase in k'.

Dielectric relaxation is the result of a movement of dipoles or electric charges due to a changing electric field in the frequency range of 10^2-10^10 Hz. This mechanism is a relatively slow process when compared with electronic transitions or molecular vibrations which have frequencies above 10^12 Hz. Only when sufficient time is allowed after the application of an electric field for the orientation to attain equilibrium will the maximum polarization, corresponding to the highest observable dielectric constant, be realized in a material. If time is allowed, then the observed dielectric constant is the static dielectric constant, Es. If the polarization is measured immediately after the field is applied, not allowing time for dipole orientation, then the instantaneous dielectric constant, e inf, is observed 3. The relaxation time occurs somewhere in between these extremes. The relaxation time for this energy-absorbing process is t= [2(pi)fmax]^-1 (See Figure 3).

Figure 2: Parallel Plate Sample Test Cells for Dielectric Measurement

Figure 3: Dielectric Relaxation Spectra

1 Application Note 1217-1, "Basics of measuring the dielectric properties of materials," Hewlett Packard literature number 5091-3300E, 1992.
2 Boiteux, Gisele, "Dielectric and Related Molecular Processes in Ion Containing Polymers," Structure and Properties of Ionomers, 227-245, D. Reidel Publishing Company, 1987.
3 Blythe, A. R., Electrical Properties of Polymers, Cambridge University Press, 1979.

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