GB2515004A - Carbon nanotube derivatives - Google Patents

Abstract

A water dispersible carbon nanotube derivative comprises: (a) a carbon nanotube structure comprised of a carbon nanotube framework having first ionic groups covalently bonded thereto; and(b) a water-dispersibility enhancing agent which is an ionic form of an organic acid substituted with an amino group and having a maximum of fourteen carbon atoms, the ionic form providing second ionic groups of opposite charge to said first ionic groups. The derivative comprises a carbon nanotube structure ionically bonded to the water-dispersibility enhancing agent by the first and second ionic groups. Preferably, the organic acid is an amino acid, such as aspartic acid, arginine or taurine. The derivative may be used in an aqueous curable phenolic resin, which may be applied to a non-woven fibrous substrate.

Description

CARBON NANOTUBE DERIVATIVES

The present invention relates to carbon nanotubes (herein also referred to as CNT5) and more particularly to derivatives thereof which are readily dispersible or soluble under aqueous conditions. The invention also relates to methods of producing such derivatives and their use in imparting desirable properties to resin compositions, particularly aqueous based resins.

Carbon nanotubes are an allotropic form of carbon which have been the subject of much research since their discovery in 1991. It is well known that carbon nanotubes have a range of interesting mechanical, electrical and thermal properties which render them useful in a variety of applications. Thus, purely by way of example, carbon nanotubes have applications in electrodes for fuel cells, enhanced electromagnetic shielding in aerospace applications composites, field-emission displays, sensors, scanning probe microscopy tips, drug delivery systems and hydrogen storage.

There is however a particular problem with carbon nanotubes in that they are relatively cohesive and tend to form agglomerates. Thus carbon nanotubes as supplied from manufacturers thereof comprise such agglomerates and for most applications the carbon nanotubes must be subjected to deagglomeration and dispersion in a liquid so that they may be further processed for incorporation into a final product. Generally this deagglomeration and dispersion step is effected by subjecting the carbon nanotube to ultrasonication. This obviously represents an additional step in the overall process as well as capital outlay for the ultrasonic equipment and associated energy costs for operation thereof.

With a view to avoiding the need for ultrasonically dispersing carbon nanotubes, research work has been directed towards the production of carbon nanotubes derivatives which (because of the properties of the derivatives) can be much more easily dispersed (or are self-dispersing") in liquids.

Chemical functionalisation of nanotube sidewalls by the covalent attachment of carboxylic acid groups to the CNT surface as been shown to increase their solubility in water. However, this often involves the use of harsh oxidizing agents such as oleum, which results in the shortening of the CNTs (Soluble Ultra-Short SWNTs, Journal of the American Chemical Society, 2006, 128, 10568-10571). Another much-utilized strategy is to attach water-soluble polymers to the nanotube-bound carboxylic acid groups, which have been introduced by oxidative acid treatment (High Aqueous Solubility of Functionalised SWNTs, Langmuir, 2004, 20, 4777-4778).

A further covalent tunctionalisation technique has involved the linking of lysine residues to the carbon nanotube via amide linkages. In this prior technique, multi-walled carbon nanotubes were treated with thionyl chloride to form nanotube derivates having acyl chloride groups. These derivatives were then reacted with lysine in the presence of pyridine so that the acyl chloride groups underwent an amidation reaction with the amino groups on lysine so as covalently to attach the amino acid to the CNT surface.

Covalent functionalisation can introduce defects to the CNT structure, affecting their desirable mechanical and electronic properties. Mild oxidation however introduces carboxylic acid groups at the initial defect sites already present therefore tends not to add significantly to the concentration of detects unless extensive and prolonged oxidation is used. (Effect of Chemical Oxidation on the Structure of SWNTs, U. Phys. Chem. B, 2003, 107, 3712-371 8) Surfactants (High Weight Fraction Surfactant Solubilisation of SWNTs in Water, Nano Letters, 2003, 3, 269-273), polymers (Diameter Selective Dispersion of SWNTs using a Water-Soluble, Biocompatible Polymer, Chemical Communications, 2006, 1425-1427 and The Preparation of Highly Water-Soluble MWNTs by Irreversible Noncovalent Functionalisation with a Pyrene-Carrying Polymer, Nanotechnology, 2008, 19, 21 5604)and biomolecules such as DNA(DNA Dissolves SWNTs in Water, Chemistry Letters, 2003, 32, 456-457) have been used to disperse CNTs by non-covalent functionalisation in aqueous solutions. Non-covalent functionalisation has the advantage that changes to the intrinsic properties of CNTs are minimized, however a physical barrier is created between the nanotubes and their environment.

Non-covalent methods of functionalisation based on the physical adsorption of polymers or surfactants have been successful at dispersing CNTs in aqueous solution and have the advantage that the electronic structure of the CNT is left intact (The Role of Surfactants in Dispersion of Carbon nanotubes, Advances in Colloid and Interface Science, 2006, 128-130, 37-46, Advances Towards Bioapplications of Carbon Nanotubes, 2004. 14, 527-541).

However the use of high concentrations of surfactants and polymers can be problematic in further processing of the CNTs e.g. in making composite materials as the surfactants are difficult to remove. Also the surfactants tend to cover / coat the surface of the nanotube therefore making it inaccessible to other functional groups or chemistry. Using surfactants the concentration of the nanotubes dispersed is relatively low.

