Functionalization of few-layer graphene sheets and carbon nanotubes for generation of hybrids and their effect on the piezoresistive properties of polymeric nanocomposites

Functionalization of few-layer graphene sheets (GSs) and multilayer carbon nanotubes (MWCNTs) for the generation of hybrid nanostructures is investigated. MWCNTs were oxidized by a mixture of nitric and sulfuric acids, while GSs were functionalized with varying concentrations of branched polyethyleneimine (PEI), investigating the role of the PEI concentration. PEI functionalization of the GSs is assessed by infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and zeta potential measurements. Successful functionalization of the GSs is conspicuously proved through new XPS bands associated with PEI, such as CONH, as well as a sign change in the zeta potential (from −12.7 mV to +11.8 mV). The concentration of 0.266 μl of PEI per ml of distilled water (0.46 mmol/L) proved to be the most effective for functionalizing the GSs, and such a concentration was further used for generating GS-PEI/oxidized MWCNT hybrids as fillers for nanocomposites. The application of such hybrid carbon nanostructures to increase the piezoresistive sensitivity of smart polymeric nanocomposites under bending loading is reported. An increase in the piezoresistive sensitivity (gauge factor) at the tensile side of the flexural coupon is observed for nanocomposites containing hybrid functionalized nanostructures.