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S. X. Drakopoulos et al.
on the chemical modification of the polymer chain to induce dipoles and (ii) the
addition of probes with strong dipole moment into the polymer matrix.
3.1 Chemical Modifications
We will first report on methods that render polyolefins dielectrically active by means
of chemical modifications, including oxidization and chlorination. The most common
is oxidization, which introduces carbonyl groups into the polymer chain that serve
as a dielectric probe. Thermally-initiated oxidization was one of the first methods to
be used, which enabled the unveiling of the dielectric spectra of PE early on [24–26].
Boyd argued that the α relaxation observed in lightly oxidized PE must involve the
screw rotation around the main chain axis of a perpendicular C=O dipole which is
yielding the necessary dielectric activity to detect the process [4, 16]. Several studies
have reported on the use of gamma radiation to oxidize PE and i-PP [12, 27–29]. As
shown in Fig. 3, the dielectric losses of some relaxations increase with increasing
doses, but after irradiation, some of the processes are not present anymore. For
example, the disappearance of the β relaxation in HDPE and i-PP upon irradiation has
been attributed to an increase in crystallinity due to chain scission and the occurrence
of crosslinking, which reduces chain mobility [12, 27, 28]. The disappearance after
irradiation of the γ process has been associated in mechanical measurements to
the increase in carbonyl content and the decreased molecular weight of the highly
oxidized PP due to chain scission [30, 31].
Artificial weathering has been proposed as an alternative to thermal oxidation,
using UV radiation in a dry environment to induce photo oxidative chain scission
Fig. 3 Dielectric loss tangent versus absorbed dose for a LDPE, b HDPE, and c i-PP. Reproduced
from Radiation-Induced Modification of Dielectric Relaxation Spectra of Polyolefins: Polyethylenes
versus Polypropylene . Polym. Bull. 2014, 71, 2317–2334, reproduced with permission
S. X. Drakopoulos et al.
on the chemical modification of the polymer chain to induce dipoles and (ii) the
addition of probes with strong dipole moment into the polymer matrix.
3.1 Chemical Modifications
We will first report on methods that render polyolefins dielectrically active by means
of chemical modifications, including oxidization and chlorination. The most common
is oxidization, which introduces carbonyl groups into the polymer chain that serve
as a dielectric probe. Thermally-initiated oxidization was one of the first methods to
be used, which enabled the unveiling of the dielectric spectra of PE early on [24–26].
Boyd argued that the α relaxation observed in lightly oxidized PE must involve the
screw rotation around the main chain axis of a perpendicular C=O dipole which is
yielding the necessary dielectric activity to detect the process [4, 16]. Several studies
have reported on the use of gamma radiation to oxidize PE and i-PP [12, 27–29]. As
shown in Fig. 3, the dielectric losses of some relaxations increase with increasing
doses, but after irradiation, some of the processes are not present anymore. For
example, the disappearance of the β relaxation in HDPE and i-PP upon irradiation has
been attributed to an increase in crystallinity due to chain scission and the occurrence
of crosslinking, which reduces chain mobility [12, 27, 28]. The disappearance after
irradiation of the γ process has been associated in mechanical measurements to
the increase in carbonyl content and the decreased molecular weight of the highly
oxidized PP due to chain scission [30, 31].
Artificial weathering has been proposed as an alternative to thermal oxidation,
using UV radiation in a dry environment to induce photo oxidative chain scission
Fig. 3 Dielectric loss tangent versus absorbed dose for a LDPE, b HDPE, and c i-PP. Reproduced
from Radiation-Induced Modification of Dielectric Relaxation Spectra of Polyolefins: Polyethylenes
versus Polypropylene . Polym. Bull. 2014, 71, 2317–2334, reproduced with permission