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Additive-free carbon nanotube dispersions, pastes, gels, and doughs in cresols Kevin Chiou a , Segi Byun a , Jaemyung Kim a , and Jiaxing Huang a,1 a Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208 Edited by David A. Weitz, Harvard University, Cambridge, MA, and approved April 23, 2018 (received for review January 8, 2018) 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 unprecedent- edly 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 hy- droxyl proton and can be removed after processing by evapora- tion or washing, without altering the surface of carbon nanotubes. Cresol solvents render carbon nanotubes polymer-like rheological and viscoelastic properties and processability. As the concentra- tion 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 pow- ders 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. cresol | carbon nanotubes | dough | solution processing | viscoelasticity C arbon nanotubes (CNTs) have been found to be attractive for applications due to their excellent electrical, thermal, and mechanical properties (15). Some types of nanotubes are al- ready mass-manufactured in the ton scale in the form of powders (3, 68). As with other industrial materials, powders are often used with solvents during processing, such as in the forms of dispersions, pastes, gels, or doughs, so that they can be made into the desirable geometries and structures (9). Solvent-based strategies that can disperse and process CNTs without contam- inating their functional surface or leaving residues would be very useful for their applications. Some types of solvents have been discovered that can produce relatively high-concentration dis- persions of CNTs, such as super acids (10), ionic liquids (11), and N-cyclohexyl-2-pyrrolidnone (12). However, most common sol- vents for nanotubes, such as N-methyl-2-pyrrolidone (NMP) (13), dimethylformamide (DMF) (13), and 1,2-dichrolobenzene (14), can only directly disperse some types of nanotubes at very low concentrations [e.g., typically <0.02 wt% for single-walled CNTs (SWCNTs)]. Here, we report that cresols, a group of in- dustrial chemicals for a number of applications (15), including for making household cleaning agents, are generic solvents for unfunctionalized CNTs of various types. They can process CNTs at concentrations up to tens of weight percent, resulting in a continuous transition from dilute dispersions, thick pastes, and free-standing gels to an unprecedented playdough-like state, as the CNT loading increases. These states exhibit polymer-like rheological and viscoelastic properties (16), which are not at- tainable with other common solvents, suggesting that the nano- tubes are indeed disaggregated and finely dispersed in cresols. Cresols can be removed after processing by heating or washing, without altering the surface of CNTs. As demonstrated below, the four nanotube/cresol states are highly processable and can be readily used in a broad array of materials-processing techniques to form desirable structures and composite materials. Results and Discussion CNTs in m-Cresol. Earlier works in the field of conjugated polymers have found that m-cresol is capable of dissolving some of the most hard-to-process conducting polymers such as polyaniline, and it interacts with the polymer chains through an effect called sec- ondary doping (1719). This inspired us to investigate the use of m- cresol as the processing solvent for CNTs, which can be viewed as highly conjugated polymers as well. Indeed, we found that powders of both SWCNTs and multiwalled CNTs (MWCNTs) can be well dispersed in m-cresol after sonication or grinding without the need for any surface functionalization. As shown in the scanning elec- tron microscopy (SEM) images (Fig. 1 A and D for MWCNTs and SWCNTs, respectively), initially the nanotubes were heavily ag- glomerated and entangled in the powders, but they became well separated after casting from the corresponding m-cresol disper- sions (Fig. 1 B, C, E, and F for MWCNTs and SWCNTs, re- spectively). These results suggest that the interaction between m- cresol and the surface of CNTs must be sufficiently strong to allow the agglomerated nanotubes to disperse. Proton NMR ( 1 H-NMR) spectroscopy was used to probe the nature of such interaction. As shown in Fig. 1G, in the presence of SWCNTs and MWCNTs, the phenolic hydroxyl proton peak shifted upfield by 0.10 ppm, while other proton peaks remained unchanged. This shift is a result of increased electron density on the phenolic hydroxyl proton, in- dicating charge-transfer interaction with the nanotubes, as is found for other Lewis acid type of solvents for CNTs (20, 21). Sonicating or grinding CNTs in m-cresol does not induce chemical changes to either the solvent or the nanotubes. This is illustrated with SWCNTs due to their higher spectroscopic Significance Carbon nanotubes can now be produced in the ton scale in the form of powders, but they need to be further processed, usu- ally 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 im- mediately usable by current materials-processing techniques for creating desirable structures or composites. Author contributions: J.H. designed research; K.C., S.B., and J.H. performed research; S.B. and J.K. contributed new reagents/analytic tools; K.C., S.B., and J.H. analyzed data; and K.C. and J.H. wrote the paper. The authors declare no conflict of interest. This article is a PNAS Direct Submission. Published under the PNAS license. 1 To whom correspondence should be addressed. Email: [email protected]. This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. 1073/pnas.1800298115/-/DCSupplemental. Published online May 14, 2018. www.pnas.org/cgi/doi/10.1073/pnas.1800298115 PNAS | May 29, 2018 | vol. 115 | no. 22 | 57035708 ENGINEERING Downloaded by guest on April 9, 2020

