Atomic force acoustic microscopy analysis of epoxy-silica nanocomposites

A DGEBA-based epoxy matrix was loaded with 10, 20 and 30 phr of fumed silica particles. Single edge notched bend (SENB) specimens were prepared and deformed to failure in three-point bending configuration. Their fracture surfaces were examined by atomic force acoustic microscopy (AFaM) in order to obtain information about the local elastic modulus of the surface at high spatial resolution. The collected information was correlated to the bulk thermo-mechanical properties of the composites. In particular, the decrease in thermo-mechanical properties like tensile modulus, yield strength, stress at break and glass transition temperature, observed for samples filled with 10 and 20 phr with respect to the unfilled matrix was found to correspond to highly heterogeneous fracture surfaces presenting a broad distribution of elastic modulus values. The AFaM data were interpreted as representative of different degrees of filler exposure on fracture surfaces and also of localized cavitation effects involved in crack propagation, both phenomena accounting for the effective plasticizing effect macroscopically observed at silica amounts of 10 and 20 phr. A substantial reduction in the exposure probability of silica particles on fracture surfaces was found for the sample filled with 30 phr of silica, which also displayed an improvement of the mechanical and thermal properties. This latter evidence was tentatively explained by supposing a physical immobilization of polymer chains at the polymer-matrix interface.