Additive-free carbon nanotube dispersions, pastes, gels, and doughs in cresols

Edited by David A. Weitz, Harvard University, Cambridge, MA, and approved April 23, 2018 (received for review January 8, 2018)
May 14, 2018
115 (22) 5703-5708

Significance

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.

Abstract

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.
Carbon 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 already 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 contaminating 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 dispersions of CNTs, such as super acids (10), ionic liquids (11), and N-cyclohexyl-2-pyrrolidnone (12). However, most common solvents 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 industrial 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 attainable with other common solvents, suggesting that the nanotubes 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 secondary 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 electron microscopy (SEM) images (Fig. 1 A and D for MWCNTs and SWCNTs, respectively), initially the nanotubes were heavily agglomerated and entangled in the powders, but they became well separated after casting from the corresponding m-cresol dispersions (Fig. 1 B, C, E, and F for MWCNTs and SWCNTs, respectively). 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 (1H-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, indicating charge-transfer interaction with the nanotubes, as is found for other Lewis acid type of solvents for CNTs (20, 21).
Fig. 1.
(AF) MWCNTs (AC) and SWCNTs (DF) before and after ultrasonication in m-cresol. (A and D) The nanotubes in their initial powder form were highly entangled and agglomerated and became well separated after being processed in m-cresol. (B and C) SEM images of MWCNTs cast from m-cresol. (E and F) SEM (E) and AFM (F) images of SWCNTs cast from m-cresol. The line scan in F shows that the height of the nanotube is ∼1 nm, consistent with the diameter of SWCNTs. (G) The 1H-NMR spectra show hydroxyl proton of m-cresol shifted upfield in the presence of either SWCNT (1, blue trace) or MWCNT (2, red trace). The CNT samples for NMR were uniformly dispersed (G, Inset). (H) FTIR spectra showing that m-cresol itself does not degrade during ultrasonication with or without SWCNTs. (I) No obvious change in the Raman spectra of pristine SWCNTs and those cast from m-cresol, suggesting that they were not damaged during sonication. The cast SWCNTs were dried and rinsed with water before taking Raman spectra.
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 sensitivity to structural changes. The Fourier-transform infrared (FTIR) spectra in Fig. 1H show that m-cresol itself does not degrade after ultrasonication with or without SWCNTs. As a relatively weak acid, m-cresol does not induce permanent chemical changes to the nanotube surface and can be removed by evaporation or washing. The Raman spectra of the pristine SWCNTs and a dried SWCNT film casted from m-cresol dispersion do not show obvious difference (Fig. 1I), suggesting that they are not damaged during processing. The absence of new bands between 400 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 in this work. While o- and p-cresol are solid at room temperature, they can also process CNTs at molten state or when blended with m-cresol at room temperature (SI Appendix, Fig. S1). This suggests that even the unrefined, crude grade of cresols, which is a liquid mixture of the three isomers, can be directly used for industrial scale processing of CNTs. Indeed, UV-visible near-IR (UV-Vis-NIR) spectra of SWCNTs dispersed in a ternary isomer mixture of cresol showed characteristic bands of well-dispersed nanotubes (SI Appendix, Fig. S2A), which was confirmed by TEM studies (SI Appendix, Fig. S2B). Industrial grades of cresols often contain phenolic impurities, and it was found that adding an additional 10 wt% of phenol into the ternary mixture did not negatively affect the stability of the nanotube dispersions (SI Appendix, Fig. S2A). The impurity tolerance and ease of removal make cresols the ideal type of nonreactive solvents for the solution processing of CNTs. Below, we demonstrate that cresol solvents render CNTs polymer-like rheological and viscoelastic properties and processability, making them immediately usable by already available material-processing techniques to create desirable structures and form factors and make composites.

