Dielectric constant, polarisation

In the case of symmetrical molecules such as carbon tetrachloride, benzene, polyethylene and polyisobutylene the only polarisation effect is electronic and such materials have low dielectric constants. Since electronic polarisation may be assumed to be instantaneous, the influence of frequency and temperature will be very small. Furthermore, since the charge displacement is able to remain in phase with the alternating field there are negligible power losses. 

In the dielectric of a condenser the dipole polarisation would increase the polarisation charge and such materials would have a higher dielectric constant than materials whose dielectric constant was only a function of electronic polarisation. 

There is an important practical distinction between electronic and dipole polarisation whereas the former involves only movement of electrons the latter entails movement of part of or even the whole of the molecule. Molecular movements take a finite time and complete orientation as induced by an alternating current may or may not be possible depending on the frequency of the change of direction of the electric field. Thus at zero frequency the dielectric constant will be at a maximum and this will remain approximately constant until the dipole orientation time is of the same order as the reciprocal of the frequency. Dipole movement will now be limited and the dipole polarisation effect and the dielectric constant will be reduced. As the frequency further increases, the dipole polarisation effect will tend to zero and the dielectric constant will tend to be dependent only on the electronic polarisation Figure 6.3). Where there are two dipole species differing in ease of orientation there will be two points of inflection in the dielectric constant-frequency curve. 

For non-polar materials (i.e. materials free from dipoles or in which the dipoles are vectorially balanced) the dielectric constant is due to electronic polarisation only and will generally have a value of less than 3. Since polarisation is instantaneous the dielectric constant is independent of temperature and frequency. Power losses are also negligible irrespective of temperature and frequency. 

With polar molecules the value of the dielectric constant is additionally dependent on dipole polarisation and commonly has values between 3.0 and 7.0. The extent of dipole polarisation will depend on frequency, an increase in frequency eventually leading to a reduction in dielectric constant. Power factor-frequency curves will go through a maximum. 

It is thus seen that for polymers in which polarisations other than electronic ones are negligible (i.e. P = P ) the dielectric constant is equal to the square of the refractive index Table 6.2). 

PTFE is an outstanding insulator over a wide range of temperature and frequency. The volume resistivity (100s value) exceeds lO Gm and it appears that any current measured is a polarisation current rather than a conduction current. The power factor is negligible in the temperature range -60°C to -i-250°C at frequencies up to lO" Flz. The polymer has a low dielectric constant similarly unaffected by frequency. The only effect of temperature is to alter the density which has been found to influence the dielectric constant according to the relationship... 

The lowest dielectric constant (1.83-1.93) of any known plastics material. (It is to be noted that this is in spite of the fact that the dielectric constant is more than the square of the refractive index, indicating that polarisations other than electronic polarisations are present—see Section 6.3). 

Because of a small dipole polarisation effect the dielectric constant is somewhat higher than that for PTFE and the polyolefins but lower than those of polar polymers such as the phenolic resins. The dielectric constant is almost... 

Wetting and capillarity can be expressed in terms of dielectric polarisabilities when van der Waals forces dominate the interface interaction (no chemical bond or charge transfer) [37]. For an arbitrary material, polarisabilities can be derived from the dielectric constants (e) using the Clausius-Mossotti expression [38]. Within this approximation, the contact angle can be expressed as ... 

At these high frequencies, the retarding effect of the ion-atmosphere on the movement of a central ion is greatly decreased and conductance tends to be increased. The capacitance effect is related to the absorption of energy due to induced polarisation and the continuous re-alignment of electrically unsymmetrical molecules in the oscillating field. With electrolyte solutions of low dielectric constant, it is the conductance which is mainly affected, whilst in solutions of low conductance and high dielectric constant, the effect is mostly in relation to capacitance. 

Mostly known for plasticizers only (see section 3.1), not for plasticized polymers or commercial polymers. The relative dielectric constant indicates the polarisability of the molecule (s of selected solvents for comparison hexane, 1.9 toluene, 2.4 chloroform, 4.8 ethyl acetate, 6.0 dichloromethane, 9.1 acetone, 20.7 water, 80.2). 

As in the solid state the share rate a of dipoles taking part in dipole orientation is much more smaller than in the liquid state, where orientation polarisation Eqs. (78) and (79) is also low. The dielectric constant even in polar polymers is not higher than 6. 

Dielectric constant of a medium is a measure of the electrostatic polarisation, which reduces the forces between charges. 

Long-wavelength refractive index [40,41] suggests a high frequency dielectric constant ex = a2 = 8.4, a value supported by recent measurement [42] and ab initio calculation [43], Ionic polarisability makes a significant contribution at low frequencies where, although measurement is presently inhibited by a high conductivity, s0 15 is recommended [43],... 

The relative dielectric constant of insulating materials (s ) is the ratio of the capacities of a parallel plate condenser measured with and without the dielectric material placed between the plates. The difference is due to the polarisation of the dielectric. It is a dimensionless quantity. 

This relationship between the dielectric constant and the molecular polarisation is known... 

For non-polar materials the relationship between the molar polarisation Pll/ the dielectric constant e and the molecular polarisability a is known as the molar Clausius-Mosotti relation and reads... 

It was shown above that the relation between the polarisation, the electric field and the dielectric constant is given by... 

As electrical forces due to polarisability and polar moment determine the cohesive energy, a certain correlation between dielectric constant and solubility parameter may be expected. Darby et al. (1967) suggested such a correlation for organic compounds. It appeared that a surprisingly simple correlation holds for polymers, viz. ... 

The Dipole Moment of Phosphine.—Molecules which are not polar in the sense of being strong acids, bases or salts, may yet show an inner polarity when investigated by certain physical methods. The use of the dielectric constant and the refractivity in calculating the polarisation of molecules is described in certain monographs and text-books, e.g. The Dipole Moment and Chemical Structure, Debye-Deans (BlacMe), 1931 Recent Advances in Physical Chemistry, 1 Glasstone (Churchill), 1931. 

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