WO2008036123A2

WO2008036123A2 - Synthetic polymer surfactants for dispersing carbon nanotubes and methods of using the same

Info

Publication number: WO2008036123A2
Application number: PCT/US2007/010037
Authority: WO
Inventors: Liwei Chen; Zi-Chen Li
Original assignee: Ohio University
Priority date: 2006-04-26
Filing date: 2007-04-26
Publication date: 2008-03-27
Also published as: WO2008036123A3

Abstract

Provided herein are synthetic polymeric surfactants suitable for dispersing SWNTs in various media. Also provided are methods for dispersing single walled carbon nanotubes (SWNTs) in various media. The methods provided herein comprise the use of one or more synthetic polymers to disperse a plurality of SWNTs in a medium. In some embodiments the SWNTs are dispersed in an aqueous solvent, whereas in other embodiments the SWNTs are dispersed in another medium such as an organic solvent or a polymer matrix.

Description

SYNTHETIC POLYMER SURFACTANTS FOR DISPERSING CARBON NANOTUBES AND METHODS OF USING THE SAME

RELATED APPLICATIONS DATA

[0001] This application claims the benefit of United States provisional patent application serial no. 60/795,045 for PROCESS FOR CARBON NANOTUBE DISPERSION USING SYNTHETIC POLYMER SURFACTANT filed April 26, 2006, the entire disclosure and the reference listed therein are fully incorporated herein by reference.

GOVERNMENT RIGHTS

[0002] The present invention was made, at least in part, under National Institutes of

Health Grant . The government may have certain rights in this invention.

BACKGROUND

[0003] Single-walled carbon nanotubes (SWNTs) exhibit extraordinary mechanical, thermal, and electrical properties due to their unique all-carbon structure. They have demonstrated great potential in applications ranging from composite materials and molecular electronics to sensors and electrochemical electrodes. The structure of a SWNT can be conceptualized as a one-atom-thick layer of graphite called graphene wrapped into a seamless cylinder. The graphene sheet is wrapped up to form a SWNT in the direction of its chiral vector, which is represented by a pair of indices (n, m). The indices (n, m) are coordinates in vector units along two axes of the plane constituting graphene's 2-dimensional honeycomb lattice structure. Certain chiral vectors are associated with specific conformations. For example, when m = 0 the SWNT has a "zigzag" conformation and when n = m the SWNT has an "armchair" conformation. For all other values of (n, m) the SWNTs have a "chiral" conformation.

[0004] Carbon nanotubes are typically produced as an agglomerated mixture

(including the various conformational species) of SWNTs bundled together. This agglomeration is due to the strong van der Waals and hydrophobic interactions that occur between individual SWNTs in aqueous environments. Thus, before further solution-phase processing can be carried out (including any separation or assembly of the as-produced SWNTs into larger structures), the SWNTs need to be de-agglomerated and successfully dispersed into either organic or aqueous solvents.

[0005] Dispersion of SWNTs into aqueous solutions can be achieved by covalently functionalizing the ends or the sidewalls of the individual nanotubes to make them hydrophilic or by using surfactants to incorporate them into micelles. A naturally occurring polymer that is very useful as a surfactant in this regard is single stranded DNA. Zheng et. al. used DNA to form SWNT-ssDNA complexes that were particularly intriguing because the complexes could be separated using ion-exchange High Pressure Liquid Chromatography (HPLC) into various fractions that were rich in either metallic or semi-conducting SWNTs. It is believed that the DNA functions as a surfactant by wrapping around the SWNTs in such a way that the aromatic bases of the DNA strand interact with the hydrophobic exterior surface of the SWNTs. Meanwhile, the charged backbone of the DNA strand is directed outwards towards the hydrophilic medium. As such, the charged backbone forms the exterior of a micelle-like structure making the SWNT-ssDNA complex soluble in water.

[0006] Zheng's work constitutes a first step in potentially isolating the different (n, m) conformations of SWNTs. However, the drawback to using DNA is its limited availability in large quantities and the limited capacity to vary the base sequences on the DNA strand. Consequently, DNA generally lacks the requisite flexibility required to further improve upon the dispersive properties is of limited utility or to tailor other properties of interest into the surfactant system.

[0007] Accordingly, a need exists to improve dispersing carbon SWNTs in solution and in other media. Such new dispersion methods should be capable of accommodating both present and future levels of SWNT production. Useful dispersion techniques should utilize readily available materials that can be made in potentially large quantities if needed. New methods should also focus on materials that can be designed or further improved in ways that will optimize the separation of different types of SWNT species from one another, including species that are known today or that will likely be discovered in the future. The most useful methods could achieve both of these goals by employing materials that are abundant, designable, and easy to synthetically modify. SUMMARY

[0008] Provided herein are polymer surfactants for dispersing SWNTs. The polymer surfactants described are synthetic polymer compositions for dispersing SWNTs in a medium comprising a polymer backbone; comprising a plurality of repeat units having chemical compatibility with the medium and optionally at least one side chain moiety attached to at least one repeat unit in the polymer backbone, wherein the side chain moiety has chemical compatibility with SWNTs.

