Dielectric Behavior of Nonpolar Polymers and Their Composites: The Case of Semicrystalline Polyolefins
Polyolefins are thermoplastic polymers used in a wide range of applications, including medical implants, insulating materials, fabrics, and packaging. The two most popular representatives, polyethylene (PE) and polypropylene (PP), present linear chemical
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Abstract Polyolefins are thermoplastic polymers used in a wide range of applications, including medical implants, insulating materials, fabrics, and packaging. The two most popular representatives, polyethylene (PE) and polypropylene (PP), present linear chemical structures that yield these materials semicrystalline (except for atactic PP). The versatility of their synthesis enables the fabrication of different grades, covering a wide range of crystallinities which can reach up to 90%. However, because of their symmetric aliphatic structure, they do not present a permanent dipole moment. Their nonpolar nature makes dielectric spectroscopy measurements challenging, as this technique relies on the relaxation of dipoles after the application of an external electric field. Here, we review different approaches that have been followed in order to introduce permanent dipoles and render polyolefins dielectrically active, including: (i) addition of probes with a permanent dipole moment, (ii) oxidation/chlorination to produce dielectrically active chains, or (iii) induced oxidation in the presence of metal oxide fillers. The introduction of dipoles, either intentionally or due to the presence of impurities, has enabled the characterization of the full relaxation spectra of polyethylene and polypropylene as well as the assignment of dielectric relaxations to their respective molecular mechanisms. We then turn our attention into PE and PP composites for electrical energy storage and insulation applications. We show how, in these materials, the effect of the polymer matrix and filler orientation has been proven key to enhance their dielectric breakdown strength. Keywords Polyolefins · Nanocomposites · Crystallinity · Orientation · Dielectric probes · Entanglement · Dielectric breakdown strength · Treeing
S. X. Drakopoulos · S. Ronca · I. Martin-Fabiani (B) Department of Materials, Loughborough University, Loughborough, Leicestershire LE11 3TU, UK e-mail: [email protected] © Springer Nature Switzerland AG 2020 T. A. Ezquerra and A. Nogales (eds.), Crystallization as Studied by Broadband Dielectric Spectroscopy, Advances in Dielectrics, https://doi.org/10.1007/978-3-030-56186-4_10
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Abbreviations PE LDPE HDPE UHMWPE PP i-PP TiO2 Al2 O3 DBANS MMT o-MMT CNF MWCNT CaCO3 BaTiO3
Polyethylene Low density polyethylene High density polyethylene Ultra-high molecular weight polyethylene Polypropylene Isotactic polypropylene Titanium oxide Aluminum oxide 4,4 -(N,N-di- butylamino)-(E)-nitrostilbene Montmorillonite Organo-montmorillonite Carbon nanofibers Multi-wall carbon nanotubes Calcium carbonate Barium titanate
1 Introduction The global plastic production has increased exponentially since 1950, when 1.5 tonnes were produced, rising to almost 348 million tonnes in 2017. In Europe alone, over 1.5 million people are directly employed in the 60,000 companies related to the plastic industry [1]. Polyolefins, consisting of saturated aliphatic hydrocarbon macromolecules, account for more than 5
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