Free‐volume effects on the thermomechanical performance of epoxy–SiO₂ nanocomposites

In this study, free‐volume effects on the thermal and mechanical properties of epoxy–SiO 2 nanocomposites were investigated. SiO 2 particles ranging from 15 nm to 2 µm were used, and the nature of the matrix–filler interphase was modified by surface grafting. Nanoparticles 15 nm in diameter yielded an increase in the glass‐transition temperature ( T g ) of the composites up to 5 °C; at the same time, they increased the storage modulus ( E ′) from 2340 to 2725 MPa. Conversely, large particles markedly decreased both T g and E ′; this suggested the pivotal role of nanoparticle size on the final properties of the nanocomposite. The functionalization of SiO 2 nanoparticles markedly improved their dispersion within the epoxy matrix. The positron annihilation lifetime spectroscopy results indicate that the free volume strongly depended on the interphase. These experimental findings obtained here could be extrapolated to industrially relevant nanocomposites and could provide a rationale for the comprehension of free‐volume effects on the thermal and mechanical properties of nanocomposite materials.