[0009] The polymer backbone may be selected from (1) styrene/maleic acid

(anhydride) alternating polymer; (2) vinyl ether/maleic acid (anhydride) alternating copolymers, in which vinyl ether is vinyl methyl, ethyl, propyl, isopropyl, or butyl ether and so forth; (3) maleic acid (anhydride), fumaric acid, and fumaric esters terpolymers; (4) acrylic polymers, which are homo- or co-polymers made from pure or mixture of acrylic acid, acrylic anhydride, acrylates, acrylamides, methacrylates, and methacrylamides; and (5) cationic backbone such as amine-containing polymers. In embodiments where the backbone is styrene, the use of side chain moieties is optional.

[0010] Side chain moieties for the synthetic polymer surfactants may be selected from derivatives of benzene, naphthalene, anthracene, tetracene, acenaphthylene, benzoanthracene, benzopyrene, benzofluoranthene, benzopyrene, benzoperylene, benzofluoranthene, benzophenanthrene, fluoranthene, fluorine, phenanthrene, acenapthene, pyrene, perylene and other polycyclic aromatic hydrocarbons (PAHs) and derivatives thereof. Side chain moieties may also be selected from organic dyes including porphyrins and porphyrin derivatives, metal porphyrin complexes and metal porphyrin derivative complexes, rhodamine family dyes, fluorescein family dyes, and other organic dyes and combinations thereof.

[0011] Synthetic polymer surfactants suitable for dispersing SWCNs in a medium may have the formula -{[X-Y]_(n-m)-[X-Y(Z)]_m}- where the ratio of (n-m):m is from about 9:1 to about 100:1. X can be derived from a monomer capable of undergoing polymerization, Y can be derived from a monomer capable of undergoing polymerization and can contain electrophilic functionalities capable of reacting with nucleophilic functionalities present on Z, and Z can be a side chain or a terminal moiety which can contain nucleophilic functionalities capable of reacting with electrophilic functionalities present on Y. Y may be attached to Z through the interaction of the electrophilic functionalities present on Y with the nucleophilic functionalities present on Z. The part of the repeat unit having the formula [X-Y] comprises a polymer backbone having chemical compatibility with the medium and Z comprises side group moieties that have chemical compatibility with the SWNTs.

[0012] Also provided herein are methods for dispersing SWNTs in a medium comprising the steps of providing at least one SWNT; providing a medium; providing a synthetic polymer, wherein the polymer comprises a polymer backbone comprising a plurality of repeat units having chemical compatibility with the medium and optionally at least one side chain moiety attached to at least one repeat unit in the polymer backbone and wherein the side chain moiety has chemical compatibility with the SWNT; and contacting at least one SWNT with the polymer for a sufficient time to disperse the SWNT in the medium. Methods are also provided herein comprising the use of one or more polymers to disperse a plurality of SWNTs in various types of media. In some examples SWNTs are dispersed in an aqueous solvent, or an organic solvent, or in mixtures thereof. In other examples SWNTs are dispersed in another medium such as a polymer matrix, or in mixtures thereof with an aqueous or organic solvent and a polymer matrix.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIGURE 1 shows a schematic representation of the synthetic polymeric surfactants described herein.

[0014] FIGURE 2 shows two representative cationic repeat units that may be used as embodiments of polymeric backbones comprising the synthetic polymeric surfactants described herein.

[0015] FIGURE 3 shows the absorption spectra (3A) and fluorescence emission spectra (3B) for a pyrene-derivatized PSMA (Polysoap) and a SWNT/Polysoap complex. For 3A, the top curve is the SWNT/Polysoap complex and the bottom curve is the Polysoap. For 3B, the top curve is the Polysoap and the bottom curve is the SWNT/Polysoap complex.

[0016] FIGURE 4 shows an AFM image of a SWNT/Polysoap complex. 4A is a

5μm><5 μm image of a SWNT/Polysoap complex; 4B is a 3-D image of the SWNT/Polysoap complex in lμmxlμm scale.

[0017] FIGURE 5 shows various carbon nanotube structures. DETAILED DESCRIPTION

General

[0018] The present invention will now be described with occasional reference to the specific embodiments of the invention. However, this invention may be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, the embodiments are provided so that the disclosure will be thorough and complete and will fully convey the scope of the invention to those skilled in the art.

[0019] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used in the description of the invention herein is for describing particular embodiments only and is not intended to be limiting of the invention. As used in the description of the invention and the appended claims, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety.

