DISPERSIONS, PASTES, GELS AND DOUGHS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to U.S. Provisional Patent Application No. 62/549,634, that was filed August 24, 2017, the entire contents of which are hereby incorporated by reference; and to U.S. Provisional Patent Application No. 62/670,150, that was filed May 11, 2018, the entire contents of which are hereby incorporated by reference.

BACKGROUND

[0002] Carbon nanotubes have been found attractive for a number of applications due to their excellent electrical, thermal and mechanical properties. In recent years, there has been significant progress in the production of carbon nanotubes. Some types of nanotubes are already mass-manufactured in the ton scale, usually in the form of powders, which must be further processed for applications. During processing, powder materials are often used with solvents, in the forms of dispersions, pastes, gels or doughs, so that they can be made into the desirable geometries and structures. (See, M. E. Fayed, et al , Handbook of Powder Science & Technology (Springer, Boston, ed. 2nd, 1997).) To make carbon nanotubes solution processable, their surfaces usually need to be modified to make them dispersible in solvents, which tend to irreversibly alter their surface properties or introduce hard-to-remove additives. Some types of solvents have been discovered that can produce relatively high concentration dispersions of carbon nanotubes, such as super acids, ionic liquids and N-cyclohexyl-2- pyrrolidnone. (See, V. A. Davis et al , True solutions of single-walled carbon nanotubes for assembly into macroscopic materials. Nat. Nanotechnol. 4, 830-834 (2009); T. Fukushima, et al. , Ionic Liquids for Soft Functional Materials with Carbon Nanotubes. Chemistry - A European Journal 13, 5048-5058 (2007); S. D. Bergin et al , Multicomponent Solubility Parameters for Single-Walled Carbon Nanotube-Solvent Mixtures. ACS Nano 3, 2340-2350 (2009).) However, most common solvents for nanotubes, such as N-methyl-2-pyrrolidone (ΝΜΡ), NN-dimethylformamide (DMF), and 1, 2-dichrolobenzene, can only directly disperse some types of nanotubes at very low concentrations. (See, C. A. Furtado et al , Debundling and dissolution of single-walled carbon nanotubes in amide solvents. J. Am. Chem. Soc. 126, 6095-6105 (2004); J. L. Bahr, et al , Dissolution of small diameter single- wall carbon nanotubes in organic solvents? Chem. Commun. 0, 193-194 (2001).)

SUMMARY

[0003] Compositions of carbon particles, such as carbon nanoparticles, in cresol solvents are provided. Also provided are methods of making the compositions and methods of forming the compositions into carbon particle-containing films, fibers, and other three- dimensional objects.

[0004] One embodiment of a composition includes carbon nanoparticles dispersed in an organic solvent comprising one or more cresols, wherein the concentration of carbon nanoparticles in the organic solvent is at least 3 mg/ml.

[0005] One embodiment of a method of making solid object from a composition that includes carbon nanoparticles dispersed in an organic solvent comprising one or more cresols, wherein the concentration of carbon nanoparticles in the organic solvent is at least 3 mg/ml, includes forming the composition into planar layer or a non-planar shape and removing the organic solvent from the composition to form a solid object, such as a coating, a film, or a three-dimensional object.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Illustrative embodiments of the invention will hereafter be described with reference to the accompanying drawings, wherein like numerals denote like elements.

[0007] FIG. 1 depicts ^-NMR spectra showing a 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.

[0008] FIG. 2 depicts FTIR spectra showing that m-cresol does not degrade during ultrasoni cation with or without SWCNTs.

[0009] FIG. 3 shows 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.

[0010] FIG. 4 shows a dispersion and a paste, two of four continuous states of MWCNTs in m-cresol, exhibiting polymer solution like rheological and viscoelastic properties. The transitions between these states are characterized by a threshold-like increase in electrical conductivity.

[0011] FIG. 5 depicts a paste and a gel, two of four continuous states of MWCNTs in m- cresol, exhibiting polymer solution like rheological and viscoelastic properties. The transitions between these states are characterized by a threshold-like increase in viscosity.

[0012] FIG. 6 shows a gel and a dough, two of four continuous states of MWCNTs in m- cresol, exhibiting polymer solution like rheological and viscoelastic properties. The transitions between these states are characterized by a threshold-like increase in compression modulus, due to the formation and gradual densification of a 3D network of dispersed nanotubes.