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Page 1: Additive-free carbon nanotube dispersions, pastes, gels ... · carbon nanotubes by charge transfer through the phenolic hy-droxyl proton and can be removed after processing by evapora-tion

Additive-free carbon nanotube dispersions, pastes,gels, and doughs in cresolsKevin Chioua, Segi Byuna, Jaemyung Kima, and Jiaxing Huanga,1

aDepartment of Materials Science and Engineering, Northwestern University, Evanston, IL 60208

Edited by David A. Weitz, Harvard University, Cambridge, MA, and approved April 23, 2018 (received for review January 8, 2018)

Cresols are a group of naturally occurring and massively producedmethylphenols with broad use in the chemical industry. Here, wereport that m-cresol and its liquid mixtures with other isomers aresurprisingly good solvents for processing carbon nanotubes. Theycan disperse carbon nanotubes of various types at unprecedent-edly high concentrations of tens of weight percent, without theneed for any dispersing agent or additive. Cresols interact withcarbon nanotubes by charge transfer through the phenolic hy-droxyl proton and can be removed after processing by evapora-tion or washing, without altering the surface of carbon nanotubes.Cresol solvents render carbon nanotubes polymer-like rheologicaland viscoelastic properties and processability. As the concentra-tion of nanotubes increases, a continuous transition of four statescan 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 pow-ders of agglomerated carbon nanotubes immediately usable by abroad array of material-processing techniques to create desirablestructures and form factors and make their polymer composites.

cresol | carbon nanotubes | dough | solution processing | viscoelasticity

Carbon nanotubes (CNTs) have been found to be attractivefor applications due to their excellent electrical, thermal, and

mechanical properties (1–5). Some types of nanotubes are al-ready mass-manufactured in the ton scale in the form of powders(3, 6–8). As with other industrial materials, powders are oftenused with solvents during processing, such as in the forms ofdispersions, pastes, gels, or doughs, so that they can be madeinto the desirable geometries and structures (9). Solvent-basedstrategies that can disperse and process CNTs without contam-inating their functional surface or leaving residues would be veryuseful for their applications. Some types of solvents have beendiscovered that can produce relatively high-concentration dis-persions of CNTs, such as super acids (10), ionic liquids (11), andN-cyclohexyl-2-pyrrolidnone (12). However, most common sol-vents for nanotubes, such as N-methyl-2-pyrrolidone (NMP)(13), dimethylformamide (DMF) (13), and 1,2-dichrolobenzene(14), can only directly disperse some types of nanotubes at verylow concentrations [e.g., typically <0.02 wt% for single-walledCNTs (SWCNTs)]. Here, we report that cresols, a group of in-dustrial chemicals for a number of applications (15), includingfor making household cleaning agents, are generic solvents forunfunctionalized CNTs of various types. They can process CNTsat concentrations up to tens of weight percent, resulting in acontinuous transition from dilute dispersions, thick pastes, andfree-standing gels to an unprecedented playdough-like state, asthe CNT loading increases. These states exhibit polymer-likerheological and viscoelastic properties (16), which are not at-tainable with other common solvents, suggesting that the nano-tubes are indeed disaggregated and finely dispersed in cresols.Cresols can be removed after processing by heating or washing,without altering the surface of CNTs. As demonstrated below,the four nanotube/cresol states are highly processable and can bereadily used in a broad array of materials-processing techniquesto form desirable structures and composite materials.

Results and DiscussionCNTs in m-Cresol. Earlier works in the field of conjugated polymershave found that m-cresol is capable of dissolving some of the mosthard-to-process conducting polymers such as polyaniline, and itinteracts with the polymer chains through an effect called sec-ondary doping (17–19). This inspired us to investigate the use ofm-cresol as the processing solvent for CNTs, which can be viewed ashighly conjugated polymers as well. Indeed, we found that powdersof both SWCNTs and multiwalled CNTs (MWCNTs) can be welldispersed in m-cresol after sonication or grinding without the needfor any surface functionalization. As shown in the scanning elec-tron microscopy (SEM) images (Fig. 1 A and D for MWCNTs andSWCNTs, respectively), initially the nanotubes were heavily ag-glomerated and entangled in the powders, but they became wellseparated after casting from the corresponding m-cresol disper-sions (Fig. 1 B, C, E, and F for MWCNTs and SWCNTs, re-spectively). These results suggest that the interaction between m-cresol and the surface of CNTs must be sufficiently strong to allowthe agglomerated nanotubes to disperse. Proton NMR (1H-NMR)spectroscopy was used to probe the nature of such interaction. Asshown in Fig. 1G, in the presence of SWCNTs and MWCNTs, thephenolic hydroxyl proton peak shifted upfield by 0.10 ppm, whileother proton peaks remained unchanged. This shift is a result ofincreased electron density on the phenolic hydroxyl proton, in-dicating charge-transfer interaction with the nanotubes, as is foundfor other Lewis acid type of solvents for CNTs (20, 21).Sonicating or grinding CNTs in m-cresol does not induce

chemical changes to either the solvent or the nanotubes. Thisis illustrated with SWCNTs due to their higher spectroscopic

