In situ synthesis of epoxy nanocomposites with hierarchical surface-modified SiO2 clusters

Abstract

Polymer nanocomposites are often produced using in situ approaches where an inorganic filler (as the dispersed phase) is synthesized directly in an organic matrix. Such an approach generally leads to improved dispersion and reduced agglomeration of the filler material. Epoxy-based nanocomposites have demonstrated promising properties for application as high-voltage insulation materials. In this work, a sol–gel based method has been adapted to synthesize surface-functionalized SiO2 in situ in epoxy. The synthesized SiO2 moieties were dispersed in clusters of 10–80 nm, and formed chemical bonds with the epoxy monomers via a silane coupling agent. Raman spectra show the formation of four-membered D1 rings, which may be part of a cage-like structure similar to that of polyhedral oligomeric silsesquioxanes (POSS). SAXS measurements indicate that the SiO2 clusters consist of a hierarchical structure with an increasing fractal dimension with increasing SiO2 content. The nanocomposites displayed improved thermal stability, while the glass transition behavior varied depending on the structure and content of the SiO2 moieties. While the relative permittivity showed no significant changes from that of pure epoxy, the onset of the dielectric relaxation changed with the SiO2 structure and content, similar to the behavior observed for the glass transition.The synthesis of surface-functionalized SiO2 in situ in an epoxy resin (DGEBA) resulted in a good dispersion and limited agglomeration of the SiO2 in the nanocomposite, with little deviation in the dielectric properties (i.e., complex permittivity). The SiO2 is suspected to have a hierarchical structure based on the SAXS measurements, with the primary structural level consisting of POSS-like cages, and an evolution in the structure from inorganic chains to cross-linked clusters with increasing SiO2 content.