Electric permittivity measurements

Also electric permittivity measurements show rather abrupt variations of dielectric parameters of PLL-Na" in the ionization range corresponding to the transition which may be related to changes in mean dimensions of polyions [15] (Figure 2). The dielectric behaviour is also in agreement with that observed for PLGA during the helix-coil transition [16]. 

Table 8-2 lists several physical properties pertinent to our concern with the effects of solvents on rates for 40 common solvents. The dielectric constant e is a measure of the ability of the solvent to separate charges it is defined as the ratio of the electric permittivity of the solvent to the permittivity of the vacuum. (Because physicists use the symbol e for permittivity, some authors use D for dielectric constant.) Evidently e is dimensionless. The dielectric constant is the property most often associated with the polarity of a solvent in Table 8-2 the solvents are listed in order of increasing dielectric constant, and it is evident that, with a few exceptions, this ranking accords fairly well with chemical intuition. The dielectric constant is a bulk property. 

An alternative electrical method that has been used in the study of glass-ionomer cements has been the measurement of dielectric properties. Tay Braden (1981, 1984) measured the resistance and capacitance of setting cements at various times from mixing. From the results obtained, relative permittivity and resistivity were calculated. In general, as these cements set, their resistivity was found to fall rapidly, then to rise again. Both these results and the results of relative permittivity measurements were consistent with the cements comprising highly ionic and polar structures. 

The dielectric constant of a polymer (K) (which we also refer to as relative electric permittivity or electric inductive capacity) is a measure of its interaction with an electrical field in which it is placed. It is inversely related to volume resistivity. The dielectric constant depends strongly on the polarizability of molecules tvithin the polymer. In polymers with negligible dipole moments, the dielectric constant is low and it is essentially independent of temperature and the frequency of an alternating electric field. Polymers with polar constituents have higher dielectric constants. When we place such polymers in an electrical field, their dipoles attempt... 

The electrical double layer has been dealt with in countless papers and in a number of reviews, including those published in previous volumes of the Modem Aspects of Electrochemistry series/ The experimental double layer data have been reported and commented on in several important works in which various theories of the structure of the double layer have been postulated. Nevertheless, many double layer-related problems have not been solved yet, mainly because certain important parameters describing the interface cannot be measured. This applies to the electric permittivity, dipole moments, surface density, and other physical quantities that are influenced by the electric field at the interface. It is also often difficult to separate the electrostatic and specific interactions of the solvent and the adsorbate with the electrode. To acquire necessary knowledge about the metal/solution interface, different metals, solvents, and adsorbates have been studied. 

As its name suggests, a liquid crystal is a fluid (liquid) with some long-range order (crystal) and therefore has properties of both states mobility as a liquid, self-assembly, anisotropism (refractive index, electric permittivity, magnetic susceptibility, mechanical properties, depend on the direction in which they are measured) as a solid crystal. Therefore, the liquid crystalline phase is an intermediate phase between solid and liquid. In other words, macroscopically the liquid crystalline phase behaves as a liquid, but, microscopically, it resembles the solid phase. Sometimes it may be helpful to see it as an ordered liquid or a disordered solid. The liquid crystal behavior depends on the intermolecular forces, that is, if the latter are too strong or too weak the mesophase is lost. Driving forces for the formation of a mesophase are dipole-dipole, van der Waals interactions, 71—71 stacking and so on. 

When applying an alternating electric field to a polymer placed between two electrodes, the response is generally attenuated and the output current is out of phase compared with the input voltage. This response stems from the polymer s capacitive component and its conductive or loss component, as represented by a complex dielectric permittivity measured frequencies f, and temperatures T ... 

However, the linear response of a dielectric to an applied field is an approximation the actual response is non-linear and is of the form indicated in Fig. 8.6. The electro-optic effect has its origins in this non-linearity, and the very large electric fields associated with high-intensity laser light lead to the non-linear optics technology discussed briefly in Section 8.1.4. Clearly the permittivity measured for small increments in field depends on the biasing field E0, from which it follows that the refractive index also depends on E0. The dependence can be expressed by the following polynomial ... 

The permanent dipole moment /r of a polar molecule, as a solute molecule in a liquid solution in a nonpolar solvent or as a molecule in a gas, can be determined experimentally from measurements of the dielectric constant k. This quantity is the ratio of the electric permittivity s of the solution or gas to the electric permittivity sq of a vacuum (8.854 X 10- Fm ) ... 

The complex electric permittivity, k = k + k , where k = C/C o is the real, and k = tan(8) / K is the complex part of the permittivity, was measured in the frequency interval 300 Hz - 1 MHz at different temperatures by a Solartron 1200 inq>edance gain analyser, using a parallel plate capacitor made of stainless steel. From the capacitance, C, and the tangent loss, tan(6), the values of k and k were calculated [2]. The temperature was controlled within O.IK using a platinum resistor Pt(lOO) as a sensor and a K30 Modinegen external cryostat coupled with a N-180 ultra-cryostat. 

In the absence of a strong dectric fidd (at = 0), the measured relative electric permittivity of an isotropic dielectric is a scalar quantity, defined as follows ... 

Figure 26 Variations Ae in electric permittivity versus E for solutions of methyl polymethacrylate in benzene (O) (0.0973 g/100 cm ) and isotactic) polystyrene in benzene (A) (0.1104 g/100 cm ) measured by Przeniczny at a frequency of 6.5 MHz at 20 °C [Measurements by Gregson have not confirmed the magnitude of these Ae E) values (Senior Reporter) ...

The number of its non-zero components will depend on the geometrical conditions of the experiment in which the induced anisotropy is measured, i.e. on the direction in which the electric permittivity is analysed (measured with regard to that of the strong applied electric field. 

High Electric Field Effects.—Determination of Variations in Electric Permittivity. The electric permittivity e of an isotropic dielectric is usually measured by means of a dowly varying dectric field, of a field strength Eoo so small as to cause only linear polarization of the medium ... 

In view of what has been said, the experimentally measured change in electric permittivity is composed of at least three effects... 

Most experimental studies involve measurements of the electrical permittivity as a function of frequency using an alternating voltage or field E. Then, the field varies with time according to the equation... 

The present research aims to develop a type of element permitting electric field relaxation, consisting of constituent materials graded for electric permittivity. The constituents used are titamium oxide and kaolin. 15 combinations of titamium oxide and kaolin in differing relative proportions were produced by a vacuum filter prcess, and tests were conducted to measure the permittivity and conductivity of each. 

Measurement of the electric permittivity, or dielectric constant, 6 of a gas over a range of temperature leads-via Debye s32 classical equation... 

The relative permittivity measures the alignment of the solvent dipoles and production of induced dipoles by an electric field. An ion produces an intense field on bound solvent molecules, and will cause partial, if not complete, alignment of the dipoles of the solvent molecules affected by the ion. This results in a drop in the observed relative permittivity of the solution relative to the pure solvent. This drop is related to the number of bound solvent molecules. Controversy exists as to whether the effect is restricted to bound molecules only, or whether other solvent molecules are involved. Both theoretical and experimental studies have been carried out. The dependence of the relative permittivity on the distance from a given ion is of fundamental importance in theories of electrolyte solutions where generally the bulk relative permittivity is used in the theoretical expressions. But it is more likely that a varying relative permittivity should be used. 

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