It is particularly desirable to provide carbon nanotubes that are readily dispersible in aqueous media without the use of ultrasonication techniques. Such water-dispersible carbon nanotubes obviously facilitate overall production processes involving incorporation of carbon nanotubes in final products under aqueous conditions. Thus, for example, such processes might involve simple" application of an aqueous dispersion of carbon nanotubes to a substrate with subsequent evaporation of the water to leave the carbon nanotubes present on the substrate, the process being facilited by virtue of the water-dispersiblity of the carbon nanotubes without the need for ultrasonication.

There is a particular need to provide water-dispersible carbon nanotube derivatives for use in aqueous based resin systems, for example aqueous based phenolic resin systems that are heat curable. Such aqueous based, curable phenolic resin systems can be used as impregnants for non-woven materials, the material subsequently being treated to effect cure of the resin to provide structure integrity for the material. Examples of fibres constituting such non-woven materials may (without limitation) be carbon fibres, glass fibres, metal-coating carbon fibres of any combination thereof. Clearly water-dispersible carbon nanotubes would facilitate preparation of products incorporating such cured resins and further incorporating carbon nanotubes (albeit in derivative form) for providing desirable properties since the need for an ultrasonication step would be avoided.

According to first aspect of the present invention there is provided a water dispersible carbon nanotube derivative comprising (a) a carbon nanotube structure comprised of a carbon nanotube framework having first ionic groups covalently bonded thereto; and (b) a water-dispersibility enhancing agent which is an ionic form of an organic acid substituted with an amino group and having a maximum of fourteen carbon atoms, said ionic form providing second ionic groups of opposite charge to said first ionic groups, whereby said derivative comprises said carbon nanotube structure ionically bonded to the water-dispersibility enhancing agent by the first and second ionic groups.

According to a second aspect of the present invention there is provided a method of preparing a water-dispersible carbon nanotube derivative comprising the steps of: (i) providing carbon nanotube structures comprised of a carbon nanotube framework and first ionic groups covalently bonded to the framework; and (ii) treating said carbon nanotube structures under aqueous conditions with a water-dispersibility enhancing agent which, under the aqueous conditions, is an ionic form of an organic acid substituted with an amino group and having a maximum of fourteen carbon atoms, said ionic form providing second ionic groups of opposite charge to said first ionic groups thereby to form the carbon nanotube derivative in which said agent is ionically bonded to the carbon nanotube structure.

The acid group of the organic acid used in the invention may, for example, be a carboxylic acid group (-000H) or a sulfonic acid group (-SO3H). The organic acid includes an amino group. . For the purposes of the present invention, a guanidine group (i.e. a -NH-C[==NH]-NH2 group) is considered to provide an amino group substituent for the organic acid.

Carbon nanotube derivatives in accordance with the invention may be readily dispersed in water without the need for ultrasonication. The derivatives are relatively simple structures in which a water-dispersibility enhancing agent is ionically bonded to a carbon nanotube structure. The derivatives may readily be prepared by treatment, under aqueous conditions, of a carbon nanotube structure in which (under the aqueous conditions) there are ionic groups covalently attached to the carbon nanotube framework with the water-dispersibility enhancing agent which (under the aqueous conditions) has ionic groups of opposite charge to those covalently attached to the carbon nanotube framework. As a result, the water-dispersibility enhancing agent becomes ionically bonded to the carbon nanotube framework via the respective ionic groups (the second ionic groups) of the amino acid and those (the first ionic groups) attached to the carbon nanotube framework.

The organic acid employed in the invention may, for example, be one having a carboxylic acid group and an amino substituent. There may be more than one amino substituent. One amino substituent may be provided on the u-carbon atom (i.e. the same carbon atom to which the -COOH group is bonded) and the other in a side chain of the organic acid.

Organic acids incorporating a carboxylic acid group and an amino group substituent for use in the invention may, for example, be one of the twenty naturally occurring amino acids, but the invention is not limited to the use of such naturally occurring amino acids. Alternatively, the acid group of the organic acid may be a sulfonic acid group. Preferably the organic acid employed in the invention will be wholly aliphatic. Alternatively or additionally the organic acid preferably has a maximum of ten carbon atoms, even more preferably a maximum of eight carbon atoms. Generally amino acids used in accordance with the invention will contain only carbon, oxygen, nitrogen and hydrogen atoms whereas sulfonic acids will generally only contain carbon, oxygen, nitrogen, sulfur and hydrogen atoms.

In one embodiment of the invention, the first ionic groups (i.e. the groups covalently bonded to the carbon nanotube framework) are anionic (i.e. negatively charged) groups and the second ionic groups are cationic (i.e. positively charged) groups. In an alternative embodiment of the invention, the first ionic groups may be cationic and the second ionic groups anionic. Consider, for example, that the organic acid is an amino acid. The amino acid may be one with an isoelectric point selected to provide second ionic groups that are cationic or anionic at the pH of the tieatment conditions. Considei, for example, that the pH of the treatment condition is less than the isoelectric point of the amino acid. In this case, the amino acid will be (predominantly) positively charged(thus providing second anionic groups that are cationic). Conversely if the amino acid with an isoelectric point less than the pH of the treatment conditions then the amino acid will be (predominantly) negatively charged (thus providing second ionic groups that are anionic).

The amino acid may for example be arginine which has an isoelectric point of 12.48. At a pH of less than 12.48 the arginine is (predominantly) positively charged and therefore provides second ionic groups that are cationic. Alternatively the amino acid may, for example, be aspartic acid which has an isoelectric point of 2.77. At higher pH values, aspartic acid is (predominantly negatively charged and therefore provides second ionic groups that are anionic). For the purposes of the present invention, specific values of isoelectric points for common amino acids are as given in "Fundamental of Organic Chemistry" (Seventh Edition) by John McMurry, page 507, Table 15.1.