Significance

Carbon nanotubes can now be produced in the ton scale in theform of powders, but they need to be further processed, usu-ally by solution-based routes, into disaggregated and moreusable forms for applications. There has been extensive effortto search and design solvents that can disperse nanotubes,which can also be easily removed afterward. Here, we reportthat m-cresol and its liquid mixtures with other isomers, whichare already manufactured for other industrial purposes, aresuch solvents. They can disperse carbon nanotube powders ofmany types at unprecedentedly high concentrations, renderingthem polymer-like rheological and viscoelastic properties, andhigh processability. This makes carbon nanotube powders im-mediately usable by current materials-processing techniquesfor creating desirable structures or composites.

Author contributions: J.H. designed research; K.C., S.B., and J.H. performed research; S.B.and J.K. contributed new reagents/analytic tools; K.C., S.B., and J.H. analyzed data; andK.C. and J.H. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

Published under the PNAS license.1To whom correspondence should be addressed. Email: [email protected].

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1800298115/-/DCSupplemental.

Published online May 14, 2018.

www.pnas.org/cgi/doi/10.1073/pnas.1800298115 PNAS | May 29, 2018 | vol. 115 | no. 22 | 5703–5708

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sensitivity to structural changes. The Fourier-transform infrared(FTIR) spectra in Fig. 1H show that m-cresol itself does not de-grade after ultrasonication with or without SWCNTs. As a rela-tively weak acid, m-cresol does not induce permanent chemicalchanges to the nanotube surface and can be removed by evapo-ration or washing. The Raman spectra of the pristine SWCNTsand a dried SWCNT film casted from m-cresol dispersion do notshow obvious difference (Fig. 1I), suggesting that they are notdamaged during processing. The absence of new bands between400 and 1,000 cm−1, where m-cresol shows strong Raman signals(22), indicates that they have been successfully removed.Among the three isomers of cresols,m-cresol is a liquid at room

temperature; therefore, it was used for most of the experiments inthis work. While o- and p-cresol are solid at room temperature,they can also process CNTs at molten state or when blended withm-cresol at room temperature (SI Appendix, Fig. S1). This sug-gests that even the unrefined, crude grade of cresols, which is aliquid mixture of the three isomers, can be directly used for in-dustrial scale processing of CNTs. Indeed, UV-visible near-IR(UV-Vis-NIR) spectra of SWCNTs dispersed in a ternary isomermixture of cresol showed characteristic bands of well-dispersednanotubes (SI Appendix, Fig. S2A), which was confirmed byTEM studies (SI Appendix, Fig. S2B). Industrial grades of cresolsoften contain phenolic impurities, and it was found that adding anadditional 10 wt% of phenol into the ternary mixture did notnegatively affect the stability of the nanotube dispersions (SI Ap-pendix, Fig. S2A). The impurity tolerance and ease of removalmake cresols the ideal type of nonreactive solvents for the solutionprocessing of CNTs. Below, we demonstrate that cresol solventsrender CNTs polymer-like rheological and viscoelastic propertiesand processability, making them immediately usable by alreadyavailable material-processing techniques to create desirable struc-tures and form factors and make composites.

Four States of MWCNTs in m-Cresol. m-cresol alone can disperseand process CNTs up to tens of weight percent, which has beenunprecedented (SI Appendix, Table S1). Since MWCNTs are themore common type of mass-produced CNTs and are much moreaffordable and available, they were chosen as the model materialfor most of the work below unless otherwise mentioned. Thephotos in Fig. 2 A–E show the as-received MWCNT powders andthe corresponding dilute dispersion, paste, gel, and a playdough-like state as the concentration in m-cresol increased. Dilute dis-persions are typically made by sonication and can remain stablefor at least many months (e.g., a photo of a 1-y-old sample isshown in SI Appendix, Fig. S3). The other higher-concentrationstates are typically made by grinding. Transitions between the fourstates are accompanied by threshold-like changes in their elec-trical, rheological, and viscoelastic properties. For example, thetransition from a dilute dispersion to a thick paste was accompa-nied by the onset of electrical conductivity ∼3 mg/mL (Fig. 2F),which can be attributed to the formation of a percolated nanotubenetwork, establishing a continuous electrically conductive pathwaythroughout the volume. At higher concentrations, increased den-sity of the MWCNT network resulted in significant changes inrheological and viscoelastic properties. For example, the transitionfrom a thick paste to a self-standing gel was marked by its inabilityto free flow ∼40–50 mg/mL, after which its viscosity increasedsignificantly (Fig. 2G). This rheological transition was similar tothe observations in a previous study of extensively oxidized CNTsin water, which can be attributed to continuous entanglement ofnanotubes (16). At concentrations >100 mg/mL, a viscoelastic,kneadable playdough-like material was obtained, which was highlycohesive and exhibited resistance to compression as characterizedby rapidly increased compression modulus (Fig. 2H).The continuous transition between these four highly process-

