Impact of postcuring and water absorption on the dielectric response of epoxy-based composites filled with MgO nanoparticles

Present work addresses the dielectric spectroscopy investigation of epoxy-based composites, filled with magnesium oxide (MgO) nanoparticles with a fillgrade of up to 10 percent by weight. An even distribution of the nanoparticles was realized via ultrasonic vibration and high shear mixing. Dielectric spectroscopy measurement was performed on samples which were cured for 4 hours. Further measurement was performed after postcuring for 48 hours. The investigation of the complex permittivity was done in the frequency domain for temperatures ranging from -20°C to 120°C. A reduction of the relative permittivity for epoxy resin-MgO composite systems was observed, with fillgrades of 0.5, 2, 5 and 10 wt.%, compared to the reference neat epoxy. The explanation for that can be found in the nature and behavior of the interface polymer layer. This modified polymer layer was created on the surface of nanoparticles by means of applying a silane coupling agent and has different properties than the bulk polymer. The interface layer acts as a third phase beside the epoxy bulk and inorganic nanofiller. The results show a reduction of the real and imaginary parts of the complex permittivity, after subjecting the samples to heat treatment for 2 days. The postcuring process leads to further polymerization of the system and also to evaporation of water, which was absorbed by the nanofiller.