[0020] Unless otherwise indicated, all numbers expressing quantities of ingredients, properties such as molecular weight, reaction conditions and so forth as used in the specification and claims are to be understood as being modified in all instances by the term "about." Accordingly, unless otherwise indicated the numerical properties set forth in the following specification and claims are approximations that may vary depending on the desired properties sought to be obtained in embodiments of the present invention. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. However, any numerical values inherently contain certain errors necessarily resulting from error found in their respective measurements.

[0021] As used herein, the terms "disperse" and "dispersion" means to provide a solution or a uniform suspension of particulates. In some embodiments, these dispersions remain stable to precipitation at room temperature for at least one month, but particular examples can have even greater stability and for such cases these conditions will be specified. As used herein, species that are chemically compatible are species that have similar molecular structures or at least one similar chemical property, (for example polarity, hydrophobicity or hydrophilicity) chemical structures or chemical properties in common.

[0022] As used herein, the term radical reversible addition-fragmentation chain transfer (RAFT) polymerization is a method for the synthesis of living radical polymers. RAFT polymerization uses an agent (such as a dithioester, dithiocarbamate, trithiocarbonate, and xanthate) or combinations thereof to mediate the polymerization via a reversible chain- transfer process. RAFT polymerization permits use of polymers with low polydispersity and high functionality. RAFT polymerization is suitable for use with a variety of monomers and for producing complex architectures such as block, star, graft, comb, and brush (co)polymers. The procedures, reaction conditions and techniques associated with RAFT polymerization are well known in the art.

Types of SWNTs and their properties

[0023] When SWNTs are made, many variety of SWNTs may be simultaneously produced, each variety possibly having distinct properties. SWNTs can be produced that have metallic-type or semiconducting-type properties, and SWNTs can be produced that have different chiral vector (n, m) conformations. Each distinct variety of SWNT (whether it be SWNTs having a unique conformation or specific electronic properties) could be useful for a given application if purified and isolated from other varieties of SWNTs. Unfortunately, there is currently no satisfactory method known for separating out and purifying one variety of SWNT from another. Existing separation techniques rely on substantial differences in the thermodynamic and kinetic properties of different chemical species in a mixture. However, when dealing with the different varieties of SWNTs, there are usually minimal differences in the thermodynamic and kinetic properties that any two varieties may exhibit. Such differences are often insufficient to allow any effective separation and purification of SWNT species using standard separation techniques. Improved dispersion of SWNT species is a first step in achieving effective separation.

Polymer compositions

[0024] The polymer compositions described herein are synthetic in nature and have surfactant properties. The polymer compositions have a backbone composed of a plurality of repeat units, where each repeat unit is a structure derived from one or more monomers that are capable of undergoing polymerization to form the repeat unit structure. The plurality of repeat units may be selected to have chemical compatibility with the medium of choice. Optionally, a side chain moiety may be attached to one or more of the repeat units in the polymer backbone. In some embodiments, these side chain moieties are selected to have chemical compatibility with the SWNTs. The polymer compositions have a basic structure which is represented by FIGURE 1.

Spacer group

[0025] According to some embodiments, a spacer group may also be present and may be attached in-between a side chain moiety and a repeat unit of the polymer backbone. Such spacer groups may have a short or an extended structure. Typical examples of spacer groups include but are not limited to the following structures and derivatives thereof: -(CH₂)_n-, where n is from 1-10; -[A(CH₂)_mB]_n-, where A and B are -N(H)- or -N(R)- (where R is an alkyl group) or -S- or -C(O)- or -C(O)O- or -CH(OH)- or -CH(R)- (where R is an alkyl group) and m is from 1-6 and n is from 1-16; and -[C₆H₄]_H-, where n is from 1-2. The longer or more extended the spacer group, the more likely it will be that the side chain moiety that is attached to it will physically extend away from the polymer backbone.

[0026] Therefore, one skilled in the art will understand that the choice of a spacer group (i.e. using a longer spacer group), may allow for increasing the influence that a side chain moiety will have over the net physical and chemical properties exhibited by the resulting polymer composition. Similarly, one skilled in the art will understand that, in comparison, polymer compositions having either short spacer groups or no spacer groups may have net physical and chemical properties dominated by the nature of the repeat unit structure, and not by the chemical and physical properties of any side group moieties present. When the choice of polymer backbone is coupled with the choice of both side group moiety and spacer group, one skilled in the art will recognize these choices may be useful for tuning the properties of a given polymer system to exhibit specific properties that are desired.

