Page 195 - Polymer-based Nanocomposites for Energy and Environmental Applications

Polymer-based nanocomposites 167

5.7 Challenges

In the modern times of technology, the need of flexible nanocomposites for high-
energy-density applications is growing at a fast speed, and it has become one of
the most attractive research fields in the recent years. In view of the above discussion,
it is evident that there is manifold improvement in dielectric parameters of ferroelec-
tric and relaxor ferroelectric-based polymer nanocomposites. However, various chal-
lenges still persist. As the overall energy density of a polymer nanocomposite depends
on the dielectric constant and the breakdown strength, the tailoring of the polarization
mechanisms, namely, electronic, ionic, orientational, and interfacial polarizations, can
give desired properties. In view of that the electronic polarization exhibits dielectric
loss in UV range, it can be investigated under power and radio-frequency ranges, so as
to achieve good dielectric constant with low dielectric loss. In case of a polymeric
system, the electronic polarization can be improved by delocalizing the electrons.
However, this is limited by the bandgap of the material. Further, ionic polarization
can be tuned by replacement of carbon with other elements in a polymer. Recently,
in a novel polymeric system, dXY 2 d (where X¼Si, Ge, and Sn and Y¼H, F,
and Cl), replacing carbon with group 14 elements enhanced the dielectric constant
of the polymer [100]. However, fabricating such types of polymers is itself challeng-
ing. Interfacial polarization is not feasible for energy storage applications because of
its long discharge time, but it persists in the film capacitors inherently. However, the
use of multilayer capacitors can give satisfactory results due to confined ion transport
[139]. But, the optimization of interfacial polarization from a single-layer film capac-
itor is still a challenge. The orientational polarization is majorly responsible for the
enhancement of energy density in polymer nanocomposites due to the limited mod-
ifications in electronic and ionic polarizations. However, intermolecular friction along
with the flipping of chain dipoles results in potential dielectric loss. So, increase in
dipole rotation and the shifting of the dipole relaxation toward GHz frequencies
can present lower dielectric loss in radio frequencies. The efficiency of ferroelectric
polymer nanocomposites is limited by their large hysteresis, which is responsible for
high energy loss, and in turn leads to early failure and heating. Therefore, the ferro-
electric polymers need to be surface-modified or converted to relaxor ferroelectrics to
achieve relatively higher energy density. The aggregation of nanoparticles, due to
their large surface area, is one of the major problems in the polymer-based
nanocomposites, causing inhomogeneity that results in breakdown at a much lower
electric field than that of pure polymer. Therefore, surface engineering of the
nanoparticles needs a special attention. The surface-modifying agent may itself induce
impurity in the nanocomposites, thereby causing more dielectric loss. Hence, the
selection of a suitable surface-modifying agent further depends on the materials used.
Polymer brushes can be used in place of modifying agents for these can be directly
bound with the fillers through a covalent bond [89,280]. The homogeneity thus
obtained improves the dielectric properties. However, large-scale manufacture of
grafting polymer brushes through surface-initiated polymerization is an issue. Gener-
ally, a higher loading is needed to get decent values of dielectric constant, which badly
affect the flexibility of the nanocomposites. Moreover, the breakdown strength