JP5119443B2 - Solubilization of carbon nanotubes using aromatic polyimide - Google Patents

Description

  The present invention belongs to the technical field of nanotechnology, and particularly relates to a novel technique for solubilizing carbon nanotubes.

  In recent years, carbon nanotubes (Non-patent Document 1) discovered in the deposits during fullerene production by Sumio Iijima et al. Have high electrical conductivity, tensile strength, heat resistance, etc. based on their unique structure, and thus are in various fields. The use of is expected.

  A major problem in the research and development of carbon nanotubes (hereinafter sometimes abbreviated as CNT) is that CNT has a bundle structure in which many tubes are bundled and is completely insoluble in a solvent. For this reason, the investigations so far relating to CNT are mainly performed from the viewpoints of physics, electronics, and the like, and approaches from chemistry and pharmaceutical medicine have been delayed. If the bundle of CNTs can be unwound and dissolved in a solvent, the application of CNTs is considered to develop dramatically.

  As a method for solubilizing carbon nanotubes, a method of initially cutting CNTs shortly and chemically modifying them at both ends was proposed (for example, Non-Patent Document 2), but this method modifies the original properties of CNTs. . Recently, the present inventors have found that specific aromatic compounds, DNA, and the like can solubilize CNTs (for example, Non-Patent Document 3, Non-Patent Document 4, and Patent Document 1). In addition to this, a method of solubilizing carbon nanotubes using a polysaccharide (β-1,3-glucan) (Patent Document 2) and a method of solubilizing by forming aqueous micelles using a surfactant (non-patent) Document 5) has also been proposed.

  If the carbon nanotubes can be solubilized together with the polymer, for example, it is expected that a composite material having an excellent function having characteristics derived from the polymer in addition to the characteristics of CNT can be obtained.

Some precedents have also been found for attempts to solubilize carbon nanotubes using polyimide to obtain composite materials. However, it is difficult to unwind the CNT bundle structure and create a state where individual tubes are dissolved independently. In fact, the CNT bundle (aggregate) may only be in a transparent colloidal dispersion state. Many. For example, Wise et al. Have studied a composite composed of a nitrile group-containing wholly aromatic polyimide and CNT (single-walled CNT) (Non-Patent Document 6). This document shows that CNT is stable in the polyimide. Only reported to be decentralized.
S. Iijima, Nature, 354, 56 (1991) J. Chen et al., Science, 282, 95 (1998) N. Nakashima et al., Trans. Mater. Research Soc. Jpn, 29 525-528 (2004) N. Nakashima et al., Chem. Lett. 32,456 (2003) RE Smalley et al., Science 298, 2361 (2002) KE Wse et al., Chem. Phys. Lett. 391, 207 (2004) JP 2005-28560 A JP 2005-104762 A

  An object of the present invention is to provide a new technique capable of contributing to effective utilization of carbon nanotubes by unwinding the bundle structure of carbon nanotubes and reliably solubilizing the carbon nanotubes.

  As a result of repeated research, the inventor has noticed that a polyimide having a specific structure is dissolved in various solvents, and found that the above objects can be achieved by using this polyimide. It was.

  Thus, the present invention provides a carbon nanotube solubilizer comprising an aromatic polyimide having a repeating unit represented by the following general formula [I].

In the formula [I], AR represents a phenyl group or a condensed polycyclic aromatic group, X may or may not exist, represents an oxygen atom or a sulfur atom, and Z represents a polar substituent for increasing solvent solubility. Or represents a nonpolar substituent.