As indicated above, the first ionic groups may be anionic groups and the second ionic groups (on the water-dispersiblity enhancing agent) may be cationic groups. The latter groups are preferably provided by a positively charged nitrogen atom. For this embodiment of the invention, the organic acid may be an amino acid incorporating a guanidine group which, in the ionic form of the amino acid for this embodiment of the invention, provides positively charged guanidinium groups. Most preferably the amino acid is arginine. In this embodiment, the anionic groups attached to the carbon nanotube structure are preferably - 000 groups. Such groups may be readily generated by oxidative treatment of the allotropic form of carbon nanotubes with a concentrated form of nitric acid, most preferably under reflux. A nitric acid concentration of about SM is suitable, but the invention is not limited to this particular concentration. This reaction proceeds generally in accordance with the following scheme: / 6M HNO3, reflux, 4,8,12 or 16 h /\ I' Although the above scheme shows only one -C00 group attached to the carbon nanotube framework, it will be appreciated that there will in fact be many such groups introduced by the nitric acid treatment.

We have found that the longer the treatment time (of the carbon nanotubes with nitric acid) the more improved is the water-dispersiblity of the derivative in accordance with the invention. We believe this to be due to an increase in the number of groups bound to the surface of the carbon nanotubes as a result of the increase in oxidation duration.

Subsequent to treatment of the carbon nanotubes with nitric acid, the oxidised carbon nanotubes are preferably washed with water to remove all traces of the nitric acid.

A particularly preferred embodiment of the method of the invention involves (in step (H)) the treatment of oxidised carbon nanotubes (prepared by treatment with nitric acid and subsequent washing, as outlined in the previous paragraph) with an aqueous solution of arginine. Since the isoelectric point of arginine is 10.76, this treatment may be effected in water without the addition of any pH modifying agent. The reaction proceeds generally in accordance with the following scheme: Arginine HEN> HO NH2 Although the above scheme shows only one -C00 group attached to the carbon nanotube framework, it will be appreciated that there will in fact be many such groups attached to a carbon nanotube molecule thus allowing association of one such molecule with many arginine residues.

As indicated above, an alternative embodiment of the invention is one in which the first ionic groups (i.e. the groups covalently bonded to the carbon nanotube framework) are cationic and the second ionic groups are anionic. In this case, the first ionic groups may, for example, be quaternary ammonium groups and the second ionic groups may be -C00 groups. The organic acid may be an amino acid having two -COOH groups, preferably aspartic acid which has an isoelectric point of 2.77. For the purposes of this embodiment of the invention, the carbon nanotube structure with positively charged first ionic groups may be obtained by the steps of: (a) oxidation of carbon nanotubes with nitric acid (as detailed above); (b) treatment of the oxidised nanotubes (after washing) with an imide; (c) treatment of the product of step (b) with an N,N-dialkyl diamine; (d) quaternisation of the product of step (c); and (e) treatment of the product of step (c) with the amino acid above its isoelectric point.

The following reaction scheme shows a particular embodiment of this process: P I,R H\ N NMe2

H NMe3

-kflNMe2 -$02 N N Me3

-

Aspartic (t, Acid b NH2 OykyOH 0 0 In an alternative embodiment of the invention, the organic acid is a sulfonic acid (substituted with an amino group). In this embodiment, the organic acid may be taurine, in which case the process of the invention proceeds generally in accordance with the following reaction scheme:

-I

\-7 / /fft ___ bJA i / Ttnne C) -J 2-' H3f 5\ / 4 / It will be noted from the above reaction scheme that the first ionic groups are the -C00 covalently bonded to the carbon nanotube framework and the second ionic groups are provided by positively charged amino groups of the taurine.

Methods in accordance with the invention may conveniently be effected at a temperature in the range of 150 to 30°C and may conveniently be effected at ambient temperature.

The carbon nanotube derivatives of the invention may be prepared from single-walled or multi-walled carbon nanotubes. Preferably the derivatives are single-walled carbon nanotubes.

The abovedescribed methods produce dispersions of the carbon nanotube derivatives which may be used without further processing.

Aqueous dispersions of carbon nanotube derivatives in accordance with the invention have a variety of applications. For example, the dispersion may be applied as is" to a substrate and the aqueous phase subsequently evaporated to leave the nanotubes adhered to the substrate. Alternatively, and this represents a particularly important feature of the present invention, the aqueous dispersions of the carbon nanotube derivatives may be admixed with water-compatible resin compositions, particularly water-compatible curable resin compositions, (e.g. water-compatible, curable phenolic resins). The dispersions are fully miscible with such water-compatible resin compositions systems thereby providing a single formulation which may be used for incorporation of both a resin and carbon nanofibres in a particular material. Such admixtures may be applied, for example, to fibrous (e.g. non-woven) substrates and subsequently cured for the dual purpose of imparting strength to the substrate as well as incorporating carbon nanofibres into the substrate. Such non-woven fibrous substrates may be produced by standard techniques (e.g. wet-laying). Without limitation, the fibres constituting the fibrous substrate may, for example, be carbon fibres, glass fibres, metal coated carbon fibres or any combination thereof.

The water-compatible resin composition may be one comprising the resin dissolved either in water, in a mixture of water and a water-compatible solvent, or in just a water-compatible solvent.

The products of treating a non-woven substrate with an aqueous based, curable resin incorporating a dispersion of carbon nanofibre derivatives in accordance with the invention and effecting subsequent curing may find application, for example, in fuel cells, enhanced magnetic shielding in aerospace applications, composites, field-emission displays, sensors, scanning probe microscopy tips, drug delivery systems and hydrogen storage.

