Compatibilizing effect of graphite oxide in graphene/PMMA nanocomposites

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The structure of single graphene sheets, which are the basal plane of graphite, can be best visualized by making a longitudinal scission on a single wall carbon nanotube (SWCNT) along the tube axis and flattening the resulting sheet. These sheets have electric conductivity, thermal conductivity, and tensile modulus values similar to SWCNTs, and can be used as alternatives to SWCNTs in various applications. Graphite shows a sharp X-ray diffraction peak at 2θ=26.5 because it has a layered structure composed of graphene sheets and the typical interlayer spacing is 3.35 A. These sheets are one-atom-thick and are composed of hexagonal carbon rings. Graphite oxide (GO), prepared by oxidizing graphite, is a graphite-derived compound with a layered structure. GO has a broad X-ray diffraction peak at lower angles than graphite, normally 2θ=10~15, because polar groups such as hydroxyl, epoxide, ether, and carboxylate groups are present on graphene sheets as a result of oxidation and expand the interlayer spacing. Recently, it has been reported that exfoliation of graphite into a single graphene sheet can be achieved from sufficiently oxidized GO, if inter-graphene spacing associated with native graphite is completely eliminated in the oxidation stage, and if adequate pressure builds up at the gallery between the GO sheets by rapid heating. The pressure results from CO2 evolved by the thermal decomposition of functional groups. This exfoliated graphite, where the inter-graphene spaces associated with GO and graphite are completely excluded after thermal expansion, has an affinity for polar solvents and polymers, as well as good conductivity, because this exfoliated graphite is composed of functionalized graphene sheets (FGSs) having polar functional groups remained even after thermal treatment. Normally, carbon nanotubes need surface treatment if they are to be used as a nanofiller for fine dispersion in a polymer matrix. However, FGS can offer comparable or better electrical conductivity enhancement than carbon nanotubes without further surface treatment, owing to the polar functional groups. Our laboratory has prepared and examined FGS nanocomposites with various polymers. During these studies, we discovered that the fine dispersion of FGS in a polymer matrix can be improved when GO is used as a compatibilizer. This paper reports the compatibilizing effect of GO in FGS/poly(methyl methacrylate) (PMMA) nanocomposites.