Phase behavior, rheology and processing of carbon nanotubes in acids
Individual carbon nanotubes (CNTs) possess extraordinary mechanical and electrical properties; our goal is to understand CNT behavior well enough to be able to process CNTs into high performance applications that retain these superior properties. Dispersing the nanotubes in a liquid makes it possible to process them into macroscopic fibers and films. Most solvents are incapable of dispersing CNTs, but our group uses superacids such as fuming sulfuric acid and chlorosulfonic acid to disperse them at high concentrations without harming or altering the CNT sidewalls. We have developed a quantitative assessment of the phase behavior of CNT dispersions as a function of acid strength and nanotube concentration; we use microscopy, spectroscopy and rheology to analyze CNT/acid dispersions. Depending on the acid strength/concentration, the dispersions form either an isotropic, biphasic, or liquid crystalline phase.
Our work with nanotube dispersions has enabled us to perform research on the production of carbon nanotube fibers using a method similar to that used in the industrial production of Kevlar fibers. Highly concentrated nanotube dispersions in acid can be extruded as a continuous filament; removal of the acid in a coagulant bath causes the dispersion to solidify into a solid fiber composed entirely of carbon nanotubes. The fact that solutions in this concentration regime are in the liquid crystalline phase promotes close packing and alignment of the nanotubes within the fiber. We are continuing to study the effects of nanotube length and alignment on the mechanical and electrical properties of the fibers; we are also studying various processing methods to improve the fiber morphology.