Four States of MWCNTs in m-Cresol.

m-cresol alone can disperse and process CNTs up to tens of weight percent, which has been unprecedented (SI Appendix, Table S1). Since MWCNTs are the more common type of mass-produced CNTs and are much more affordable and available, they were chosen as the model material for most of the work below unless otherwise mentioned. The photos in Fig. 2 AE show the as-received MWCNT powders and the corresponding dilute dispersion, paste, gel, and a playdough-like state as the concentration in m-cresol increased. Dilute dispersions are typically made by sonication and can remain stable for at least many months (e.g., a photo of a 1-y-old sample is shown in SI Appendix, Fig. S3). The other higher-concentration states are typically made by grinding. Transitions between the four states are accompanied by threshold-like changes in their electrical, rheological, and viscoelastic properties. For example, the transition from a dilute dispersion to a thick paste was accompanied by the onset of electrical conductivity ∼3 mg/mL (Fig. 2F), which can be attributed to the formation of a percolated nanotube network, establishing a continuous electrically conductive pathway throughout the volume. At higher concentrations, increased density of the MWCNT network resulted in significant changes in rheological and viscoelastic properties. For example, the transition from a thick paste to a self-standing gel was marked by its inability to free flow ∼40–50 mg/mL, after which its viscosity increased significantly (Fig. 2G). This rheological transition was similar to the observations in a previous study of extensively oxidized CNTs in water, which can be attributed to continuous entanglement of nanotubes (16). At concentrations >100 mg/mL, a viscoelastic, kneadable playdough-like material was obtained, which was highly cohesive and exhibited resistance to compression as characterized by rapidly increased compression modulus (Fig. 2H).
Fig. 2.
Four continuous states of MWCNTs in m-cresol exhibiting polymer solution like rheological and viscoelastic properties. (AE) The nanotube powders (A) can be processed in m-cresol to yield dilute dispersion (B), thick paste (C), self-standing gel (D), and finally kneadable dough (E). (FH) The transitions between these states are characterized by a threshold-like increase in electrical conductivity (F), viscosity (G), and compression modulus (H), due to the formation and gradual densification of a 3D network of dispersed nanotubes.
The continuous transition between these four highly processable polymer solution-like states suggests that the nanotubes were dispersed and outstretched in m-cresol, forming a cohesive network that densifies at increasing concentrations. If the nanotubes were still agglomerated as in their powders, the corresponding high-concentration products would not be cohesive due to segregated domains, resulting in poor processability (see schematic illustrations in SI Appendix, Fig. S4 and related discussion in the legend and below). These four states have been observed for all of the CNTs tested (e.g., unfunctionalized single-walled or multiwalled tubes of various sizes). As demonstrated by the examples below, m-cresol indeed offers unprecedented versatility for processing CNTs for existing and new applications.

Dilute Dispersion and Langmuir-Blodgett Assembly.

Both SWCNTs and MWCNTs can disperse at higher concentrations in m-cresol than in other common solvents such as NMP and DMF (SI Appendix, Fig. S5). The m-cresol dispersion can be directly applied to Langmuir–Blodgett (LB) assembly for making monolayer thin films. Successful LB monolayer assembly requires high-quality nanotube dispersions without other surface-active materials to disrupt their packing on water surface, which is challenging for additive-based CNT dispersions. Since m-cresol can gradually dissolve in water, it dissipated into the subphase after spreading the nanotubes on the water surface, leaving clean nanotubes on the water surface (Fig. 3A). The water-supported monolayers could be further densified by closing two barriers, yielding a positive surface pressure (Fig. 3B), which could then be transferred to a substrate by dip-coating (Fig. 3 B, Inset). Fig. 3C is a low-magnification SEM overview of a MWCNT film on glass slide collected at a surface pressure of 30 mN/m, which appears to be continuous, uniform, and cohesive. Since many of the starting MWCNTs were curled, twisted, or even kinked (Fig. 1C) and could not lay flat, the near-monolayer thickness of the film (Fig. 3D) also confirms that the heavily agglomerated MWCNTs in the starting powders (Fig. 1A) indeed have been well separated in m-cresol. Strong van der Waals attraction at the tube–tube junctions contributed to the continuity and cohesiveness of the MWCNT monolayer.
Fig. 3.
Monolayers of MWCNTs from dilute dispersion by LB assembly. (A) After spreading, a uniform semitransparent film is observed over the entire area of the trough. (B) Isothermal compression of the monolayer increases its surface pressure, indicative of higher nanotube density. B, Inset shows a dip-coated film on glass. (C and D) SEM image showing the film is a continuous, uniform, paper-like monolayer (C) made of a network of nanotubes (D). (E) Sheet resistances and the corresponding transparencies of MWCNT layers on PET substrate made by repetitive dip-coating.
Transferring the nanotube monolayer onto soft plastic substrates such as poly(ethylene terephthalate) formed a flexible transparent conductor (Fig. 3E). Sheet resistance and optical transparency of the nanotube coating could be fine-tuned further by precisely controlling the number of deposited layers, as well as the packing density within each monolayer. For example, a sheet resistance of 90 kΩ/square was obtained at 72% of optical transparency, which is already comparable to the best values obtained with films made from MWCNT powders (23). As shown in Fig. 1, using m-cresol as a processing medium did not damage the surface of nanotubes or leave hard-to-remove residues, which resulted in satisfying conductivity of the LB films without the need for extensive further annealing steps. Similarly, LB assembly of SWCNT monolayers has been achieved (SI Appendix, Fig. S6).