Polymer backbone

[0027] According to some embodiments, polymer backbones are contemplated where the at least one monomer used to produce the repeating unit is styrene or vinyl ether. Other polymer backbones are contemplated where at least two monomers are used to produce the repeat unit and wherein one of the monomers is styrene or vinyl ether and the other monomer is selected from the group consisting of acrylic acid, acrylic anhydride, acrylamide, acrylate, methacrylate, methacrylamide, methyl-methacrylate, fumaric acid, fumaric anhydride, maleic acid, and maleic anhydride, and mixtures thereof. Exemplary embodiments of polymer backbones may also include a styrene/maleic acid (anhydride) alternating polymers; vinyl ether/maleic acid (anhydride) alternating copolymers, in which vinyl ether is vinyl methyl, ethyl, propyl, isopropyl, or butyl ether and etc; other copolymers containing maleic acid (anhydride), fumaric acid, and fumaric esters in the backbone; acrylic polymers, which are either homo- or co-polymers made from pure or mixture of acrylic acid, acrylic anhydride, acrylates, acrylamides, methacrylates, and methacrylamides.

[0028] In some embodiments, polymer backbones are capable of carrying a cationic charge. In this regard, amine or amide-containing polymers may be useful. Examples of such amine or amide-containing polymers include poly(lysine) and polymers with backbones containing either amines or amides or mixtures thereof as part of the repeating unit. Specifically, embodiments of polymers may have the repeat unit structures shown in FIGURE 2.

[0029] In some embodiments, polymer backbones and salts thereof are contemplated wherein the backbone comprises a repeat unit having the formula -{[X-Y]_(n-m)-[X-Y(Z)]_m}-. In this formula, X represents a species derived from a monomer capable of undergoing polymerization and Y represents a species derived from a monomer capable of undergoing polymerization that also has a chemical functionality or chemical functional groups present that are electrophilic in character, hi this formula, Z represents a side chain moiety having a chemical functionality or chemical functional groups present that are nucleophilic in character. Polymer backbones of the type contemplated, having a repeat unit represented by - {[X-Y](n_-m)-[X-Y(Z)]_m}-₅ may be made when X and Y are reacted and undergo polymerization to form an X-Y bond. Note also that Y may be chemically bonded to Z. The Y-Z chemical bond forms when a polymer having a repeat unit represented as -[X-Y]- is reacted with Z. In such reactions the electrophilic functional groups present on Y interact with the nucleophilic functional groups present on Z to form the Y-Z bond.

[0030] In some embodiments, polymers having the above formula include polymers where the electrophilic functionality of Y arises from either an acid or acid anhydride group, while Z is a species derived from an aromatic hydrocarbon and the nucleophilic functionality of Z arises from an amine group. Examples of such embodiments of these polymers include polymers where Z is a species derived from an aromatic hydrocarbon selected from the group consisting of benzene, naphthalene, anthracene, tetracene, acenaphthylene, bezoanthracene, benzopyrene, benzofluoranthene, benzofluoranthene, benzophenanthrene, fluoranthene, fluorine, phenanthrene acenapthene, pyrene, perylene and mixtures thereof.

[0031] In various embodiments, the amount of substitution (represented as m) of side chain moieties Z onto the total number (n) of repeat units (where n = ([X-Y]_(n._m)) + ([X- Y(Z)]_m) that constitute the polymer backbone (as represented by the formula -{[X-Y](_n-m)-[X- Y(Z)]_m}-) is determined by the ratio (n-m):m. Examples of such embodiments of polymer substitution with side chain moieties Z include those where the ratio of (n-m):m may be from about 9:1 to about 100:1. The component of the polymer repeat unit formula represented as [X-Y] is the structural part of the polymer that gives rise to the polymer having chemical compatibility with a given medium. Similarly, Z represents the structural part of the polymer (the side group moieties) that gives rise to the polymer having chemical compatibility with SWNTs.

Polymerization of the polymer backbone

[0032] Any suitable polymerization method may be used to make the polymer backbone part of the polymer compositions contemplated and claimed herein. One useful polymerization method is radical reversible addition-fragmentation chain transfer (RAFT) polymerization. Polymer compositions having repeat units that are derived from at least two monomers capable of undergoing RAFT polymerization can be made when the two monomers undergo RAFT polymerization to form the repeat units. In one example, RAFT polymerization synthesis to provide a polymer that has alternating styrene and maleic acid monomers in the backbone. SCHEME 1 shows a polystyrene-maleic acid (PSMA) polymer synthesized by reaction of styrene (PS) monomers with maleic acid (MA) monomers. The resulting PSMA-polymer has repeat unit that contains a styrene (PS) species and a maleic acid (MA) species. SCHEME 1

[0033] SCHEME 1 also shows how an alternating styrene (PS) maleic acid (MA) polymer (PSMA) may be further converted into a polymeric surfactant or "Polysoap". The Polysoap can be synthesized by subjecting the PSMA polymer to further functionalization with an amino-containing pyrene or pyrene derivative. Functionalization of the polymer in this manner involves the electrophilic maleic acid anhydride groups present on PSMA undergoing a condensation reaction with the nucleophilic amine groups present on the amino- pyrene derivative.

