Page 160 - Polymer-based Nanocomposites for Energy and Environmental Applications

Polymer-based nanocomposites 135

5.2.1 Nature of the filler and the polymer

The combination of the filler and the polymer presents improved dielectric proper-
ties depending on the type and behavior of polymer matrices and fillers [76]. Various
polymers have been used based on their distinct properties including epoxy [64,
77-83], polyimides [84-88], poly(methyl methacrylate) (PMMA) [75,89-91],
polydimethylsiloxane (PDMS) [92], to synthesize polymer nanocomposites. For
example, epoxy resins have been observed to show high reactivity, easier processing,
light weight, and low cost [93].
Based on the mean dipole moment, polymers can be grouped into polar and non-
polar polymers [94-96]. The individual dipole moments cancel out each other due to
symmetry in the nonpolar polymer and are hence responsible for the lower dielectric
constant. A few nonpolar polymers are low-density polyethylene (LDPE), poly-
tetrafluoroethylene (PTFE), polyolefins, etc. However, the dipoles usually do not can-
cel out each other giving rise to reinforcement of individual dipole moments in the
polar polymers and hence show comparatively larger dielectric constant than nonpolar
polymers. The polar nature of the polymers is acquired due to the presence of polar
groups and chain geometry. For example, out of various forms of poly(vinylidene
fluoride) (PVDF), dipoles are in one direction giving rise to polar behavior in all-trans
β-PVDF. Generally, PVDF has a dielectric constant of about 10 with higher break-
3
down strengths. It has a discharged energy density of 2.8 J/cm at 2400 kV/cm
(Table 5.1).
Thus, PVDF has a better energy density in comparison with the other polymers like
poly(arylene ether nitrile) and polyimides, due to comparatively higher dielectric con-
stant and breakdown strength. The proper selection of a polymer along with proper
fillers of different types and shapes also improves the electric properties of the poly-
mer nanocomposites. Generally, two different types of fillers, namely, nonconducting
and conducting fillers, have been utilized to prepare different polymer-based
nanocomposites. Nonconducting fillers contain ceramics like barium titanate
(BaTiO 3 ) [114-116], strontium titanate (SrTiO 3 ) [117],Bi 2 O 3 -ZnO-Nb 2 O 5 [118],
and calcium titanate (CaTiO 3 ) [119]. These ceramics are generally insulators due
to their high bandgap and that the accumulation of charges occurs only on the appli-
cation of an electric field [120,121]. Simultaneously, conducting fillers like graphene
[122,284], carbon black (CB) [123,124], and carbon nanotubes (CNTs) [125-127] are
also largely used. Although, higher values of dielectric constant can be easily acquired
by using conducting fillers at smaller concentrations than nonconducting fillers, but
the properties change abruptly near the percolation threshold, which is the critical con-
centration above which continuous channels begin to arise over the whole system,
thereby discouraging further addition of fillers [128]. The shapes and sizes of the
fillers are also important factors that can further enhance the dielectric properties
of the polymer nanocomposites [48,129]. Various shapes like zero-, one-, and two-
dimensional and sizes cause different percolation limits, which in turn influences
the flexibility and the loading of the polymer nanocomposites. For higher-dimensional
fillers, the percolation limits can be achieved earlier as compared with lower-
dimensional fillers for the same loading concentration. The dielectric constant and