[0013] FIG. 7 is schematic drawings illustrating carbon nanotubes in a powder starting materials that are heavily agglomerated and take the form of segregated particles. Top row: Well-dispersed CNTs can form continuous transition in a cresol solvent as concentration increases, starting from (1) a dilute dispersion, where nanotubes are individually separated, to (2) a thick paste, where the nanotubes form a percolated network, then (3) a self-supporting gel, in which the CNT network becomes denser and hinders the free flow, but that can still deform under external stress, and (4) a viscoelastic dough, where the even denser CNT network shows resistance to compression. The network of disaggregated nanotubes should be uniform for generating the desirable rheological (e.g. , for spreading, coating, blending with powders and extruding) and viscoelastic properties (e.g. , for kneading and molding). Bottom row: Aggregated CNTs cannot form the aforementioned states with desirable rheological and viscoelastic properties. For example, at low concentration, the dispersion would consist of clusters of entangled CNTs; while at higher concentrations, the corresponding products would have heavily segregated domains, leading to poor processability. Impact on processabilitv: A good dispersion can be processed into near-monolayer CNT thin films, while a poor dispersion would only yield a film made of clusters of aggregated tubes. A good paste can spread uniformly on substrates and yield a continuous CNT coating, while a poor paste would result in a discontinuous film with segregated domains. A good gel allows 3D printing using fine nozzles, while a poor gel would clog the nozzle due to the segregated blobs. A good dough is highly cohesive and can be kneaded and rolled while maintaining its continuity, while a poor dough has many weak boundaries between the segregated domains, and would be too fragile to handle. [0014] FIG. 8 shows an isotherm obtained during compression of monolayers of MWCNTs from dilute dispersion by LB assembly. Isothermal compression of the monolayer increases its surface pressure, indicative of higher nanotube density.

[0015] FIG. 9 depicts monolayers of MWCNTs from dilute dispersion by LB assembly. The SEM image shows the film as a continuous, uniform, paper-like monolayer.

[0016] FIG. 10 shows monolayers of MWCNTs from dilute dispersion by LB assembly. The SEM image shows the film as a continuous, uniform, paper-like monolayer made of a network of nanotubes.

[0017] FIG. 11 depicts monolayers of MWCNTs from dilute dispersion by LB assembly. Sheet-resistances and the corresponding transparencies of MWCNT layers on PET substrate made by repetitive dip-coating are depicted.

[0018] FIG. 12 shows MWCNT paste exhibits shear-thinning behavior at all the concentrations tested, which is typical for a polymer dissolved in good solvents. From II to VI, the concentrations are increased from 10, 20, 30, 40, and 50 mg/ml. Pure m-cresol (I) does not exhibit this behavior.

[0019] FIG. 13 shows the yield stress of the MWCNT paste increases relatively slowly as the nanotube concentration increases, until it reaches the range of the gel state (V).

[0020] FIG. 14 shows continuous and patterned blade coating of MWCNTs from the thick paste. Blade-coating creates a continuous and uniform nanotube film on glass after drying, which is free of cracks (SEM image) that are typically seen for coatings made with other solvents, such as NMP (see Figure S7).

[0021] FIG. 15 shows MWCNT gels have increasingly solid-like behavior as the nanotube concentration increases, based on the results of storage moduli.

[0022] FIG. 16 shows gels have increasingly solid-like behavior as the nanotube concentration increases, based on the results of loss moduli measurements.

DETAILED DESCRIPTION

[0023] Compositions of carbon particles, such as carbon nanoparticles, in cresol solvents are provided. Also provided are methods of making the compositions and methods of forming the compositions into carbon particle-containing films, fibers, and other three- dimensional objects.

[0024] The cresols may be the only solvent in the composition or may be mixed with additional organic solvents to provide a cresol solvent mixture. Depending upon the concentration of the carbon particles in the solvents, the compositions may take on the forms of a liquid dispersion, a paste, a gel, or a dough. The cresols, along with any other solvents present, can be removed from the compositions after processing. The use of cresols is advantageous because they do not alter the surface properties of the carbon particles. As such, the compositions offer versatile processability for bulk quantities of carbon particles, without negatively altering their pristine properties. This makes compositions readily usable by existing and emerging materials processing techniques used in industry, including Langmuir- Blodgett assembly, spin coating, drop casting, doctor blading, screen-printing, 3D printing, molding and rolling to create desirable forms or composites for a wide array of applications.

