Dielectric and relaxation properties of composites of epoxy resin and hyperbranched-polyester-treated nanosilica

Hyperbranched polyester is effective for enhancing molecular bond strength and improving the mechanical behavior of nanofilled polymers. This study examines the dielectric and polarization relaxation characteristics of epoxy resin composites filled with nanosilica 30 nm in diameter, which is treated by terminal carboxyl hyperbranched polyester. TEM and SEM analysis indicate that the nanosilica surface is grafted with a functional polymer layer ranging in thickness from several to tens of nanometers, and the nanosilica agglomeration in epoxy resin is remarkably inhibited. Measurements of thermally stimulated depolarization current and differential scanning calorimetry show that, deep traps with an energy of 1.09 eV are present in the nanocomposites, and the glass transition temperature (T_g) is increased by 11 °C at most at filler concentrations from 1 to 7 wt%. Moreover, the room-temperature relative permittivity and dielectric loss factor of the composites at 50 Hz are decreased by 0.22 and 1.3‰, respectively. Conductivity at 10 mHz to 1 kHz and dc conductivity are also significantly decreased when the operating temperature is below T_g. The polarization relaxation process of the nanocomposite is dominated by regional carrier migration, interfacial and dipole polarization. The relaxation frequency of dipole polarization at high temperature (>T_g) is transformed to satisfy the Vogel-Tammann-Fulcher law. This research suggests that both the dielectric and the polarization relaxation properties of the epoxy resin composites can be modified by filling hyperbranched-polyester-treated nanosilica, because it enhances the bond strength of the inorganic-organic interface and enlarges the molecular scale of the composites via cross-linking reactions.