Electric polarisability

PVDF is mainly obtained by radical polymerisation of 1,1-difluoroethylene head to tail is the preferred mode of linking between the monomer units, but according to the polymerisation conditions, head to head or tail to tail links may appear. The inversion percentage, which depends upon the polymerisation temperature (3.5% at 20°C, around 6% at 140°C), can be quantified by F or C NMR spectroscopy [30] or FTIR spectroscopy [31], and affects the crystallinity of the polymer and its physical properties. The latter have been extensively summarised by Lovinger [30]. Upon recrystallisation from the melted state, PVDF features a spherulitic structure with a crystalline phase representing 50% of the whole material [32]. Four different crystalline phases (a, jS, y, S) may be identified, but the a phase is the most common as it is the most stable from a thermodynamic point of view. Its helical structure is composed of two antiparallel chains. The other phases may be obtained, as shown by the conversion diagram (Fig. 7), by applying a mechanical or thermal stress or an electrical polarisation. The / phase owns ferroelectric, piezoelectric and pyroelectric properties. 

External stress changes the Si—O—Si bond angles between tetrahedra so that the dipole moments no longer cancel and the crystal has a net electrical polarisation. The effect in a-quartz is small the output electrical energy is only 0.01 of the input... 

Some piezoelectric crystals are electrically polarised in the absence of mechanical stress one example is gem-quality tourmaline crystals. Normally, this effect is unnoticed because the crystal does not act as the source of an electric field. Although there should be a surface charge, this is rapidly neutralised by charged particles from the environment and from the crystal itself. However, the polarisation decreases with increasing temperature and this can be used to reveal the polar nature of the crystal. If tourmaline is heated its polarisation decreases and it loses some of its surface charges. On rapid cooling it has a net polarisation and will attract small electrically charged particles such as ash. Such crystals are known as pyroelectric, and ferroelectric crystals are a special subclass of pyroelectric crystals. 

Electric polarisation (P ef deformation, Por orientation polarisation) Momentum coordinate Spatial coordinate... 

In order to generate the second harmonic of an electromagnetic wave, one needs to make use of some device which has a non-linear property. In the case we are considering, the non-linear relationship made use of is that between applied electric field and electric polarisation. One can write... 

C.J.F. Bottcher, Theory of electric polarisation (Elsevier, Amsterdam, 1952) Chapter 3... 

Electric polarisation waves can be classified in terms of their normal modes. In general, these normal modes form bands, though in some cases isolated modes may also exist. The lowest frequency of all these modes is different from zero. A band of normal modes covering a frequency range between oo and to 2, ... 

In the proposed configuration, a large modification of atomic propagation will be due to electric polarisability of lithium, but no phase shift should result if the apparatus is symmetric. Because this symmetry cannot be perfect, this will be the main systematic effect. Another possible arrangement, which also takes advantage of the Aharanov-Bohm effect [7], will be considered in this... 

Bottcher CJF and Bordewijk P, "Theory of Electric Polarisation", Vol. 2, "Dielectrics in Time-dependent Fields", Elsevier, Amsterdam, 2nd Ed, 1973. 

Bennett and Mitchell found AEa an additive property of normal liquids, but this result is only another form of Kopp s law of additivity of molecular volumes ( 2.VIII B), or of the additivity of molecular refractions, since AEa is proportional to the electric polarisability, which is additive. According to Einstein, no heat content is to be ascribed to the surface film and the whole of its energy is potential. 

Lenard suggested that the molecules in the surface of a liquid are oriented, and Langmuir and others gave experimental evidence of this by measurement of the area of insoluble ffims on liquid surfaces. Jung, by assuming electrically polarisable molecules of polarisability a, found ... 

Graphite was known from the 60 s to become metallic under suitable doping with potassium. It was then normal to try and dope with alkali metals the fullerenes Qq molecules discovered in 1985 by H.W. Kroto and R.E. Smalley, and to extend this treatment to balloons or tubes-like C molecules later developed in great quantities. The superconductivities observed here, often at fairly high temperatures, seem of a normal BCS type, with notable electron-phonon coupling and also with electron-electron repulsions weakened by the electrical polarisability of these big and easily excited molecules. Here again, more remains to be done on the detailed characteristics of conduction and superconductivity. 

Bottcher, C.J.F. (1952) Theory of Electric Polarisation, Elsevier, Amsterdam. 

In a series of publications by Ishii et al. [54-57], effects of the electric field on structural changes in the amorphous regions, accompanied by an additional relaxation process, were discussed. These effects are reflected in the angular dependence of the second moment at different temperatures. The separation of any orientational effects due to poling from stretching effects were made by the preparation of different sample types. The complications for such a separation arose from the facts that (i) mechanically induced effects on chain orientations are much larger than that of the (electric) dipole reorientation and (ii) after poling only a small irreversible electric polarisation remains. 

McHugh, M.A. Krukonis, V.J. "Supercritical Fluid Extraction Principles and Practice," Butterworths Boston, MA, 1986. Bottcher, C.J., "Theory of Electric Polarisation," Elsevier Amsterdam, 1952, 63-74, 133-139. 

Bbttcher, C. J. F. Theory of Electric Polarisation Amsterdam Elsevier 1952... 

For a mode to be IR-active the molecule must have an oscillating electric dipole moment, called the transition dipole or transition moment, when it vibrates in this mode. For a vibration to be Raman-active the molecule must have an oscillating electrical polarisability (see section 9.2) when it vibrates in the mode. This means essentially that the shape of the molecule must be different in opposite phases of the vibration. These requirements constitute the selection rules for the two kinds of spectroscopy. Figure 2.10 shows the four modes of vibration of the CO2 molecule and their IR and Raman activities. 

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