Two-way CO2-responsive dispersions of carbon nanotubes in water

Abstract

The “grafting to” method was used to functionalize carbon nanotubes (CNTs) with poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) by applying an atom transfer radical reaction. The obtained CNT-g-PDMAEMA was subjected to consecutive quaternization and alkaline hydrolysis to transform PDMAEMA on the surface of CNTs into poly(methacrylic acid) (PMAA). It was found that both CNT-g-PDMAEMA and CNT-g-PMAA could be easily dispersed in an aqueous media and also exhibited the pH- and CO₂-responsive behavior. While CNT-g-PDMAEMA remained stably dispersed in an acidic environment and readily precipitated at high pH values, CNT-g-PMAA displayed a reversed direction of responsiveness, staying dispersed in the basic environment and aggregating when the solution's pH was lowered. Most importantly, we demonstrated that CNTs grafted with PDMAEMA could undergo CO₂-triggered redispersion, and on the other hand, CNTs grafted with PMAA could be easily switched to the agglomerated state by bubbling CO₂ in the basic medium. Elimination of CO₂ from the solutions by purging with an inert gas reverted the original appearance of CNTs. Thus, a unique two-way gas responsive system of manipulating CNT dispersibility was created through a sequence of simple chemical modifications. Furthermore, because CNTs acquired the ability to respond to the CO₂ trigger, the addition of acids and bases to the solutions to cause coagulation was no longer required, and as a result, the accumulation of salts in the system could be prevented; that altogether establishes a green and environmentally friendly way of manipulating CNT dispersions.