Self-organization of PEO-graft-single-walled carbon nanotubes in solutions and Langmuir-Blodgett films

Poly(ethylene oxide) (PEO), soluble in both water and many organic solvents, is grafted onto singlewalled carbon nanotubes (SWNTs), and aggregation behaviors of the resulting PEO-graft-SWNT in solutions and in Langmuir-Blodgett (LB) films are investigated. SWNTs, cleaved by acid, are dispersed relatively well in DMF and water, but poorly in chloroform and THF. PEO-graft-SWNT was synthesized by treating acid-cut SWNTs with SOCl₂, followed by a reaction with monoamine-terminated PEO in a DMF and water medium. Atomic force microscopy reveals that PEO and SWNT segments take expanded and extended conformations when freshly prepared PEO-graft-SWNTs are cast from water. When PEO-graft-SWNTs are dispersed in chloroform, each SWNT segment collapses into a globular aggregate. Aging the chloroform dispersion produces self-organized structures detectable by light scattering. Langmuir-Blodgett films made from this aged solution afford a surface-micelle structure in which the coagulated collapsed SWNT core is surrounded by extended PEO patches. Addition of DMF to this chloroform solution re-expands the SWNT segments, although not completely. These results demonstrate that the conformation of SWNTs can be controlled by solvent quality as if they are ordinary hydrocarbon-based block copolymers. Yet, the conformational change is not completely reversible, and coagulation, rather than entanglement, becomes the major event even at locally concentrated regions.