Water‐Redispersible Isolated Single‐Walled Carbon Nanotubes Fabricated by In Situ Polymerization of Micelles

Single-walled carbon nanotubes (SWNTs) have remarkable mechanical, electrical, and thermal properties and have a wide range of potential applications, such as nanoscale electronic devices, energy storage, and reinforcement for materials. In many cases, however, insolubility and bundling of SWNTs in aqueous solution because of their strong hydrophobicity and van der Waals attractions, respectively, are problematic for practical applications. Recently there have been many studies on achieving the homogenous dispersion of isolated SWNTs in aqueous solution by using amphiphilic molecules such as surfactants, polymers, lipids, or DNAs as dispersants. However, because of the dynamic nature of the self-assembled structures of these amphiphilic molecules the SWNT dispersions are sensitive to changes in external conditions, such as temperature and concentration, and can be destroyed during material processing, forming bundled aggregates and making additional processing very difficult. For example, once surfactant-aided dispersions of isolated SWNTs are freeze-dried they can not be redispersed in water, instead forming aggregates of bundled SWNTs. Here, we report a new type of noncovalently (i.e., no covalent bonding between the dispersants and the SWNTs) functionalized isolated SWNT powder that is easily dispersible in water by only ten minutes of mild vortex mixing. This was achieved by i) dispersing SWNTs in water by using a cationic surfactant that has polymerizable counterions, and ii) permanently fixing the surfactant monolayer on the SWNTs by in situ free-radical polymerization of the counterions, followed by freeze-drying. To our knowledge, this is the first demonstration of noncovalently functionalized SWNTs that are made readily redispersible in water even after harsh processing such as freeze-drying. HiPco (high-pressure CO conversion) SWNTs (2 mg mL) were mixed in water with the cationic surfactant cetyltrimethylammounium 4-vinylbenzoate (CTVB, 5 mg mL), which has polymerizable counterions. Super-purified HiPco SWNTs (purity > 98 wt %) were purchased from Carbon Nanotechnologies Inc., and the CTVB molecule was synthesized in-house. The mixture of SWNTs and CTVB was sonicated for one hour to exfoliate bundled SWNTs and produce individually isolated SWNTs with an adsorbed layer of CTVB. The CTVB monolayer on the SWNT surface was “locked in” by in situ free-radical polymerization of the counterions of CTVB at 60 °C (by injecting the free-radical initiator 2,2′-azobis[2-(2-imidazolin-2-yl) propane] dihydrochloride (VA-044), Fig. 1). NMR measurements of the polymerized mixture showed that peaks corresponding to vinyl groups were almost completely absent, indicating nearly 100 % polymerization of the counterions (see Supporting Information). To separate the isolated SWNTs from the bundled ones, the polymerized mixture was ultracentrifuged (ca. 111 000 g) for 4 h, and the upper ca. 70 % of the solution, which showed homogeneous dispersion of SWNTs, was decanted. The decanted solution contained isolated SWNTs coated with polymerized CTVBs (p-SWNT) and polymerized CTVB micelles (p-CTVB) which were not involved in SWNT coating. To reduce the content of p-CTVB, the decanted mixture was filtered through a membrane with pores of 0.22 lm. The UV-vis-NIR (NIR: near-IR) spectra of the filtered mixture showed sharp van Hove transition peaks (Fig. 2b) that are typical for individually isolated SWNTs in solution. The absorption spectra were also used to estimate the concentration of SWNTs (0.08 mg mL) and CTVB (2.6 mg mL) in the solution after filtering. This p-SWNT dispersion was diluted or concentrated for further characterization. To test the stability and redispersibility of the p-SWNTs, the decanted and filtered dispersion of p-SWNTs (SWNT 0.13 mg mL) was freeze-dried, resulting in a black powder of p-SWNT. The p-SWNT powder was easily dissolved in water at the same concentration as before freeze-drying with only ten minutes of mild vortex mixing. It should be noted that the resulting dispersion of p-SWNTs did not form any visible aggregates, whereas the CTVB-aided dispersion of SWNTs without polymerization (u-SWNT) was not cleanly redisC O M M U N IC A IO N