Guide Polymeric Foams: Mechanisms and Materials (Polymeric Foams Series)

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Polymer Dielectric Materials

Find Rare Books Book Value. Sign up to receive offers and updates: Subscribe. Quantitative treatment of a dielectric in an electric field can be summarized using Clausius—Mossotti equation 1. This equation shows that dielectric constant is dependent on polarisability and free volume of the constituents element present in the materials.

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Polarisability refer to the proportionality constant for the formation of dipole under the influence of electric field. Thus its value is typical for each different type of atom or molecule. The dependency of free volume of relative permittivity thus originate from the volume involved in one mole of the material.

Again the molar volume is characteristic of each different type of atom or molecule. Molar polarization therefore is obtained if the molar volume is introduced into these derivations leading to Clausius—Mossotti equation. Physically, polarisability is induced when there is electric field applied onto the materials.

In the absence of electric field, the electrons are distributed evenly around the nuclei. When the electric field is applied the electron cloud is displaced from the nuclei in the direction opposite to the applied field. This result in separation of positive and negative charges and the molecules behave like an electric dipole. There are three mode of polarizations [ 10 ]:. Orientational polarization — For polar molecules, there is a tendency for permanent dipole to align by the electric field to give a net polarization in that direction.

However if the field changes as when alternating electric current is applied, polarization will also oscillate with the changing electric field. All three modes of polarization contributing to the overall dielectric constant will be dependent on the frequency of the oscillating electric field.

Obviously the electronic polarization is instantaneous as it is able to follow in phase with the changing electric field compared to atomic polarization which in turn better able to follow the oscillating electric field compared to the orientational polarization. Certain structures and elements display a higher polarisibility than the others. Aromatic rings, sulphur, iodine and bromine are considered as highly polarisable. The present of these groups induced an increase in dielectric constant. Therefore it is easily polarized.

For large size atoms like bromine and iodine, the electron cloud is so large and further apart from the influence of electrostatic attraction of the positive nucleus. It is expected to display a high polarisibility.

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This is as oppose to fluorine which has small atomic radius and concentrated negative charge. It is able to hold the electron cloud much tightly resulted in a low polarisability. This will induce a lower dielectric constant. Free volume is also an important factor in determining the dielectric constant. Free volume is defined as the volume which is not occupied by the polymeric material. The addition of pendant groups, flexible bridging units, and bulky groups which limit chain packing density have been utilised to enhance free volume. A higher fractional free volume means that the density of the material will be lower resulting in a lower polarisible group per unit volume.

Replacement of hydrogen with fluorine result in lowering of dielectric constant since fluorine occupies higher volume. Thus beside being low polarisability, introduction of fluorine induce a significant decrease of dielectric constant through an increase in free volume.

Polymers can be polar or non-polar. This feature affect significantly the dielectric properties. Under alternating electric field, polar polymers require sometime to aligned the dipoles. At very low frequencies the dipoles have sufficient time to align with the field before it changes direction. At very high frequencies the dipoles do not have time to align before the field changes direction.

At intermediate frequencies the dipoles move but have not completed their movement before the field changes direction and they must realign with the changed field. The electronic polarization and to some extent atomic polarization, is instantaneous weather at high or low frequency for both polar and non polar polymers. Therefore, polar polymers at low frequencies eg 60 Hz generally have dielectric constants of between 3 and 9 and at high frequencies eg Hz generally have dielectric constants of between 3 and 5.

For non-polar polymer the dielectric constant is independent of the alternating current frequency because the electron polarization is effectively instantaneous hence they always have dielectric constants of less than 3. The chain geometry determines whether a polymer is polar or non-polar. If the polymer is held in a fix confirmation, the resulting dipole will depend whether their dipole moments reinforce or cancell each other. In the case of extended configuration of PTFE, the high dipole moment of —CF 2 - units at each alternating carbon backbone cancelled each other since their vector are in opposite directions. Its dielectric constant therefore is low 2. On the other hand, PVC has its dipole moment directing parallel to each other resulting in reinforcement of dipole. Its dielectric constant is 4. This is illustrated as in Figure 2. The designing of dielectric material so as to achived the desired dielectric properties should take careful consideration of net polarity of the structure.

This has been exemplified by the opposite effect in indiscriminately subsitituting fluorine atom into a polyimide chain resulting in an increase in otherwise low dielectric constant material. This different mode of mechanism lead to the resonance spectra in the case of electronic polarization which occur at frequency beyond 10 12 Hz.

At below this frequency, the relaxation spectra prevail relating to the behavior of dipole polarization. This observation can best be summarized as in the following Figure 3 :. Dielectric constant and loss dispersion of dielectric materials against frequency adapted from Wikipedia. Relative permittivity can be express in complex form as in Equation 4 below:.

1. Introduction

It consist of the real part which is dielectric constant and the imaginary part which is the dielectric loss. Dielectric loss result from the inability of polarization process in a molecules to follow the rate of change of the oscillating applied electric field. It does not occur instatntaneously but the polarization diminished exponentially.

If the relaxation time is smaller or comparable to the rate of oscillating electric field, then there would be no or minimum loss. However when the rate of electric field oscillate well faster than the relaxation time, the polarization cannot follow the oscillating frequency resulting in the energy absorption and dissipated as heat. Dipole polarisability is frequency dependent and can be shown as in Equation 5. It normally occur in the microwave region. Figure 3 above shows the variation in real dielectric constant with the imaginary dielectric loss.

This maximum represent the complete failure for the dipole to follow the oscillating electric field beyond which the dipole remain freeze with no effective contribution to the dielectric constant. The mechanism for electronic and atomic polarization occur at higher frequency shorter wavelength eg. This region involved excitation of electrons which is characterized by the quantized energy level hence is known as resonance behaviour.

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The dielectric constant display a maximum before a symmetrical drop about a certain frequency. These maximum and minimum represent the optimum polarization in phase with the oscillating frequency. At this point the frequency of applied electric field is at resonant with the natural frequency hence there is a maximum absorption. Temperature affect dielectric properties.

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As the temperature is increased the intermolecular forces between polymer chains is broken which enhances thermal agitation. The polar group will be more free to orient allowing it to keep up with the changing electric field. At lower temperature, the segmental motion of the chain is practically freezed and this will reduce the dielectric constant. At sufficiently higher temperature, the dielectric constant is again reduced due to strong thermal motion which disturb the orientation of the dipoles. At this latter stage the polarization effectively contribute minimal dielectric constant.

Beside the kinetic energy acquired, free space in the polymer matrix is of necessity so as to induce segmental movement.