Besides fillers, the polymer matrix also plays a decisive role in the dielectric composites. The glass transition temperature and thermal decomposition temperature of the matrix usually determine the service temperature range of the composites.39−41 Conventional polymers generally have a low glass transition temperature and low dielectric constant, which limits the application area of the dielectric composites. Therefore, thermostable engineering plastics with high dipole units (such as urea, sulfone, and thiourea6,42) are ideal polymer matrix for high-k nanocomposites.