Small-Angle Neutron Scattering from Aqueous Dispersions of Single-Walled Carbon Nanotubes with Pluronic F127 and Poly(vinylpyrrolidone)

Amphiphilic block copolymers are excellent dispersants for single-walled carbon nanotubes (SWCNT) in aqueous environments, where their noncovalent attachments do not affect the pi chemical bonding. In this small-angle neutron scattering (SANS) study, we investigate whether the coverage of Pluronic F127 polymers around the CNTs depends on the solution concentration in the range of 1-6% (w/w). The observations indicate that at these concentrations the SWCNT surface is fully saturated at about 14 chains per unit length of 100 angstrom. Furthermore, we seek to verify whether the unusual effect observed in a previous study by contrast variation, interpreted as being due to a dense hydration layer around the polymer chains, also appears using a homopolymer (polyvinylpyrrolidone - PVP) that does not contain poly(ethylene oxide) (PEO) units. The SANS patterns showed again a minimal intensity value at much higher solvent composition (75% D2O) than the expected value of 29% D2O. The minimum scattering curve exhibited a nearly q(-1) power law at small angles, an indication of rodlike entities. A model of a CNT thin bundle with loosely adsorbed polymer chains around it (core-chains) was reasonably well fitted to the data. The polymer chains are assumed to be surrounded by a water layer with a slightly higher density than bulk water, having partial selectivity for D2O.