Page 191 - Polymer-based Nanocomposites for Energy and Environmental Applications

164 Polymer-based Nanocomposites for Energy and Environmental Applications

matrix interface is achieved as the polymer chains are robustly bonded on the nano-
particle surfaces. In addition, the nanoparticle concentration can be tuned by adjusting
the feed ratio of the monomer and the initiator-functionalized nanoparticles. Finally, a
broad range of monomers can be polymerized through this route. Huang and Jiang
[65,276] have used ATRP and RAFT to prepare core-shell high-dielectric-constant
PMMA@BaTiO 3 and PS@BaTiO 3 nanocomposites, respectively (Fig. 5.10).
Both, the ATRP and the RAFT, bring homogeneous nanoparticle dispersion for
highly filled nanocomposites. In case of PS@BaTiO 3 nanocomposites, there was
an enhance in the dielectric constant of the nanocomposite with c. 48 vol% BaTiO 3
to 24 from the value of 2.8 found in case of the pure PS, while the dielectric loss
was almost equal to that for the pure PS. In such systems, the dielectric constant
and the dielectric loss remain stable over a wide range of frequencies, which is desir-
able for the functional devices. Biaxially oriented polypropylene (BOPP) film-based
capacitors have been extensively exploited because of a low energy loss, high break-
down strength, low capacity loss under high frequencies, sealability, lightweight, and
flexibility and have found various applications in consumer electronics, electric grids,
electric vehicles, and pulse-power systems. Never the less, the smaller dielectric con-
stant of PP limits the utilization of BOPP in large-scale applications. Marks et al. have
reported an efficient method for the preparation of PP-based, high-permittivity, and
low-loss nanocomposites [53,60,62,74,273,278]. The nanoparticles (e.g., BaTiO 3 ,
TiO 2 , ZrO 2 , SrTiO 3 , MgO, Ba 0.5 Sr 0.5 TiO 3 , and Al) were first coated with a
methylaluminoxane (MAO) cocatalyst (Fig. 5.10C), which caused the formation of
AldO covalent bonding. Subsequently, the MAO-treated nanoparticles were put to
reaction with a metallocene olefin polymerization catalyst (ac-ethylenebisindenyl)
zirconium dichloride, (EBIZrCl 2 ). The functionalization of the MAO-treated
nanoparticles by EBIZrCl 2 leads to polymerization-active species that could start
the in situ propylene polymerization and finally give isotactic PP (isoPP)
nanocomposites. The isoPP nanocomposites had a higher dielectric constant and very
high breakdown strength and hence good energy storage capacity as compared with
the pure PP. Although only a few monomers are compatible for this method, it has
several unique benefits: (i) It has high yield; (ii) the growth of the propagating poly-
olefin chains from the activated catalyst centers can lead to large local hydrostatic
pressures, which may disrupt nanoparticle aggregation in the final nanocomposites;
and (iii) it minimizes the electric mismatch between the nanoparticles and the matrix.

5.6.2 Core-shell nanoparticles synthesized by the
“grafting-to” method

This method involves grafting the presynthesized polymer chains onto the nanopar-
ticle surface via an interaction between the polymer end groups and the functional
groups present on the nanoparticle surfaces. Unlike the “grafting from” strategy,
the “grafting to” strategy can control the molecular composition and the molecular
weight of the polymer chains to get the desired efficiency of the final nanocomposites.
Click chemistry is a celebrated method used to link reaction partners that exhibits high