Scalable Solution Processing of Pristine Carbon Nanotubes for Self-Assembled, Tunable Materials with Direct Application to Space Technologies

Some of the most readily adoptable applications for CNT materials are as flexible and electrically conductive films. Solution processing CNTs into films using superacid solvents such as chlorosulfonic acid (CSA) has inherent processing challenges due to the reactivity of the solvent with ambient moisture. This issue was overcome by developing a low cost method for preparing CNT aligned films from superacid solutions with variable thickness. This shearing method can be performed quickly in a fume hood while removing the need for expensive filters or large volumes of solution. The films have anisotropic electrical, mechanical, and optical properties. Such a simple method for preparing films will help evaluate CNT material electrical properties and has already found applications in transparent electrodes, sensors, and nanocomposites. The long-range homogeneous alignment of the shear aligned films also enables a novel technique for preparing high performance fibers that are ~7 cm long. Substantial CNT alignment allows each film to be peeled and twisted into multiple fibers with well-defined diameters; the new method lowers the CNT mass requirement for fiber production by three orders of magnitude (down to ~0.2 mg), allowing rapid experimentation and fast feedback to synthesis methods. This method was utilized to investigate the structure-property relationships that govern CNT fiber properties and indicate that the internal packing density and CNT aspect ratio currently dominate fiber tensile strength. By maximizing the internal packing density, fiber tensile strength was increased by 60% relative to what has been achieved with full scale solution spinning. Coupled with high flexibility and excellent electrical conductivity, these CNT fibers are unmatched in multifunctionality, making them well-suited for applications in wearable electronic devices. Because the mechanical and electrical properties are expected to continue improving as higher aspect ratio CNTs become available, these CNT materials may prove invaluable in other engineering applications such as lightweight wiring, structural composites, or robotics. A new characterization problem arises when evaluating specific mechanical and electrical properties on the small scale CNT fibers, as linear density measurements require a relatively large quantity of material for massing. For this reason, in collaboration with NASA LaRC, a new instrument was designed for measuring CNT fiber linear density on short fibrils. Fiber linear density can be evaluated by measuring the resonant frequency of a known length of CNT fiber under tension in the presence of a magnetic field by passing an AC through the fiber and measuring the AC resistance and phase shift induced from vibration. Alternatively, the instrument can record the sound produced by the vibrating fiber to continuously monitor vibrational resonance during a tensile test. The tension-dependent frequencies are then used to accurately calculate the linear density of the fiber. This method is shown to be accurate on linear densities covering a broad range (0.77 dtex to 12 dtex) and can be performed on significantly less material than what is required for the microbalance method with a simpler and more accurate resonance identification strategy than what is employed by FAVIMAT. Finally, this grant has further developed the ability to solution process CNTs in non-corrosive acids such as methanesulfonic, phosphoric, and p-toluenesulfonic acid. The material-friendly nature of these acid solvents enables adoption of common printing and coating techniques. CNT thin films were prepared by Mayer rod coating onto flexible substrates with electrical conductivities equivalent to what is achieved with CSA (and meet the standard for LCD and touch screen applications). Full CNT individualization and liquid crystal formation can be obtained at concentrations as high as 10 mg/mL. The unique capabilities of p-toluenesulfonic acid were further demonstrated by using it as a drop-in ink for benchtop silk-screen printing and 3D-printing using commercially available equipment. This is the first realization of neat CNT 3D objects by additive manufacturing. These solutions provide a SWCNT platform with tailorable phase behavior, chemical compatibility, and rheological properties which will enable the fabrication of advanced polymer composites and novel device development for wearable electronics.