Commercial phenolic resins that may be used in accordance with the invention may be water miscible per se, miscible with a mixture of water and a water miscible organic solvent, or miscible with just a water-compatible organic solvent. Examples are commercial phenolic resins that may be used include Durez 37082 and Durez 33121, both supplied by Sumitomo Bakelite. These resins generally require dissolution with a mixture of water and a water miscible organic solvent (e.g. acetone). Other options for phenolic resins that may be used in accordance with the invention include SC-bOB supplied by Borden and PSE1001 supplied by Barrday Composite Solutions. These resins may be diluted with a water miscible solvent (e.g. iso-propyl alcohol) for admixture with an aqueous dispersion of the carbon nanotube derivatives.

The invention will be illustrated by the following non-limiting Examples and the accompanying drawings, in which: Fig. 1 shows the Raman spectra of carbon nanotubes both before and after refluxing with nitric acid in accordance with the procedure of Example 1; Fig. 2 shows a photograph of two vials, the left-hand vial containing the product of Example 1 in water and the right-hand vial containing that product further treated with arginine in accordance with the procedure of Example 2; Fig. 3 is a photograph of the samples shown in Fig. 3 after sonication, dilution and filtration in accordance with the procedure of Example 2; :ii Fig. 4 shows the UV-vis-NIFI spectra of the sample shown in Fig. 3; Fig. 5 shows a photograph of two vials, the left one containing non-oxidised CNTs and the right one containing oxidised CNT's in water with arginine present in accordance with the procedure of Example 3; Fig. 6 shows the same samples as in Fig. 6 after sonication, dilution and filtration, in accordance with the procedure of Example 3; Fig. 7 shows plots of Thermogravimetric Analysis (IGA) data for oxidised CNTs produced in accordance with Example 4; Fig. S shows Transmission Electron Microscopy images of CNTs prepared in accordance with the procedure of Example 4; Fig. 9 shows photographs of the products obtained in accordance with Example 4 in water containing arginine, the vials having been allowed to stand for various periods of time; Fig. 10 is similar to Fig. 9 but without arginine present; Fig. 11 shows photographs of vials in which, in each photograph, the left hand vial shows a sample prepared with oxidised CNTs in water without arginine and the right hand vial shows a sample prepared with the same oxidised CNT in arginine, all samples having been ultrasonically sonicated, filtered and diluted; Fig. 12 shows photographs of two vials, the left one containing an aqueous sample of a phenolic resin and the right one containing the same resin to which oxidised CNTs and arginine have been added; and Figs. 13 and 14 show Raman spectra obtained in accordance with the procedure of Example 10.

In all the Examples, the carbon nanotubes used were Single-Wall Carbon Nanotubes available under the designation Elicarb from Thomas Swan.

ExamDle 1 This Example demonstrates oxidation of carbon nanotubes using nitric acid to generate -C00 groups aftached to the carbon nanotube framework in accordance with the following generalised reaction scheme. Q4_

SM HNO3, reflux, 16 h Wet carbon nanotubes were added to 6 M nitric acid (100 mL) in a round bottomed flask.

The solution was heated to 120 °C and refluxed for 16 h, after which time it was allowed to cool to room temperature. The carbon nanotubes (CNTs) were filtered through a polycarbonate membrane (0.2 pm, Whatman) and washed with a copious amount of water until pH neutral. The functionalized CNTs were then redispersed and filtered using methanol (10 mL) and N,N-dimethylformamide (10 mL), and dried overnight at 100 ¶3.

The Raman spectra of the CNTs before and after refluxing in nitric acid are given in Fig. 1.

The increase in the intensity of the D band relative to the 3 band increases after the ref lux so that the 1D'10 ratio increases from 11 to 49%, which indicates the presence of carboxylate anion groups on the surface.

ExamDle 2 This Example demonstrates the solubility of the oxidised CNTs produced in accordance with Example 1 in water both in the absence and in the presence of arginine, the latter becoming ionically associated with the -COO-groups in accordance with the following generalised reaction scheme: Arginine HEN> HO NH2 Oxidised CNTs (5 mg) as prepared in accordance with Example 1 were added to 2 mL of a solution of arginine in water (0.871 g/mL) in a screw top vial.

For the purposes of comparison, oxidised CNTs (5 mg) as prepared in accordance with Example 1 were added to 2 mL of water in a screw top vial.

Both samples were allowed to stand for 5 minutes and the vials then photographed. The result is shown in the photograph of Fig. 2.

It can be seen in Fig. 2 that the left sample tube, corresponding to the sample without arginine, appears clear with debris of undissolved CNTs left at the bottom, while the right sample tube, corresponding to the sample with arginine, appears cloudy -indicating that the CNTs had dissolved.

Ultrasonic sonication for 30 minutes resulted in the dispersion of both samples in water to form very black solutions. In order to carry out UV-vis-NIR measurements the samples were diluted by a factor of 25 and filtered. This also allowed the clear difference in solubility between them to be observed (Fig. 3).

The UV-vis-NIR spectrum of both samples was recorded and the solubility determined from the absorbance values at 700 nm. The spectrum is appended in Fig. 4 for which the upper curve is the sample with arginine present and the lower curve is the sample without arginine present.

The UV-vis-NlR spectra (Fig. 4) show the solubility of the oxidised ONTs in water is 0.36 mg mU. In the presence of arginine, however, the solubility of the oxidised CNTs in water is increased to 2.55mg mL*

Example 3

This Example provides a comparison of the solubility of (non-oxidised) CNTs and oxidised CNTs (the latter being prepared in accordance with Example 1 (in water) in the presence of arginine.

