By using an inexpensive, already mass produced, simple solvent called cresol, Huang has discovered a way to make disperse carbon nanotubes at unprecedentedly high concentrations without the need for additives or harsh chemical reactions to modify the nanotubes. In a surprising twist, Huang also found that as the nanotubes’ concentrations increase, the material transitions from a dilute dispersion to a thick paste, then a free-standing gel and finally a kneadable dough that can be shaped and molded.
“Because of their exceptional mechanical, thermal and electrical properties, carbon nanotubes have attracted a lot of attention for a number of applications,” said Huang, professor of materials science and engineering in Northwestern’s McCormick School of Engineering. “But after decades of research and development, some of the excitement has faded.”
The reason? Carbon nanotubes are notoriously tricky to process — especially in large quantities. About 10,000 times thinner than a human hair, the wiry, tube-shaped structures are said to be stronger than steel and conduct heat and electricity far better than copper. But when mass produced — usually in the form of powders — the tubes twist and clump together. This complication is a major barrier to the material’s widespread applications.
“Aggregated tubes are hard to disperse in solvents,” Huang said. “And if you cannot get a good dispersion, then you won’t be able to make high-quality nanotube thin films that many applications rely on.”
In order to bypass this problem, previous researchers used additives to coat the nanotubes, which chemically altered their surfaces and forced them to separate. Although these methods do work, they leave behind residues or alter the nanotubes’ surface structures, which can blunt their desirable properties.
By contrast, Huang’s team found that cresol does not deteriorate carbon nanotubes’ surface functions. And, after separating the entangled tubes, researchers can simply remove the chemical by washing it off or heating it until it evaporates.
Finding unexpected kneads
After unlocking a new way to make carbon nanotubes in higher and higher concentrations, Huang and his team discovered new forms of the material. As the concentration of carbon nanotubes increases, the material transitions from a dilute dispersion to a spreadable paste to a free-standing gel and finally to a kneadable dough. These various forms can be molded, reshaped or used as conductive ink for 3D printing.
“The dough state of nanotubes is fascinating,” said Kevin Chiou, a graduate student in Huang’s laboratory and first author of the paper. “It can be readily shaped and molded into arbitrary structures just like playdough.”
“Essentially, this solvent system now makes nanotubes behave just like polymers,” Huang said. “It is really exciting to see cresol-based solvents make once hard-to-process carbon nanotubes as usable as common plastics.”
Carbon nanotubes can now be produced in the ton scale in the form of powders, but they need to be further processed, usually by solution-based routes, into disaggregated and more usable forms for applications. There has been extensive effort to search and design solvents that can disperse nanotubes, which can also be easily removed afterward. Here, we report that m-cresol and its liquid mixtures with other isomers, which are already manufactured for other industrial purposes, are such solvents. They can disperse carbon nanotube powders of many types at unprecedentedly high concentrations, rendering them polymer-like rheological and viscoelastic properties, and high processability. This makes carbon nanotube powders immediately usable by current materials-processing techniques for creating desirable structures or composites.
Cresols are a group of naturally occurring and massively produced methylphenols with broad use in the chemical industry. Here, we report that m-cresol and its liquid mixtures with other isomers are surprisingly good solvents for processing carbon nanotubes. They can disperse carbon nanotubes of various types at unprecedentedly high concentrations of tens of weight percent, without the need for any dispersing agent or additive. Cresols interact with carbon nanotubes by charge transfer through the phenolic hydroxyl proton and can be removed after processing by evaporation or washing, without altering the surface of carbon nanotubes. Cresol solvents render carbon nanotubes polymer-like rheological and viscoelastic properties and processability. As the concentration of nanotubes increases, a continuous transition of four states can be observed, including dilute dispersion, thick paste, free-standing gel, and eventually a kneadable, playdough-like material. As demonstrated with a few proofs of concept, cresols make powders of agglomerated carbon nanotubes immediately usable by a broad array of material-processing techniques to create desirable structures and form factors and make their polymer composites.