Thick Paste, Blade Coating, and Screen-Printing.

Increasing the loading of MWCNTs up to 40 mg/mL resulted in a more viscous paste, which exhibited relatively high viscosity and shear thinning behavior (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 a continuous film by these techniques, the paste must be sufficiently cohesive so that the coating does not break up under the shear during spreading or crack by the capillary action during drying. Therefore, the nanotubes need to be interconnected throughout the paste without extensively segregated domains (see SI Appendix, Fig. S4 and related discussion). Fig. 4C illustrates blade coating of the paste. The oven-dried coating on glass is continuous and free of cracks over the entire area (Fig. 4D). SEM images show that it is made of an interwoven, continuous, and high-density network of nanotubes (Fig. 4E). As a comparison, a coating casted with NMP at the same concentration resulted in discontinuous islands (see SEM image in SI Appendix, Fig. S7). Segregated MWCNTs in NMP resulted in an incohesive paste that could not maintain a continuous layer after blading and during drying. Similar to blade coating, industrial screen-printing can directly use the MWCNT paste to generate functional patterns. A proof-of-concept demonstration of interdigitated electrode patterns printed on paper is shown in Fig. 4F. Blade coating is commonly used to make electrodes for energy storage devices from slurries, which often use CNTs as conductive binder for active materials (8). Highly cohesive, additive-free pastes with well-dispersed nanotubes are readily compatible with these slurry-processing techniques and could directly benefit this large-scale application of CNTs.
Fig. 4.
Blade coating of MWCNTs from the thick paste. (A) The MWCNT paste exhibits shear-thinning behavior at all of the concentrations tested, which is typical for polymer dissolved in good solvents. From II to VI, the concentrations 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 paste increases relatively slowly as the nanotube concentration increases, until it reaches the range of the gel state (V). (CE) Blade-coating (C) creates a continuous and uniform nanotube film on glass after drying (D), which is free of cracks (E, SEM image) that are typically seen for coatings made with other solvents, such as NMP (SI Appendix, Fig. S7). (F) Patterns of interdigitated electrodes screen-printed on paper.

MWCNT Pastes for Polymer Composites.

Polymer nanocomposite is another area that uses a very large scale of CNTs (3, 7). The paste state 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 commodity polymers such as poly(methyl methacrylate) (PMMA), nylons, polyethylene terephthalate, polystyrene, and phenolic resins (24), which helps the blending process. Using the paste also drastically reduces the amount of solvent needed for manufacturing and greatly shortens the baking time needed for solvent removal. Fig. 5 shows a proof-of-concept experiment, where PMMA powders were directly mixed with the paste by mortar and pestle (also see SI Appendix, Fig. S8). The product was rolled into a flexible and highly plastic sheet, which sustained >800% of tensile strain. Upon thermal curing at 150 °C, the sheet hardened (Fig. 5B) due to partial removal of m-cresol. At 1 wt% loading of MWCNTs in PMMA, the Young’s modulus of the composite (1.46 GPa) increased by 24% in comparison with a similarly processed PMMA sheet (1.17 GPa). SEM observation confirmed that the MWCNTs had been finely dispersed in the PMMA matrix (Fig. 5C). Such soft–hard transition is critical for industrial forming techniques, which turn materials into desirable geometries and form factors. The additive-free CNT pastes in cresols could be useful for accelerating the development and manufacturing of polymer nanocomposites.
Fig. 5.
MWCNTs/polymer nanocomposite by direct mixing using the paste. (A) Photos showing MWCNT/PMMA composite sheet made by direct 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 nanocomposite.

