We
report in this paper the successful production of stable high-concentration
graphene dispersions in low-boiling-point, low-polarity conventional
organic solvents (chloroform and THF) by liquid-phase noncovalent
exfoliation of graphite assisted with a hyperbranched polyethylene
(HBPE) as the stabilizer. In the exfoliation process, HBPE adsorbs
onto the surface of exfoliated graphene flakes, providing steric stabilization
against their restacking. A systematic investigation on the effects
of exfoliation conditions, including the solvent and the amounts of
graphite and HBPE, has been conducted. Graphene dispersions with the
concentration up to 0.18 mg/mL in chloroform and 0.045 mg/mL in THF
have been obtained. It is also demonstrated that the dispersions can
be further concentrated by solvent evaporation to give highly concentrated
stable dispersions at 3.4 mg/mL. Through their characterizations with
transmission electron microscopy, atomic force microscopy, and Raman
spectroscopy, the majority of the graphene products is found to be
high-quality, defect-free, few-layer graphene flakes with the layer
number between 2 and 4 and the lateral dimension in the range of 0.2–0.5
μm. The dispersions can be fabricated into flexible conductive
free-standing graphene films and be used to prepare graphene/ethylene
copolymer composites through solution blending, which show significant
enhancements in both thermal and mechanical properties.
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