Page 506 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Testing and Spectrometric Characterization of Polymers 469

13.9 ELECTRIC MEASUREMENTS

Material response is typically studied using either direct (constant) applied voltage (DC) or alter-
nating applied voltage (AC). The AC response as a function of frequency is characteristic of a
material. In the future, such electric spectra may be used as a product identification tool, much

like IR. Factors such as current strength, duration of measurement, specimen shape, temperature,
and applied pressure affect the electric responses of materials. The response may be delayed
because of a number of factors, including the interaction between polymer chains, the presence
within the chain of specific molecular groupings, and effects related to interactions in the specifi c

atoms themselves. A number of properties, such as relaxation time, power loss, dissipation factor,
and power factor, are measures of this lag. The movement of dipoles (related to the dipole polar-
ization, P) within a polymer can be divided into two types: an orientation polarization (P′) and a
dislocation or induced polarization.
The relaxation time required for the charge movement of electronic polarization E to reach equi-
–15
librium is extremely short (about 10 s) and this type of polarization is related to the square of the
–3
index of refraction. The relaxation time for atomic polarization A is about 10 s. The relaxation
time for induced orientation polarization P’ is dependent on molecular structure and it is tempera-
ture dependent.
The electric properties of polymers are also related to their mechanical behavior. The dielec-
tric constant and dielectric loss factor are analogous to the elastic compliance and mechanical
loss factor. Electric resistivity is analogous to viscosity. Polar polymers, such as ionomers, possess
permanent dipole moments. These polar materials are capable of storing more electric energy
than nonpolar materials. Nonpolar polymers are dependent almost entirely on induced dipoles for
electric energy storage. Thus, orientation polarization is produced in addition to the induced polar-
ization when the polar polymers are placed in an electric field. The induced dipole moment of a

polymer in an electric field is proportional to the field strength, and the proportionality constant

is related to the polarizability of the atoms in the polymer. The dielectric properties of polymers
are affected adversely by the presence of moisture, and this effect is greater in hydrophilic than in
hydrophobic polymers.
The Clausis–Mossotti equation 13.8 shows that the polarization of a polymer, P, in an electric

field is related to the dielectric constant, e, the molecular weight, M, and the density, ρ.

P = (e − 1/e + 2) M/ρ (13.8)

At low frequencies, the dipole moments of polymers are able to remain in phase with changes in

a strong electric field resulting in low power losses. However, as the frequency increases the dipole

reorientation may not occur sufficiently rapid to maintain the dipole in phase with the electric fi eld
and power losses occur.

13.9.1 DIELECTRIC CONSTANT
As previously note, the dielectric constant, e (ASTM D-150–74), is the ratio of the capacity of a
condenser made with or containing the test material compared with the capacity of the same con-
denser with air as the dielectric. Polymers employed as insulators in electrical applications, should
have low dielectric constants while those used as semiconductors or conductors should have high
dielectric constants.
The dielectric constant is independency of electrical frequency at low to moderate frequencies
but varies at higher frequencies. For most materials, the dielectric constant is approximately equal
to the square of the index of refraction and to one-third the solubility parameter.

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