Gel and 3D Printing.

Above 40 mg/mL, the MWCNT network in m-cresol was sufficiently dense to hinder free flow, leading to a freestanding gel. As the nanotube concentration increased, the gel became 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 extrusion type of processing (Fig. 6B). Therefore, the MWCNT gel could deform and reconnect easily. Fig. 6C shows a MWCNT gel being extruded to form self-supporting fibers through a 0.5-mm-diameter needle. Since the gel is cohesive, extrusion can be continuously operated even with finer needles (e.g., 0.1-mm diameter). In contrast, fiber extrusion cannot be performed with other solvents such as NMP in similar range of concentrations. Instead, jetting (for 0.5-mm needle) and clogging (for 0.1-mm needle) occurred due to jamming of the nozzles by blobs of segregated nanotubes (SI Appendix, Fig. S4 and related discussion). This again reflects that the nanotubes were uniformly dispersed by m-cresol and outstretched like polymers in the gel, rendering it suitable rheological properties for continuous, unhindered extrusion. This gel is immediately usable for programmed and automated printing (Fig. 6D, inner diameter of 0.1 mm). As a proof of concept, a cup-shaped structure was 3D-printed from the gel (Fig. 6E). The base of the cup was made of two criss-cross layers of close-packed fibers, and the side was made of vertically stacked rings. After drying, the cup structure shrank slightly isotropically but maintained its shape, resulting in a stiff solid object that could be further handled (Fig. 6F).
Fig. 6.
Extrusion and 3D printing using the MWNCTs gel. (A and B) The gel shows increasingly solid-like behavior as nanotube concentration increases, based on the results of storage moduli (A) and loss moduli (B) measurements. (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 a programmable stage (inner diameter of 0.1 mm; D). (E and F) A 3D printed cup made of MWCNTs (E), which maintains its shape after drying (F).

MWCNT Dough.

The last state of MWCNT/m-cresol is a viscoelastic dough (>100 mg/mL), which can be kneaded or rolled without fracture. 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 nanotubes in the densely woven 3D network, which prevents them from leaving residues on paper. Control experiments were also done with other solvents, such as NMP, at similar concentrations. However, the resulting mixtures were too fragile to manipulate and broke into pieces upon kneading. Since the nanotube/m-cresol dough was kneadable and stain-free (Fig. 7A), it must be highly cohesive and free of mechanically weak boundaries between segregated grains of CNTs (see SI Appendix, Fig. S4 and related discussion), as seen in the starting powders (SEM images in Fig. 1 A and D). As with a bread dough, the MWCNT dough could be cut into pieces and rejoined when pressed together or molded into arbitrary shapes without altering its viscoelastic properties. Fig. 7C shows a thick film cold-rolled from the dough, which was still soft and plastic (Fig. 7D) and could be reshaped by using a mold (Fig. 7E). The MWCNT doughs could be hardened to fix their shapes after heating at >200 °C to remove m-cresol. The hardened structures could then be returned to the soft dough state by absorbing m-cresol. The playdough-like processability should open up opportunities to fabricate arbitrarily shaped 3D solids of neat CNTs for a range of electronic, thermal, and energy applications.
Fig. 7.
Playdough-like MWCNTs/m-cresol solid. (A and B) Kneading a nanotube 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). (CE) The dough can be transformed into arbitrary geometries, such as a freestanding strip by cold rolling (C and D) and other arbitrary shapes defined by a mold (E).

