Direct covalent modification of thermally exfoliated graphene forming functionalized graphene stably dispersible in water and poly(vinyl alcohol)

The syntheses of water-dispersible graphene via graphene oxide colloid dispersion and/or using functionalizations that disrupt the pi-bond system of graphene or contaminate a graphene surface with big amounts of undesired impurities face some challenges in practical applications. Approaches based on thermally exfoliated graphene might be promising for many applications in which flat and perfect single-layer graphene is not mandatory and productivity is more than important. In this paper, for the first time, we report a simple and effective method to prepare water-dispersible graphene directly from thermally exfoliated graphene by covalent modification utilizing the inherent defects of graphene as active sites. That is, the epoxide groups on graphene were reacted with ethanolamine and then with n-butyl bromide to prepare the graphene decorated with cationic ammonium ions (alkylated graphene, AAG). Elemental analysis, thermogravimetry, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy demonstrated that the reactions have proceeded as designed. The Raman spectra showed that the pi-electronic system of sp (2)-bonded carbons of the graphene was not damaged by the modification. The homogeneous colloidal dispersion of AAG in water remained stable for at least 6 months, showing that the wrinkled nature of the graphene as well as the electrostatic repulsion and steric hindrance between the graphene sheets caused by the bulky ammonium moieties on the graphene's surface efficiently prevented the graphene from restacking and aggregating. The AAG dispersed stably in a poly(vinyl alcohol) matrix produced an extraordinarily high modulus increase of 236 % with just 1 phr (about 0.5 vol%) of AAG.