Page 167 - Polymer-based Nanocomposites for Energy and Environmental Applications

Polymer-based nanocomposites 141

the main chain. However, γ-relaxation occurs at much lower temperature and is asso-
ciated with movement of small units of the main chain or side chain [153].

5.2.4 Dielectric breakdown phenomena in polymers
One more important factor imperative in determining the dielectric properties of the
polymer nanocomposites is breakdown strength. Breakdown is detrimental for high-
energy-density applications as it severely affects the performance of the capacitor. It is
the exclusive failure of a dielectric in an external applied field that causes mechanical
damage and subsequent electric conduction.
For the better understanding of this phenomenon, the mechanisms involved in the
electric breakdown in solid dielectrics need to be studied. The conduction phenome-
non in the solid dielectrics include currents caused due to orientation and interfacial
polarization and also due to the electronic and ionic motions. According to Vorobev,
the electric breakdown in solid dielectrics is a solid-to-plasma-phase transition phe-
nomena gaining energy from the external field [154]. The common modes of electric
breakdown for solid dielectrics are thought to be intrinsic thermal and ionization
mechanisms [155,156]. The intrinsic electric strength determines the true strength
of a material, and it mainly depends on the material and the temperature. It is assumed
that an insulator reaches its intrinsic strength when the field applied is such that it
brings electronic transition in it. It is determined under strictly controlled test condi-
tions so as to produce a high stress at the center of the test specimen to assure zero
external discharges. Another type is the thermal breakdown, which is the failure
due to localized nonuniform fields and polarization in the electric field. This comes
into act at a critical current density and can be controlled under special experimental
conditions. One most common is the avalanche mechanism, and it occurs when free
electrons in polymers achieve high energy by accelerating along the mean free path to
knock out other electrons. This results into conduction due to multifold increase in
number of electrons in the conduction band. Simultaneously, the amorphous nature
of polymers provides a free path for the electrons to accelerate under an external field.
This forms the basis of free-volume theory proposed by Artbauer [157]. As mentioned
by Sabuni and Nelson, there was a decrease in dielectric strength on increasing the
temperature with a sharp reduction at a critical temperature, indistinguishable from
the glass transition temperature, when the effect of plasticizers on electric strength
of polystyrene at different temperatures was studied [158]. Thus, the plasticizers play
an important role in the dielectric strength and the critical temperature. They attributed
the breakdown to higher mobility of free electrons at higher temperatures. Wiacek, on
the other hand, has studied the addition of cycloaliphatic hydrocarbon units to fluo-
rinated polyesters [159]. He explored the higher-order hydrocarbons that are similar to
diamond in their molecular structure and electric properties. He found that poly
9,9-bis(4-hydroxyphenyl) fluorine end-capped with 4,9-diamantane had the highest
breakdown strength and subsequent high energy density. Furthermore, the introduc-
tion of a cross-linking agent, cyclotene, and subsequent curing enhanced the break-
down strength by 16%. As such, various modifications have been made in