The reaction scheme for synthesize | combining the example of the aromatic polyimide used by this invention is shown. The visible-near infrared absorption spectrum of the DMSO solution of SWNT / aromatic polyimide measured in the solubilization test of the carbon nanotube according to the present invention is illustrated. The AFM image of SWNT / polyimide obtained by this invention is illustrated. 2 illustrates near infrared photoluminescence two-dimensional mapping results measured for a DMSO solution of SWNT / aromatic polyimide according to the present invention. The absorption spectrum measured by mixing another solvent with the DMSO solution of SWNT / aromatic polyimide according to the present invention is illustrated. The measurement result of FT-IR of one example of the polyimide used by this invention is shown. The measurement result of ¹ H-NMR of one example of the polyimide used in the present invention is shown. The measurement result of FT-IR of another example of the polyimide used in the present invention is shown. The measurement result of ¹ H-NMR of another example of the polyimide used in the present invention is shown. The measurement result of FT-IR of another example of the polyimide used by this invention is shown. The measurement result of FT-IR of the other example of the polyimide used by this invention is shown. The measurement result of < ¹ > H-NMR of the other example of the polyimide used by this invention is shown. The visible-near infrared absorption spectrum of DMSO solution of SWNT / aromatic polyimide measured in the carbon nanotube solubilization test according to the present invention is illustrated. The visible-near infrared absorption spectrum of the aqueous solution of SWNT / aromatic polyimide measured in the carbon nanotube solubilization test according to this invention is illustrated. 2 illustrates near infrared photoluminescence two-dimensional mapping results determined for a DMSO solution of SWNT / aromatic polyimide according to the present invention.

  As described above, in the formula [I] representing the repeating unit of the polyimide constituting the carbon nanotube solubilizer of the present invention, AR represents a phenyl group or a condensed polycyclic aromatic group, and X may not exist. When present, it represents an oxygen atom or a sulfur atom, and Z represents a polar substituent or a nonpolar substituent for enhancing solvent solubility. Among these, preferred examples of the aromatic group represented by AR include tetravalent functional groups (substituted) such as a benzene ring, a naphthalene ring, an anthracene ring, a phenanthrene ring, a pyrene ring, a perylene ring, and a naphthacene ring. May be included).

  Further, in the formula [I], the polar substituent represented by Z is a functional group that imparts solubility to a polar solvent to the polyimide, and preferable examples include a sulfonic acid group, a phosphoric acid group or a sulfuric acid group, or Examples thereof include, but are not limited to, trialkylamine salts. Here, the alkyl of the trialkylamine salt has 1 to 18 carbon atoms, and particularly preferably has 1 to 12 carbon atoms. The polyimide of the formula [I] having such a polar substituent is soluble in a polar solvent. The polar solvent is not limited to an aprotic solvent but may be a protic polar solvent (for example, water, methanol, ethanol, propanol, butanol, etc.), but in general, an aprotic polar solvent (for example, dimethyl sulfoxide). (DMSO), dimethylformamide (DMF), m-cresol, acetonitrile) are preferred.

The aromatic polyimide represented by the formula [I] which is the subject of the present invention may have a nonpolar substituent as Z. Preferable examples of the nonpolar substituent include a long chain alkyl group having 8 to 18 carbon atoms (which may be branched), but are not limited thereto. The polyimide of the formula [I] having such a polar substituent is soluble in a polar solvent (for example, hexane, benzene, toluene, diethyl ether, chloroform, etc.).
Thus, preferred polyimides constituting the carbon nanotube solubilizer of the present invention include those having a repeating unit represented by the following formula [P1], [P2], [P3], or [P4]. It is not limited.

In the formulas [P1], [P2], [P3], and [P4], Et represents an ethyl group.

The aromatic polyimide composed of the repeating unit represented by the formula [I] can be synthesized by devising various reactions. FIG. 1 shows a synthesis scheme of an aromatic polyimide composed of repeating units represented by the above formulas [P1], [P2], [P3], and [P4]. As shown in the figure, in general, m-cresol was used as a solvent, and the raw materials, acid dianhydride and diamine, were heated in the presence of triethylamine and benzoic acid at 80 ° C. for 4 hours and at 180 ° C. for 20 hours. The desired aromatic polyimide is obtained by performing reprecipitation in acetone and collecting it (see Non-Patent Document 7). The molecular weight of the aromatic polyimide thus obtained and used in the present invention is generally about 15000 to 50000.
J. Fang et al., Macromolecules, 35, 9022 (2002)

