Dipole orientation polarisation

If the frequency of the alternating voltage is too high, there is not enough time for the dipoles to rotate, there is no polarisation, and consequently the permittivity is low. So the permittivity falls off with increasing frequency just as did the compliance. 

The dimensionless / q against log frequency curves superimpose on those of X/Jq. So, too, do the curves against temperature. 

It will have been noticed in the above that it is the dimensionless normalised curves that are identical. Of course, the amplitude factors and have different units and may be very different in magnitude. Thus a transition that is very large in compliance may be very small in dielectric permittivity (small dipole moments involved). Conversely, a transition that is difficult to spot in compliance measurements may have large amplitude in the permittivity relaxation (large dipole moments orientating). So dielectric relaxation can be used to measure the polymer transitions in exactly the same way as can dynamic mechanical relaxation. 

A significant aspect of the change in permittivity is seen because the real permittivity, , is in fact the dielectric constant. So a polymer will have a high dielectric constant if the chain has high dipole moments and is free to undergo internal rotation. However, when the rotation cannot take place, the dielectric constant is much lower. So a polymer glass will always have a lower dielectric constant than a polymer rubber of the same molecular polarity. 

This concept of the working range applies to polymers between the glass to rubber and melt transitions just the same as between the a (glass to rubber) and P transitions. In practice, for high quaHty capacitors, it is essential to ensure that there are no mobile charges in the polymer. During the synthesis process, ionic catalysts are often used and residues of these can enable ionic conduction. Removal of these residues ensures that the measured frequency response arises purely from movement of the molecular dipoles. 

---

I Introduction to molecular motion in polymers 12.4 Dipole orientation 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. 

Pq the dipole or orientation polarisation P itself is defined by the Clausius-Mosotti Equation... 

Fig. 14. Orientation polarisation of dipole moments in an electric field Eot, (1), (2) possible orientation angles 8 and rr — 8 and (3), (4) stochastic orientation (angles 6j and 82)...

Besides the orientation polarisation we observe a deformation polarisation, identical to that of an induced dipole moment. In a spatial unit the polarisation caused by deformation (due to a shift of the electrical charges) will be... 

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. 

Molecular polarisability is the result of two mechanisms (a) distortional polarisation and (b) orientation polarisation. Distortional polarisation is the result of the change of electric charge distribution in a molecule due to an applied electric field, thereby inducing an electric dipole. This distortional polarisation is coined ad. Permanent dipoles are also present in the absence of an electric field. At the application of an electric field they will orient more or less in the direction of the electric field, resulting in orientation polarisation. However, the permanent dipoles will not completely align with the electric field due to thermal agitation. It appears that the contribution of molecular polarisability from rotation is approximately equal to p2/(3kT). Accordingly, the total molecular polarisability is... 

When the conductivity in question is very low, there are other experimental difficulties too. On application of a step voltage across a specimen, the initial current may be dominated by a displacement current due to polarisation of the material. Since some dipole orientation may be very slow to reach equilibrium, the displacement current can swamp a small conduction current for a long... 

From the simulations, we conclude that two hydrogen bonding force constants are a basic requirement for reproducing the measured spectrum. If a water-water potential generates sufficiently large force constant differences for the different proton configurations (or the different relative dipole-dipole orientations in water or ice), it should produce the same effect as seen in the LR model. The anisotropic properties of the classic potentials are a result of charge interaction and this anisotropy should increase in the polarisable potentials and hence they produce a broad optic peak. This broad peak indicates that the orientational variation of the potential function has been increased considerably but it may still be less than the critical value of 1.5 as we indicated in the section 6.1. One would, therefore, expect that a better polarisable potential would, eventually, be able to reproduce the split optic peaks in the measured INS spectrum. 

Surface potentials arise from electronic polarisation and (in a polar solvent) dipole orientation of the solvent molecules at the free surface of the solution. 

Even without an electric field, permanent dipole moments exist as a consequence of the dissymmetry of the charge distribution over a molecule. When an electric field is applied, those dipoles orient themselves. It is the dipolar polarisation described by Debye for dipoles in solution. 

A number of common molecules, including water, carry a permanent dipole. If such molecules are exposed to an electric field they will try to orient the dipole along the field (Figure 11.4c). As the movement of molecules in solids is restricted, orientational polarisability is more often noticed in gases and liquids. 

Molecular polarisability is discussed in detail in chapter 9. Orientational polarisation involves the rotation, under the influence of an electric field, of molecules, or parts of molecules, that have permanent electric dipoles. This effect clearly leads to a possible mechanism for orienting molecules and, in particular, for reorienting the domains of liquid crystals or LCPs that have permanent dipoles. It is less obvious that deformational polarisation can lead to orientation. If, however, the polarisability of a molecule is anisotropic, as is usually the case, the polarisability and hence the induced electric dipole will differ for different orientations of the molecule in the field. The interaction energy with the field is therefore reduced if the molecule rotates so that its direction of maximum polarisability coincides with the field direction. Electric-field orientation is largely of importance in device applications. 

Fig. 12.2 Orientation of dipoles by polarisation (a) corona poling (b) electrode poling system (c) random orientation of polar domains (d) application of high DC electric field (polarisation) (e) remnant polarisation after the electric field is extinguished.

When a (polar) molecule is placed in an electric field, two types of molecule/field interactions take place, namely reversible storage and irreversible dissipation of field energy. The first interaction is a capacitive effect, caused by the polarisability of a molecule. Molecules placed in an electric field are polarised. Various polarisation mechanisms are distinguished (atomic, electrical and macroscopic polarisation, and dipole orientation). When the electric field is removed the molecules will return to... 

Ferroelectrics In some ionic dielectrics with one axis of symmetry and a specified Polarization direction, small domains (lO -lO " m) with opposing Polarization directions (Fig. 5.5a) are formed, then initially the material is not polarised mac-roscopically, but after being subjected to an external EM field, shows properties typical for ferroelectrics. The external EM field can here easily change its direction of Polarization through the growth of domains with dipoles oriented according to... 

For isolated atoms, the polarisability is isotropic - it does not depend on the orientation of fhe atom with respect to the applied field, and the induced dipole is in the direction of the electric field, as in Equation (4.51). However, the polarisability of a molecule is often anisotropic. This means that the orientation of the induced dipole is not necessarily in the same direction as the electric field. The polarisability of a molecule is often modelled as a collection of isotropically polarisable atoms. A small molecule may alternatively be modelled as a single isotropic polarisable centre. 

In the case of polymer molecules where the dipoles are not directly attached to the main chain, segmental movement of the chain is not essential for dipole polarisation and dipole movement is possible at temperatures below the glass transition temperature. Such materials are less effective as electrical insulators at temperatures in the glassy range. With many of these polymers, e.g., poly(methyl methacrylate), there are two or more maxima in the power factor-temperature curve for a given frequency. The presence of two such maxima is due to the different orientation times of the dipoles with and without associated segmental motion of the main chain. 

---