Thermodynamically stable, size selective solubilization of carbon nanotubes in aqueous solutions of amphiphilic block copolymers

Abstract

Two molecular modes of amphiphilic block copolymer-carbon nanotube interactions have been identified in the literature, one involving the adsorption of individual block copolymer molecules on the carbon nanotubes and the other involving the adsorption of multimolecular, spherical micelles. In both cases, the nature of stability imparted to the dispersion of carbon nanotubes in the aqueous medium is kinetic, controlled by the steric barrier imposed by the adsorbed individual block copolymer molecules or the adsorbed micelles. In this study, we propose another mode of molecular interaction, wherein the block copolymer molecules self-assemble around the nanotube, generating aggregates in which the nanotubes are solubilized. In this case, the resulting system is a thermodynamically stable nanocolloidal solution, similar to aqueous surfactant solutions, in contrast to the kinetically stabilized nanotube dispersions. To examine whether such solubilization of nanotubes is possible, we have constructed a simple phenomenological theory for the free energy change associated with solubilization and have performed illustrative numerical simulations based on the theory. The calculated results for the commercially available symmetric PEO-PPO-PEO triblock copolymers and for the PEO-PPO diblock copolymers having the same composition and molecular weight as the triblocks show that indeed the block copolymer molecules are capable of solubilizing the carbon nanotubes. While the block copolymers whose natural curvature is cylindrical are the best candidates to solubilize the nanotubes, other block copolymers whose natural curvature is spherical or lamellar, are also found capable of solubilizing the nanotubes. Most interestingly, the solubilization is found to be size specific suggesting that this can be developed into a practical method to fractionate carbon nanotubes by their diameter. These results are applicable to both single-walled and multiwalled carbon nanotubes and the general conclusions are valid also for other diblock and triblock copolymers.