[0034] For some embodiments, pyrene is one of many useful side group moieties because its fused aromatic ring structure is graphene-like, which resembles the molecular structure of SWNTs and gives pyrene a chemical compatibility with the SWNTs. Resultantly, pyrene has a strong tendency to adsorb onto SWNTs. Thus, when pyrene is used as side groups in a polymer the resulting polymer composition that is formed will have the capacity to interact with SWNTs in solution. There are also residual acid functionalities present on the maleic acid monomers that constitute the polymer backbone. As a result of these residual acid functionalities, at elevated pH the polymer backbone can become negatively charged. This capacity to become negatively charged makes the resulting polymer composition that is formed soluble in water. Thus, polymers synthesized according to SCHEME 1 may have sufficient surfactant-like properties that it will mimic the behavior of ssDNA, and wrap around SWNTs in order to disperse them into an aqueous solution. Side chain moieties

[0035] Side chain moieties are optional, and one skilled in the art will recognize that the amount of side chain substitution selected to provide desired SWNT dispersive properties may depend on the nature of the polymer backbone. For example, when the polymer backbone is PSMA, the benzene-like structures (arising from the styrene monomers used to synthesizing the polymer backbone) are sufficiently compatible with SWNTs that the underivatized PSMA polymer alone is capable of dispersing SWNTs into solution even though no SWNT compatible side chains moieties are incorporated into the polymer structure.

[0036] Side chain substitution may occur in a manner such that a side chain moiety is attached to a repeat unit located at the end of a polymer backbone. In some embodiments, attachment of a side chain moiety at this position effectively terminates further propagation or growth of the polymer chain. A side chain moiety attached at this terminal location on a polymer backbone constitutes an end group on the backbone of a polymer chain.

[0037] In some embodiments, polymer compositions are contemplated that may contain side chain moieties that are derived from organic dyes. Useful organic dyes can include structures such as porphyrins, porphyrin derivatives, metal porphyrin complexes, metal porphyrin derivative complexes, rhodamine dyes, fluorescein dyes, any other type of organic dyes, and combinations thereof. The side chain moiety may also be derived from aromatic hydrocarbons. Useful aromatic hydrocarbons can include aromatic hydrocarbons, substituted aromatic hydrocarbons, and derivatives thereof selected from the group consisting of benzene, naphthalene, anthracene, tetracene, acenaphthylene, bezoanthracene, benzopyrene, benzofluoranthene, benzofluoranthene, benzophenanthrene, fluoranthene, fluorine, phenanthrene, acenapthene, pyrene, perylene any other polycyclic aromatic hydrocarbons (PAHs), and mixtures thereof.

Methods for dispersing SWNTs

[0038] Methods for dispersing SWNTs described herein are useful for dispersing

SWNTs that then may be used in a variety of functions and applications. For example, these methods can be used to purify SWNTs from raw products, to dissolve or disperse SWNTs into aqueous or non-aqueous solutions, and to disperse SWNTs into polymer matrices and into composite materials. [0039] In some embodiments, the methods may also be used or may play a role in the preparation of SWNT-based electrochemical electrodes; or in making SWNT dispersions that are then further used in solar energy conversion (in the form of photovoltaic cells or photoelectrochemical solar cells), in solar fuel production, and in solar hydrogen production. Likewise, these methods may also be used or may play a role in making SWNT dispersions that are then used to prepare optoelectronic devices, including light emitting diodes, light- emitting transistors, photo-detecting diodes, and transistors. Because methods for dispersing SWNTs are likely to be the important first step separating and isolating species of SWNTs, additional applications will become apparent.

[0040] Methods for dispersing SWNTs in various media are provided. One such method comprises the steps of providing at least one SWNT in a medium; providing a synthetic polymer comprising a polymer backbone having a plurality of repeat units that have chemical compatibility with the medium and that optionally contain at least one side chain moiety attached to at least one of the repeat units in the polymer backbone, wherein the side chain moiety has chemical compatibility with the SWNT; and contacting the SWNT with the polymer for a sufficient time to disperse the SWNT into the medium. Furthermore, methods are provided wherein the step of providing a polymer composition is contemplated to involve providing one or more polymers or a mixture of polymers that may be useful for dispersing SWNTs.

[0041] Polymers that are useful for carrying out the method of dispersing SWNTs may have side chain moieties that can interact with the SWNTs to aid in dispersion of the SWNTs into the medium. The polymers may have a backbone that is soluble in a medium that is provided. Useful media may be selected from the group consisting of water, organic solvents, polymer matrices and mixtures thereof.