Conclusion

Cresol-based CNT dispersions, pastes, gels, and doughs exhibit polymer-like rheological and viscoelastic properties, rendering them polymer-like processability. Cresols work generically for unfunctionalized CNTs of many types and can be conveniently removed from the final products without negatively altering their pristine properties. Cresols are abundantly produced, relatively inexpensive, and quite stable to handle at room temperature and ambient atmosphere. These advantages make cresols an ideal class of processing solvents for CNTs, especially for their mass-produced powder form. It should help to overcome many aspects of the processability problems of CNTs, which has been one of the greatest hurdles preventing their widespread industrial applications. The surprise that solvents with such simple molecular structures work so well is also likely to inspire many more discoveries about the interactions between organic molecules and graphitic surface, as well as in new material and engineering technologies based on CNTs and other graphitic nanostructures.

Materials and Methods

Materials.

CNT powders of various types, sources, and levels of purities from three vendors were tested, and all dispersed well in m-cresol and its liquid mixtures with other isomers. These included: (i) CoMoCAT MWCNTs (98% carbon content), CoMoCat SWCNTs (90% carbon content, 90% semiconducting), and double-walled CNTs [90% carbon content, made by chemical 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 lengths of 50 µm), low-density SWCNTs (TNSR; 95% purity, approximate lengths of 5–30 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), and short MWCNTs (TNSM2; 95% purity, approximate lengths of 0.5–2 µm) were all made by CVD and obtained from TimesNano.
P2 SWCNTs and MWCNTs (CoMoCat) were used for demonstrating LB assembly (Fig. 2 and SI Appendix, Fig. S6). The results of the pastes, gels, and doughs shown in the work were demonstrated with CoMoCat MWCNTs as the model material, although other types of MWCNTs work as well.
Other chemicals were purchased from Sigma-Aldrich and used as received, including m-cresol (99%), o-cresol (99%), p-cresol (98%), toluene (99.9%), phenol (>99%), DMF (99.8%), NMP (anhydrous, 99.5%), PMMA [200,000 molecular weight (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 or SWCNTs were first mixed with m-cresol by using a mortar and pestle, then sonicated in pulse mode (2 s on/2 s off cycles for a total of 1 h) by using a Qsonica Q125 sonicator rated at 125 W, equipped with a 1/4-inch standard tapered tip at 90% power. After sonication, the dispersion was subject to exhaustive high-speed centrifugation at 11,000 rpm for 1 h by using an Eppendorf 5804 desktop centrifuge (equivalent to a relative centrifugal force of 15,557 × g). The supernatant was recovered and used. Samples for making 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 a muffled furnace at 250 °C for 1 h.
All parts of the LB system (Nima Technology) were thoroughly cleaned with acetone before use. By using a glass syringe, 1 mL of m-cresol dispersion (SWCNT or MWCNT) was carefully spread onto the air–water interface. A tensiometer with a Wilhelmy plate was used to monitor surface pressure while closing the barriers. At surface pressures of ∼40 mN/m for SWCNTs and 30 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 films were annealed at 150 °C for 30 min before subsequent LB deposition to produce multilayered films.

Blade-Coating and Screen Printing.

MWCNT paste in m-cresol (100 mg/mL) was made by direct mixing using a mortar and pestle, then diluted to 40 mg/mL and hand-ground further to yield a spreadable thick paste. Glass slides were first silanized with 5 wt% methyltrichlorosilane in toluene for 10 min and then washed thoroughly by using toluene followed by acetone. Two strips of Kapton tapes were attached to the sides of the silanized glass slide as spacers to control the thickness of the coating. Approximately 0.3 mL of MWCNT paste was deposited onto the shallow trough created by the Kapton tapes. A razor blade was used to drag the paste to coat the slide. The coating was left to dry at 150 °C for 2 h. Control experiments were done by using NMP instead of m-cresol as the solvent at the same nanotube concentration. Screen-printing was 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 was then flattened by cold rolling, which turned flexible and rubbery after being air-dried (Fig. 5 and SI Appendix, Fig. S6). Curing at 150 °C for 2 h significantly hardened the piece and fixed its shape.