  The aromatic polyimide of the formula [I] is soluble in various solvents at room temperature, and by utilizing this property, the carbon nanotube can be solubilized as follows: Dissolve in a polar or non-polar solvent as described above. Next, carbon nanotubes are added to the solvent solution of the aromatic polyimide thus obtained. Furthermore, ultrasonic irradiation is performed on the solvent solution of the obtained aromatic polyimide and carbon nanotube. If necessary, the solution after ultrasonic irradiation is subjected to centrifugal separation, whereby bundled carbon nanotubes are reliably removed. In addition, even if it is an aromatic polyimide which has a repeating unit as represented by Formula [I], the polyimide in which the site | part corresponding to Z is a hydrogen atom does not melt | dissolve in any organic solvent.

  When the above CNT solubilization is performed using an aromatic polyimide according to the present invention, it is visible to near-infrared absorption that individual carbon nanotubes are dissolved independently (isolated) over a wide concentration range of CNTs. It has been confirmed by measuring the spectrum. That is, as the CNT concentration increases, the solution gradually increases in viscosity and eventually forms a gel, but the bundle structure unwinds in any region where a low concentration solution forms a viscous solution or even a gel. It is recognized that the state is maintained (see Examples 2 and 3 described later).

  Another feature of the solubilization method of the present invention using an aromatic polyimide having an aromatic moiety is that it can selectively solubilize nanotubes of several structures with specific chiral vectors. For example, when an aromatic polyimide having a repeating unit represented by the above-described formula [P2] is used, unlike a case where a normal surfactant is used, a single-walled carbon nanotube having a chiral index of (8, 6) Can be solubilized in particular (see Example 4 below). This is presumably because the solubilization mechanism is different from that in the solubilization method (Non-Patent Document 5) in which a micelle aqueous solution is formed using a surfactant.

  According to the present invention, a solution (solvent solution) or gel composed of aromatic polyimide and carbon nanotubes is obtained by performing the solubilization method as described above. Such a solution or gel can be directly used for a film forming process, an extrusion molding process, or the like.

  Furthermore, the aromatic polyimide / CNT solution obtained by solubilizing CNTs once by the above solubilization operation can be mixed with other solvents, particularly polar solvents such as water, ethanol, and acetonitrile. Even when diluted with these solvents, the individual CNTs remain independently dissolved (see Example 5 below). Therefore, it is also possible to prepare a composite system in which a soluble component is added to these solvents, for example, using this property.

  As is well known, carbon nanotubes include single-walled carbon nanotubes (abbreviated as SWCNT or SWNT) and multi-walled nanotubes (abbreviated as MWCNT or MWNT). The principle of the present invention can be applied to multi-walled nanotubes, but is particularly suitable for single-walled carbon nanotubes. The “carbon nanotube” or “CNT” used in connection with the present invention mainly refers to a single-walled carbon nanotube.

Examples are given below to illustrate the features of the present invention more specifically, but the present invention is not limited to these examples.
The materials and measuring devices used in the examples are as follows.
Carbon nanotubes: Purified SWNT (HiPco) Purchased from Carbon Nanotechnologies Co.
Ultraviolet-visible-near infrared absorption spectrum measurement: Spectrophotometer JASCO, V-570.
Near-infrared fluorescence spectrum measurement: Horiba fluorescence spectrometer
Spex Fluorolog: NIR.
Intermolecular force microscope: Nanoscope IIIa
(Veeco Instruments).

Synthesis of Fully Aromatic Polyimide According to the reaction scheme of (A), (B), (C) and (D) shown in FIG. 1, the formulas [P1], [P2], [P3], and [P4] An aromatic polyimide having the following repeating unit (hereinafter referred to as P1, P2, P3, P4 (aromatic) polyimide, or simply expressed as P1, P2, P3, P4) was synthesized. The product was identified by ¹ H-NMR and FT-IR measurements.
As identification data, FIG. 6a shows P1 FT-IR, FIG. 6b shows P1 ¹ H-NMR, FIG. 7a shows P2 FT-IR, FIG. 7b shows P2 ¹ H-NMR, and FIG. 8 shows P3 FT-IR. FIG. 9a shows the FT-IR measurement results of P4, and FIG. 9b shows the ¹ H-NMR measurement results of P4.