able polymer solution-like states suggests that the nanotubes weredispersed and outstretched in m-cresol, forming a cohesive net-work that densifies at increasing concentrations. If the nanotubes

Fig. 1. (A–F) MWCNTs (A–C) and SWCNTs (D–F)before and after ultrasonication in m-cresol. (A andD) The nanotubes in their initial powder form werehighly entangled and agglomerated and becamewell separated after being processed in m-cresol.(B and C) SEM images of MWCNTs cast from m-cre-sol. (E and F) SEM (E) and AFM (F) images of SWCNTscast from m-cresol. The line scan in F shows that theheight of the nanotube is ∼1 nm, consistent with thediameter of SWCNTs. (G) The 1H-NMR spectra showhydroxyl proton of m-cresol shifted upfield in thepresence of either SWCNT (1, blue trace) or MWCNT(2, red trace). The CNT samples for NMR were uni-formly dispersed (G, Inset). (H) FTIR spectra showingthat m-cresol itself does not degrade during ultra-sonication with or without SWCNTs. (I) No obviouschange in the Raman spectra of pristine SWCNTs andthose cast from m-cresol, suggesting that they werenot damaged during sonication. The cast SWCNTswere dried and rinsed with water before takingRaman spectra.

5704 | www.pnas.org/cgi/doi/10.1073/pnas.1800298115 Chiou et al.

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were still agglomerated as in their powders, the correspondinghigh-concentration products would not be cohesive due to se-gregated domains, resulting in poor processability (see schematicillustrations in SI Appendix, Fig. S4 and related discussion in thelegend and below). These four states have been observed for all ofthe CNTs tested (e.g., unfunctionalized single-walled or multi-walled tubes of various sizes). As demonstrated by the examplesbelow, m-cresol indeed offers unprecedented versatility for pro-cessing CNTs for existing and new applications.

Dilute Dispersion and Langmuir-Blodgett Assembly. Both SWCNTsand MWCNTs can disperse at higher concentrations in m-cresolthan in other common solvents such as NMP and DMF (SIAppendix, Fig. S5). The m-cresol dispersion can be directly ap-plied to Langmuir–Blodgett (LB) assembly for making mono-layer thin films. Successful LB monolayer assembly requireshigh-quality nanotube dispersions without other surface-activematerials to disrupt their packing on water surface, which ischallenging for additive-based CNT dispersions. Since m-cresolcan gradually dissolve in water, it dissipated into the subphaseafter spreading the nanotubes on the water surface, leaving cleannanotubes on the water surface (Fig. 3A). The water-supportedmonolayers could be further densified by closing two barriers,yielding a positive surface pressure (Fig. 3B), which could thenbe transferred to a substrate by dip-coating (Fig. 3 B, Inset). Fig.3C is a low-magnification SEM overview of a MWCNT film onglass slide collected at a surface pressure of 30 mN/m, whichappears to be continuous, uniform, and cohesive. Since many ofthe starting MWCNTs were curled, twisted, or even kinked (Fig.1C) and could not lay flat, the near-monolayer thickness of thefilm (Fig. 3D) also confirms that the heavily agglomeratedMWCNTs in the starting powders (Fig. 1A) indeed have beenwell separated in m-cresol. Strong van der Waals attraction atthe tube–tube junctions contributed to the continuity and co-hesiveness of the MWCNT monolayer.Transferring the nanotube monolayer onto soft plastic sub-

strates such as poly(ethylene terephthalate) formed a flexibletransparent conductor (Fig. 3E). Sheet resistance and opticaltransparency of the nanotube coating could be fine-tuned furtherby precisely controlling the number of deposited layers, as well asthe packing density within each monolayer. For example, a sheetresistance of 90 kΩ/square was obtained at 72% of optical trans-parency, which is already comparable to the best values obtainedwith films made from MWCNT powders (23). As shown in Fig. 1,usingm-cresol as a processing medium did not damage the surface

of nanotubes or leave hard-to-remove residues, which resulted insatisfying conductivity of the LB films without the need for ex-tensive further annealing steps. Similarly, LB assembly of SWCNTmonolayers has been achieved (SI Appendix, Fig. S6).

