We report the preparation of pH-responsive graphene nanosheets functionalized with polymer brushes. The following steps were employed for the preparation of graphene-polymer nanocomposites: (1) Graphite oxide (GO) was prepared by the modified Brodie method from commercially available graphite powder. (2) Thermal reduction of GO yielded functionalized graphene sheets (FGSs) in bulk quantities. (3) ATRP initiators were then introduced onto the surface of FGSs (FGS-Br). (4) Poly(t-butyl acrylate) (PtBA) brushes were grown directly from FGS-Br through surface-initiated atom transfer radical polymerization (SI-ATRP). (5) Water-dispersable FGS-poly(acrylic acid) (FGS-PAA) hybrids were prepared by subsequent hydrolysis of FGS-PtBA tert-butyl groups. This highly controllable polymerization method affords hybrid FGS-PtBA with tailorable lengths of polymer brushes possessing average molecular weights (M-n) ranging from 1.0 x 10(4) to 2.4 x 10(4) and narrow molecular weight distributions (M-W/M-n = 1.11-1.33). The resulting FGS-PtBA and FGS-PAA were carefully characterized using a number of techniques, including thermogravimetric analysis, FT-IR, gel permeation chromatography, Raman spectroscopy, and transmission electron microscopy, all of which supported the successful growth of PtBA brushes onto the surface of FGS and the successful transformation to PAA brushes by hydrolysis. We demonstrated that the dispersion and aggregation states of FGS-PAA hybrid materials in water were controlled by pH. This tailored control of dispersion state that can be altered simply by a change in the solution pH is anticipated to enhance the properties of hybrid graphene-based materials and to encourage their potential use for advanced electronic, energy, and sensor applications. (C) 2012 Elsevier Ltd. All rights reserved.