[0025] The compositions include carbon particles (for example, carbon nanoparticles) dispersed in a solvent that includes one or more cresols. The cresols, which are a group of methylphenols, include the isomers m-cresol, o-cresol, and p-cresol, and combinations of two of more thereof. The carbon particles can include carbon nanotubes, carbon black particles, graphite particles, fullerenes, single sheet graphene particles (e.g., graphene flakes), reduced graphene oxide (r-GO) particles (e.g., r-GO flakes), or a combination of two or more thereof. The carbon nanoparticles are so called because they have at least one dimension (i.e., length, width, or height) that is 1000 nm or less; depending upon the shape of the nanoparticles, they may have two or three dimensions that are 1000 nm or less. Some embodiments of the nanoparticles have length, width, and/or height dimensions that are 500 nm or less, some have length, width and/or height dimensions that are 100 nm or less, and some have length, width, and/or height dimensions that are 10 nm or less. The carbon nanotubes can be single- walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs) (i.e., nanotubes having an averaged number of walls great than 1), or a mixture thereof. The carbon nanotubes can be semiconducting, metallic, or a combination of semiconducting and metallic carbon nanotubes. The carbon nanotubes may have functional groups, such as carboxylic acid functionalities, attached to their surfaces, or they may be free of surface functionalities. Carbon nanotubes having a wide range of lengths can be dispersed in the cresols. The lengths of the carbon nanotubes that can be dispersed are not particularly limited. By way of illustration, carbon nanotubes having lengths in the range from 5 nm to 10,000 nm can be used. The diameters of the carbon nanotubes that can be dispersed are not particularly limited. By way of illustration, carbon nanotubes having diameters in the range from about 0.8 nm to 50 nm can be used.

[0026] Organic solvents that can be mixed with the one or more cresols to form a cresol solvent mixture include solvents that are miscible with the cresols, do not undergo hydrogen bonding, and have a dielectric constant of 10 or lower. These include, for example, hexane, cyclohexane, chloroform, toluene, other saturated hydrocarbons and their isomers (for example, pentane, heptane, decane, dodecane), xylenes, styrenes, dichloromethane, 1,4- butadiene, and mixtures of two or more thereof. In the cresol solvent mixtures, these miscible, non-hydrogen bonding solvents may be the majority solvent, making up more than 50 vol. % of the cresol solvent mixture. This includes embodiments of the cresol solvent mixtures in which the additional organic solvents make up at least 70 vol. %, at least 90 vol. %, and at least 99 vol. % of the cresol solvent mixture. In such embodiments, the cresol solvents act as dispersing agents for the carbon particles in relatively dilute carbon nanoparticle dispersions. For example, these solvent mixtures can be used to form dispersions having a carbon nanoparticle concentration of less than 3 mg carbon nanoparticle/ml of organic solvent, less than 1 mg carbon nanoparticle/ml of organic solvent, less than 1 mg carbon nanoparticle/ml of organic solvent, or less than 0.5 mg carbon nanoparticle/ml of organic solvent.

[0027] Other organic solvents that can be mixed with the one or more cresols and, if present, the other organic solvents recited immediately above, include phenolic compounds (that is - compounds that have a phenol group), such as phenol bisphenols and bisphenol derivatives (e.g., bisphenol-A, bisphenol-S, bisphenol-AF, bisphenol-F, bisphenol-M, bisphenol B, bisphenol C), phloroglucinol, resorcinol, 4-chloro-3-methylphenol, biphenol, naphthol isomers, naphthlenediol isomers, naphthlenetriol isomers, xylenols, other halogenated phenols and isomers thereof (such as, 4-bromophenol, 3,4-fluorophenol and 4- iodophenol), and mixtures of two or more thereof. These phenolic solvents may also be present as the majority solvent (> 50 vol.%) in the cresol solvent mixture. However, they can also be used at smaller concentrations. By way of illustration, some embodiments of the cresol solvent mixtures contain less than 50 vol. % of the phenolic solvents, less than 30 vol. % of the phenolic solvents, less than 10 vol. % of the phenolic solvents, less than 1 vol. % of the phenolic solvents, or less than 0.1 vol. % of the phenolic solvents. Phenolic solvent - cresol solvent mixtures that become liquid upon mixing or that can be liquified by heating, can be used to form carbon nanoparticle dispersions, pastes, gels, and doughs by adjusting the cresol solvent mixture content of the composition. Other embodiments of the cresol solvent mixtures contain at least 50 vol. % of the phenolic solvents, at least 70 vol. % of the phenolic solvents, at least 90 vol. % of the phenolic solvents, or at least 99 vol. % of the phenolic solvents.

