Water-dispersible graphene designed as a Pickering stabilizer for the suspension polymerization of poly(methyl methacrylate)/graphene core–shell microsphere exhibiting ultra-low percolation threshold of electrical conductivity

The microsphere of poly(methyl methacrylate) (PMMA)/graphene composite, having a core–shell structure, is effectively prepared by Pickering suspension polymerization using a water-dispersible sulfonated graphene alone as a Pickering stabilizer. This is the first case where a graphene is designed as a Pickering stabilizer for suspension polymerization. The sulfonated graphene is prepared using a novel covalent modification method that utilizes the epoxide groups on graphene, which are the inherent defects of thermally reduced graphene, to react with potassium 2-aminoethanesulfonate, without any additional damage to the intrinsic conjugated sp2 carbon network of the graphene. The size of the microsphere is tunable by the amount of graphene from several tens to several hundreds of micrometers. The thermal stability of PMMA at the core is substantially improved by the thermally stable graphene shell enclosing it. The compression molded composite of the microspheres displays an excellent electrical conductivity of 15.7 S m−1 at a graphene content of 5.33 phr (2.78 vol.%) and an ultra-low percolation threshold of 0.04 phr (0.02 vol.%), which is the lowest value ever reported for polymer/graphene composites, because the graphene shells form a well-defined, three-dimensional conductive graphene network throughout the composite.