Solubilization test Carbon nanotubes were solubilized using the aromatic polyimide synthesized in Example 1. Each aromatic polyimide was dissolved in DMSO to prepare a DMSO solution having a concentration of 1 mg / mL of each aromatic polyimide. Purified SWNTs were added to this DMSO solution and subjected to ultrasonic treatment for 15 minutes. After visual observation and measurement of an absorption spectrum, the same operation was repeated by increasing the concentration of SWNTs. The concentration of SWNTs was in the range of 0.1 to about 3 mg / mL (0.1 to 3 weight ratio of polyimide).

  In either case, it was observed that the solution gradually increased in viscosity as the concentration of SWNT increased, and a gel was formed above a certain concentration. That is, in the polyimide of P1, the solution began to become viscous when the SWCN concentration was about 0.98 to the polyimide weight ratio, and gel formation was observed when the ratio exceeded about 1.7. When P2 polyimide was used, viscosity started to appear in the solution at a weight ratio of about 1.4 to the polyimide, and a gel was formed when it exceeded about 1.8. Similarly, when P3 polyimide is used, the viscosity starts to increase from about 1.7 to about 1.5, and gel formation is observed when the weight ratio exceeds 2.5, and when P4 polyimide is used, When the polyimide weight ratio was about 1, viscosity began to appear in the solution, and when it exceeded about 2, a gel was formed.

  From the measurement of the visible-near infrared absorption spectrum performed during the solubilization test as described above, it was confirmed that in any case, the unbundled state of the bundle structure was maintained. As a representative example, FIG. 2 shows a visible-near infrared absorption spectrum measured during a solubilization test conducted using P2 polyimide.

Near-infrared absorption spectrum measurement is one of the means for indicating the presence of carbon nanotubes in which a bundle structure is unwound and dissolved individually in a solution or dispersion (Non-Patent Document 8). As shown in FIG. 2, when carbon nanotubes are solubilized using a wholly aromatic polyimide according to the present invention, a characteristic spectrum due to the individual dissolution of SWNTs over a wide concentration range of SWNTs. Is observed in the near infrared region. That is, the waveform and peak position of the spectrum measured for the viscous solution (spectrum b in FIG. 2) and the gel (spectrum c in FIG. 2) are as follows: This is substantially the same as that measured for a sample (spectrum d in FIG. 2) obtained by subjecting the DMSO solution to centrifugation (10000 g). This confirms that the viscous solution and the gel of SWNT are formed from individually dissolved SWNTs, and the bundle structure is maintained in the viscous solution and the gel. . The same applies to, for example, the visible-near infrared absorption spectrum (see FIG. 10) of a solution obtained by adding SWNT to a DMSO solution of P4 polyimide.
Furthermore, if the aromatic polyimide of the present invention is used, the SWNT solution with the bundle structure unraveled in water can obtain a gel. It is clear from the spectrum (FIG. 11).
RE Smalley et al., Science 297, 593 (2002)

AFM Observation AFM (atomic force microscope) observation of the DMSO solution of SWNT / aromatic polyimide composite obtained in Example 2 was performed. The sample was prepared by immersing the mica substrate in the solution, washing, and then vacuum drying.
As a typical example, FIG. 3 shows an AFM image when an aromatic polyimide of P2 is used. The SWNT has a diameter of 95% or more in the range of 0.7 to 2.0 nm, and it is shown that most SWNTs exist in a dissolved state. Is consistent with

It can be confirmed that the SWNTs are dissolved in the near-infrared photoluminescence two-dimensional mapping solution and the individual SWNTs are dissolved by emitting fluorescence in the near-infrared region, and excited with respect to the wavelength of the emission spectrum. It is known that the chiral vector of SWNT can be determined by mapping the spectrum wavelength (Non-patent Document 8).

  Thus, near-infrared photoluminescence two-dimensional mapping was determined for the DMSO solution of SWNT / aromatic polyimide obtained in Example 2. As an example, FIG. 4 shows the results when P2 aromatic polyimide is used, and FIG. 12 shows the results when P4 aromatic polyimide is used. Samples were prepared by subjecting the solution to centrifugation at 10000 g for 3 hours.