Thick Paste, Blade Coating, and Screen-Printing. Increasing the load-ing of MWCNTs up to 40 mg/mL resulted in a more viscous paste,which exhibited relatively high viscosity and shear thinning be-havior (Fig. 4A) with yield stress in the range of 1–10 Pa (Fig. 4B),making it suitable to use by brushing or painting. To make acontinuous film by these techniques, the paste must be sufficientlycohesive so that the coating does not break up under the shearduring spreading or crack by the capillary action during drying.Therefore, the nanotubes need to be interconnected throughoutthe paste without extensively segregated domains (see SI Appendix,Fig. S4 and related discussion). Fig. 4C illustrates blade coating of

Fig. 2. Four continuous states of MWCNTs in m-cresol exhibiting polymer solution like rheologicaland viscoelastic properties. (A–E) The nanotubepowders (A) can be processed in m-cresol to yielddilute dispersion (B), thick paste (C), self-standing gel(D), and finally kneadable dough (E). (F–H) Thetransitions between these states are characterizedby a threshold-like increase in electrical conductivity(F), viscosity (G), and compression modulus (H), dueto the formation and gradual densification of a 3Dnetwork of dispersed nanotubes.

Fig. 3. Monolayers of MWCNTs from dilute dispersion by LB assembly.(A) After spreading, a uniform semitransparent film is observed over theentire area of the trough. (B) Isothermal compression of the monolayer in-creases its surface pressure, indicative of higher nanotube density. B, Insetshows a dip-coated film on glass. (C and D) SEM image showing the film is acontinuous, uniform, paper-like monolayer (C) made of a network ofnanotubes (D). (E) Sheet resistances and the corresponding transparencies ofMWCNT layers on PET substrate made by repetitive dip-coating.

Chiou et al. PNAS | May 29, 2018 | vol. 115 | no. 22 | 5705

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the paste. The oven-dried coating on glass is continuous and freeof cracks over the entire area (Fig. 4D). SEM images show that it ismade of an interwoven, continuous, and high-density network ofnanotubes (Fig. 4E). As a comparison, a coating casted with NMPat the same concentration resulted in discontinuous islands (seeSEM image in SI Appendix, Fig. S7). Segregated MWCNTs inNMP resulted in an incohesive paste that could not maintain acontinuous layer after blading and during drying. Similar to bladecoating, industrial screen-printing can directly use the MWCNTpaste to generate functional patterns. A proof-of-concept dem-onstration of interdigitated electrode patterns printed on paper isshown in Fig. 4F. Blade coating is commonly used to make elec-trodes for energy storage devices from slurries, which often useCNTs as conductive binder for active materials (8). Highly co-hesive, additive-free pastes with well-dispersed nanotubes arereadily compatible with these slurry-processing techniques andcould directly benefit this large-scale application of CNTs.

MWCNT Pastes for Polymer Composites. Polymer nanocomposite isanother area that uses a very large scale of CNTs (3, 7). The pastestate offers a number of potential advantageous for manufacturing.To start, the paste can be easily mixed with powders of polymers,which is one of the most common forms of industrial polymers.Moreover, m-cresol itself is a known solvent for many commoditypolymers such as poly(methyl methacrylate) (PMMA), nylons,polyethylene terephthalate, polystyrene, and phenolic resins (24),which helps the blending process. Using the paste also drasticallyreduces the amount of solvent needed for manufacturing and

greatly shortens the baking time needed for solvent removal. Fig. 5shows a proof-of-concept experiment, where PMMA powders weredirectly mixed with the paste by mortar and pestle (also see SIAppendix, Fig. S8). The product was rolled into a flexible and highlyplastic sheet, which sustained >800% of tensile strain. Upon ther-mal curing at 150 °C, the sheet hardened (Fig. 5B) due to partialremoval of m-cresol. At 1 wt% loading of MWCNTs in PMMA,the Young’s modulus of the composite (1.46 GPa) increased by24% in comparison with a similarly processed PMMA sheet(1.17 GPa). SEM observation confirmed that the MWCNTs hadbeen finely dispersed in the PMMA matrix (Fig. 5C). Such soft–hard transition is critical for industrial forming techniques, whichturn materials into desirable geometries and form factors. Theadditive-free CNT pastes in cresols could be useful for acceleratingthe development and manufacturing of polymer nanocomposites.

