activity, forming branched polymer chain in late transition metal catalyst cases,
which reduce polymer crystallinity and melting temperature.
Many current commercial functionalization processes are based on the postpolymerization process [13, 14]. Chemical modifications of the pre-formed
polyolefins have been usually carried out in situ during the fabrication process to
reduce the production cost, as well as to relieve concern (in many cases)
surrounding the reduction of the processibility of polyolefins after the functionalization reaction. However, the combination of the inert nature of the polyolefin
(requiring high energy) and a short reaction time (during processing) causes a great
deal of difficulty in controlling the polymer composition and structure. There is no
facile reaction site in the saturated PE, PP, and (ethylene propylene) EP polymers.
The only successful tactic is to use a free radical initiator or radiation to activate the
polymer by breaking some stable C–H bonds and forming free radicals along the
polymer chain. The resulting polymeric radicals then undertake chemical reactions
with some polar chemical reagents, such as maleic anhydride, acrylic acid, etc.
However, such free radical chemistry is usually accompanied by many undesirable
side reactions (crosslinking, degradation, oligomerization, etc.) and by-products.
Overall, the current commercial process is far from ideal.
Despite its scientific challenges, energy storage has also long been an industrially important area and is an integral part of energy utilization efficiency [15]. Polyolefin (PE and PP) films are commonly used as separators in Li-ion batteries and
dielectrics in capacitors for energy storage applications. Capacitors [16, 17] are
passive electronic devices that store energy in the form of an electrostatic field. In
their simplest form, capacitors consist of two conducting plates (positive and
negative electrodes) separated by an insulating material called the dielectric,
which can be air, ceramic, polymer, etc. In contrast to batteries, which have high
energy density and low power density, capacitors usually exhibit high power
density but very low energy density. The inherent scientific challenge is to increase
the energy density of the capacitor, which is governed by the dielectric that
separates the opposite static charges on two electrode surfaces. Recently, metallized polymer film capacitors [18, 19] have attracted a great deal of attention due to
their desirable properties, such as light weight, low cost, and excellent processability for forming thin films with a large surface area. They also demonstrate flexibility
and toughness under stress and the ability to be packaged into a desirable configuration. Currently, state-of-the-art polymer film capacitors are based on BOPP
(biaxial-oriented PP) thin films [20, 21]. Despite the low energy density, in the
range of 2–3 J/cm
3 , BOPP shows almost no energy loss during charging–discharging cycles and self-healing after a film puncture, which merely results in
a gradual loss of capacitance so that they can be operated near the breakdown
voltage with good long-term reliability. Many BOPP capacitors are currently used
as pulse power capacitors in commercial and military devices.
Based on the energy density equation (Section 4), it is theoretically possible to
increase the energy density of BOPP capacitors by increasing the PP
dielectric constant (ε) and/or the applied electric field (E) controlled by breakdown
strength. However, recent experimental results, studying several high dielectric
Functional Polyolefins: Synthesis and Energy Storage Applications
235
which reduce polymer crystallinity and melting temperature.
Many current commercial functionalization processes are based on the postpolymerization process [13, 14]. Chemical modifications of the pre-formed
polyolefins have been usually carried out in situ during the fabrication process to
reduce the production cost, as well as to relieve concern (in many cases)
surrounding the reduction of the processibility of polyolefins after the functionalization reaction. However, the combination of the inert nature of the polyolefin
(requiring high energy) and a short reaction time (during processing) causes a great
deal of difficulty in controlling the polymer composition and structure. There is no
facile reaction site in the saturated PE, PP, and (ethylene propylene) EP polymers.
The only successful tactic is to use a free radical initiator or radiation to activate the
polymer by breaking some stable C–H bonds and forming free radicals along the
polymer chain. The resulting polymeric radicals then undertake chemical reactions
with some polar chemical reagents, such as maleic anhydride, acrylic acid, etc.
However, such free radical chemistry is usually accompanied by many undesirable
side reactions (crosslinking, degradation, oligomerization, etc.) and by-products.
Overall, the current commercial process is far from ideal.
Despite its scientific challenges, energy storage has also long been an industrially important area and is an integral part of energy utilization efficiency [15]. Polyolefin (PE and PP) films are commonly used as separators in Li-ion batteries and
dielectrics in capacitors for energy storage applications. Capacitors [16, 17] are
passive electronic devices that store energy in the form of an electrostatic field. In
their simplest form, capacitors consist of two conducting plates (positive and
negative electrodes) separated by an insulating material called the dielectric,
which can be air, ceramic, polymer, etc. In contrast to batteries, which have high
energy density and low power density, capacitors usually exhibit high power
density but very low energy density. The inherent scientific challenge is to increase
the energy density of the capacitor, which is governed by the dielectric that
separates the opposite static charges on two electrode surfaces. Recently, metallized polymer film capacitors [18, 19] have attracted a great deal of attention due to
their desirable properties, such as light weight, low cost, and excellent processability for forming thin films with a large surface area. They also demonstrate flexibility
and toughness under stress and the ability to be packaged into a desirable configuration. Currently, state-of-the-art polymer film capacitors are based on BOPP
(biaxial-oriented PP) thin films [20, 21]. Despite the low energy density, in the
range of 2–3 J/cm
3 , BOPP shows almost no energy loss during charging–discharging cycles and self-healing after a film puncture, which merely results in
a gradual loss of capacitance so that they can be operated near the breakdown
voltage with good long-term reliability. Many BOPP capacitors are currently used
as pulse power capacitors in commercial and military devices.
Based on the energy density equation (Section 4), it is theoretically possible to
increase the energy density of BOPP capacitors by increasing the PP
dielectric constant (ε) and/or the applied electric field (E) controlled by breakdown
strength. However, recent experimental results, studying several high dielectric
Functional Polyolefins: Synthesis and Energy Storage Applications
235