Oxidised CNTs (5 mg) as prepared in accordance with Example 1 were added to 2 mL of a solution of arginine in water (0.871 gImL) in a screw top vial.

Wet CNTs (0.140 g) (i.e. non-oxidised CNTs) were added to 2 mL of a solution of arginine in water (0.871 gImL) in a screw top vial.

Both vials were allowed to stand it was noted that the oxidised CNTs dissolved spontaneously whereas the wet CNTs did not dissolve. The two vials were photographed after 5 minutes and the result is shown in Fig. 5 in which the left hand vial contains the sample prepared from the wet CNTs and the right hand vial contains the samples prepared from the oxidised ONTs.

Both samples underwent ultrasonic sonication for 30 minutes, followed by dilution by a factor of 25 and then filtration. The two samples were again photographed and the result is shown in Fig. 6 in which, once again, the left hand vial shows the result obtained for the sample originally prepared with wet CNTs and the right hand vial shows the result for the sample prepared with oxidised CNTs. An obvious difference in solubility between them is observed between the two samples, which indicates that an interaction between the negatively charged carboxylate anions and positively charged arginine is necessary to achieve solubility of the CNTs in water.

Example 4

This Example demonstrates the effect of effecting oxidation of CNTs by refluxing with nitric acid for different periods of time.

As received (AR) wet Elicarb CNTs (1 g) were added to 6 M nitric acid (50 mL) in a round bottomed flask. The solution was heated to 120 °C and refluxed for 4, 8, 12 or 16 h, after which time it was diluted with high purity water and filtered through a polycarbonate membrane (0.2 pm, Whatman). The sample was then washed with a copious amount of water until pH neutral. The functionalized CNTs were then redispersed and filtered using methanol (10 mL) and N,N-dimethylformamide(10 mL), and dried overnight at 100 ¶3.

The procedure described in the previous paragraph was carried out a total of 3 times for each of the reflux periods (4, 8, 12 and 16 hours) for the purposes of comparison.

The degree of functionalisation of the CNT surface was estimated using thermogravimetric analysis (TGA) in an inert atmosphere (helium) since any covalent modifications are labile or decompose upon heating. By assuming that all the weight loss is due to the modification, an estimation of the degree of functionalisation was made by comparison with the TGA of the starting CNT material.

Fig. 7 represents one of the TGA results for each reflux time of the CNTs overlaid with AR-CNTs dried by ethanol dispersion for comparison. The weight loss at 600 °C for each sample and the corresponding approximate number of functional groups present are shown in the Table 1 below.

Table 1

Reflux % Weight Loss at Approx. No. of Carbon atoms per Time 600 ¶2 1 functional group Modification 10.8 31 3.23 4 10.5 32 3.13 6.4 55 1,82 8 13.1 25 4.02 8.9 38 2.60 10.6 32 3.16 12 11.23 30 3.37 10.6 32 3.16 10.3 33 3.06 16 10.2 33 3.03 11.8 28 3.57 Transmission electron microscopy (TEM) images of CNTs that have been refluxed in 6M nitric acid are shown in Fig. 8. The images show that tube-like structures remain after 4, 8 and 12 hours of oxidation.

Example 5

This Example demonstrates the solubility of the oxidised CNTs obtained in accordance with Example 4 in water both in the presence and absence of arginine.

Samples of oxidised CNTs as prepared in accordance with Example 4, with varying lengths of time spent in reflux, were placed in capped vials containing water and arginine. These samples were photographed at 30 seconds, 2 minutes, 10 minutes and 30 minutes to illustrate both the change in solubility depending on reflux time and the rate of dissolution (Fig. 9). The results are shown in Fig. 9 in which the left-and-right-hand photographs in the top row show the vials after 30 seconds and 2 minute respectively whereas the left-and-right-hand photographs in the bottom row show the vials after 10 minutes and 30 minutes standing respectively.

For the purposes of comparison, the oxidised CNTs prepared in Example 4 at ref lux times of 4, 8, 12 and 16 hours did not spontaneously dissolve in water (in the absence of arginine) even after 30 minutes -see Fig. 10 in which, going from left to right, the samples are for CNTs refluxed at 4, 8, 12 and 16 hours.

Example 6

Demonstrates properties of samples prepared in accordance with Example 5 after sonication, filtration and dilatation.

The eight samples prepared in Example 5 (i.e. two samples for each of the 4, 8, 12 and 16 hour reflux times, one with arginine and one without) were subjected to ultrasonic sonication for 30 minutes. This resulted in the dispersion of all samples to form very black dispersions.

These dispersions were then filtered and diluted by a factor of 60. The two samples for each reflux time (one with arginine and the other without) were then photographed side-by-side and the result is shown in Fig. 11 in which, in the top row, the left-and-right-hand photographs show the results for the CNTs refluxed for 4 and 8 hours respectively and in the bottom row the left-and-right-hand photographs show the results for the CNT's refluxed for 12 and 16 hours respectively. In each photograph, the left hand vial is for the sample prepared without arginine. It will be appreciated that Fig. 11 shows the clear effect arginine had on the solubility of the oxidised CNTs in water.

Example 7

This Example provides a quantitative demonstration of the solubility of the carbon nanotubes in the various samples produced in accordance with Example 6.

