Page 168 - Polymer-based Nanocomposites for Energy and Environmental Applications

142 Polymer-based Nanocomposites for Energy and Environmental Applications

polymers to improve the dielectric breakdown strength of the materials. Mixed ratios
of linear and branched polyethylene (PE) were studied by Hosier et al., and they
observed that PE mixtures showed a higher breakdown strength than linear PE
[160]. They proposed that the thick lamellae structure network influenced local charge
transport processes that result in high breakdown strength for blends. Schneuwly et al.
also worked on polypropylene (PP) films and succeeded to enhance their breakdown
strength by >25%, implementing impregnation with rapeseed oil [161]. They argued
that the voids in the amorphous region of the polymer were covered by the oil that
caused high overall dielectric strength. However, temperature had a negative effect
when small amounts of low-molecular-weight organic and inorganic additives were
added into the polymer. They asserted that the amorphous regions softened on
increase in temperature that created more free volume that lead to lower breakdown
strength. Apart from this, chemical modifications were also tried. Job et al. [162]
improved the breakdown strength of poly(ethylene terephthalate) (PET) films by
in situ polymerization of a layer of polyaniline (PANI). They filled the voids of
PET by nonconductive PANI, and a 30% increase in dielectric strength was found.
Ieda et al. [163] also discussed a detailed analysis of breakdown processes in
polymers.

5.2.4.1 Breakdown behavior in polymer nanocomposites

In order to improve the dielectric properties of polymers, the addition of inorganic
fillers to the polymers was explored to acquire effective dielectric constant and energy
density [164]. In the conventional composites, typical filler particles are often larger in
size that depletes breakdown strength [165]. Aggregation of filler particles was pro-
posed to cause defect centers that distort and enhance the local electric field, leading to
reduced breakdown strength. The difference in permittivities of the filler and the poly-
mer matrix are primarily responsible for the field distortion under AC conditions.
Under DC conditions, field distortion occurs exclusively due to the difference in con-
ductivities [166]. Further, the particle size is also detrimental as it enhances the field
probability. Polymer nanocomposites with filler particles of nanometer dimensions
have been explored to overcome the limitations of conventional composites
[167,168]. Nanocomposites present better properties than microcomposites as they
contain lesser amounts of fillers and have large filler-polymer interfacial area. The
desired properties can be obtained from the bulk polymer by changing it into an inter-
facial polymer, when a few weight percent of anisotropic fillers with high aspect ratio
are added. In case of nanocomposites, the composite properties depend on the dom-
inant role of the interface, whereas for microcomposites, the composite properties are
generally a weighted average of its constituents [13].

5.2.5 Role of interfaces in the polymer nanocomposites
To understand the role of interfaces in the polymer nanocomposites, two important
models were hypothesized, namely, Lewis’s model and Tanaka’s model, which are
discussed in the following subsections [167,169-171].