Dielectrics polarisability

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 ... 

TABLE 11.1 Group contributions to molar dielectric polarisation (P) in isotropic polymers (cmVmol)... 

Relation between dielectric polarisability and optical dispersion... 

The main features (simplified) of dielectric polarisability as a function of frequency are shown in Fig. 11.6. At low frequencies the total polarisation manifests itself completely. However, the orientation of the polar groups is relatively slow and as the frequency increases the orientation lags behind. When the frequency reaches a value of about 1012 c/s, the dipoles are unable to follow the oscillations of the field (Pdip/ i-e. our Ps, disappears). Only random orientations are left and these do not contribute to the resultant... 

The measure of lag between dielectric polarisation and changes in electric field is the dielectric loss factor defined as ... 

Here all the dielectric polarisation has been subsumed in e and the SCP manifested in p. Thus the bulk dielectric properties of the material are accounted for by the intensive quantity e which will here be taken, as usual, independent of p and E. The present work will, for simplicity, deal with a homogeneous, isotropic material, usually in a one-dimensional approximation. It will be assumed that all frequency dispersions associated with e occur appreciably above all SCP frequencies of interest. This criterion requires that the lowest-frequency dielectric dispersion region occurs appreciably above the radial frequency, = tj), associated with the dielectric relaxation time of the bulk material. 

The effect of the electric field upon a polymer could be to cause ionic or electronic conductance, dielectric loss or breakdown. Common polymers are good insulators, some values of volume resistivity appear in Table 2.3, but problems can arise with hydrophilic polymers such as the polyamides. Nylon 66 has a specific resistance of about 10 ft cm when dry, but on equilibration with saturated air at room temperature this substance absorbs about 8% of water and its specific resistance falls to about lO ft cm. When a polymer is placed in an electric field, the effect is to displace the centres of gravity of electronic and nuclear charges so that the material becomes dielectrically polarised. Further... 

C. J.F. Bdttcher, P. Bordewijk [ Theory of Dielectric Polarisation vol.l (Elsevier, Amsterdam, 1973)]... 

---