Epoxy-alumina functionally graded nanocomposites: gradation and morphological effect of alumina on impact strength and viscoelastic properties

This article presents an experimental investigation of low-velocity impact and dynamic mechanical testing of epoxy-aluminium oxide (alumina), functionally graded nanocomposite for the above two different directions of loading. Two different morphologies (rod and spherical) of alumina nanoparticles were diffused in epoxy resin by ultrasonication technique. Functionally graded polymer nanocomposites (FGPNCs) were prepared by varying the weight percentage (% by weight) of nanoparticles in the thickness direction. Sequential casting was adopted for synthesizing the nanocomposite layers having 0%, 0.25%, 0.5%, 0.75% and 1% (by weights) of nanoparticles in a vertical acrylic mould. Transmission electron micrographs showed a uniform dispersion of alumina nanoparticles within the FGPNCs. FGPNC containing nanorods and spherical nanoparticles exhibited improvement of 11% and 8%, respectively, compared to neat epoxy when impacted from the direction of the nanocomposite layer. Whereas, when the impact was from the direction of the neat epoxy layer, the impact strength of FGPNC having nanorods improved by 7% while only a slight increment in the impact strength of FGPNC having spherical nanoparticles was observed in comparison to neat epoxy. Field emission scanning electron micrographs (FESEM) of the fractured surfaces revealed the responsible toughening mechanisms of FGPNCs for different impact loadings. Gradation and addition of alumina nanoparticles in epoxy had a stronger effect on the storage modulus in the rubbery region compared to the glassy region. In the rubbery region, the storage modulus of FGPNC (nanorods) and FGPNC (spherical) was recorded three times and two times higher than that in the glassy region, respectively, when the samples were loaded from the direction of the nanocomposite layer.