Gel and 3D Printing. Above 40 mg/mL, the MWCNT network inm-cresol was sufficiently dense to hinder free flow, leading to afreestanding gel. As the nanotube concentration increased, the gelbecame more solid-like with increased storage modulus (Fig. 6A).The loss modulus increased more slowly than the storage modulus,rendering the gel a sufficient level of liquid character for extrusiontype of processing (Fig. 6B). Therefore, the MWCNT gel coulddeform and reconnect easily. Fig. 6C shows a MWCNT gel beingextruded to form self-supporting fibers through a 0.5-mm-diameter needle. Since the gel is cohesive, extrusion can be con-tinuously operated even with finer needles (e.g., 0.1-mm di-ameter). In contrast, fiber extrusion cannot be performed withother solvents such as NMP in similar range of concentrations.Instead, jetting (for 0.5-mm needle) and clogging (for 0.1-mmneedle) occurred due to jamming of the nozzles by blobs of seg-regated nanotubes (SI Appendix, Fig. S4 and related discussion).This again reflects that the nanotubes were uniformly dispersed bym-cresol and outstretched like polymers in the gel, rendering itsuitable rheological properties for continuous, unhindered extru-sion. This gel is immediately usable for programmed and auto-mated printing (Fig. 6D, inner diameter of 0.1 mm). As a proof ofconcept, a cup-shaped structure was 3D-printed from the gel (Fig.6E). The base of the cup was made of two criss-cross layers ofclose-packed fibers, and the side was made of vertically stackedrings. After drying, the cup structure shrank slightly isotropically

Fig. 4. Blade coating of MWCNTs from the thick paste. (A) The MWCNTpaste exhibits shear-thinning behavior at all of the concentrations tested,which is typical for polymer dissolved in good solvents. From II to VI, theconcentrations are increased from 10, 20, 30, 40, to 50 mg/mL. Pure m-cresol (I) does not exhibit this behavior. (B) The yield stress of the pasteincreases relatively slowly as the nanotube concentration increases, until itreaches the range of the gel state (V). (C–E ) Blade-coating (C) creates acontinuous and uniform nanotube film on glass after drying (D), which isfree of cracks (E, SEM image) that are typically seen for coatings made withother solvents, such as NMP (SI Appendix, Fig. S7). (F ) Patterns of in-terdigitated electrodes screen-printed on paper.

Fig. 5. MWCNTs/polymer nanocomposite by direct mixing using thepaste. (A) Photos showing MWCNT/PMMA composite sheet made by di-rect mixing of polymer powders with the paste, followed by cold rolling(also see SI Appendix, Fig. S8). (B) The uncured composite sheet (1.0 wt%MWCNTs) is highly ductile and hardens upon curing at 150 °C for 2 h. (C )SEM image shows well-dispersed MWCNTs embedded within the nano-composite.

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but maintained its shape, resulting in a stiff solid object that couldbe further handled (Fig. 6F).

MWCNT Dough. The last state of MWCNT/m-cresol is a viscoelasticdough (>100 mg/mL), which can be kneaded or rolled withoutfracture. In contrast to a gel (Fig. 7B), when kneaded on paper,the dough did not leave any stain mark (Fig. 7A and SI Appendix,Fig. S9). This is due to the strong attraction between the nano-tubes in the densely woven 3D network, which prevents them fromleaving residues on paper. Control experiments were also donewith other solvents, such as NMP, at similar concentrations.However, the resulting mixtures were too fragile to manipulateand broke into pieces upon kneading. Since the nanotube/m-cresoldough was kneadable and stain-free (Fig. 7A), it must be highlycohesive and free of mechanically weak boundaries between seg-regated grains of CNTs (see SI Appendix, Fig. S4 and relateddiscussion), as seen in the starting powders (SEM images in Fig. 1A and D). As with a bread dough, the MWCNT dough could becut into pieces and rejoined when pressed together or molded intoarbitrary shapes without altering its viscoelastic properties. Fig. 7Cshows a thick film cold-rolled from the dough, which was still softand plastic (Fig. 7D) and could be reshaped by using a mold (Fig.7E). The MWCNT doughs could be hardened to fix their shapesafter heating at >200 °C to remove m-cresol. The hardenedstructures could then be returned to the soft dough state by ab-sorbing m-cresol. The playdough-like processability should openup opportunities to fabricate arbitrarily shaped 3D solids of neatCNTs for a range of electronic, thermal, and energy applications.

ConclusionCresol-based CNT dispersions, pastes, gels, and doughs exhibitpolymer-like rheological and viscoelastic properties, rendering

them polymer-like processability. Cresols work generically forunfunctionalized CNTs of many types and can be convenientlyremoved from the final products without negatively altering theirpristine properties. Cresols are abundantly produced, relativelyinexpensive, and quite stable to handle at room temperature andambient atmosphere. These advantages make cresols an idealclass of processing solvents for CNTs, especially for their mass-produced powder form. It should help to overcome many as-pects of the processability problems of CNTs, which has beenone of the greatest hurdles preventing their widespread in-dustrial applications. The surprise that solvents with suchsimple molecular structures work so well is also likely to inspiremany more discoveries about the interactions between organicmolecules and graphitic surface, as well as in new material andengineering technologies based on CNTs and other graphiticnanostructures.