[0028] Some embodiments of the compositions are free of polymers, including polymer resins, organic monomers and oligomers that are precursors of organic polymers (i.e., molecules that react to form polymers), and initiators (e.g., free radical initiators, such as peroxides) that induce the polymerization and of precursors of organic polymers, the crosslinking of polymers in the composition, or the crosslinking of the carbon nanoparticles themselves. By way of illustration only, polymers, polymer resins, and precursors of polymers that have been used as binders in other carbon nanoparticle composites, but may be excluded from the present compositions, include polyacrylates, polyamides, polyalkylene oxides, polyurethanes, epoxide polymers, cellulose and their precursors. In addition, embodiments of the compositions may be free of any chemicals that act to increase the dispersibility of the carbon particles in the cresols, the cresol solvent mixtures and/or in water. Such chemicals include surfactants molecules and other dispersants. However, the compositions may contain other components, including components that are present as impurities in the starting materials. For example, some embodiments of the composition may include phenolic molecules that are present as impurities in some commercially available cresols. If the other components are present as impurities they are typically present at small concentrations - typically 1 wt. % or less, including 0.1 wt. % or less. However, phenolic compounds need not be present only as impurities; they can be added in selected amounts to provide solvent mixture.

[0029] Some embodiments of the compositions contain only the carbon particles, the cresols, and the miscible organic non-hydrogen bonding solvents and/or phenolic solvents. Some embodiments of the compositions contain only the carbon particles, the cresols and the phenolic solvents. Some embodiments of the compositions contain only the carbon particles and the cresols. [0030] In other embodiments of the compositions, chemical components other than carbon particles, cresols, miscible organic non-hydrogen bonding solvents, and phenolic compounds can be present. By way of illustration only, in various embodiments of the compositions, these additional chemical components may account for up to 10 wt.% of the composition. This includes embodiment of the compositions in which these additional chemical components account for up to 5 wt. % of the composition, up to 2 wt.% of the composition, up to 1 wt.% of the composition, or up to 0.1 wt. % of the composition.

[0031] The compositions can be made by combining the carbon particles (e.g., for example, a carbon nanotube sample, which may include entangled and agglomerated carbon nanotubes), with the one of more cresols of the cresol solvent mixture followed by mixing. The degree and type of mixing will depend on the desired form of the composition. For example, a fine dispersion of carbon nanoparticles in the cresol solvents or solvent mixtures can be formed by mixing a carbon nanoparticle powder in the solvent using ultrasonication. Alternatively, thicker compositions, such as pastes, gels, and doughs, can be made by grinding a carbon nanoparticle powder in the cresols or the cresol solvent mixtures using, for example, a mortar and pestle.

[0032] Low viscosity, liquid dispersions of the carbon nanotubes can be formed by using relatively low concentrations of the carbon nanotubes. The dispersions may be characterized by low viscosities and low electrical conductivities, which reflect the lack of a percolating carbon nanotube network. For example, some embodiments of the carbon nanotube dispersions have a viscosity near that of one or more cresols (for example, differing from the viscosity of the cresols or cresol mixture by no more than a factor of 0.05, including by no more than a factor of 0.01) at 298K and a conductance of 10 ^"4 S or lower. This includes carbon nanotube dispersions having a conductance of 10 ^"5 S or lower. Typical carbon nanotube concentrations in the dispersions include concentrations in the range from near zero (> 0; for example, about 0.01 or 0.1 mg/ml) up to about 3 mg/ml, although this range can vary depending on the types and morphologies of the carbon nanotubes being used. As used herein, mg/ml refers to the mg of carbon particles per ml of the solvent being used; thus, if the solvent contains only cresols, mg/ml refers to mg of carbon nanoparticles per ml of cresol. In some embodiments of the dispersions the carbon nanotube concentration is less than 1 mg/ml. Dispersions of other carbon particles can also be formed, although the concentration ranges can vary depending on the types and morphologies of the carbon particles being used.

[0033] The carbon nanotube dispersions (or dispersions of other carbon nanoparticles) can be formed into thin films, including monolayer thin films. Langmuir-Blodgett assembly is an example of a technique that can be used to make monolayer thin films, as illustrated in the Example. The sheet resistance and optical transparency of the thin films can be fine-tuned by precisely controlling the number of deposited layers, as well as the carbon particle packing density within each monolayer. For example, thin optically transparent films of the carbon nanotubes having a sheet resistance of at least 70 kQ /sq, including at least 90 kQ /sq, and an optical transparency of at least 60%, including at least 70%, can be formed.

[0034] At higher concentrations, carbon nanotube pastes (or pastes of other carbon particles) can be formed. The transition from a dispersion to a paste can be characterized by a marked increase in the electrical conductivity of the composition, which can be attributed to the formation of a percolating carbon network. The pastes are also more viscous than the dispersions and can exhibit shear thinning behavior. For example, some embodiments of the carbon nanotube pastes have a viscosity in the range up to about 800 Pa*S 298K and a conductance of 10 ^"3 S or higher. This includes carbon nanotube pastes having a conductance of 10 ^"2 S or higher. Typical carbon nanotube concentrations in the pastes include concentrations in the range from about 3 mg/ml up to about 40 mg/ml, although this range can vary depending on the types and morphologies of the carbon nanotubes being used. Pastes of other carbon particles can also be formed, although the concentrations ranges can vary depending on the types and morphologies of the carbon particles being used.

