Colossal dielectric constant polymer nanocomposites: Role of charge injection at matrix–filler interfaces

Abstract

Recent experiments show that certain self-assembled polymer nanocomposites exhibit colossal dielectric constants (∼4000 at 20 Hz), as compared to eight for the polymer matrix, even for small filler volume fractions (∼7%). However, reasons for this are not well understood. In this work, two possible explanations are systematically evaluated: (i) presence of interface zones with dielectric constants much greater than that of the matrix and (ii) charge injection from filler surfaces into the dielectric at matrix–filler interfaces. Linear, isotropic composites with periodic and random arrangements of spherical, conducting nanofillers are considered. While finite element analyses on unit cells are used to compute quasi-static dielectric properties of periodic composites, standard homogenization theories are used for investigating random composites. A novel two-parameter, non-local model is used to describe the charge injection. It is shown that both the effects improve the overall dielectric constants but in very different ways. While charge injection leads to a drastic increase in effective dielectric constant even for small filler volume fractions (∼1%), high dielectric constant interface zones significantly alter the overall properties only at percolation volume fractions. Charge injection appears to be the likely reason for the colossal dielectric constants observed experimentally in self-assembled nanocomposites.