Electrical and thermomechanical properties of epoxy-POSS nanocomposites

Two types of the epoxy-POSS nanocomposites were prepared and their electrical/dielectric and thermomechanical properties were determined. The mono- and octa-epoxyfunctionalized POSS (POSS,E1 and POSS,E8) were covalently incorporated in the epoxy network matrix DGEBA-3,3′-dimethyl-4,4′-diaminocyclohexylmethane (Laromin C260) as pendant units or as polyhedral junctions, respectively. While the POSS junctions are well dispersed in the hybrid network DGEBA-Laromin-POSS,E8, the pendant POSS aggregate to form large crystalline POSS domains.The properties of the nanocomposites correlate with the morphology. The nanocomposites with inhomogeneously dispersed large aggregates of pendant POSS,E1 show poor properties, including low T_g and thermal stability, and the high dielectric loss factor at higher temperatures. On the contrary, the well homogeneous nanocomposites containing POSS,E8 dispersed units display improved electrical properties in the range of 1–10 wt.% POSS. The hybrid exhibits the high resistivity (1 × 10¹⁵ Ω m) and polarization index p_i1 characterizing electrical relaxation, as well as the low dielectric loss factor tan δ, mainly at temperatures above 50 °C. Moreover, the material shows improved thermo oxidative stability and enhanced thermomechanical behavior. Consequently, this nanocomposite was proved to be a prospective insulation material particularly for a high temperature electrotechnical application. In contrast to electrical properties being the best at a low POSS content, the thermomechanical properties, such as rubbery modulus and T_g, are gradually improving with increasing POSS content due to growing crosslinking density.