Preparation of CO₂-switchable graphene dispersions and their polystyrene nanocomposite latexes by direct exfoliation of graphite using hyperbranched polyethylene surfactants

Herein the preparation of CO₂-switchable graphene dispersions by noncovalent exfoliation of graphite in water using a CO₂-switchable star copolymer surfactant is described. The copolymer is composed of a hyperbranched polyethylene (HBPE) core and multiple poly[(2-(dimethylamino)ethyl methacrylate)-co-(2-(diethylamino)ethyl methacrylate)] (P(DMAEMA-co-DEAEMA)) arms synthesized through sequential Pd-catalyzed chain walking copolymerization of ethylene and 2-(2-bromoisobutyryloxy)ethyl acrylate followed by atom transfer radical copolymerization (ATRcoP) of DMAEMA and DEAEMA. The effects of the HBPE core and P(DMAEMA-co-DEAEMA) arms on CO₂-switchable efficiency were investigated. Transmission electron microscopy (TEM), atomic force microscopy (AFM), and X-ray diffraction (XRD) measurements demonstrated the formation of graphene with 90% of them having lateral dimensions of 0.5–2 μm and a thickness of 2.2–5.2 nm (2–5 layers of graphene) using HBPE-P(DMAEMA-co-DEAEMA)s as the dispersant. The average conversion of graphite to graphene was approximately 5%. The graphene dispersions could be readily aggregated by N₂ bubbling and redispersed through CO₂ introduction. These CO₂/N₂ reversible aggregation and redispersion processes were repeatable for multiple cycles. The graphene dispersions could be directly used to produce stable styrene emulsions. CO₂-switchable graphene/polystyrene nanocomposite latexes were synthesized via emulsion polymerization producing latexes that are destabilized with N₂ bubbling.