Surface functionalization of graphene oxide (GO) sheets using polymers has emerged as a subject of enormous scientific interest due to the wide applications of GO in polymer composites and functional graphene-based materials. In this study, we grafted GO sheets with polystyrene (PS) and poly(styreneisoprene) (PSI) using GO itself as a cationic initiator for homopolymerization of styrene and copolymerization of styrene and isoprene. The resultant GO-g-PS and GO-g-PSI composites displayed enhanced dispersibility in common organic solvents. With increasing the loading of isoprene in the copolymerization reaction, the glass transition temperature of the obtained products gradually decreased, combining the increased roughness of the GO-g-PSI sheets compared with the GO-g-PS sheets, which indicated the increased phase separation between the PS and PI segments in the PSI layer. Therefore, the packing of the GO-g-PS sheets, as well as the GO-g-PSI sheets, was not as compact as that of the GO sheets, leaving gradually increased quantity of pores in the films prepared with GO-g-PS and GO-g-PSI. Capitalizing on these tunable characters, hybridized membranes prepared by depositing GO sheets, GO-g-PS sheets, and the GO-g-PSI sheets obtained with gradually increased loading of isoprene in the copolymerization on the surfaces of commercially available polytetrafluoroethylene membranes displayed gradually increased gas permeability.