[0042] Monomers that may be useful for making the polymer backbones when the medium selected is water include (but are not limited to) styrene, maleic acid, and vinyl ether. In other situations when the medium selected is either a non-aqueous solvent or a polymer matrix, one skilled in the art will understand that monomers can be chosen to comprise the backbone that have solubility or other chemical compatibility with the non-aqueous solvent or matrix of interest. Furthermore, useful monomers for making polymers for dispersing SWNTs according to the methods described herein can be monomers that are capable of undergoing polymerization, including RAFT polymerization, to form the desired repeat unit structure. Examples of such polymers include structures where the repeat unit is formed using two monomers; one monomer being either styrene or vinyl ether while the other monomer may be selected from the group consisting of acrylic acid, acrylic anhydride, acrylamide, acrylate, methacrylate, methacrylamide, methyl-methacrylate, fumaric acid, fumaric anhydride, maleic acid, and maleic anhydride, and mixtures thereof.

[0043] Other polymers useful for carrying out the dispersion methods described herein may have a charged backbone that aids in dissolving the resultant polymer in water. These monomers may also have a side chain moiety or side chain moieties that interact with the SWNTs. Examples of such structures include polymers capable of developing a cationic charge, where the repeat unit contains amine or amide functional groups or mixtures thereof. Specific embodiments contemplated include poly(lysine), and polymers having repeat unit structures that are shown in FIGURE 2.

[0044] Other polymers contemplated as useful for carrying out the dispersion methods herein described include water soluble polymers that may have a Lower Critical Solution Temperature (LCST) in the temperature range from about 25 to about 35 'C. In some embodiments, such polymers may have a LCST in the range from about 30-32 'C. The LCST is the temperature at which a polymer dissolved in aqueous solution undergoes a phase transition, going from one phase (a homogeneous solution) to a two-phase system (a polymer rich phase and a water rich phase). Polymers that change from a one to two phase system as the temperature increases are characterized as having inverse solubility and are called temperature sensitive polymers. As used in some embodiments, temperature sensitive polymers undergo solubilization quickly and exhibit highly dispersive properties in cold water but remain relatively inert in warmer water.

[0045] Useful temperature sensitive polymers may include polymers synthesized from n-alklyacrylamide-based monomers. For example, polymers synthesized from isoproplymethacrylamide, diethylacrylamide, proplyacrylamide, ethylproplyacrylamide, n- and tert-butlyacrylamide and ethoxyethylacrylamide may be useful temperature sensitive polymers in this regard. Other useful temperature sensitive polymers may include polymers selected from poly(lysine), poly(N-isopropylacrylamide), poly(N-(3-ethoxypropyl)acryl- amide), or derivatives thereof. [0046] It is contemplated that temperature sensitive polymers having backbone structures based on monomers of the type described herein may not need to be further derivatized with side group moieties in order to be useful. However, such temperature sensitive polymers may be further converted if it is so desired by attaching any of the various side group moieties described herein onto the backbone structures of the temperature sensitive polymers using conventional chemical synthesis techniques.

[0047] A variety of chemical structures having chemical compatibility with or a capacity to interact with the SWNTs may be useful as side chain moieties. For example, the side chain moiety may be derived from an organic dye or the side chain moiety may be derived from an aromatic hydrocarbon. Useful derivatives of aromatic hydrocarbons may be selected from the group consisting of benzene, naphthalene, anthracene, tetracene, acenaphthylene, bezoanthracene, benzopyrene, benzofiuoranthene, benzofluoranthene, benzophenanthrene, fluoranthene, fluorine, phenanthrene acenapthene, pyrene, perylene and mixtures thereof. The side chain moieties may be attached to a repeat unit located anywhere along the polymer backbone, including an end repeat unit of a polymer backbone. A side chain moiety attached at such an end location on a polymer backbone would constitute a polymer end group.

[0048] Any suitable polymerization method may be used to make these polymers.

One useful polymerization method in this regard may be RAFT polymerization. As such, embodiments of this method include using any of the polymers described herein, where the polymers used have a repeat unit that is derived from at least two monomers that undergo RAFT polymerization to form the repeat unit.

Examples

[0049] The examples disclosed herein are for illustrative purposes, only and are not meant to limit the scope of the invention.

PSMA preparation

[0050] PSMA was synthesized as shown in SCHEME 1. 2.08 g (0.02 mol) of styrene (PS) and 1.96 g (0.02 mol) maleic anhydride (MA) monomers were polymerized using typical RAFT polymerization techniques, resulting in an alternating polystyrene-maleic acid polymer (PSMA) with 80% yield. The molecular weight of the PSMA was measured using standard Gel Permeation Chromatography (GPC) techniques to be 8,400 daltons with a molecular weight distribution of (M_w/M_n~1.2). This molecular weight distribution is much narrower than the molecular weight distributions that simple radical copolymerization products typically yield.

