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Polymer-based nanocomposites 145

Fig. 5.5 Multicore model for
Bonded layer
(~1 nm) nanoparticle-polymer interfaces.
Reprinted with permission from Prateek
Thakur VK, Gupta RK. Recent progress
Nano Bound layer on ferroelectric polymer-based
particle (2–9nm) nanocomposites for high energy density
capacitors: synthesis, dielectric
Loose layer properties, and future aspects. Chem
(several tens of nm) Rev 2016;116:4260 317. Copyright
2016 American Chemical Society.
Diameter of nanoparticle: 20–50nm
Diameter of nanoparticle: 20–50nm
Thickness of layers: 10–30nm
Thickness of layers: 10–30 nm
Inter-particle distance in polymer matrix
Inter-particle distance in polymer matrix
:
nm
( (Surface to surface): 40–100 nm
Surface to surface
)
40–100
The interface has been proposed to consist of a bonded bound and a loose layer. The
bonded layer is produced when a polymer is tightly bonded to the particle as generally
observed in case of nanocomposites. The bound layer that overlaps the bonded layer
represents a strong bond between the polymer chains and the first layer. The third layer
is a “loosely coupling interaction” layer that possesses free volume and the crystalline
region of the polymer matrix. On the basis of this multicore model, Smith et al. gave a
mechanistic hypothesis of the interface structure [33]. Due to the multilayer structure,
a gradient in the charge mobility is created. The surface charge on the nanoparticle
accumulates due to the differences in the Fermi levels between the nanoparticles
and the polymer. And, a redistribution of charge occurs at the interface, forming a
Helmholtz or Stern layer to maintain charge neutrality at the interface. A diffuse dou-
ble layer of charge exists far away from the interface in the matrix polymer. To under-
stand the positive role of the interfacial structure on the dielectric properties of
nanocomposites, it has been proposed that the diffuse double layer is a region of higher
charge mobility and strongly effects the dispersion and dielectric properties of the
nanocomposite. The charge in the Stern layer influences this double layer, and hence,
the suitable engineering of the interface results in changes in the free volume, mobil-
ity, and trap sites for charge carriers. Ma et al. explained this phenomenon. They found
a decrease in the charge mobility in titanium dioxide modified with a polar silane cou-
pling agent [41]. The filler reduces overlapping of local conductive regions at lower
concentrations and thus prevents premature dielectric breakdown. Ma et al. observed
that the surface functionalization of nanofillers does not always enhance the disper-
sion of fillers. Although, the addition of polar silane-modified TiO 2 nanoparticles in a
polyethylene matrix lead to agglomeration, a higher breakdown strength was found in
surface-modified TiO 2 nanocomposites than in the unmodified. The reason proposed
was a decline in the degree of polymer crystallinity and increased electron scattering
by the polar interfacial groups. Contrarily, Kim et al. modified the surface with phos-
phonic acid groups and decreased the aggregation of BaTiO 3 nanoparticles [40]. They
acquired better properties for the nanocomposite compared with the base polymer.
Li et al. also got the same results who argued that, the improved energy density of