Dielectric study

During photodegradation and photo-oxidation, both polar groups e.g. carbonyl, which has an appreciable dipole moment, and polarizable groups, e.g. conjugated C=C sequences, are introduced into the polymer these reactions result in the deterioration of the useful dielectric properties of polymers. 

Dielectric measurements (relative dielectric constants, dielectric strength and dielectric loss factor) have been made for photodegraded polyethylene [1284], polystyrene [840, 2227, 2228, 2248] poly(ethylene terphthalate) [1040], polyamides and polyimides [120] and polycarbonates [1738]. 

Dielectric constants generally increase on photodegradation and photooxidation these increases reflect the incorporation of polar groups into the polymer. A good correlation between the carbonyl dipole relaxation strength and carbonyl concentrations is observed. The relaxation strength measurements provides quantitative kinetic information for polymer oxidation. 

Dielectric dissipation factors for polyamides and polyimides increase rapidly on exposure to ultraviolet (290-400 nm) radiation in air [120] short-wave UV radiation (254 nm) has a similar effect on poly(vinylidene chloride) [1658]. Photodegradation of polyethylene rapidly increases both 

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Bone S, Pethig R (1985) Dielectric studies of protein hydration and hydration-induced flexibility. JMol Biol 181 323-326. 

In the past, impedance or dielectric studies have been examined as an experimental technique to monitor the flow properties, effects of composition, and the advancement of a reaction during cure [1]. Until a paper by Zukas et al [2], little emphasis had been placed on the frequency dependence except to note the shift in position and magnitude of impedance maxima and minima. Furthermore, most measurements on curing systems reported results in terms of... 

The authors group has recently been engaged in more detailed dielectric studies in the helix-coil transition region, using well-fractionated samples of PBLA and PCBL. 

Tma, which is directly measured in dielectric studies, may be obtained by substituting (2.7) into (2.4) ... 

DSC is increasingly being applied to the study of epoxy resin cure in combination with other analytical methods such as nuclear magnetic resonance and Fourier transform infra-red spectroscopy, chromatographic methods, and dynamic mechanical or dielectric studies. It is probably as part of such combined investigations that DSC can be used most effectively in basic research, and in quality control and assessment. 

In all the above three polymers only a single process is apparently observed in the time window for PCS (10-6 to 100 s). The shape of the relaxation function is independent of temperature. The temperature dependence of (r) follows the characteristic parameters observed for mechanical or dielectric studies of the primary (a) glass-rubber relaxation. Relaxation data obtained by many techniques is collected together in the classic monograph of McCrum, Read and Williams41. The data is presented in the form of transition maps where the frequencies of maximum loss are plotted logarithmically... 

The transition has also been detected in dielectric studies at frequencies from 103 to 107 cps (Mikhailov, Kabin, and Smolyanskii). The activation energy for the transition is 18 kcal per mole, and the activation entropy is 45 entropy units per mole. From the breadth of the loss peak in Fig. 8, it has been determined that more than a single relaxation time is required. 

Johari, G R, S. Kim, and R. M. Shanker. 2005. Dielectric studies of molecular motions in amorphous solid and ultraviscous acetaminophdrPharm ScB4 2207-2223. 

Many dielectric studies of epoxy-amine systems have been reported. It appears that the change in dielectric properties during cure may be due to three main contributions ... 

Fig. 1 Scheme illustrating the preparation of thin polymer films for dielectric studies... 

Lawless, G. W. High temperature dielectric studies of epoxy resin, Polym. Eng. Sci., 20, 546 (1980)... 

Sheppard, Jr., N. F., Coin, M. C. W., Senturia, S. D. A dielectric study of the time-temperature-transformation (TTT) diagram of DGEBA epoxy resins cured with DDS, p. 1243, Proc. 29th SAMPE Symp., 1984... 

Lane, J. W., Bachmann, M. S., Seferis, J. C. Dielectric Studies of the cure of epoxy matrix systems, J. Appl. Polym. Sci., in press... 

All the examples described above show that confinement in different cases may be responsible for nonmonotonic relaxation kinetics and can lead to a saddle-like dependence of relaxation time versus temperature. However, this is not the only possible reason for nonmonotonic kinetics. For instance, work [258] devoted to the dielectric study of an antiferromagnetic crystal discusses a model based on the idea of screening particles. Starting from the Arrhenius equation and implying that the Arrhenius activation energy has a linear dependence on the concentration of screening charge carriers, the authors of Ref. 258 also obtained an expression that can lead to nonmonotonic relaxation kinetics under certain conditions. However, the experimental data discussed in that work does not show clear saddle-like behavior of relaxation time temperature dependence. The authors of Ref. 258 do not even discuss such a possibility. 

D Angelo, M., Fioretto, D., Onori, G., Palmieri, L., and Santucci, A. (1996), Dynamics of water-containing sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles A high-frequency dielectric study, Phys. Rev. E, 54(1), 993-996. 

The broad-band dielectric study of highly filled PDMS is complementary to the NMR study of molecular motions in filled PDMS. The dielectric experiments were performed in the frequency range of 10" -10 Hz [27], A combined analysis of the dielectric spectra both for filled PDMS and the pure components of the mixtures was used to assign the dielectric losses to motions of adsorbed and non-adsorbed PDMS chain units. As discussed above, the interpretation of the results is based on a two-phase model assiuning the exchange of chain units at the surface of Aerosil between adsorbed and non-adsorbed states. 

Busing, D., Jenau, M., Reuter, J., Wurflinger, A. and Tamarit, J. L. (1995). Differential thermal-analysis and dielectric studies on 2-methyl-2-nitro-propoane under high-pressure. Z. Natur. A. J. Phys. Sci., 50, 502-4. [239]... 

Such, briefly, is the content of the chapters which follow. Apart from thoroughly recommending them to others, it only remains for this first reader to offer his sincere thanks—not only for the very ready collaboration of the authors, but also for their valuable contributions to the not excessive corpus of significant literature on dielectric studies. The interest and help of the Chemical Society editors are also much appreciated. 

The diathermic applications of dielectric studies are not solely biological, although the most important are medicinal and culinary. There also exist industrial diathermic applications, concerned mostly with fast drying and... 

The newly established methods in time-domain dielectric studies are reviewed by Dr. Suggett in Chapter S. In general principle these are extensions of the step- or ramp-voltage methods which in various forms have been in occasional use over some thirty years, but the differences in the speed of operation are such as to make them qualitativdy different. For the high-frequency r on the first system was desoibed by Fellner-Feldegg in 1969, whilst a spectrometer operating from 10 Hz to 10 Hz... 

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