[0035] Coatings of the carbon nanotube pastes (or pastes of other carbon particles) can be applied to a surface via, for example, brushing, doctor blading, inkjet printing, or screen- printing. The coatings can then be heated to drive off the cresols and other solvents, leaving a film composed of a high-density network of nanoparticles. The coatings and films can be patterned or unpatterned (e.g., continuous). The pastes can also be made into polymer composites by mixing polymers into the pastes. Polymers that can be incorporated into the paste include poly(methyl methacrylate) (PMMA), nylons, polyethyelene terephthalate (PET), polystyrene, and phenolic resins. Other materials that can be incorporated into the pastes to form composite materials include metals and oxides, such as metal oxides and ceramics. The polymers and other materials can be incorporated into the pastes by pre- forming the paste and then combining the paste with the other particles by, for example, shear-mixing, co-grinding, and/or ball-milling. By way of illustration a polymeric composite can be made by combining particles of a polymer (e.g., a polymer powder) with the paste to form a composite of the polymer and the paste. If the polymer is curable, the polymer in the paste can be cured to form a crosslinked polymeric matrix in which the carbon nanotubes (or other carbon particles) are dispersed. Alternatively, a polymeric composite can be formed by combining the paste with curable polymer precursors, such as organic monomers or oligomers and then curing the precursors to form a polymeric matrix.

[0036] The polymer composites can be formed into films that can be rolled into flexible and highly plastic sheets, or made into other three-dimensional objects. Upon thermal curing, the sheets or objects can be hardened due to the removal of the cresols and, if present, other solvents. The inclusion of carbon nanotubes in the polymers can provide an increased Young's modulus. The loading of carbon nanoparticles in the polymer composites and the hardened objects made therefrom can be varied based on their intended applications. By way of illustration, embodiments of the polymer composites and/or hardened objects can have a carbon nanotube loading from 0.2 wt. %, to at least 30 wt. %, based on the total weight of the carbon nanotubes and the polymer. Some embodiments of the hardened films made from the carbon nanotube pastes can sustain a tensile strain of 500 % or more, including a tensile strain of 800% or more.

[0037] At still higher concentrations, carbon nanotube gels (or gels of other carbon particles) can be formed. The transition from a paste to a gel can be marked by a sharp increase in the viscosity of the composition. For example, some embodiments of the carbon nanotube gels have a viscosity of at least 1000 Pa*S 298K. This includes carbon nanotube gels having a viscosity of at least 1200 Ps*S and at least 1400 Pa*S at 298 K. Typical carbon nanotube concentrations in the gels include concentrations in the range from about 40 mg/ml up to about 100 mg/ml, although this range can vary depending on the types and

morphologies of the carbon nanotubes being used. Gels of other carbon particles can also be formed, although the concentration ranges can vary depending on the types and morphologies of the carbon particles being used. The gels are free-standing and can be extruded into self- supporting fibers, which maintain their shape after removing the cresol solvents, to form stiff solid objects via 3D (i.e., extrusion) printing or extrusion. [0038] At even higher concentrations, carbon nanotube doughs (or doughs of other carbon particles) can be formed. The transition from a gel to a dough is characterized by an increase in the compression modulus. The doughs are generally stiff, viscoelastic materials. For example, some embodiments of the carbon nanotube doughs have a stiffness of 0.125 kPa or higher. This includes carbon nanotube doughs having a stiffness of 0.15 or higher and 0.2 or higher. Typical carbon nanotube concentrations in the dough include concentrations in the range from about 100 mg/ml up to about 140 mg/ml - or even higher, although this range can vary depending on the types and morphologies of the carbon nanotubes being used. Doughs of other carbon particles can also be formed, although the concentrations range can vary depending on the types and morphologies of the carbon particles being used. The doughs are viscoelastic and highly cohesive. They can be kneaded or rolled, cut into pieces and recombined when pressed together, and/or molded into arbitrary shapes and obj ects. For example, the high carbon nanoparticle doughs can be used to fabricate pure carbon nanoparticle devices, sculpt irregular shapes, and form flexible thin films. By way of illustration, embodiments of the doughs can be placed into a mold and molded into macroscopic three-dimensional objects, including objects having length, width, and/or height dimensions of at least 1 mm, including one or more dimensions of at least 1 cm. Such components include, for example, electronic components or mechanical parts. The cresols can be removed from the doughs before or after the dough is released from the mold.