To determine the effect of reflux time on the solubility of the oxidised CNTs in water, with and without arginine present, the UV-vis-NIR spectrum for each of the sonicated samples prepared in Example 6 was recorded. Using the absorbance value at 700 nm and accepting the molar extinction coefficient to be 28.9 mL mg cm* the solubility was determined using the Beer-Lambert law and the results are shown in the Table 2 below.

Table 2

Solubility without Average Solubility Solubility with Average Solubility Reflux Arginine without Arginine Arginine with Arginine Time (mglmL) (mglmL) (mglmL) (mg/mL) 4 0.083 089 4 0.094 075 0.88 4 o.io ______________ 101 ____________ 8 0.11 208 8 0.14 0.116 2.40 2.32 8 0.10 _________________ 248 ______________ 12 0.13 0.126 345 3.71 12 0.11 __________________ 3.36 ________________ 12 0.14 4.34 16 0.15 7.58 16 0,28 0.24 7.71 7.36 16 0.29 _________________ 6.79 _______________ It can be seen from Table 2 that, in every case, the 16 hour reflux provided the highest solubility of the ONTs in water with an average value of 7.36 mg/mL obtained when arginine was present and 0.24 mg/mL when absent.

Example 8

This Example demonstrates the stability of dispersions of oxidised CNTs in water both in the presence and absence of arginine.

Aqueous dispersions of oxidised CNTs produced in accordance with Example 4 with a 16 hour reflux period were prepared both with and without arginine. The dispersions were monitored over a period of one week to assess stability. The results are shown in the Table 3 below.

Table 3

Time from initial UV measurement Solubility with Arginine Solubility without (days) (mg/mL) Arginine (mg/mL) 0 6.79 0.29 1 6.06 0.3 5.77 0.29 7 5.72 0.29 As can be seen from the above Table, the solubility of the oxidised CNTs in water without arginine remained constant over seven days at ca. 0.29 mg/mL. However, in the presence of arginine the solubility of the oxidised CNTs in water was not as stable. A significant decrease of 1 mg/mL was found to have occurred by the end of the week from the initial measurement.

Example 9

This Example demonstrates dispersion of CNTs in an aqueous-based phenolic resins in the presence of arginine.

CNTs (10 mg) prepared in accordance with Example 4 using a reflux time of 8 hours were dispersed in a 5 mL solution of arginine in water (0.0871 g/mL). The dispersed CNTs were added to the phenolic resin (10 g) (Cellobond 501008) dropwise with constant mixing at 200 rpm of the resin.

Fig. 12 shows photographs comparing the resin before (left hand photograph) and after (right hand photograph) addition of the oxidised CNTs. Before addition, the Cellobond SC100B resin was a clear mid-brown liquid. After addition, the resin dispersion was very black, indicating dispersion of the CNTs in the resin.

Example 10

This Example demonstrates the effect of purification of CNTs before oxidiation on their solubility in arginine/water. The CNTs used for this Example were Elicarb CNTs 94890/591 and Elicarb CNTs K5087.

Elicarb CNTs 94890/591 (4 g) were purified by heating in air at 120 °C for 1 h, then at 450 00 for 1 h. Using ultrasonic sonication the CNTs were dispersed in water (100 mL) for 15 mins.

Hydrochloric acid (100 mL) was then added to the dispersion to obtain a 6 M solution and left to stir overnight. The purified CNTs were isolated by filtration over a polycarbonate membrane (0.2 pm, Whatman) and washed with copious amounts of high purity water until pH neutral. The purified CNTs were dried overnight at 100 °C.

Elicarb CNTs K5087 (500 mg) were purified by ref luxing in GM nitric acid (70 mL) for 2 hours, This treatment serving simultaneously to purify the CNTs and introduce carboxylic acid groups. The CNTs were then filtered and washed with high putity water until pH neutral. A stock solution of 0.01 M sodium hydroxide in water was prepared. The CNTs (75 mg, mmol) were then dispersed in the NaOH solution (25 mL) using an ultrasonic bath (Ultrawave U50, 30-40 kHz) for 15 mm. The resulting dispersion was filtered through a nylon membrane (0.2 pm, Whatman) and a brown filtrate was produced. The sonication in sodium hydroxide and filtration was repeated until the filtrate was colourless. The purified CNTs were washed with high purity water until pH neutral and dried overnight at 100 CC.

The purity of the thus treated CNTs was then investigated using Raman spectroscopy.

The three most important features the Raman spectra of CNTs display are the radial breathing mode (RBM) between 100-300 cm1, the tangential mode (the G band) between 1500-1600 cm1 and the disorder mode (D band) between 1200-1400 cm1. The D band arises due to the presence of amorphous carbon impurities or because of the presence of defect sites on the CNT sidewall. Therefore a decrease in the intensity of the D band relative to the G band (l/l ratio) can be correlated with a decrease in the amount of amorphous carbon present and consequently can be used to estimate the relative purity of the CNTs.

Fig. 13 shows the overlaid Raman spectra of 94890/591 CNTs before and after purification.

As expected the CNTs that have undergone the purification procedure show a decrease in the intensity of the D band and therefore a decrease in the ID/la ratio from 12% to 8% which is consistent with the removal of non-CNT carbon material.

The Raman spectra of the K5087 CNTs before and after purification are shown in Error! Reference source not found.. The intensity of the D band (H290 cm) is observed to decrease upon heating so that the ID/la ratio decreases from 6% to 5%. As the D band indicates the presence of defects, this change in the ID/la ratio is indicative of an improvement in sample purity.

After purification treatment the solubility of purified sample 94890/591 in water/arginine was 0.022 mg/mL, while for K5087 it was 0.20 mg/mL. Both purified samples were subjected to an 8 hour reflux in 6 M nitric acid and the UV spectra of these CNTS in arginine solution recorded several times over the course of one week to determine if the dispersions produced were stable. The results are displayed in Table 4.

