Page 178 - Polymer-based Nanocomposites for Energy and Environmental Applications

Polymer-based nanocomposites 151

polymers with confirmations of TGTG for α- and δ-phases, TTT for β-phase, and
0
T3GT3G for γ-phases.
0
As per polarization, β-phase of the PVDF possesses highest dipole moment of
2.10 D in perpendicular to chain length, while α-, γ-, and δ-phases possess dipole
moments of 1.20 and 1.02 D in perpendicular and parallel to the direction of chain
length, respectively [207]. Among various kinds of PVDF, α-phase is nonpolar as
the dipole moments cancel out each other, while δ-, γ-, and β-phases possess high
polarizability due to the presence of net dipole moments and hence contain high
dielectric constants as compared with α-phase [208]. The dielectric constant of PVDF
lies in the range of 6–12 [209]. Moreover, the direction of c-axes of PVDF molecule
with respect to the applied electric field also contributes in high dielectric properties.
Relatively larger dielectric constant can be acquired for the dipoles that are arranged in
parallel direction to the field (viz., CH 2 CF 2 ). Contrarily, if the dipoles are present in
perpendicular direction to the field, the orientation of the dipoles could be restricted
resulting in relatively lower dielectric constant. The dielectric loss of PVDF is rela-
tively high (0.02 at 1 kHz) as compared with BOPP (0.0002 at 1 kHz) [210]. Further,
PVDF polymers also exhibit high DC breakdown strength, 7000 kV/cm (for 10 μm,
capacitor grade films), but relatively smaller discharged energy density of 2–3 J/cm 3
at 2000 kV/cm [106,210,211]. Relatively smaller discharged energy density is due to
the attainment of saturation polarization at a much lower electric field. Out of the
above mentioned PVDF, only β-phase is ferroelectric in behavior with thermodynam-
ically stable spontaneous polarization.

5.3.2 PVDF based nanocomposites
Various types of polymer matrix-based nanocomposites have been synthesized using
different nanofillers and polymers. However, among all the high-dielectric-constant
polymers, PVDF-based copolymers and blends have been cited to be the best because
of their relatively higher dielectric constant.

5.3.2.1 Polymer nanocomposites based on non-conducting fillers
This section focuses on the synthesis and physical and electric properties of non-
conducting fillers/PVDF and PVDF-based copolymer/terpolymer nanocomposites.
The effect of morphology of different nanostructures like spherical and one-
dimensional fillers in two-phase and three-phase nanocomposites has also been
mentioned.

Two-phase composites based on spherical fillers
Two-phase polymer nanocomposites composed of spherical fillers embedded in
PVDF matrix are discussed in this section. Among different nonconducting fillers,
ceramic nanofillers, especially BaTiO 3 , have been extensively used. Dielectric prop-
erties of various BaTiO 3 -based nanocomposites have been studied in detail. Mao et al.
discussed the effect of size on the dielectric properties of the BaTiO 3 /PVDF
nanocomposites [193]. At 60 vol%, the dielectric constant of BaTiO 3 /PVDF was