Dielectric function polyethylene

Figure 4. Polyethylene the normal incidence reflectivity (----------) and optical absorption coefficient (-----) computed from the dielectric function of Figure 3 (1)...

In the copolyamides under consideration, the dipoles that are responsible for the dielectric relaxations are associated with the C = 0 groups of the amide functions. Due to the quasi-conjugated character of the CO - NH bond, the amide group takes on a rigid plane conformation in such a way that the dielectric relaxations of copolyamides should correspond to motional modes that involve amide groups and not only the carbonyls, in contrast to what happens with the ester groups encountered in polyethylene fere-phthalalc (Sect. 4.1.2). 

In addition to high breakdown strength, the electrical insulators for superconducting magnets must have excellent dielectric properties at cryogenic temperatures. Chant reported the results of measurements on dielectric constant and loss tangent (tan 5) for several polymers over the temperature range from 4.2 to 300 K [83], The variation of dielectric constant of samples as a function of temperature is shown in Fig. 15. The dielectric constants of nonpolar polymers, such as polyethylene, polypropylene and polytetrafluoroethylene, are substantially independent of temperature, whereas those of polar polymers except polyimide decrease by a maximum of 20% as the temperature is reduced. The values of tan 8 at the frequency of 75 cps for nonpolar polymers decreased by... 

The dielectric strength of polyethylene has been measured as a function of additive concentration while in stabilised polymers the depth of degradation was found to be uniform . HALS have been found to be effective stabilisers for UV cured coatings and do not influence the cure rate . Bleached wood pulps can be effectively stabilised by ascorbic acid and wool by hydroxy-benzotriazoles and HALS . Hydroxybenzotriazole stabilisers also protect wood pulp and polyurethanes by co-reaction . The spectroscopic properties of monomeric and polymeric benzotriazoles have also been compared . UV absorbers are effective in clear coats and when grafted to wood . Dihydroxybenzophenone stabilisers inhibit the chain scission in the photodegradation of poly(methoxyacrylophenone) while tin stabilisers have also been found to photostabilise PVC . 

As previously mentioned, further irradiation of polyethylene leads to cross-linking, which eliminates the normal melting point, and, above a temperature of 113°C, the material exhibits a rubber-like elasticity and, as a result, does not flow. The irradiated polethylene still possesses excellent electrical properties and is therefore capable of functioning as a dielectric between the normal operating temperature of conventional... 

Takala M, Ranta H, Nevalainen P, Pakonen P, Pelto J, Karttunen M, Virtanen S, Koivu V, Pettersson M, Sonerud B, Kannus K (2010) Dielectric properties and partial discharge endurance of polypropylene-silica nanocomposite. IEEE Trans Diel Electr Insul 17 1259-1267 Tanaka T, Kozako M, Fuse N, Ohki Y (2005) Proposal of a multi-core model for polymer nanocomposite dielectrics. IEEE Trans Diel Electr Insul 12 669-681 Vaughan AS, Swingler SG, Zhang Y (2006) Polyethylene nanodielectrics the influence of nanoclays on structure formation and dielectric breakdown. Trans lEE Jpn 126 1057-1063 Venkatesulu B, Thomas MJ (2010) Erosion resistance of alumina-filled silicone rubber nanocomposites. IEEE Trans Diel Electr fiisul 17 615-624 Weibull W (1951) A statistical distribution function of wide applicability. J Appl Mech Trans ASME 18 293-297... 

Figure 7.49 Dielectric loss as a function of log frequency at constant temperature, 109.3°C, for two polyethylene terephthalate samples showing the process. Fully amorphous (filled circles) and a sample with 50% crystallinity (open circles). Drawn after Cobum and Boyd (1986).

FIG. 39 The development of dielectric loss measured at room temperature as a function on carbonyl content during the oxidation of various polyethylene. White LDPE, about 20 CH3/IOOOC, p = 0.925 g cm gray HDPE, about 4 CH3/IOOOC, p = 0.957 g cm hatched HDPE, about 2 CH3IOOOC, p = 0.974 g cm . (Adapted from Ref. 73.)... 

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