Polyamide 6/Graphene composites: The effect of in situ polymerisation on the structure and properties of graphene oxide and reduced graphene oxide

Nanocomposites were prepared via in situ, ring opening polymerisation of e-caprolactam in the presence of single layer graphene oxide (GO) and chemically reduced graphene oxide (rGO). The structure and properties of the graphene before and after polymerisation were investigated, alongside the properties of the composites, using atomic force microscopy (AFM), Fourier transform infra-red spectroscopy (FTIR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Reduction of the Graphene Oxide (GO) during polymerisation was observed along with functionalisation of graphene flakes with polyamide 6 (PA6) chains. FTIR and XRD show the retention of some oxygen functionalities after reduction, specifically doubly bound oxygen species. The functionalised graphene sheets were analysed after centrifugal removal from composites using formic acid as a solvent. AFM imaging indicated a sheet height of 0.75 nm for single layer graphene oxide with an increase to 4 nm for graphene sheets after polymerisation. This suggests that the graphene acts as a base for polymerisation with polymer chains propagating from its surface, promoting interfacial interaction. More so in PA6/GO composites as there is a greater amount of PA6 bound to GO when compared to rGO. Raman data depicts a slight restoration of sp2 hybrid for functionalised-GO (f-GO) which does not occur in functionalised-rGO (f-rGO). XRD suggests no restacking of GO sheets occurred during the polymerisation process whilst graphitisation of rGO occurred. Thermal analysis measurements revealed that nanocomposites are thermally stable and graphene inclusion influenced both crystallinity and the molecular weight of the polymer.