Table 4

________________ 94890/591 K5087 Without Arginine Arginine Without Arginine Arginine ________________ (mQ/mL) (mQ/mL) (mçj/mL) (mQ/mL) After Purification 0 0.022 0.079 0.2 8 h Reflux 0.087 1.14 0.29 1.85 8h Reflux 1 day 0.093 1.1 0.31 1.74 Sh Reflux 5 days 0.09 1.04 0.3 1.74 Sh Reflux 7 days 0.086 1.04 0.29 1.73 ExamDle 11 Elicarb CNTs (K5087) purified as in Example 10 and refluxed for 4 orB hours were dispersed in solutions of arginine/water with either 43.6 g/L or 87.1 g/L of arginine present. Solubility was measured at time periods of 0, 1, 5 and 7 days and the results are shown in Table 5.

Table 5

Time from initial UV Arginine Concentration Reflux Time Solubility measurement (gIL) (hours) mglmL 4 0.32 __________________ 8 1.08 Odays 4 063 87.1 8 1.85 43:6 ___________ __________ 1 day 4 0.62 87.1 6 L74 4 031 _________________ 8 095 5days 4 059 ___________________ 87.1 ___________ __________ 43.6 8 O90 7days 4 061 67.1 8 1.73 ExamDle 12 This Example demonstrates production of carbon nanotube derivatives using taurine. CNTs (2 g) were refluxed in 6 M HNO3 (140 rnL) for 4 h. The CNTs were then isolated by filtration over a polycarbonate membrane (0.2 FIm, Whatman) and washed with high purity water until neutral and dried in a vacuum oven overnight at 100 °C. Oxidised CNTs (5-7 mg) were dispersed in high punty water (3 rnL), 0.25 M taunnelution (3 rnL) and 0.5 M taunne solution (3 rnL) by ultrasonic sonication for 30 minutes. The solutions were filtered and then diluted by a factor of 30. The tJv-vis-NIR spectra were recorded and the solubility of the CNTs deternilned from the absorbance value at 700 nm.

The oxidised CNTs were soluble in water; however interaction with taurine increased the solubility of the CNTs further (Table 4). The interaction with taurine did not increase the solubility of the CNTs to the same extent as the interaction with arginine, but the taurine solutions are pH 7 as opposed to the pH 11 of the arginine solutions.

Table 4

Soliitic'n Sohihility in cmg rnL litter - 0.25 \I Taut me II C 0.5 M lauriue 15 (1.25 M.&tniiiie 0.5 MAiiuute I fi Figure 1 shows an overlaid Raman spectra (532 rim) of Elicarb CNTs before and after refluxing with nitric acid normalised at the C-band. The D band is marked with Figure 2 shows a photograph of oxidised Elicarb CNTs in water without arginine present (left) and with arginine present (right). Samples were left to stand for 5 minutes.

Figure 3 shows a photograph of the oxidised CNTs in water without arginine present (left) and with arginine present (right) after solication, dilution x25, and filtration, Figure 4 shows a UV-vis NIR spectra of oxidised CNTs without arginine present (lower trace) and with arginine present (upper trace).

Figure 5 shows a photograph of Elicarb CNTs (left) and oxidised ONTs in water with arginine present (right). Samples were left to stand for 5 minutes.

**fl..

:::: Figure 6 shows a photograph of Elicarb CNTs (left) and oxidised CNTs (right) in water with arginine present after sonication, dilution and filtration.

Figure 7 shows TGA data of CNTs; d1ed by ethanol dispersion (1), refluxed for 4 hours (2), 8 hours (3), 12 hours (4) and 16 hours (5).

Figure 8 shows transmission electron microscopy images of CNTs that have been refluxed in 6M nitric acid for 4, 8 and 12 hours.

Figure 9 shows photographs of oxidised CNTs spontaneously dissolving in water with arginine present after (top l-r) 30 seconds and 2 minutes (bottom l-r) 10 mins and 30 mins.

Figure 10 shows a photograph of oxidised CNTs refluxed for 4, 8, 12 and 16 h (l-r) after standing for 30 mins in water without arginine present.

Figure 11 shows photographs of oxidised CNTs refluxed for (top l-r) 4 and 8 h (bottom I-r) 12 and 16 h dispersed in water without arginine present (I) and with arginine present (r). Samples have been ultrasonically sonicated for 30 minutes1 filtered and diluted by a factor of 60.

Figure 12 shows photographs of the phenolic resin (left) and after addition of oxidised CNTs in arginine/water.

Figure 13 shows an overlaid Raman spectra (532 nm) of 94890.591 AR-CNTs before and after purification normalised at the G-band. D band is marked with * Figure 14 shows an overlaid Raman spectra (532 nm) of K5087 AR-CNTs before and after purification normalised at the G-banc. D band is marked with ** 0** e.... * C * * * C. * * C. * S S *** C * *** *

Claims (46)

