Dielectric Measurement Methods

As mentioned previously, the complex dielectric permittivity (g>) can be measured by DS in the extremely broad frequency range 10-6-1012 Hz (see Fig. 1). However, no single technique can characterize materials over all frequencies. Each frequency band and loss regime requires a different method. In addition to the intrinsic properties of dielectrics, their aggregate state, and dielectric permittivity and losses, the extrinsic quantities of the measurement tools must be taken into account. In this respect, most dielectric measurement methods and sample cells fall into three broad classes [3,4,91] ... 

The theory of dielectric relaxation is briefly summarized in another chapter Dielectric relaxation and dielectric strength of polypropylene and its composites in this book. The thermally stimulated current (TSC) technique is a special subclass of dielectric measuring methods. Nevertheless, because of its versatility and specific features, it is worth treating separately. 

Dielectric Measurement Methods Applied to Thermoset Cure... 

The 2eta potential (Fig. 8) is essentially the potential that can be measured at the surface of shear that forms if the sohd was to be moved relative to the surrounding ionic medium. Techniques for the measurement of the 2eta potentials of particles of various si2es are collectively known as electrokinetic potential measurement methods and include microelectrophoresis, streaming potential, sedimentation potential, and electro osmosis (19). A numerical value for 2eta potential from microelectrophoresis can be obtained to a first approximation from equation 2, where Tf = viscosity of the liquid, e = dielectric constant of the medium within the electrical double layer, = electrophoretic velocity, and E = electric field. 

Volta potentials are measured by means of voltaic cells, i.e., systems composed of conducting, condensed phases in series, with a gas, liquid dielectric (e.g., decane) or a vacuum (in the case of solid conductors such as metals) gap situated between two condensed phases. The gap, g, may contain a gas such as pure air or nitrogen, saturated with vapors of the liquids present. Owing to the presence of a dielectric, special methods are necessary for the investigation of voltaic cells (see Section IV). 

The most familiar method of evaluating is by dielectric dispersion experiments, in which the real and imaginary parts of the complex dielectric constant over those of the solvent are determined as functions of frequency. It is the value of referring to the state of vacuum that can be correlated with the molecular structure of the solute. Polymers cannot be dispersed in the gaseous state. Furthermore, solvents effective for polypeptides are usually polar, and only approximate theories are presently available for the estimate of vacuum < 2> from dielectric measurements with polar solvents. Therefore the dipolar information about polypeptides is always beset with ambiguity in absolute magnitude as well as in interpretation. 

The RIS model, coupled with the Flory matrix method, is applied to the calculation of the unperturbed mean-square end-to-end distance in polylcyclohexene sulphone) as a function of several parameters. The calculations are performed for atactic, isotactic and syndiotactic chains the tacticity arises from the two possible ways, D and L, in which the rings can be attached to the main chain, assuming that the C—C bonds are all in the trans conformation, as indicated by dielectric measurements. 

The behavior of plasticizers in a polymer has already been examined by Wiirstlin (16) with dielectric measurements. However, this method is not sensitive enough to detect the plasticizer behavior in the system at small plasticizer concentrations. Luther and Weisel (12) have also pointed out this fact as a result of their DC measurements of PVC-plasticizer systems. The following measurements deal with the behavior of plasticizers in polystyrene. 

Dielectric measurements in non-polar solutions of polymers having a unbalanced dipole moment along the chain therefore provide a simple method for obtaining t , . 

Summarizing the information from the optical, electrooptical, and dielectric measurements we can draw several structures which are consistent with the crystallographic data but differ in the tilt and polarization distribution (Fig. 9). There is no direct method which would enable one to determine unambiguously which structure is correct, and we are left with some freedom for speculation. It seems... 

Hakki, B.W. and Coleman, P.D. (1960) A dielectric resonator method of measuring inductive capacities in the millimeter range, I.R.E. Trans. Microwave Theory Tech. 8, 402-10. 

Equation 1.79 is the basis for a measurement method of the dielectric response function /ft). Upon connecting the switch of the circuit shown in Figure 1.28, a polarization current, ipolft), through the capacitor can be recorded, according to the following equation... 

Dielectric test methods are used to measure the cure of epoxy adhesives between two conducting electrodes. This method is especially appropriate for metal-to-metal joints because the substrates themselves can be used as the electrode. The adhesive is treated as a capacitor during the test. Its response (dielectric constant, dissipation factor, etc.) over a range of electrical frequencies is measured as a function of curing time. 

Measurements of dielectric properties have been used to monitor chemical reactions in organic materials for more than fifty years. In 1934, Kienle and Race 11 reported the use of dielectric measurements to study polyesterification reactions. Remarkably, many of the major issues that are the subject of this review were identified in that early paper the fact that ionic conductivity often dominates the observed dielectric properties the equivalence between the conductivity measured with both DC and AC methods the correlation between viscosity and conductivity early in cure the fact that conductivity does not show an abrupt change at gelation the possible contribution of orientable dipoles and sample heterogeneities to measured dielectric properties and the importance of electrode polarization at low frequencies. 

Still another method for obtaining the dipole moment of a molecule involves the study of the variation of its rotation spectra (in the micro-wave region) caused by the application of an external electric field. Although this method gives a value far more precise than that obtained by dielectric measurements, it has to date been applied to only a small number of volatile compounds. 

Since the late nineteenth century, dielectric spectroscopy has been used to monitor dynamical properties of solid and liquid materials. At that time, dielectric measurements were performed either at a single frequency or in a very limited frequency range now, however, measurement technique and instrumentation have developed to such an extent that dielectric spectroscopy is today a well-established method to probe molecular dynamics over a broad range in frequency or time (cf. reviews by Johari [1], Bottcher and Bordewijk [34], Williams [35,36], and Kremer and Schonhals [37]), even with commercially available equipment. Including the latest developments, one can even say that nowadays dielectric spectroscopy is the only method that is fully able to realize the idea of 0- to 1-THz spectroscopy. In data sets that cover the range of up to 10 6—1013 Hz—that is, from ultra-low frequencies up to the far infrared—the full range of reorientational dynamics in... 

The dimerization constants of carboxylic acids determined by the partition method are usually lower than the values obtained by IR spectroscopy, cryoscopy, or dielectric measurements (100). The correction for hydration gives dimerization constants expected from values in dry solvents by spectroscopic or dielectric measurements (29, 30). 

Fig. 5.11 The basic electrical circuit of the Hartshorn and Ward (1936) resonance method of dielectric measurement.

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