Large enhancement of dielectric properties in polyetherimide-based nanocomposites

In two composition ranges of nanocomposites, anomalous enhancement of the dielectric property of polyetherimide (PEI, dielectric constant similar to 3.2) by adding Al2O3 nanoparticles (dielectric constant similar to 9-10), which have dielectric properties comparable to PEI, was found. At a low filling ratio (< 0.8 vol%), the dielectric constant of the nanocomposites exhibits a maximum value (similar to 3.8) in composites with 0.3 vol% nanoparticles. When the volume fraction of nanofillers exceeds 0.8 vol%, the dielectric properties can be continuously enhanced by adding Al2O3; in composites containing 20 vol% Al2O3, the dielectric constant can reach 8.5, which is more than twice that of the pristine polymer. The dielectric loss of the composites studied remains low (< 0.02). When Al2O3 nanoparticles are replaced with MgO nanoparticles, the behavior persists, indicating that it is a general phenomenon in PEI nanocomposites. The experimental findings suggest that the improved dielectric properties observed in nanocomposites with high nanoparticle loading can be attributed to increased free volume. The unexpected expansion of the nanocomposite volume shows that adding nanoparticles may introduce more free volume into the polymer, boosting the activity of polar functional groups in the polymer and resulting in a more robust dielectric response.