Synthesis, Dispersion, and Viscosity of Poly(ethylene glycol)-Functionalized Water-Soluble Single-Walled Carbon Nanotubes

Abstract

The carbon nanotube−polyethylene glycol (PEG) graft copolymer was synthesized by covalent functionalization of electric arc single-walled carbon nanotubes (SWNTs) with the monofunctional, tetrahydrofurfuryl-terminated polyethylene glycol PEG-THFF (MW∼200), to give a material composed of 80 wt % SWNTs. We show that the sequential processing of the resulting material by ultrasonication and high-shear mixing provides a means to disperse the SWNT-PEG-THFF macromolecules on two different length scales and leads to highly viscous solutions; at a concentration of 10 mg/mL the kinematic viscosity (ν) of an aqueous SWNT-PEG-THFF dispersion reaches a value of ν > 1000 cSt (for water ν ∼ 1 cSt). Analysis of this procedure by means of viscosity measurements and atomic force microscopy (AFM), shows that ultrasonication is effective in disrupting the SWNT bundles, while the high shear mixing disperses the individual SWNTs. The kinematic viscosity of aqueous dispersions of SWNT-PEG-THFF was measured as a function of nanotube concentration and compared to that of SWNT-PEG dispersions. The viscosity and AFM measurements show that the SWNT-PEG-THFF and SWNT-PEG graft copolymers form aqueous dispersions with distinct viscous characteristics; the use of monofunctional PEG-THFF for covalent functionalization of the SWNTs prevents cross-linking of the SWNTs in the final product, and this allows the production of more completely dispersed SWNTs than in the case of the SWNT-PEG graft copolymer, which is synthesized from a bifunctional glycol.