[0039] Unless otherwise indicated, quantitative values disclosed herein that are temperature and/or pressure dependent, refer to those values as measured at room

temperature (e.g., 23 °C) and atmospheric pressure (e.g., 1 atm).

[0040] Methods for measuring the viscosity, conductance, and stiffness (compression modulus) of the carbon nanoparticle compositions are provided in the Example below.

EXAMPLE

[0041] This example demonstrates the use of cresols as generic solvents for processing various kinds of carbon nanotubes, and further demonstrates that they can also be easily removed afterwards by washing or evaporation. Most strikingly, cresols can process carbon nanotubes over a very broad range of concentrations, reaching the level of tens of weight percent. As the concentration of carbon nanotubes increases, a continuous transition of four distinctive states ~ namely dilute dispersions, thick pastes, freestanding gels and viscoelastic doughs ~ is observed, all of which are readily usable by a wide array of material processing techniques.

[0042] The results of the studies described in this example demonstrate that powders of both single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) can be well dispersed in m-cresol after sonication or grinding without the need for any surface functionalization. As verified by scanning electron microscopy (SEM) images, initially the nanotubes were heavily agglomerated and entangled in the powders, but they became well separated after casting from the corresponding m-cresol dispersions. These results indicate that the interaction between m-cresol and the surface of the carbon nanotubes must be sufficiently strong to allow the agglomerated nanotubes to disperse. Proton nuclear magnetic resonance ^H-NMR) spectroscopy was employed to probe the nature of such interaction. As shown in FIG. 1, 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 was a result of increased electron density on the phenolic hydroxyl proton, indicating charge-transfer interaction with the nanotubes.

[0043] Sonicating or grinding carbon nanotubes 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. 2 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 cast from m-cresol dispersion do not show obvious differences (FIG. 3), suggesting that the pristine SWCNTs are not damaged during processing. The absence of new bands between 400 to 1000 cm ^"1 , where m-cresol shows strong Raman signals, indicates that m-cresol have been successfully removed.

[0044] Among the three isomers of cresols, m-cresol is a liquid at room temperature; therefore, it is used for most of the experiments in this example. While o-cresol and />-cresol are solid at room temperature, they can also process carbon nanotubes at a molten state, or when blended with m-cresol at room temperature. This indicates 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 carbon nanotubes. Indeed, UV-Vis-NIR spectra of SWCNTs dispersed in a ternary isomer mixture of cresol show characteristic bands of well dispersed nanotubes, which is confirmed by TEM studies. Industrial grade cresols often contain phenolic impurities, and it is found that adding an additional 10 wt.% of phenol into the ternary mixture would not negatively affect the stability of the nanotube dispersions. The impurity tolerance and ease of removal make cresols the ideal type of non-reactive solvents for the solution processing of carbon nanotubes. In the sections below, cresol solvents are demonstrated to render carbon nanotubes with polymer-like rheological and viscoelastic properties and processability, making them immediately usable by already available material processing techniques to create desirable structures, form factors, and composites.

[0045] Four states of MWCNTs in /n-cresol. m-cresol alone can disperse and process carbon nanotubes up to tens of weight percent. Since MWCNTs are the more common type of mass-produced carbon nanotubes, and are much more affordable and available, they were chosen as the model material for most of the work in the following sections unless otherwise mentioned. Dilute dispersions were typically made by sonication, and could remain stable for at least many months. The other higher concentration states were typically made by grinding. Transitions between the four states were 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 around 3 mg/ml (FIG. 4), 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 MWCNTs 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 an inability to free flow around 40-50 mg/ml, after which the viscosity increased significantly (FIG. 5). At concentrations above 100 mg/ml, a viscoelastic, kneadable play dough-like material was obtained, which was highly cohesive and exhibited resistance to compression, as characterized by a rapidly increased compression modulus (FIG. 6).

[0046] The continuous transition between these four highly processable, polymer solution like states indicates 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 of agglomerated nanotubes, resulting in poor processability (see schematic illustrations in FIG. 7). These four states were observed for all the carbon nanotubes 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 carbon nanotubes for existing and new applications.

[0047] Dilute dispersion and Langmuir-Blodgett assembly. 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 the water surface, which is challenging for additive-based carbon nanotube dispersions. Since m-cresol can gradually dissolve in water, it dissipated into the sub-phase after spreading the nanotubes on the water surface, leaving clean nanotubes on the water surface. The water-supported monolayers could be further densified by closing two barriers, yielding a positive surface pressure (FIG. 8), which could then be transferred to a substrate by dip-coating. FIG. 9 is a low magnification SEM overview of a MWCNT film on a glass slide collected at a surface pressure of

30 mN/m, which is continuous, uniform and cohesive. Since many of the starting MWCNTs were curled, twisted or even kinked, and could not lay flat, the near-monolayer thickness of the film (FIG. 10) also confirmed that the heavily agglomerated MWCNTs in the starting powders indeed had 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.