Materials and MethodsMaterials. CNT powders of various types, sources, and levels of purities fromthree vendors were tested, and all dispersed well in m-cresol and its liquidmixtures with other isomers. These included: (i) CoMoCAT MWCNTs (98%carbon content), CoMoCat SWCNTs (90% carbon content, 90% semi-conducting), and double-walled CNTs [90% carbon content, made bychemical vapor deposition (CVD)] were obtained from Sigma-Aldrich; (ii)SWCNTs (P2, 90% purity) and carboxylic functionalized SWCNTs (P3, 90%purity) were made by arc-discharge and obtained from Carbon Solution Inc.;and (iii) graphitized MWCNTs (TNGM2; 99.9% purity, approximate lengthsof 50 μm), low-density SWCNTs (TNSR; 95% purity, approximate lengths of5–30 μm, 0.027 g/cm3), high-density SWCNTs (TNST; 95% purity, 0.14 g/cm3),short SWCNTs (TNSSR; 95% purity, approximate lengths of 1–3 μm), andshort MWCNTs (TNSM2; 95% purity, approximate lengths of 0.5–2 μm) wereall made by CVD and obtained from TimesNano.

P2 SWCNTs and MWCNTs (CoMoCat) were used for demonstrating LBassembly (Fig. 2 and SI Appendix, Fig. S6). The results of the pastes, gels, and

Fig. 6. Extrusion and 3D printing using the MWNCTs gel. (A and B) The gelshows increasingly solid-like behavior as nanotube concentration increases,based on the results of storage moduli (A) and loss moduli (B) measure-ments. (C and D) MWCNT gel can be continuously extruded from a needle(inner diameter of 0.5 mm; C), which allows patterning of nanotubes using aprogrammable stage (inner diameter of 0.1 mm; D). (E and F) A 3D printedcup made of MWCNTs (E), which maintains its shape after drying (F).

Fig. 7. Playdough-like MWCNTs/m-cresol solid. (A and B) Kneading ananotube dough (>100 mg/mL) only leaves traces of solvent on paper (A),while a stiff nanotube gel (<100 mg/mL) leaves extensive stains of nanotubes(B; also see SI Appendix, Fig. S9). (C–E) The dough can be transformed intoarbitrary geometries, such as a freestanding strip by cold rolling (C and D)and other arbitrary shapes defined by a mold (E).

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doughs shown in the work were demonstrated with CoMoCat MWCNTs asthe model material, although other types of MWCNTs work as well.

Other chemicals were purchased from Sigma-Aldrich and used as received,includingm-cresol (99%), o-cresol (99%), p-cresol (98%), toluene (99.9%), phenol(>99%), DMF (99.8%), NMP (anhydrous, 99.5%), PMMA [200,000 molecularweight (Mw)], and methyltrichlorosilane (99%). Ternary isomer mixture of cresol(>99%wt, 1:1:1 ratio) was purchased from Fisher Scientific and used as received.

LB Assembly and Transparent Conductive Thin Films. Powders of MWCNTs orSWCNTs were first mixed with m-cresol by using a mortar and pestle, thensonicated in pulse mode (2 s on/2 s off cycles for a total of 1 h) by using aQsonica Q125 sonicator rated at 125 W, equipped with a 1/4-inch standardtapered tip at 90% power. After sonication, the dispersion was subject toexhaustive high-speed centrifugation at 11,000 rpm for 1 h by using anEppendorf 5804 desktop centrifuge (equivalent to a relative centrifugalforce of 15,557 × g). The supernatant was recovered and used. Samples formaking transparent conductors were first purified by a nonoxidative route,including washing in 3 M HCl at 65 °C for 4 h, followed by baking in amuffled furnace at 250 °C for 1 h.

All parts of the LB system (Nima Technology) were thoroughly cleanedwith acetone before use. By using a glass syringe, 1 mL ofm-cresol dispersion(SWCNT or MWCNT) was carefully spread onto the air–water interface. Atensiometer with a Wilhelmy plate was used to monitor surface pressurewhile closing the barriers. At surface pressures of ∼40 mN/m for SWCNTs and30 mN/m for MWCNTs, monolayer films were dip-coated onto a substrate(typically glass slides) with a pull speed of 2 mm/min. The obtained LB filmswere annealed at 150 °C for 30 min before subsequent LB deposition toproduce multilayered films.

