Dielectric Behavior of Nonpolar Polymers and Their Composites …
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and is not reactive at room temperature, making it suitable for use in reusable bags
and food packaging films. Because of its very reduced amount of branching, crystallinity in high density polyethylene (HDPE) can reach 70–80%, resulting in higher
tensile strength and thermal resistance than LDPE. HDPE is used in toys, milk, or
shampoo bottles. Ultra-high molecular weight polyethylene (UHMWPE) is characterized by its extremely long chains, reaching molecular weights in the order of 10
6
g/mol. It embodies the advantages of HDPE and it has great chemical resistance,
as well as having the best impact strength among thermoplastics. These properties
make UHMWPE an excellent material for personal armor and medical implants. PP
generally presents better mechanical properties and thermal resistance than PE, but
its chemical resistance is lower. The most widely used is isotactic PP (i-PP), in which
the methyl group is on the same side of the main chain (Fig. 1b), with a crystallinity
in between that of LDPE and HDPE. PP is widely used in food packaging, bank
notes, and sweet or snack wrappers.
In most of these products, the fundamental understanding of the relationship
between the internal structure and dynamics of the polymer and the final properties
are key to optimize performance. One of the most valuable techniques to obtain
such understanding at a molecular and chain level is dielectric spectroscopy, which
measures the relaxation of dipoles present in the material after the application of an
external electric field of varying frequency [3]. This technique relies on the presence
of polar molecules, whose orientation can be affected by the external field and their
reorientation back to equilibrium can be detected. However, the chemical structure
of polyolefins is nonpolar, resulting in a lack of dipoles which renders them dielectrically inactive in the absence of impurities. Different approaches have been studied
to introduce dipoles in polyolefins, involving either chemical modifications to introduce polar groups such as direct oxidization, chlorination, or indirect oxidization
through the introduction of inorganic fillers, as well as the addition of probes with a
large permanent dipole moment. In this chapter, we will first discuss representative
dielectric spectra of the most common variations of PE and PP, followed by a discussion on the different approaches used to render them dielectrically active. Then, we
will present a summary of the studies on polyolefin composites for electrical energy
storage and insulation applications.
2 Dielectric Spectra Overview
Dielectric relaxation dynamics of PE and i-PP have been extensively studied for
decades, either relying on the presence of dipoles originated by impurities or sample
preparation, or actively introducing dipoles in the polymer by different methods
which will be reviewed in the following section. In this section, as a background for
the rest of the chapter, we will give an overview of the different relaxations found
in the different grades of PE and i-PP, as well as the current knowledge on their
molecular origins.
245
and is not reactive at room temperature, making it suitable for use in reusable bags
and food packaging films. Because of its very reduced amount of branching, crystallinity in high density polyethylene (HDPE) can reach 70–80%, resulting in higher
tensile strength and thermal resistance than LDPE. HDPE is used in toys, milk, or
shampoo bottles. Ultra-high molecular weight polyethylene (UHMWPE) is characterized by its extremely long chains, reaching molecular weights in the order of 10
6
g/mol. It embodies the advantages of HDPE and it has great chemical resistance,
as well as having the best impact strength among thermoplastics. These properties
make UHMWPE an excellent material for personal armor and medical implants. PP
generally presents better mechanical properties and thermal resistance than PE, but
its chemical resistance is lower. The most widely used is isotactic PP (i-PP), in which
the methyl group is on the same side of the main chain (Fig. 1b), with a crystallinity
in between that of LDPE and HDPE. PP is widely used in food packaging, bank
notes, and sweet or snack wrappers.
In most of these products, the fundamental understanding of the relationship
between the internal structure and dynamics of the polymer and the final properties
are key to optimize performance. One of the most valuable techniques to obtain
such understanding at a molecular and chain level is dielectric spectroscopy, which
measures the relaxation of dipoles present in the material after the application of an
external electric field of varying frequency [3]. This technique relies on the presence
of polar molecules, whose orientation can be affected by the external field and their
reorientation back to equilibrium can be detected. However, the chemical structure
of polyolefins is nonpolar, resulting in a lack of dipoles which renders them dielectrically inactive in the absence of impurities. Different approaches have been studied
to introduce dipoles in polyolefins, involving either chemical modifications to introduce polar groups such as direct oxidization, chlorination, or indirect oxidization
through the introduction of inorganic fillers, as well as the addition of probes with a
large permanent dipole moment. In this chapter, we will first discuss representative
dielectric spectra of the most common variations of PE and PP, followed by a discussion on the different approaches used to render them dielectrically active. Then, we
will present a summary of the studies on polyolefin composites for electrical energy
storage and insulation applications.
2 Dielectric Spectra Overview
Dielectric relaxation dynamics of PE and i-PP have been extensively studied for
decades, either relying on the presence of dipoles originated by impurities or sample
preparation, or actively introducing dipoles in the polymer by different methods
which will be reviewed in the following section. In this section, as a background for
the rest of the chapter, we will give an overview of the different relaxations found
in the different grades of PE and i-PP, as well as the current knowledge on their
molecular origins.