Polyethylene dielectric studies

Due to its practical importance as a cable material and due to its special position as a model polymer, polyethylene has been the subject of numerous dielectric studies. In coimnon with poly(vinylidene fluoride), polyethylene shows a complicated relaxation behaviour. Since the chain is intrinsically non-dipolar the material is oxidized, dil( inated or is made by copolymerizing carbon monoxide with ethylene, so that d les can be introduced to act as a probe on the motions of the parent material. Extensive accounts of the eady eiqjerimental wodc are available (McCrum et al.,... 

Banford et al. studied the radiation effects on electrical properties of low-density polyethylene (LDPE) at 5 K with the use of a 60Co gamma source and a thermal nuclear reactor [86]. They reported that both the electrical conductivity and the dielectric breakdown strength of LDPE at 5 K were not significantly affected by radiation absorbed doses up to 10s Gy, but an erratic pulse activity under high applied fields was observed in the sample irradiated at 106 Gy. 

Dynamic mechanical and NMR investigations of crystals grown from dilute solutions for polymers other than linear polyethylene have been much less extensive. Studies have been reported for the linear polymers polyoxy methylene (3, 40, 94), poly (ethylene oxide) (3, 78), and nylon 6 (42), and the branched polymers polypropylene (40), poly-l-butene (19, 95), poly(4-methyl-l-pentene) (33), poly (vinyl alcohol) (78), and branched polyethylene (78). In addition, dielectric loss measurements have been made on crystal aggregates of poly (ethylene oxide) (23), poly (vinyl alcohol) (68), and polyoxymethylene (3) and mechanical loss measurements have been carried out on polyoxymethylene formed by solid state polymerization (94). 

Chiu (116) used the apparatus previously described to study the thermal decomposition of selected polymers such as polyethylene terephthalate), po y(vinyl fluoride), po y(vinylidene fluoride), and others. The dielectric constant curves of a group of fluorocarbon polymers are shown in Figure 11.33. As illustrated, the more polar polymers such as poly(vinylidinefiuoride) (PVDF) and poly(vinyl fluoride) (PVF) show characteristic dielectric loss peaks that are distinguishable from the relatively featureless and low-loss curves of the other polymers. For PVF, the low-temperature process is due... 

Microwave-assisted esterification by a heterogeneous acid catalyst has been studied in a low dielectric constant medium (see Scheme 35) [64]. A continuous-flow setup has been devised in the system and the heterogeneous acid catalyst (Amberlyst A15 sulphonic acid cation-exchange resin) 61 localized in a polyethylene active flow cell. Use of a low dielectric constant medium (hexane) ensured absorption of microwave radiation only to the reacting species. In this case, the findings suggest a comparable esterification reaction under both microwave and thermal conditions. Furthermore, the presence of water in the catalytic resin resulted in a reduction of the reaction rate irrespective of the type... 

The area of commercial interest in this study is the manufacture and use of crossllnked polyethylene as a dielectric material in high voltage wire and cable applications. A typical cable configuration is shown in Figure 2. ( 2)... 

This chapter treats principally the vibrational spectra determined by infrared and Raman spectroscopy. The means used to assign infrared absorption bands are outlined. Also, the rationale for the selection of permitted absorption bands is described. The basis for the powerful technique of Fourier Transform Infrared (FTIR) is presented in Appendix 6A. Polyethylene is used to illustrate both band assignment and the application of selection rules because its simple chain structure and its commercial importance have made polyethylene the most thoroughly studied polymer. The techniques of nuclear magnetic resonance, neutron inelastic scattering and ultraviolet spectroscopy are briefly described. The areas of dielectric loss and dynamic mechanical loss are not presented in this chapter, but material on these techniques can be found in Chapters 5. 

Broadband dielectric spectroscopy is a powerful tool to investigate polymeric systems (see [38]) including polymer-based nanocomposites with different nanofillers like silica [39], polyhedral oligomeric silsesquioxane (POSS) [40-42], and layered silica systems [43-47] just to mention a few. Recently, this method was applied to study the behavior of nanocomposites based on polyethylene and Al-Mg LDH (AlMg-LDH) [48]. The properties of nanocomposites are related to the small size of the filler and its dispersion on the nanometer scale. Besides this, the interfacial area between the nanoparticles and the matrix is crucial for the properties of nanocomposites. Because of the high surface-to-volume ratio of the nanoparticles, the volume fraction of the interfacial area is high. For polyolefin systems, this interfacial area might be accessible by dielectric spectroscopy because polyolefins are nonpolar and, therefore, the polymeric matrix is dielectrically invisible [48]. 

The effects of air or oxygen plasma on polymer films have been reported. In a comparative study with polypropylene (PP) and polyethylene (PE), higher levels of oxygen incorporation were achieved in PP than with PE. In this method, the initial step involves formation of radicals on the top of the layer of the polymer surface. These can react with each other to initiate cross linking or branching or result in surface oxidation. The use of nonthermal plasma treatment to polymer surfaces to enhance wettability and adhesion has been reported. The use of Corona discharge and dielectric discharge has also been reported for polymer modification [78]. 

Thomas and King s and our own studies on polyethylene and a recent investigation of polypropylene by Gilchrist, we believe that the loss peak at below 4.2 K in these four films is due to the presence of an antioxidant, whose contribution to the dielectric loss in the 4 to 10 K range was not eliminated by annealing. 

The intrinsic dielectric losses of pure polyethylene and polypropylene are very small at 4.2 K (i.e., 5 x 10 ) The higher values of tan 6 measured for commercially produced polyolefins are due to the presence of additives placed in the polymer during the manufacturing process to protect the polymer in its intended air environment. Early work by King and Thomas disclosed that the antioxidant may be one of the major sources of dielectric loss at temperatures of 6 to 8 K. A subsequent study of the effects of antioxidant on tan 6, carried out jointly by Battelle Columbus Laboratories (BCL), the National Bureau of Standards (NBS), and BNL, also showed that the 60 Hz loss tangent of polyethylene, in the region of 4 to 10 K, was strongly dependent upon both type and... 

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