copolymer, even under a relatively low electric field condition. At the same time, all
PP–OH copolymers also exhibit a similar dielectric loss spectra (resembling that of
BOPP); the loss maintains very low (tan δ < 0.001) frequency between the 1,000
and 1,000,000 Hz range and at temperatures between À20 and 100
C.
Figure 20 shows the D–E loops (top) and energy density (bottom) of several
PP–OH copolymers containing 0, 0.7, 1.8, and 4.2 mol% OH comonomer units, as
well as the BOPP reference [69]. All PP–OH copolymers exhibit similar linear and
slim D–E loops (resembling BOPP); the slope of the D–E loop increases with the
OH content, consistent with the dielectric results. In addition, the slope stays
constant over a wide range of applied electric fields, up to E ¼ 600 MV/m. The
charge displacement of PP–OH-3 reaches 2.4 μC/cm
2 at 600 MV/m, which is
double that of PP under the same applied electric field. Evidently, the dielectric
loss remains very small, with even the PP–OH copolymers exhibiting significantly
higher dielectric activities. Figure 20 (bottom) compares the energy density of the
same four PP and PP–OH polymers. The energy density (U e ¼
R
EdD discharing ) is
estimated from the discharging cycle in Fig. 20 (top), which clearly increases with
the OH content and exponentially increases with the applied electric field. At the
applied electric field E ¼ 600 MV/m, the energy density for PP–OH-3 reaches
7.42 J/cm
3 , which is more than double that shown in BOPP capacitors. Most
importantly, the increase in energy density does not cause an increase in energy
loss (the area enclosed by the charging–discharging cycle), which remains very low
(similar to PP) for all PP–OH copolymers.
The OH groups in the flexible side chains clearly contribute to the polarizability
of the PP–OH at an unexpectedly large scale, which may originate from the induced
electronic polarization of OH groups along with the local dipole orientation. It is
peculiar to observe polar group reversible polarization for the first time. As
illustrated in Fig. 21, the flexible OH groups may form interchain H-bondings,
with a network structure that provides reversible segment stability even when the
temperature rises up to 100
C. A FTIR spectrum of PP–OH-3 also shows a broad
OH absorption band peaked at 3,300 cm
À1 , indicating H-bonding between OH
groups in the PP–OH copolymer.
Figure 22a compares the breakdown strength of three x-PP copolymers,
containing 0.7, 1.4, and 2.1 mol% crosslinkers units, and the corresponding linear
PP polymer [70]. Figure 22b shows their Weibull distributions, with the estimated α
and β values. All polymer films (thickness: 10–15 μm) were prepared by solution
casting before thermal crosslinking. The chemical crosslinking has a significant
effect on the breakdown strength and breakdown distribution – the higher the
crosslinking density, the higher the breakdown strength (α value) and narrower
the distribution (β value). The x-PP-3 thin film shows a breakdown strength
between 620 and 670 MV/m, with a α value of 645 MV/m, which is almost the
same as those of BOPP films that are carefully conditioned (through stretching and
annealing) to increase chain orientation and crystallinity and to reduce defects. In
addition, the x-PP-3 film exhibits a very narrow breakdown distribution with an
exceptionally high β value of 42, indicating excellent dielectric reliability, which is
Functional Polyolefins: Synthesis and Energy Storage Applications
271
PP–OH copolymers also exhibit a similar dielectric loss spectra (resembling that of
BOPP); the loss maintains very low (tan δ < 0.001) frequency between the 1,000
and 1,000,000 Hz range and at temperatures between À20 and 100
C.
Figure 20 shows the D–E loops (top) and energy density (bottom) of several
PP–OH copolymers containing 0, 0.7, 1.8, and 4.2 mol% OH comonomer units, as
well as the BOPP reference [69]. All PP–OH copolymers exhibit similar linear and
slim D–E loops (resembling BOPP); the slope of the D–E loop increases with the
OH content, consistent with the dielectric results. In addition, the slope stays
constant over a wide range of applied electric fields, up to E ¼ 600 MV/m. The
charge displacement of PP–OH-3 reaches 2.4 μC/cm
2 at 600 MV/m, which is
double that of PP under the same applied electric field. Evidently, the dielectric
loss remains very small, with even the PP–OH copolymers exhibiting significantly
higher dielectric activities. Figure 20 (bottom) compares the energy density of the
same four PP and PP–OH polymers. The energy density (U e ¼
R
EdD discharing ) is
estimated from the discharging cycle in Fig. 20 (top), which clearly increases with
the OH content and exponentially increases with the applied electric field. At the
applied electric field E ¼ 600 MV/m, the energy density for PP–OH-3 reaches
7.42 J/cm
3 , which is more than double that shown in BOPP capacitors. Most
importantly, the increase in energy density does not cause an increase in energy
loss (the area enclosed by the charging–discharging cycle), which remains very low
(similar to PP) for all PP–OH copolymers.
The OH groups in the flexible side chains clearly contribute to the polarizability
of the PP–OH at an unexpectedly large scale, which may originate from the induced
electronic polarization of OH groups along with the local dipole orientation. It is
peculiar to observe polar group reversible polarization for the first time. As
illustrated in Fig. 21, the flexible OH groups may form interchain H-bondings,
with a network structure that provides reversible segment stability even when the
temperature rises up to 100
C. A FTIR spectrum of PP–OH-3 also shows a broad
OH absorption band peaked at 3,300 cm
À1 , indicating H-bonding between OH
groups in the PP–OH copolymer.
Figure 22a compares the breakdown strength of three x-PP copolymers,
containing 0.7, 1.4, and 2.1 mol% crosslinkers units, and the corresponding linear
PP polymer [70]. Figure 22b shows their Weibull distributions, with the estimated α
and β values. All polymer films (thickness: 10–15 μm) were prepared by solution
casting before thermal crosslinking. The chemical crosslinking has a significant
effect on the breakdown strength and breakdown distribution – the higher the
crosslinking density, the higher the breakdown strength (α value) and narrower
the distribution (β value). The x-PP-3 thin film shows a breakdown strength
between 620 and 670 MV/m, with a α value of 645 MV/m, which is almost the
same as those of BOPP films that are carefully conditioned (through stretching and
annealing) to increase chain orientation and crystallinity and to reduce defects. In
addition, the x-PP-3 film exhibits a very narrow breakdown distribution with an
exceptionally high β value of 42, indicating excellent dielectric reliability, which is
Functional Polyolefins: Synthesis and Energy Storage Applications
271