Non-covalently modified graphene sheets by imidazolium ionic liquids for multifunctional polymer nanocomposites

Chemical reduction of graphite oxide (GO) to produce graphene nanosheets often results in irreversible agglomeration and precipitation. Herein, stable well-dispersed graphene sheets in solvents were obtained by simultaneous functionalization and reduction of GO under alkaline conditions, in the presence of sodium borohydride and imidazolium ionic liquids (Imi-ILs) containing two vinyl-benzyl groups. In this case, positively charged imidazolium groups of Imi-ILs underwent ion-exchange with negatively charged GO sheets and were linked to their edges, while Imi-ILs were non-covalently attached onto the large surfaces of graphene through π-π and/or cation-π stacking interactions. The vinyl-benzyl reactive sites were then copolymerized in situ with methyl methacrylate to fabricate graphene/poly(methyl methacrylate) (PMMA) composites. Functionalized graphene sheets were uniformly dispersed in the PMMA matrix and contributed to large increases in storage modulus (+58.3%) and glass transition temperature (+19.2 °C) at 2.08 vol.% loading. High electrical conductivity was also achieved at graphene loading levels beyond 1 vol.% (ca. 2.55 Sm ^-1) with a low percolation threshold (0.25 vol.%) for the composites. Hence, a general methodology which facilitates the development of a multifunctional advanced material has been successfully established. This can be extended to other vinyl polymer-based composites containing graphene.