Three-Dimensional Printing.

MWCNTs/m-cresol gel was made by direct mixing using a mortar and pestle at a concentration of 120 mg/mL The resulting mixture was diluted to 80 mg/mL and ground further. The gel was loaded into a syringe and manually extruded from needles with diameters of 0.1 and 0.5 mm, which can be fitted onto a 3D printer (Hyrel 30M). Printed 3D structure can be removed from the glass substrate after being air-dried for 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 a mortar and pestle at a concentration of 300 mg/mL or higher. The mixture was then 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 nanotube dough does not stain the substrate, while doing so with a gel or paste would result in significant staining. A kneaded dough was sandwiched between two stainless 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-casted onto 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 with an Agilent 8453 UV/Vis spectrometer. NIR spectra were taken by using a PerkinElmer LAMBDA 1050 spectrometer. TEM images were taken with a JEOL ARM300F GrandARM transmission electron microscope. Drop-cast SWCNTs were air-dried and rinsed with water and ethanol before Raman spectroscopy measurement (WITec Alpha 300; 532-nm excitation). FTIR spectra were recorded on a PerkinElmer Instrument spectrometer (Spectrum Spotlight 300). The 1H-NMR spectra were acquired on a 400-MHz Agilent DD MR-400 NMR system. The samples were prepared by adding 100 µL of SWCNT or MWCNT dispersions in m-cresol in 1 mL of CDCl3. The nanotubes were found to be stably dispersed in the entire duration of NMR experiments. Transparency of the 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 probe equipped with a Keithley 2400 source meter. Viscoelastic and rheological properties were measured by using an Anton Paar MCR 502 rheometer using a cone-on-plate configuration. The cone has a 25-mm diameter with a 5° gap angle. Viscosity vs. concentration measurements in Fig. 2G were measured with a rotation speed of 1°/s. Yield stresses were obtained by using a Herschel–Bulkley regression included in the Anton Paar software package. Shear-thinning viscosities of Fig. 4A were measured with a linear ramping shear rate between 0.01 and 100 rad/s. Storage and loss moduli were measured simultaneously by using the same rheometer setup at an amplitude of 1%. Tensile and compression tests were done on a Bose electroforces 5500 tester. The composite films were cut into dog-bone shapes and pulled at a rate of 0.05 mm/s until failure. Only the results from samples that failed in the middle were considered. Gel and dough samples for compression tests were first molded into cylindrical shapes and carefully transferred to the tester. Compression was done at 0.005 mm/s until the sample ruptured. The slope of the first linear 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 student fellowship supported by Basic Science Research Program through the National Research 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.

Supporting Information

Appendix (PDF)

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Information & Authors

Information

Published in

Go to Proceedings of the National Academy of Sciences
Go to Proceedings of the National Academy of Sciences
Proceedings of the National Academy of Sciences
Vol. 115 | No. 22
May 29, 2018
PubMed: 29760075

Classifications

Submission history

Published online: May 14, 2018
Published in issue: May 29, 2018

Keywords

  1. cresol
  2. carbon nanotubes
  3. dough
  4. solution processing
  5. viscoelasticity

Acknowledgments

We thank Dr. J. Luo for providing some CNT samples. K.C. is a NSF Graduate Research Fellow. S.B. acknowledges a visiting student fellowship supported by Basic Science Research Program through the National Research 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.

Notes

This article is a PNAS Direct Submission.

Authors

Affiliations

Kevin Chiou
Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208
Segi Byun
Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208
Jaemyung Kim
Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208
Jiaxing Huang1 [email protected]
Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208

Notes

1
To whom correspondence should be addressed. Email: [email protected].
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.

Competing Interests

The authors declare no conflict of interest.

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    Additive-free carbon nanotube dispersions, pastes, gels, and doughs in cresols
    Proceedings of the National Academy of Sciences
    • Vol. 115
    • No. 22
    • pp. 5619-E5253

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