Preparation and characterization of epoxy composites filled with functionalized nanosilica particles obtained via sol–gel process

To investigate the interfacial effect on properties of epoxy composites, uniform sized silica particles (S) were synthesized by sol–gel reaction and then modified either by substituting surface silanol groups into epoxide ring (S–epoxide), amine (S–NH2) or isocyanate (S–NCO) groups or by calcinating them to remove surface silanol groups (CS). The modified particles are identified by infrared and raman spectroscopy, differential scanning calorimetry (DSC), and particle size analyzer. It has been found that surface modified particles can be chemically reacted with epoxy matrix, which is confirmed by exothermic peaks in DSC thermograms. In scanning electron micrographs of fractured composites, it is observed that the particle dispersion and interface are considerably affected by functional groups of fillers. Weak interfaces and aggregation of particles are observed for composites filled with CS or S–NCO. However, the aggregation of fillers is highly suppressed in composites filled with S–epoxide and S–NH2 particles. Generally, the coefficients of thermal expansion (CTE) of composites are reduced with an increase of filler contents. Moreover, composites with strong interface exhibit an additional reduction of CTEs. Composites with weak interface show essentially no change in glass transition temperature (Tg) and damping with filler contents, while composites with strong interface show an increase of Tg and a decrease of damping with filler content.