Characterization, Processing, and Synthesis of Extreme-Performance Continuous Carbon Nanotube Network Composites

Abstract

Continuous carbon nanotube (CNT) networks are an emerging, hierarchically-structured, and commercially available nanomaterial built from countless CNT nanocrystals. These macroscopic yarn materials promise to bridge the gap between microscopic CNT fibers – which are well-known for their superlative material properties – and human-scale fiber reinforcements for extreme-performance composites. Yet because the constituent CNTs interact only via intermolecular forces, network properties fall short of their building blocks. Although these materials show promise as reinforcement in composites, the networks’ low-permeability and tortuous nanoporous structure renders imbibition with liquids like a polymer matrix or surface functionalizing agents challenging. Thus, traditional composite fabrication strategies can be ineffective when applied to CNT yarns, especially commercial products subject to proprietary microstructural manipulation.

Using commercially-available CNT yarns fabricated through floating-catalyst chemical vapor deposition (FCCVD) as model systems, we first explore yarn characteristics which are unique to their hierarchical, bundled-fiber structure, placing focus on the oxygen-rich amorphous carbon phase found in pre-densified, chemically-stretched yarns. A green hydrothermal technique is explored to remove this phase from the surface level inward, allowing for purification and improved infiltrability. However, we find this phase is distinct from previously-reported amorphous carbons found in CNTs, showing it behaves as a matrix which may improve polymer bonding. An analysis of imbibition and fluid transport in these CNT yarns finds that while infiltration of low-viscosity liquids like water is thermodynamically-favored, it is limited when surpassing the threshold of capillary pore percolation. Nevertheless, infiltration in lower-density networks is not only observed, but exploited through the demonstration of dielectric heating in a microwave reactor, where we show fluid imbibed within the network can be boiled to induce swelling and exfoliation of CNT bundles (or conversely, this may be avoided) through optimization of the heating parameters and solvent.

Next, with a firm understanding of the yarn networks’ properties and the impact of various processing effects, we demonstrate two techniques of producing polymer in-situ using dissolved monomers to side-step slow infiltration. The first technique is in-situ interfacial polymerization (ISIP), which is adapted to the yarns studied in this work to yield polyetherimide–CNT yarn composites. When applied to chemically-stretched yarn, specific strengths as high as 2.2 GPa/(g-cm3) are achieved in the flexible and durable yarn composite. We show parameters and conditions which maximize tensile properties and challenges associated with the rapid nature of the process, concluding with the successful demonstration of a roll-to-roll fabrication scheme for producing arbitrary amounts of polymer.

In our second technique, we produce extreme-performance polyimide and polybenzimidazole composites through green in-situ polymerizations (ISSP) in CNTs and macroscopic fiber networks. This approach utilizes superheated water and alcohol as a powerful medium to disperse monomers and initiate polymerization of high-performance coatings within a porous network. We demonstrate ISSP-CNT composites with variable coating morphologies (conformal, shish-kebab, etc.), in-air stability to over 500°C, and doubled specific stiffness and specific strength. Finally, we validate the multifunctional behavior of polyimide-CNT composites by showing a strong, flexible composite can store energy and behave as a free-standing battery electrode.