A green method to prepare nanosilica modified graphene oxide to inhibit nanoparticles re-aggregation during melt processing

A graphene oxide-silica nanohybrid (GOS), self-assembled into a lasagna-like structure, was prepared in water and used as a filler for the melt preparation of polyamide 6 (PA6)-based nanocomposites. For sake of comparison, PA6-based materials were prepared under the same processing conditions by adding GO only or a physical mixture of GO and silica (GO+S). All the materials were characterized from a morphological, spectroscopic, thermal, dynamic-mechanical (DMA) and mechanical point of view. For all the nanocomposites, the interphase was studied either by analyzing loss factor plots coming from DMA measurements and by implementing a novel approach, i.e. combining solvent extraction, SEM and EDX measurements. While GO and GO+S showed an almost negligible effect on the macroscopic features of PA6, due to lack of dispersion, PA6-GOS ternary nanocomposites displayed an outstanding enhancement of mechanical and thermo-mechanical performance. This feature is likely due to the formation of an extended and strong interphase. In fact, the silica layers intercalated between GO lamellae played as an exfoliating agent, thus proving to be particularly efficient in avoiding self-aggregation of GO sheets, while those covalently attached to basal planes of GO acted as a rivet, capable to interlock the surrounding polymer chains. Finally, with respect to other techniques commonly adopted for the fabrication of polymer-graphene nanocomposites, the green route herein proposed does not involve any toxic solvent nor time-consuming protocols, and allows achieving remarkable improvements in stiffening (up to +180%), strengthening (up to +210%) and toughening (up to +210%) at extremely low filler contents (0.25% or 0.5%).