Preparation of pyrene-derivatitized PMSA (Polysoaps 1-4)

[0051] All examples of pyrene-derivatized PMSA (Polysoaps 1-4) were prepared using the same reaction conditions and procedure now described for Polysoap 1. TABLE 1 summarizes results of the pryene content of Polysoaps 1-4 and the (n-m):m side-group substitution ratios that are calculated.

[0052] Polysoap 1: Ig of PSMA was allowed to react with 20 mg of 1-aminopyrene in a solution with 1,4-dioxane (Sigma- Aldrich), at a temperature of 90 'C for 24 hours. The reaction mixture was then cooled to room temperature and the pyrene-derivatized PSMA polymer was worked up by precipitating the polymer out of the reaction mixture into methanol. The polymer was isolated, dissolved into excess acetone, and then precipitated again into methanol. This procedure was repeated until the polymer product was sufficiently purified. The product was isolated and dried to give a PSMA-backboned Polysoap having a pyrene side chain content of about 1.15 % and a yield of 95 %. The reaction product had an (n-m):m side-group substitution ratio of about 99:1.

[0053] Polysoap 2: Ig of PSMA was allowed to react with 50 mg of 1-aminopyrene in a solution with 1,4-dioxane. The reaction lead to a PSMA-backboned Polysoap having a pyrene side chain content of about 2.30 % and a yield of 94 %. The reaction product has an (n-m):m side-group substitution ratio of about 49:1.

[0054] Polysoap 3: 1 g of PSMA was allowed to react with 120 mg of 1- aminopyrene in a solution with 1,4-dioxane. The reaction lead to a PSMA-backboned Polysoap having a pyrene side chain content of about 5.0 % and a yield of 90 %. The reaction product has an (n-m):m side-group substitution ratio of about 19:1.

[0055] Polysoap 4: 1 g of PSMA was allowed to react with 250 mg of 1- aminopyrene in a solution with 1,4-dioxane. The reaction lead to a PSMA-backboned Polysoap with a pyrene side chain content of about 10 % and a yield of 90 %. The reaction product has an (n-m):m side-group substitution ratio of about 9:1. SWNT/Polysoap complexes

[0056] All examples of SWNT dispersed in pyrene-derivatized PSMA (Polysoaps 1-

4) were carried out using the same using the same reaction conditions and procedure now described. 4 mg of Polysoap (any of Polysoaps 1-4) was diluted in 4 ml of aqueous 60 mM NaOH in a test tube. After standing overnight, 1.6 mg of purified HiPCO SWNT powder (as supplied by Carbon nanotechnology Inc., TX) was added to the aqueous Polysoap/NaOH mixture. This mixture was then sonicated using a VCXl 30 sonicator (Sonics & Materials, Inc., Connecticut) for one hour at ~ 5 W power and 75% amplitude. The resulting suspension was centrifuged for 2 hours at 13,500 g to give an aqueous solution of SWNT/Polysoap complex in the supernatant.

SWNT/Poly(lysine) complexes

[0057] SWNT/Poly(lysine) complexes in aqueous solution were obtained by mixing 1 mg of purified HiPCO SWNT powders (as supplied by Carbon Nanotechnology Inc, TX) with 2 ml of 0.1% w/v Poly-L-Lysine aqueous solution (Sigma- Aldrich). This mixture was then sonicated using a VCX130 sonicator (Sonics & Materials, Inc., Connecticut) for one hour at ~ 5 W power and 75% amplitude. The mixture was kept in an ice-bath during sonication. Afterward, the solution was centrifuged for 10 minutes at 9,300 g. Any precipitate was removed from the supernatant to yield SWNT/Poly(lysine) complexes in aqueous solution.

SWNT/ Poly(N-isopropylacrylamide) complexes

[0058] SWNT/Poly(N-isopropylacrylamide) complexes in aqueous solution were obtained by mixing 1 mg of purified HiPCO SWNT powders (as supplied by Carbon Nanotechnology Inc, TX) with 3 mL of an aqueous solution that was 10 mg/mL of PNIPAAm (Mn = 20,000-25,000, Aldrich) and 0.02 M in NaOH. This mixture was then sonicated using a VCXl 30 sonicator (Sonics & Materials, Inc., Connecticut) for 90 minutes at ~ 5 W power and 75% amplitude. The mixture was kept in an ice-bath during sonication. Afterward, the solution was centrifuged for 5 minutes at 800 g and 3 minutes at 2,300 g. Any precipitate was removed from the supernatant to yield SWNT/Poly(N-isopropylacrylamide) complexes in aqueous solution. Dispersion and solubility of SWNTs

[0059] The solubility of SWNTs in the Polysoap containing solutions (Polysoaps 1-4) was evaluated by adding THF to the aqueous SWNT/Polysoap complex solutions. Since THF is a good solvent for the Polysoaps, addition of THF to aqueous SWNT/Polysoap solutions will result in the destabilization of any micelle-like complexes that may be formed. Destabilization of the micelle-like complexes causes the SWNTs to agglomerate and precipitate out of solution. After THF was added, the sample was centrifuged and the precipitated SWNTs were collected. The precipitate was then dried and weighted. The solubility of SWNT was then calculated using the measured weight of precipitate and results were expressed as a mass ratio of SWNT:Polysoap. TABLE 1 summarizes results for Polysoaps 1-4.

