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.

Critical parameters in exfoliating graphite into graphene

“…Among the Pluronics dispersants we find that Pluronics with 40 PO monomers is optimal for GS dispersion with negligible dependence of the EO block's size, in line with previous experimental 24 and numerical 25 findings for CNT dispersions. In contrast, Hersam and coworkers found only a weak dependence on the PO chain length and much stronger effect of the hydrophilic chain with an optimum around 75 EO monomers (6.5 kDa).…”

The science of dispersing carbon nanotubes with surfactants

“…We always try to establish links to experimental observations and strive to intensify the symbiosis between experiment, simulation and theory. 17 2 Why is dispersion difficult?…”

Functionalization of Single-walled Carbon Nanotubes with Thermo-reversible Block Copolymers and Characterization by Small-angle Neutron Scattering

“…It has been predicted that the stabilization of carbon nanotubes using amphiphilic block copolymers shows size selectivity so that carbon nanotubes with a specific diameter can be selectively solubilized. 25 In this work, SANS analysis reveals that the SWNT core in the synthesized nanorods has a diameter of about 7.2 Å on average. This implies that poloxamer 407 could even solubilize SWNTs with a relatively small diameter around 7-8 Å among HiPCo (high-pressure carbon monoxide) SWNTs, which are known to have a diameter of 7-14 Å.…”