[0048] Transferring the nanotube monolayer onto soft plastic substrates such as poly(ethylene terephthalate) formed a flexible transparent conductor. 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 (FIG. 11). For example, a sheet resistance of 90 kQ /sq was obtained at 72% of optical transparency. Using m-cresol as a processing medium did not damage the surface of the nanotubes nor leave hard-to-remove residues, and resulted in satisfying conductivity of the LB films without the need for extensive further annealing steps. Similarly, LB assembly of SWCNT monolayers was achieved.

[0049] 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. 12), with yield stress in the range of 1 to 10 Pa (FIG. 13), making it suitable for use by brushing or painting. In order to make a continuous film using these techniques, the paste should 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 should be interconnected throughout the paste without extensively segregated domains (FIG. 7). The paste was applied by blade coating. The oven-dried coating on glass was continuous and free of cracks over the entire area. SEM images show that it was made of an interwoven, continuous and high-density network of nanotubes (FIG. 14). Similar to blade coating, industrial screen-printing can directly use the MWCNT paste to generate functional patterns. Blade coating is commonly used to make electrodes for energy storage devices from slurries, which often use carbon nanotubes as a conductive binder for active materials. 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 carbon nanotubes.

[0050] MWCNT pastes for polymer composites. Polymer nanocomposite is another area that uses a very large scale of carbon nanotubes. 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 solvent for many commodity polymers such as poly(methyl methacrylate) (PMMA), nylons, polyethyelene terephthalate, polystyrene, and phenolic resins, 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. A proof-of-concept experiment was conducted where PMMA powders were directly mixed with the paste by mortar and pestle. The product was rolled into a flexible and highly plastic sheet, which sustained over 800% of tensile strain. Upon thermal curing at 150 °C, the sheet hardened 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 to a similarly processed PMMA sheet (1.17 GPa). SEM observation confirmed that the MWCNTs had been finely dispersed in the PMMA matrix. Such a soft-hard transition is critical for industrial forming techniques, which turn materials into desirable geometries and form factors. The additive-free carbon nanotube pastes in cresols can be useful for accelerating the development and manufacturing of polymer nanocomposites.

[0051] 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 increases, the gel became more solid-like with an increased storage modulus (FIG. 15). The loss modulus increased more slowly than the storage modulus, rendering the gel a sufficient level of liquid character for an extrusion type of processing (FIG. 16).

Therefore, the MWCNT gel could deform and reconnect easily. A MWCNT gel was extruded to form self-supporting fibers through a 0.5 mm diameter needle. Since the gel was cohesive, extrusion could be continuously operated even with finer needles (e.g., 0.1 mm diameter). 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 was immediately usable for programmed and automated printing. As a proof-of-concept, a cup-shaped structure was 3D printed from the gel. 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 isotopically, but maintained its shape, resulting in a stiff solid object that could be further handled.

[0052] MWCNT dough. The last state of MWCNTs/m-cresol composition was a viscoelastic dough (>100 mg/ml), which could be kneaded or rolled without fracture. In contrast to a gel, when kneaded on paper, the dough did not leave any stain mark. This was due to the strong attraction between the nanotubes in the densely woven 3D network, which prevented them from leaving residues on the paper. Since the nanotube/m-cresol dough was highly kneadable and stain-free, it must be highly cohesive and free of mechanically weak boundaries between segregated grains of carbon nanotubes. 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. A thick film was cold-rolled from the dough, which was still soft and plastic, and could be reshaped using a mold. The

MWCNTs doughs could be hardened to fix their shapes after heating at above 200 °C to remove the m-cresol. The hardened structures could then be returned to the soft dough state by absorbing m-cresol. This playdough-like processability is useful for the fabrication of arbitrarily shaped 3D solids of neat carbon nanotubes for a range of electronic, thermal and energy applications.