  As shown in FIG. 4, the presence of (8,6), (9,5), (12,1) (14,0) and (14,9) SWNTs is recognized, and (8,6) SWNT The strength is particularly great. Further, from FIG. 11, the polyimide of P4 has the chirality of (7,6), (9,4), (8,6), (8,7), (9,5) and (10,5). It is understood that the SWNTs that are possessed are isolated and dissolved. These are different from SWNT (Non-patent Document 5) present in the micellar aqueous solution when the surfactant SDS (sodium dodecyl sulfate) is used, and it is assumed that the carbon nanotubes according to the present invention proceed by a unique mechanism. Is done.

Mixing test with other solvent Water, acetonitrile or ethanol was mixed with the DMSO solution of SWNT / aromatic polyimide obtained by the operation of Example 2. As a representative example, FIG. 5 shows the results of absorption spectrum measurement when water is mixed in a DMSO solution of SWNT / polyimide using the P1 aromatic polyimide. In the figure, (a) is a stock solution before mixing, (b) is a case of water: DMSO solution = 1: 1, and (c) is a case of water: DMSO solution = 9: 1. In any case, no substantial change in the absorption spectrum was observed, and it was confirmed that SWNTs were individually dissolved even when the solubilized solution was diluted with water. The SWNTs were still uniformly solubilized by visual observation. Similar results were obtained when diluted with acetonitrile or ethanol.

Protonation of polyimide The aromatic polyimide synthesized in Example 1 was protonated to convert the triethylamine salt portion into a sulfonic acid group. Protonation was performed by immersing the P1 and P2 membranes washed in hot methanol in 1N hydrochloric acid for 8 to 12 hours, washing with ultrapure water, and drying.
Thus, the polyimide obtained by protonating P1 and P2 still maintained solubility in DMSO. Therefore, a solubilization test of SWNT was performed in a DMSO solution of protonated P1 and P2 in the same manner as in Example 2, and a visible-near infrared absorption spectrum was measured. Even when protonated polyimide is used, SWNT can be solubilized in the same manner as when P1 and P2 are used, and substantially the same absorption spectrum is obtained, and the triethylamine salt structure of P1 and P2 is solubilized. It was understood that there was no effect.

Claims (7)

A carbon nanotube solubilizing agent comprising an aromatic polyimide having a repeating unit represented by the following general formula [I].
(In the formula [I], AR represents a phenyl group or a condensed polycyclic aromatic group, X may or may not exist, represents an oxygen atom or a sulfur atom, and Z represents a polar substitution for enhancing solvent solubility. Represents a group or a non-polar substituent.)   The carbon nanotube solubilizer according to claim 1, wherein the polar substituent is a sulfonic acid group, a phosphoric acid group, or a sulfuric acid group, or a trialkylamine salt thereof (the alkyl has 1 to 18 carbon atoms). .   The carbon nanotube solubilizer according to claim 1, wherein the non-polar substituent is a long-chain alkyl group (the alkyl group has 8 to 18 carbon atoms). The carbon nanotube solubilizer according to claim 1, comprising an aromatic polyimide having a repeating unit represented by the following formula [P1], [P2], [P3], or [P4].
(In the formulas [P1], [P2], [P3], and [P4], Et represents an ethyl group.)   A method for solubilizing carbon nanotubes using a solubilizing agent comprising an aromatic polyimide according to claim 1, wherein the aromatic polyimide is dissolved in a polar or nonpolar solvent, and the resulting aromatic polyimide solvent is obtained. A method comprising the steps of adding carbon nanotubes to a solution, and irradiating the solvent solution of the obtained aromatic polyimide and carbon nanotubes with ultrasonic waves.   The method for solubilizing carbon nanotubes according to claim 5, further comprising a step of subjecting the solution after ultrasonic irradiation to centrifugation.   A solution or gel obtained by the method of claim 5 and comprising an aromatic polyimide and carbon nanotubes.