polymers and composites, show many complications. It is very difficult to find a
new polymer that can fulfill all the required properties (i.e., high energy density,
low energy loss, good processability, self-healing, long-term reliability, low cost,
etc.). For example, the ferroelectric vinylidene fluoride (VDF)-based terpolymer
with a high ε of over 60 can exhibit a high energy density of ~25 J/cm
3 at
E ¼ 500 MV/m [25–28], but this energy is accompanied by the hysteresis loops
during polarization–depolarization cycles. This irreversible polarization behavior
causes a minimum energy loss of ~30% that is transformed to heat, which must be
instantaneously removed from the capacitor to prevent melting of the polymer thin
film. In a large-scale energy storage device, the acceptable dielectric loss is less
than 1%. In light of this, one of our approaches has been to adopt the newly
developed PPs as the dielectrics, with the objective of improving the dielectric
properties and maintaining the desirable properties of PP (i.e., low energy loss,
good processability, self-healing, long-term reliability, low cost, etc.). In this
chapter, I will summarize our experimental results on the application of
hydroxylated polypropylene (PP–OH) to increase the dielectric constant (ε) and
crosslinked polypropylene (x-PP) to increase breakdown strength (E) in capacitors.
2 New Functionalization Approach
In the early 1980s, the discovery of homogeneous single-site metallocene catalysts
[22–25] provided an excellent opportunity to explore new functionalization
approaches to circumvent the chemical difficulties in direct and postpolymerization processes. Comparing with heterogeneous (multiple active sites)
Ziegler–Natta catalysts, one major advantage of single-site metallocene catalysts is
the superior capability in the copolymerization reaction to form copolymers with
narrow distributions of molecular weight and composition. In addition, the combination of tunable active site and well-controlled polymerization mechanism allows
effective incorporation of large comonomers, including styrenic and cyclic
comonomers, into PE and PP copolymers with a broad range of copolymer
compositions. In the late 1980s, we were thinking about how to apply this new
metallocene-mediated copolymerization capability into our functionalization
chemistry [3, 29]. Scheme 1 illustrates the general reaction scheme, now called
the “reactive” copolymer approach [30–33].
A comonomer that contains a “reactive” group should exhibit three essential
properties: (1) good stability with metallocene catalyst, (2) good solubility with the
reaction media, and (3) facile interconversion of reactive group into functional
(polar) groups, such as OH and NH 2 groups, after polymerization [3, 34]. In
previous years, we had identified three suitable “reactive” comonomers, i.e., borane
monomer, p-methylstyrene (p-MS), and divinylbenzene (DVB), as illustrated in
Scheme 1. In concert with the selected metallocene catalysts, the copolymerization
reactions take place to form well-controlled “reactive” polyolefin copolymers with
various reactive sites in the side chains. Subsequently, the incorporated reactive
236
T.C.M. Chung
new polymer that can fulfill all the required properties (i.e., high energy density,
low energy loss, good processability, self-healing, long-term reliability, low cost,
etc.). For example, the ferroelectric vinylidene fluoride (VDF)-based terpolymer
with a high ε of over 60 can exhibit a high energy density of ~25 J/cm
3 at
E ¼ 500 MV/m [25–28], but this energy is accompanied by the hysteresis loops
during polarization–depolarization cycles. This irreversible polarization behavior
causes a minimum energy loss of ~30% that is transformed to heat, which must be
instantaneously removed from the capacitor to prevent melting of the polymer thin
film. In a large-scale energy storage device, the acceptable dielectric loss is less
than 1%. In light of this, one of our approaches has been to adopt the newly
developed PPs as the dielectrics, with the objective of improving the dielectric
properties and maintaining the desirable properties of PP (i.e., low energy loss,
good processability, self-healing, long-term reliability, low cost, etc.). In this
chapter, I will summarize our experimental results on the application of
hydroxylated polypropylene (PP–OH) to increase the dielectric constant (ε) and
crosslinked polypropylene (x-PP) to increase breakdown strength (E) in capacitors.
2 New Functionalization Approach
In the early 1980s, the discovery of homogeneous single-site metallocene catalysts
[22–25] provided an excellent opportunity to explore new functionalization
approaches to circumvent the chemical difficulties in direct and postpolymerization processes. Comparing with heterogeneous (multiple active sites)
Ziegler–Natta catalysts, one major advantage of single-site metallocene catalysts is
the superior capability in the copolymerization reaction to form copolymers with
narrow distributions of molecular weight and composition. In addition, the combination of tunable active site and well-controlled polymerization mechanism allows
effective incorporation of large comonomers, including styrenic and cyclic
comonomers, into PE and PP copolymers with a broad range of copolymer
compositions. In the late 1980s, we were thinking about how to apply this new
metallocene-mediated copolymerization capability into our functionalization
chemistry [3, 29]. Scheme 1 illustrates the general reaction scheme, now called
the “reactive” copolymer approach [30–33].
A comonomer that contains a “reactive” group should exhibit three essential
properties: (1) good stability with metallocene catalyst, (2) good solubility with the
reaction media, and (3) facile interconversion of reactive group into functional
(polar) groups, such as OH and NH 2 groups, after polymerization [3, 34]. In
previous years, we had identified three suitable “reactive” comonomers, i.e., borane
monomer, p-methylstyrene (p-MS), and divinylbenzene (DVB), as illustrated in
Scheme 1. In concert with the selected metallocene catalysts, the copolymerization
reactions take place to form well-controlled “reactive” polyolefin copolymers with
various reactive sites in the side chains. Subsequently, the incorporated reactive
236
T.C.M. Chung