Materials and methods

[0053] Materials. Carbon nanotube 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 include: (1) CoMoCAT® MWCNTs (98% carbon content), CoMoCat® SWCNTs (90% carbon content, 90% semi-conducting), and double- walled carbon nanotubes (90% carbon content, made by chemical vapor deposition (CVD)) were obtained from Sigma- Aldrich; (2) SWCNTs (P2, 90% purity) and carboxylic functionalized SWCNTs (P3, 90% purity) were made by arc-discharge and obtained from Carbon Solution Inc.; (3) Graphitized MWCNTs (TNGM2, 99.9% purity, approximate lengths of 50 μηι), low density SWCNTs (TNSR, 95% purity, approximate lengths of 5- 30 μηι, 0.027 g/cm ³ ), high density SWCNTs (TNST, 95% purity, 0.14 g/cm ³ ), short

SWCNTs (TNSSR, 95% purity, approximate lengths of 1-3 μιη), and short MWCNTs (TNSM2, 95% purity, approximate lengths of 0.5-2 μηι) were all made by CVD and obtained from TimesNano.

[0054] P2 SWCNTs and MWCNTs (CoMoCat®) were used for demonstrating LB assembly. The results of the pastes, gels and doughs were demonstrated with CoMoCat® MWCNTs as the model material, although other types of MWCNTs work as well.

[0055] Other chemicals were purchased from Sigma- Aldrich and used as received, including m-cresol (99%), o-cresol (99%), / cresol (98%), toluene (99.9%), phenol (>99%), N,N-dimethylformamide (DMF) (99.8%), N-methyl-2-pyrrolidone (NMP) (anhydrous, 99.5%), poly(methylmethacrylate) (PMMA, 200,000 Mw) and methyltrichlorosilane (99 %). Ternary isomer mixture of cresol (>99 wt.%, 1 : 1: 1 ratio) was purchased from Fisher Scientific and used as received.

[0056] LB assembly and transparent conductive thin films. Powders of MWCNTs or SWCNTs were first mixed with m-cresol using a mortar and pestle, then sonicated in pulse mode (2s on/ 2s off cycles for a total of 1 hour) 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 11000 rpm for 1 hour using an Eppendorf 5804 desktop centrifuge. The supernatant was recovered and used. Samples for making transparent conductors were first purified by a non-oxidative route, including washing in 3M HC1 at 65 °C for 4 hours, followed by baking in a muffled furnace at 250 °C for 1 hour.

[0057] All parts of the LB system (Nima Technology) were thoroughly cleaned with acetone before use. 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 the surface pressure while closing the barriers. At surface pressures of around 40 mN/m for SWCNTs and 30 mN/m for MWCNTs, monolayers 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 multi- layered films.

[0058] Blade-coating and screen printing. MWCNTs 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 minutes, and then washed thoroughly 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. About 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 hours. Control experiments were done using NMP instead of m-cresol as the solvent at the same nanotube concentration. Screen-printing was done on paper through a mask using a paste of 10 mg/ml.

[0059] 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) using a mortar and pestle for 10 minutes. The composite was then flattened by cold rolling, which turned flexible and rubbery after being air-dried. Curing at 150 °C for 2 hours significantly hardened the piece and fixed its shape.

[0060] 3D 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 could be fitted onto a 3D printer (Hyrel 30M). The printed 3D structure could be removed from the glass substrate after being air-dried for 12 hours, and could be further hardened by baking to remove m-cresol.

[0061] MWCNTs dough. MWCNT/m-cresol dough was made by direct 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 a final thickness of 200 μιτι, which could be cut into various shapes with a razor blade or cookie cutters.

[0062] 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 using a Perkin Elmer 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). 1H-NMR spectra were acquired on a 400 MHz Agilent DD MR-400 NMR system. The samples were prepared by adding 100 μΐ of SWCNT or MWCNT dispersions in m-cresol in 1 ml of CDCh. The nanotubes were found to be stably dispersed during the entire duration of the NMR experiments. The transparency of the LB films was measured using an Agilent 8453 UV/Vis spectrometer. The sheet resistance of the films was obtained using an in-line four-point probe equipped with a Keithley 2400 source meter. Viscoelastic and rheological properties were measured using an Anton Paar MCR 502 rheometer using a cone-on-plate configuration. The cone had a 25 mm diameter with a 5° gap angle. Viscosity versus concentration measurements were measured with a rotation speed of 1 s. Yield-stresses were obtained using a Herschel-Bulkley regression included in the Anton Paar software package. Shear-thinning viscosities were measured with a linear ramping shear rate between 0.01 to 100 rad/s. Storage and loss moduli were measured simultaneously using the same rheometer setup at an amplitude of 1%. Tensile and compression tests were done on a Bose electroforce 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.

[0063] The word "illustrative" is used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as "illustrative" is not necessarily to be construed as preferred or advantageous over other aspects or designs. Further, for the purposes of this disclosure and unless otherwise specified, "a" or "an" means "one or more."

[0064] The foregoing description of illustrative embodiments of the invention has been presented for purposes of illustration and of description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and as practical applications of the invention to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.