Blade-Coating and Screen Printing. MWCNT paste in m-cresol (100 mg/mL) wasmade by direct mixing using a mortar and pestle, then diluted to 40 mg/mLand hand-ground further to yield a spreadable thick paste. Glass slides werefirst silanized with 5 wt%methyltrichlorosilane in toluene for 10 min and thenwashed thoroughly by using toluene followed by acetone. Two strips ofKapton tapes were attached to the sides of the silanized glass slide as spacersto control the thickness of the coating. Approximately 0.3 mL ofMWCNT pastewas deposited onto the shallow trough created by the Kapton tapes. A razorblade was used to drag the paste to coat the slide. The coating was left to dryat 150 °C for 2 h. Control experiments were done by using NMP instead ofm-cresol as the solvent at the same nanotube concentration. Screen-printingwas done on paper through a mask by using a paste of 10 mg/mL.

Polymer Composite. To make MWCNT/PMMA nanocomposite, a MWCNTs/m-cresol paste (40 mg/mL) was ground directly with powders of PMMA(200,000 Mw) by using a mortar and pestle for 10 min. The composite wasthen flattened by cold rolling, which turned flexible and rubbery after beingair-dried (Fig. 5 and SI Appendix, Fig. S6). Curing at 150 °C for 2 h signifi-cantly hardened the piece and fixed its shape.

Three-Dimensional Printing.MWCNTs/m-cresol gel was made by direct mixingusing a mortar and pestle at a concentration of 120 mg/mL The resultingmixture was diluted to 80 mg/mL and ground further. The gel was loaded

into a syringe and manually extruded from needles with diameters of0.1 and 0.5 mm, which can be fitted onto a 3D printer (Hyrel 30M). Printed3D structure can be removed from the glass substrate after being air-driedfor 12 h, which can be further hardened by baking to remove m-cresol.

MWCNT Dough. MWCNT/m-cresol dough was made by directly mixing using amortar and pestle at a concentration of 300 mg/mL or higher. The mixture wasthen diluted to 150 mg/mL and ground further to yield a dough-like material,which was kneaded to the shape of a ball. Kneading or rolling a nanotubedough does not stain the substrate, while doing so with a gel or paste wouldresult in significant staining. A kneaded dough was sandwiched between twostainless steel foils and cold rolled to a film with final thickness of 200 μm,which can be cut into various shapes with a razor blade or cookie cutters.

Characterization. Dispersions of carbon materials in m-cresol were drop-castedonto silicon wafers and dried at 200–250 °C, before SEM (FEI Nova 600 system)and AFM (Park Systems XE-100, tapping mode). UV/vis spectra were taken withan Agilent 8453 UV/Vis spectrometer. NIR spectra were taken by using a Per-kinElmer LAMBDA 1050 spectrometer. TEM images were taken with a JEOLARM300F GrandARM transmission electron microscope. Drop-cast SWCNTswere air-dried and rinsed with water and ethanol before Raman spectroscopymeasurement (WITec Alpha 300; 532-nm excitation). FTIR spectra were recor-ded on a PerkinElmer Instrument spectrometer (Spectrum Spotlight 300).The 1H-NMR spectra were acquired on a 400-MHz Agilent DD MR-400 NMRsystem. The samples were prepared by adding 100 μL of SWCNT or MWCNTdispersions in m-cresol in 1 mL of CDCl3. The nanotubes were found to bestably dispersed in the entire duration of NMR experiments. Transparency ofthe LB films was measured by using an Agilent 8453 UV/Vis spectrometer.Sheet resistance of the films was obtained by using an in-line four-point probeequipped with a Keithley 2400 source meter. Viscoelastic and rheologicalproperties were measured by using an Anton Paar MCR 502 rheometer using acone-on-plate configuration. The cone has a 25-mm diameter with a 5° gapangle. Viscosity vs. concentration measurements in Fig. 2G were measuredwith a rotation speed of 1°/s. Yield stresses were obtained by using a Herschel–Bulkley regression included in the Anton Paar software package. Shear-thin-ning viscosities of Fig. 4A were measured with a linear ramping shear ratebetween 0.01 and 100 rad/s. Storage and loss moduli were measured simul-taneously by using the same rheometer setup at an amplitude of 1%. Tensileand compression tests were done on a Bose electroforces 5500 tester. Thecomposite films were cut into dog-bone shapes and pulled at a rate of0.05 mm/s until failure. Only the results from samples that failed in the middlewere considered. Gel and dough samples for compression tests were firstmolded into cylindrical shapes and carefully transferred to the tester. Com-pression was done at 0.005mm/s until the sample ruptured. The slope of the firstlinear region of the stress-strain curve was taken as the compression modulus.

ACKNOWLEDGMENTS. We thank Dr. J. Luo for providing some CNT samples.K.C. is a NSF Graduate Research Fellow. S.B. acknowledges a visiting studentfellowship supported by Basic Science Research Program through the NationalResearch Foundation of Korea funded by the Ministry of Education, Science,and Technology. J.H. was supported by an earlier Guggenheim Fellowship,part of which was applied to purchase some materials.

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5708 | www.pnas.org/cgi/doi/10.1073/pnas.1800298115 Chiou et al.

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