Page 190 - Polymer-based Nanocomposites for Energy and Environmental Applications

Polymer-based nanocomposites 163

nanoparticles. The shells serve as buffer layers to minimize the local electric field
enhancement that enhance the breakdown strength of the nanocomposites. One more
benefit is that quantum confinement and coulomb blockade effects of ultrasmall
metallic nanoparticles decorated on the surface of high-dielectric-constant particles
can be exploited to limit the leakage currents and increase the breakdown strength
of the high-dielectric-constant nanocomposites. Also, the disappearance of electric
percolation transition in polymer nanocomposites filled with electrically conductive
particles as acquired by core-shell strategies shows high dielectric constant, low
dielectric loss, and satisfactory breakdown strength. Marks et al. found that the PP
nanocomposites with Al 2 O 3 @Al core-shell nanoparticles have a high breakdown
strength (viz., 75.8 MV m 1 for PP nanocomposites with 12.4 vol% Al 2 O 3 @Al
core-shell nanoparticles) along with low dielectric loss and enhanced dielectric
constant, even at an elevated metal nanoparticle concentrations [273]. This laid the
basis for metallic nanoparticles filled nanocomposites to be adopted for energy storage
3
applications (e.g., the discharged energy density is 13.4 J/cm for PP nanocomposites
with 12.4 vol% Al 2 O 3 @Al core-shell nanoparticles). It was also found that the
nanocomposites synthesized directly by polymer@BaTiO 3 nanoparticles from the
starlike unimolecular block copolymer micelles have much improved dielectric con-
stant than those of nanocomposites synthesized by commercially available BaTiO 3
nanoparticles [274]. Further, the core-shell strategies easily control the critical factors
including interface thickness, interface interactions, and electric mismatch at the inter-
faces, which significantly affect the electric properties of the high-dielectric-constant
nanocomposites. Hence, an insight regarding the roles of the interfaces on the
electric properties of the high-dielectric-constant nanocomposites is obtained by this
unique strategy; however, many challenges are still underway. Technologically, the
incorporation of nanoparticles having suitable nanoeffects in the shells and inno-
vative fabrication technologies for core-shell nanoparticles may overcome these
limitations. Finally, the processing details and typical property requirements of
high-dielectric-constant nanocomposites are to be explored for the development of
advanced high-dielectric-constant nanocomposites [275].

5.6.1 Core-shell nanocomposites synthesized by the
“grafting from” route

This method involves the in situ polymerization of monomers on initiator-
functionalized nanoparticle surfaces, and the introduction of a sufficient quantity of
initiating sites on the nanoparticle surfaces is the basis of this method. A few well-
adopted “grafting from” techniques are controlled radical polymerization, such as
atom transfer radical polymerization (ATRP) and reversible addition-fragmentation
chain transfer (RAFT) polymerization that provide many positivities
[65,89,276,277]. One is that the shell layer protects nanoparticle aggregation. Sec-
ondly, the nanocomposites can be prepared directly from core-shell nanoparticles
using the shell layer as a matrix that leads to high-quality highly filled nanocomposites
that are free of defects, such as voids and pores. Moreover, a strong nanoparticle/