Conductive polypyrrole/tungsten oxide metacomposites with negative permittivity

Polypyrrole (PPy) nanocomposites reinforced with tungsten oxide (WO ₃) nanoparticles (NPs) and nanorods (NRs) are fabricated by a surface-initiated polymerization method. The electrical conductivity is observed to depend strongly on the particle loadings, molar ratio of oxidant to pyrrole monomer, and the filler morphology. The electron transportation in the nanocomposites follows a quasi-three-dimensional variable range hopping (VRH) conduction mechanism as evidenced by the temperature-dependent conductivity function. Unique negative permittivity is observed in both pure PPy and its nanocomposites, and the switching frequency (frequency where the real permittivity switches from negative to positive) can be tuned by changing the particle loading, ratio of oxidant to pyrrole monomer, and the filler morphology. The extent of charge carrier localization calculated from the VRH mechanism is well-correlated to the dielectric properties of the nanocomposites. WO₃ NRs are observed to be more efficient in improving the electrical conductivity, dielectric permittivity, and thermal stability of the resulting nanocomposites as compared to those with WO₃ NPs. The microstructures of pure PPy and its nanocomposites are observed by scanning electron microscopy and transmission electron microscopy. Powder X-ray diffraction analysis demonstrates the crystalline structure of WO₃ nanostructures, as well as their corresponding nanocomposites. Thermogravimetric analysis reveals a significantly enhanced thermal stability with the addition of nanofillers.