Dielectric Behavior of Nonpolar Polymers and Their Composites …
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amorphous regions [6]. Dielectric [7] and mechanical [8] studies prove that the α
process in i-PP is multicomponent, involving two or three subprocesses. The next
process in order of increasing temperature is the β relaxation, which is related to the
glass transition in both PE [9] and i-PP [6]. This process is more intense in LDPE
than in HDPE, as the crystallinity is lower for LDPE and there are more amorphous
chains available to contribute to the glass transition. Then we have the γ relaxation,
which in PE is assigned to the movement of certain parts of chains in the amorphous
regions in the vicinity of the crystalline lamellae. Several γ subprocesses have been
reported, but their assignment to specific molecular dynamics is not fully clear yet
[10–12]. In i-PP, the γ process has been related to the movement of chain ends or
branches, in a crankshaft-type fashion, in mechanical and dielectric studies [8, 13,
14]. A fourth relaxation has been reported at very low temperatures in both PE and
i-PP, the δ process. This relaxation has been correlated with hindered rotation of
CH 3 groups [14, 15] and it is generally weak or absent. It is expected to be especially
weak in the case of PE, where the CH 3 groups can only be found at the chain ends.
We believe it is of importance to mention that the matter of polyethylene’s glass
to rubber transition temperature has been an ongoing matter for decades and some
researchers still do not fully agree on its assignment. Various works over the years
have proposed that either the β-relaxation [16, 17] or the γ-relaxation [18, 19] correspond to the dynamic glass to rubber transition process. It has even been suggested
that polyethylene has two glass transitions, γ-relaxation for linear polyethylene and
β-relaxation for branched polyethylene [20, 21]. Generally, in semicrystalline polymers, the β-relaxation appears to be broadened when compared with amorphous
samples and considerably less prominent than the β-relaxation, supporting the idea
that the β-relaxation is the dynamic glass to rubber transition process [16]. On the
other hand, the γ-relaxation has been found to increase in intensity with decreasing
crystallinity, and has been assigned to mobile groups of amorphous chains in the
vicinity of crystalline lamellae [22, 23]. For these reasons, and as described before,
here we will follow the most widespread approach in the community of assigning the
β relaxation to the glass transition of PE and the γ relaxation to a process associated
to movement of localized amorphous chain segments.
3 Methods to Introduce Polar Groups
The unveiling of the dielectric spectra of polyolefins, whose nonpolar structures result
in the absence of a permanent dipole moment, has only been possible thanks to the
introduction of dipoles. For the polymer to be dielectrically active and enable the
measurement of its relaxation spectra, such introduction of dipoles must have taken
place, either unintentionally (e.g. oxidization due to impurities or sample preparation
conditions) or intentionally. We will now review the different methods used to render
PE and i-PP dielectrically active, as well as the influence these procedures have on
their final dielectric spectra. Methods are classified into two types: (i) those that rely
247
amorphous regions [6]. Dielectric [7] and mechanical [8] studies prove that the α
process in i-PP is multicomponent, involving two or three subprocesses. The next
process in order of increasing temperature is the β relaxation, which is related to the
glass transition in both PE [9] and i-PP [6]. This process is more intense in LDPE
than in HDPE, as the crystallinity is lower for LDPE and there are more amorphous
chains available to contribute to the glass transition. Then we have the γ relaxation,
which in PE is assigned to the movement of certain parts of chains in the amorphous
regions in the vicinity of the crystalline lamellae. Several γ subprocesses have been
reported, but their assignment to specific molecular dynamics is not fully clear yet
[10–12]. In i-PP, the γ process has been related to the movement of chain ends or
branches, in a crankshaft-type fashion, in mechanical and dielectric studies [8, 13,
14]. A fourth relaxation has been reported at very low temperatures in both PE and
i-PP, the δ process. This relaxation has been correlated with hindered rotation of
CH 3 groups [14, 15] and it is generally weak or absent. It is expected to be especially
weak in the case of PE, where the CH 3 groups can only be found at the chain ends.
We believe it is of importance to mention that the matter of polyethylene’s glass
to rubber transition temperature has been an ongoing matter for decades and some
researchers still do not fully agree on its assignment. Various works over the years
have proposed that either the β-relaxation [16, 17] or the γ-relaxation [18, 19] correspond to the dynamic glass to rubber transition process. It has even been suggested
that polyethylene has two glass transitions, γ-relaxation for linear polyethylene and
β-relaxation for branched polyethylene [20, 21]. Generally, in semicrystalline polymers, the β-relaxation appears to be broadened when compared with amorphous
samples and considerably less prominent than the β-relaxation, supporting the idea
that the β-relaxation is the dynamic glass to rubber transition process [16]. On the
other hand, the γ-relaxation has been found to increase in intensity with decreasing
crystallinity, and has been assigned to mobile groups of amorphous chains in the
vicinity of crystalline lamellae [22, 23]. For these reasons, and as described before,
here we will follow the most widespread approach in the community of assigning the
β relaxation to the glass transition of PE and the γ relaxation to a process associated
to movement of localized amorphous chain segments.
3 Methods to Introduce Polar Groups
The unveiling of the dielectric spectra of polyolefins, whose nonpolar structures result
in the absence of a permanent dipole moment, has only been possible thanks to the
introduction of dipoles. For the polymer to be dielectrically active and enable the
measurement of its relaxation spectra, such introduction of dipoles must have taken
place, either unintentionally (e.g. oxidization due to impurities or sample preparation
conditions) or intentionally. We will now review the different methods used to render
PE and i-PP dielectrically active, as well as the influence these procedures have on
their final dielectric spectra. Methods are classified into two types: (i) those that rely