Dipole movements

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. 

At low frequencies when power losses are low these values are also low but they increase when such frequencies are reached that the dipoles cannot keep in phase. After passing through a peak at some characteristic frequency they fall in value as the frequency further increases. This is because at such high frequencies there is no time for substantial dipole movement and so the power losses are reduced. Because of the dependence of the dipole movement on the internal viscosity, the power factor like the dielectric constant, is strongly dependent on temperature. 

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. 

On the basis of an Onsager cavity (23) model of dielectrics applied to a polar solute with an intrinsic dipole movement /xr° in its rth electronic state, Mazurenko gives an equation for the orientational free energy of the solute molecule in a pure polar solvent environment, which can be identified as equivalent to u/jlpe chem, thus 2... 

Kaliszan, R. and H. D. Hdtje. 1982. Gas chromatographic determination of molecular polarity and quantum chemical calculation of dipole movements in a group of substituted phendleChromatogr. 234 303-311. 

The moleeules or atoms comprising the dielectric exhibit a dipole movement distanee. An example of this is the stereochemistry of covalent bonds in a water molecule, giving the water molecule a dipole movement. Water is the typical case of non-symmetric molecule. Dipoles may be a natural feature of the dielectric or they may be induced. Distortion of the electron cloud aroimd non-polar molecules or atoms through the presence of an external electric field can induce a temporary dipole movement. This movement generates friction inside the dielectric and the energy is dissipated subsequently as heat[l]. 

Dipole-dipole forces are the forces created by the permanent dipole movement of a polar molecule. For example, acetone ((CHsliC = O) has electronegative oxygen that causes a shift in the electron density toward the oxygen. This distribution of electrons leads to the oxygen having a partial negative charge (8 —) and the... 

Detailed examination of the relaxations requires isothermal scans of relative permittivity and dielectric loss factor as a function of frequency/ so that effective dipole movements and activation energies of relaxation times may be obtained. A typical pair of plots of d and e" values against log/is shown in Fig. 3.7. Graphs of dielectric data of this kind are sometimes called, rather... 

Figure 6.25. Decrease in chemisorbed CO stretching frequency on Rh(111) with increasing dipole movement of coadsorbed donors [40].

Piezoelectricity links the fields of electricity and acoustics. Piezoelectric materials are key components in acoustic transducers such as microphones, loudspeakers, transmitters, burglar alarms and submarine detectors. The Curie brothers [7] in 1880 first observed the phenomenon in quartz crystals. Langevin [8] in 1916 first reported the application of piezoelectrics to acoustics. He used piezoelectric quartz crystals in an ultrasonic sending and detection system - a forerunner to present day sonar systems. Subsequently, other materials with piezoelectric properties were discovered. These included the crystal Rochelle salt [9], the ceramics lead barium titanate/zirconate (pzt) and barium titanate [10] and the polymer poly(vinylidene fluoride) [11]. Other polymers such as nylon 11 [12], poly(vinyl chloride) [13] and poly (vinyl fluoride) [14] exhibit piezoelectric behavior, but to a much smaller extent. Strain constants characterize the piezoelectric response. These relate a vector quantity, the electrical field, to a tensor quantity, the mechanical stress (or strain). In this convention, the film orientation direction is denoted by 1, the width by 2 and the thickness by 3. Thus, the piezoelectric strain constant dl3 refers to a polymer film held in the orientation direction with the electrical field applied parallel to the thickness or 3 direction. The requirements for observing piezoelectricity in materials are a non-symmetric unit cell and a net dipole movement in the structure. There are 32-point groups, but only 30 of these have non-symmetric unit cells and are therefore capable of exhibiting piezoelectricity. Further, only 10 out of these twenty point groups exhibit both piezoelectricity and pyroelectricity. The piezoelectric strain constant, d, is related to the piezoelectric stress coefficient, g, by... 

When dipoles are present but not frozen-in, as in water or in polychloroprene, the dipoles will tend to align with the field and increase the capacity of a dielectric over and above that due to electron polarization, the effect being known as dipole polarization. In a low frequency, alternating current field it is usually possible for the dipole movement to keep in phase with changes in the alignment of the electric field. As the frequency is increased the molecules do not have time to complete their alignment before the field changes direction. This reduces the amount of dipole polarization and there is a reduction in the dielectric constant. Eventually a frequency is reached where there is no time for any dipole movement and the dielectric constant is due to electron polarization only. 

If the temperature is raised the reduction in the viscosity of the system makes dipole movement easier and there is a shift in the... 

In the range of frequencies where dipole movement is finite but incomplete there is some internal friction as the movement of dipoles fails to keep in step with the change in field. This causes loss of electric power and some building up of heat in the dielectric. This can be characterized by various parameters such as power factor and loss factor. The peak in the power factor-frequency curves coincides with the point of inflection in the dielectric constant-frequency curves and is also shifted by raising the temperature (Fig. 4.4). 

In this technique one measures the intensity fluctuation of scattered light by the droplets as they undergo Brownian motion [13]. When a light beam passes through a colloidal dispersion, an oscillating dipole movement is induced in the... 

The translation of the center of gravity of a molecule is a vector, and so is the dipole movement. Let us consider a vector with x, y, and z components and investigate its transformation properties when symmetry operations are performed on it. If such a vector is rotated about the z axis, the z component will remain unchanged. In Fig. 3.12 is illustrated the projection of this vector on the xy plane. The vector projection r starts in position A and is rotated through an angle (f) to position B. From Fig. 3.12 we can write the following relationships ... 

The absorption of infrared radiation occurs to a maximum degree when the direction of the electric vector of the radiation is parallel to the direction of the dipole moment changes involved in the various vibrational modes of the absorbing molecule. If the direction of the electric vector lies at an angle to the direction of the dipole movement change, the component of the former resolved along the latter direction is involved in the absorption process, which thus occurs less strongly. This is the directional property that makes infrared spectroscopy useful for orientation studies. 

The dielectric anisotropy Ae of LC-materi-als is defined by Ae= j -ex, where and ej. are the dielectric constants parallel and perpendicular to the director. From the Maier and Meier theory it can be seen that both the polarizability anisotropy Aa and the permanent dipole movement /i of the LC molecule determine the dielectric anisotropy... 

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