Polymers have long been known as insulating materials and are often used to insulate cables and electrical devices. They are also called dielectrics. Traditionally, dielectric materials are made from inorganic materials such as porcelain, mica, and quartz. However, polymers are also used as dielectric materials. They have the advantage that they can be easier processed, are flexible, cheaper, and can be better tailored for specific applications. They also have excellent chemical resistance. Some disadvantages of polymeric materials are their much lower upper service temperature, their susceptibility to atmospheric and hydrolytic degradation and their flammability. They also have a much larger coefficient of thermal expansion compared to ceramic materials.
An important property of an insulator is its relative permitivity or dielelectric constant (εr). It describes the tendency of a material to polarize in response to an applied electrical field. It is a dimensionless parameter, defined as the ratio of the external (applied) field E0 (the field without the dielectric) to the electric field inside the dielectric, E:
εr = E0 / E
The field inside the dielectric, E, is the vector sum, E = E0 + Ep, of the external field E0 and the field Ep caused by the substance polarization P
Ep = - P / ε0
where ε0 is the permittivity of the vacuum. The polarization is usually proportional to the macroscopic field, particularly if the field is weak:
Ep = - χ E,
εr = 1 + χ
χ is the macroscopic susceptibility of the dielectric which is another dimensionless parameter that depends on the material.
The dielectric properties (susceptibility, polarizability, permitivity, etc.) of any polymer depend on its chemical composition and structure. Non-polar plastics with a symmetrical structure and truly covalent bonds are the best insulators. Since there are no dipoles present in the repeat units, an applied field cannot cause any dipoles to allign in field direction which would weaken the electrical field. The electrical field can, however, push the elctrons slightly in the directions of the positive electrode which creates some polarization. This effect is instantaneous, i.e. the frequency of the electrical field will not much affect this form of polarization, or in other words, the dielelctric constant of non-polar materials is independant of the frequency.
Examples of non-polar plastics are PTFE, PP, PE, and PIB. These materials have very high resistivity and low dielelctric constants.
Most other plastics have polarized covalent bonds, that is, the electrons are drawn closer to the more electronegative atoms. These dipoles will orient themselves to lign up with the electrical field, very similar to a compass needle that attempts to lign up with the earth's magnetic field. The result is a much stronger dipole polarization and a higher relative permeability (dielectric constant) and lower resistivity compared to non-polar polymers.
The polarization of a dielectric material, P (C/m2), is defined as the dipole moment per unit volume. On a microscopic scale, the degree of polarization depends on the polarizability of the molecules (repeat units) and the local electric field, Eloc,
P = Σi ni αi Eloc
where αi is the polarizability of a structural element i and ni is its concentration (number of structural elements i per unit volume),
The relationship between the polarization, P, and the applied electric field E is
P = ε0 E (ε - 1)
And the relationship between the local field, Eloc, and the polarization, P, is
Eloc = E + P / (3 ε0)
Combining all three expression gives the Clausius-Mosotti relation:
The total polarization associated with atoms, ions, and molecules has three different contribution:
Electronic polarization due to displacement of the local electronic charge clouds around each nucleus by the electric field.
Ionic polarization in ionic materials due to displacement of cations and anions in opposite directions.
Orientational polarization due to alignment of permanent electric dipoles
Both ionic and orientational polarization depend on the temperature and field frequency whereas the electronic polarization is more or less independent of the frequency and shows a much weaker dependency on the temperature (instantaneous polarization of non-polar plastics).
Since the electrical forces due to polarizability and polar moments determine the cohesive energy, a correlation between the dielectric constant and the cohesive energy (or solubility parameter) can be expected. The relationship between these two parameters is shown in the figure below. It appears that a simple linear correlation exists:
δ ≈ 6.8 ε