Influence of graphene oxide with different oxidation levels on the properties of epoxy composites

In this study, a series of GO with different oxidation levels were prepared via a modified Hummers' method, in which the oxidant/graphite ratio and oxidation time were varied. The change of GO structure as a function of oxidation level was investigated by X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, and X-ray diffraction measurements. The prepared GO were used for reinforcing epoxy resins, and the surfaces of GO were functionalized by the amine hardener during the dispersion process. Microscopic observations show that the oxidation level has a significant impact on the dispersion state of GO in the epoxy matrix. Both GO with the low and high oxidation levels tend to have low degree of dispersion, even aggregate. The influences of oxidation level on the performances of epoxy/GO composites, including curing behaviors, rheological, thermal and mechanical properties, were investigated in detail. Notably, the GO with optimum level of oxidation (denominated as GO-4) shows superior toughening effects on the epoxy matrix, compared to the reported epoxy/GO composites. The incorporation of merely 0.2 wt% GO-4 would yield 56 and 128% enhancement in the critical stress intensity factor (KIC) and the critical strain energy release rate (GIC), respectively.