Mechanism of carbon nanotube dispersion and precipitation during treatment with poly(acrylic acid)

Carbon nanotubes have been shown to be easily dispersed within an acidic aqueous solution of poly(acrylic acid) but precipitate when the pH is increased. Transmission electron microscopy showed that the nanotubes were more exfoliated under the acidic condition but highly aggregated under the basic condition. Carbon K-edge NEXAFS spectroscopy showed that the carbon nanotubes did not chemically react with poly(acrylic acid) during the dispersion or precipitation and that the dispersion mainly involved physical adsorption of poly(acrylic acid) onto the nanotubes. Together with the carbon K-edge NEXAFS spectra, the cobalt L3,2-edge NEXAFS spectra suggested that under the basic condition, the cobalt impurity within the nanotubes strongly reacted with poly(acrylic acid) resulting in complex formation. Cobalt reduces the adsorption of poly(acrylic acid) onto the nanotubes, which then reduced the nanotube dispersion and resulted in the precipitation.