1. CLAIMS1. A water dispersible carbon nanotube derivative comprising (a) a carbon nanotube structure comprised of a carbon nanotube framework having first ionic groups covalently bonded thereto; and (b) a water-dispersibility enhancing agent which is an ionic form of an organic acid substituted with an amino group and having a maximum of fourteen carbon atoms, said ionic form providing second ionic groups of opposite charge to said first ionic groups, whereby said derivative comprises said carbon nanotube structure ionically bonded to the water-dispersibility enhancing agent by the first and second ionic groups.
2. 2. A derivative as claimed in claim 1 wherein the organic acid is wholly aliphatic.
3. 3. A derivative as claimed in claim 1 or 2 wherein the organic acid has a maximum of ten carbon atoms.
4. 4. A derivative as claimed in claim 3 wherein the organic acid has a maximum of eight carbon atoms.
5. 5. A derivative as claimed in any one of claims 1 to 4 wherein the first ionic groups are anionic groups.
6. 6. A derivative as claimed in claim 5 wherein the first ionic groups are -000 groups.
7. 7. A derivative as claimed in claim 5 or 6 wherein the second ionic groups comprise positively charged nitrogen atoms.
8. 8. A derivative as claimed in claim 7 wherein the second ionic groups are -NH-C(NH2)-NH2 groups.
9. 9. A derivative as claimed in any one of claims 1 to 8 wherein the organic acid is an amino acid.
10. 10. A derivative as claimed in claim 9 wherein the amino acid is arginine.
11. 11. A derivative as claimed in any one of claims 1 to 8 wherein the organic acid is a sulfonic acid.
12. 12. A derivative as claimed in claim 11 wherein the sulfonic acid is taurine.
13. 13. A derivative as claimed in any one of claims 1 to 3 wherein the first ionic groups are cationic groups.
14. 14. A derivative as claimed in claim 13 wherein the first ionic groups comprise positively charged nitrogen atoms.
15. 15. A derivative as claimed in claim 14 wherein the first ionic groups are quaternary ammonium groups.
16. 16. A derivative as claimed in any one of claims 13 to 15 wherein the second ionic groups comprise -C00 groups.
17. 17. A derivative as claimed in claim 16 wherein the organic acid is aspartic acid.
18. 18. A derivative as claimed in any one of claims 1 to 17 wherein carbon nanotube is a Single Walled Nanotube.
19. 19. An aqueous solution comprised of carbon nanotube derivatives as claimed in any one of claims ito 18.
20. 20. An aqueous resin composition comprising a resin dissolved in the aqueous phase and further comprising a carbon nanotube derivative as claimed in any one of claims 1 to 18 dissolved in the aqueous phase.
21. 21. A resin composition as claimed in claim 20 wherein the resin is a curable resin.
22. 22. A resin composition as claimed in claim 21 wherein the curable resin is a phenolic resin.
23. 23. A treatment process comprising applying to a substrate a dispersion as claimed in claim 19 or a resin composition as claimed in any one of claims 20 to 22.
24. 24. A process as claimed in claim 23 wherein the substrate is a fibrous substrate.
25. 25. A process as claimed in claim 24 wherein the fibrous substrate is a non-woven fibrous substrate.
26. 26. A process as claimed in any one of claims 23 to 25 using a resin composition as claimed in claim 18 or 19, wherein said process further comprises effecting curing of the resin.
27. 27. A method of preparing a water-dispersible carbon nanotube derivative comprising the steps of: (i) providing carbon nanotube structures comprised of a carbon nanotube framework and first ionic groups covalently bonded to the framework; and (ii) treating said carbon nanotube structures under aqueous conditions with a water-dispersibility enhancing agent which is an ionic form of an organic acid substituted with an amino group and having a maximum of fourteen carbon atoms, said ionic form providing second ionic groups of opposite charge to said first ionic groups thereby to form the carbon nanotube derivative in which said agent is ionically bonded to the carbon nanotube structure.
28. 28. A method as claimed in claim 27 wherein the organic acid is wholly aliphatic.
29. 29. A method as claimed in claim 27 or 28 wherein the organic acid has a maximum of ten carbon atoms.
30. 30. A method as claimed in claim 29 wherein the organic acid has a maximum of eight carbon atoms.
31. 31. A method as claimed in any one of claims 27 to 30 wherein the first ionic groups are anionic groups.
32. 32. A method as claimed in claim 31 wherein said first ionic groups in the carbon nanotube structure are -000 groups.
33. 33. A method as claimed in claim 32 wherein the carbon nanotube structure is provided by effecting oxidation of the allotropic form of a carbon nanotube to oxidise carbon atoms in the framework thereof to said -C00 groups.
34. 34. A method as claimed in claim 33 wherein said oxidation is effected by treating the allotropic form of the nanotube with nitric acid.
35. 35. A method as claimed in any one of claims 27 to 35 wherein the organic acid is an amino acid.
36. 36. A method as claimed in any one of claims 27 to 35 wherein the organic acid is a sulfonic acid.
37. 37. A method as claimed in any one of claims 31 to 36 wherein the second ionic groups comprise positively charged nitrogen atoms.
38. 38. A method as claimed in claim 37 wherein the second ionic groups are -NH-C(NH2)-NH2 groups.
39. 39. A method as claimed in claim 38 wherein the organic acid is arginine.
40. 40. A method as claimed in any one of claims 27 to 31 wherein the first ionic groups are cationic groups.
41. 41. A method as claimed in claim 40 wherein the first ionic groups comprise positively charged nitrogen atoms.
42. 42. A method as claimed in claim 32 wherein the first ionic groups are quaternary ammonium groups.
43. 43. A method as claimed in any one of claims 40 to 42 wherein the second ionic groups comprise -C00 groups.
44. 44. A method as claimed in claim 43 wherein step (ii) of the method is effected at a pH greater than the isoelectric point of the amino acid.
45. 45. A method as claimed in claim 43 or 44 wherein the amino acid is aspartic acid.
46. 46. A method as claimed in any one of claims 27 to 45 wherein carbon nanotube is a Single Walled Nanotube.