High dielectric constant rutile-polystyrene composite with enhanced percolative threshold

TiO2 (rutile) nanocrystals, obtained by hydrothermal synthesis, are coated with polystyrene, grown by RAFT polymerization, and are dispersed into a polystyrene matrix at various concentrations. The morphology of both the polystyrene coating shell and TiO2 filler particles dispersed in the polymeric matrix is investigated. The polymer molecules attached to the surfaces of TiO2 nanoparticles exist in a "brush" regime; rutile nanoparticles self-assemble in chestnut-burr aggregates whose number increases with the filler amount. By increasing the filler concentration, the composites display a high dielectric constant, which is ascribed to the self-assembling of rutile nanoparticles in chestnut-burr aggregates, where a number of rutile crystals share the lateral faces and form capacitive microstructures. The crystals in these aggregates are separated by a polymer thin layer and allow a high percolative threshold, 41% v/v of filler amount, before the formation of a continuous network responsible for the sudden change of the dielectric characteristics. Despite the high content of inorganic filler, the dissipation factor remains low, even approaching the lower frequencies. The material is easily processable because of its polymeric nature and good reproducibility, thanks to the morphology control of the filler particles and their aggregates.