Mild acid-based surfactant-free solutions of single-walled carbon nanotubes: Highly viscous, less toxic, and humidity-insensitive solutions

Abstract

Owing to their exceptional physical properties, single-walled carbon nanotubes (SWNTs) are widely applicable if translated into macroscopically assembled SWNT materials. The assembly method involves SWNT solutions prepared using surfactants or superacids. As both variations of the method are difficult to industrialize, a novel SWNT solution is prepared herein using mild polyphosphoric acid (PPA) without surfactant. Although the dissolution of SWNTs in PPA is thermodynamically unfavorable because of lower acidity compared to superacids, high viscosity kinetically contributes to phase stabilization when SWNTs are de-bundled upon high shearing. Interestingly, the SWNT/PPA solution is humidity-insensitive, which is beneficial for handling. In the 0.01–2.0 wt% range, the critical concentration and saturation point, at which phase transitions of isotropic-liquid crystalline (LC) and LC-gel states occur, are observed at 0.1 and 1.0 wt%, respectively. According to the steady shear rheology, the optimal concentrations for fiber spinning are 0.3–0.7 wt%. Polarized Raman and WAXD analyses show that the wet-spun SWNT fibers from the 0.5 wt% solution exhibit the highest fiber orientation, correlating with the lowest extrusion shear stress of the solution. The KevlarTM p-aramid dissolved in PPA and blended with SWNTs shows a negligible effect on the solution rheology but higher fiber modulus at high fractions.