TABLE l

[0060] The solubility results shown in TABLE 1 were compared with the known solubility of SWNTs in ssDNA d(GT)₂₀ as a control. The solubility of SWNTs in ssDNA d(GT)₂o has previously been determined to be less than 0.4:1. Overall, the SWNT dispersive properties of both non-derivatized PSMA and pyrene-derivatized PSMA are much greater than ssDNA d(GT)₂₀.

[0061] The SWNT dispersion power of non-derivatized PSMA and pyrene- derivatized PSMA may also be compared by examining TABLE 1. Results show that non- derivatized PSMA was effective in dispersing SWNTs, even though it contained no pyrene side groups. Comparing non-derivatized PSMA with Polysoap 1, which has a 1.1% pyrene content and (n-m):m side-group substitution ratio of 99:1, the SWNT solubility increased from 0.7:1 in PSMA to 1.2:1 in Polysoap 1. Comparing non-derivatized PSMA with Polysoap 2, which has a 2.3% pyrene content and (n-m):m side-group substitution ratio of 49:1, the SWNT solubility further increased from 0.7:1 in PSMA to 1.3:1 in Polysoap 2.

Spectra for SWNT/Poly-soap complexes

[0062] Absorption spectra shown in FIGURE 3A and fluorescence excitation spectra shown in FIGURE 3B indicate that SWNTs are dispersed in the solution in the form of SWNT/Polysoap complexes. Absorption peaks in the visible region of a SWNT/Polysoap complex solution (top curve of FIGURE 3A) agrees well with spectra taken for aqueous SWNT mixtures reported in the literature. By contrast, the absorption spectra of just the Polysoap in solution (bottom curve of FIGURE 3A) does not have any absorption features exhibited by SWNTs.

[0063] Empirical proof that the micelle-like complexes have been destabilized due to the Polysoap desorbing from the SWNTs (but yet the Polysoap remains dissolved in the solution) is found by measuring the UV fluorescence of an aqueous SWNT/Polysoap complex solution before and after THF has been added. The fluorescence spectra of an aqueous Polysoap solution that does not contain any SWNTs (top curve of FIGURE 3B) displays the characteristic fluorescence of pyrene in an aqueous environment. However, when the SWNTs are mixed into this Polysoap solution (bottom curve of FIGURE 3B) the fluorescence of the pyrene is completely quenched. This indicates that the SWNTs mix with the Polysoap polymers in solution to form SWNT/Polysoap complexes. However, it was observed that after THF was added, the UV fluorescence attributed to the pyrene side group moieties present in the samples was no longer quenched. These results correlate with previous speculation that the SWNTs are stabilized in the aqueous solution as complexes that form through interactions between the SWNTs and the pyrene side chain moieties present on the Polysoap polymers.

[0064] The structures of SWNT/Polysoap complexes were studied further through imaging with atomic force microscopy (AFM). A drop of a SWNT/Polysoap solution was deposited onto a Mica substrate using spin-coating techniques. The substrate was then dried under a stream of nitrogen gas. The sample was then imaged with an Asylum Research (Santa Barbara, CA) MFP3D AFM under ambient conditions. The SWNTs imaged in FIGURE 4A appear as individual SWNTs, implying that the SWNT's were no longer agglomerated in the aqueous solution but rather had been successfully dispersed. The zoom- in 3D image in FIGURE 4B, shows that the height profile along the axis of the imaged SWNT complex has quasi-regular corrugation. This type of variation is not typically seen with bare SWNT samples that are prepared by CVD (growth on the same mica substrate). Additionally, this variation is too great to be attributable to mere noise generated by the mica background. As such, this data leads to a speculation that the height corrugation arises from a quasi-periodic wrapping of the poly-soap around the SWNTs.

Claims

What is claimed

1) A synthetic polymer composition for dispersing single walled carbon nanotubes (SWNTs) comprising: a. a polymer backbone; comprising a plurality of repeat units wherein the repeat units are derived from at least two monomers, wherein i. one monomer is styrene or vinyl ether, and ii. one monomer is selected from the group consisting of acrylic acid, acrylic anhydride, acrylamide, acrylate, methacrylate, methacrylamide, methyl-methacrylate, fumaric acid, fumaric anhydride, maleic acid, and maleic anhydride and mixtures thereof, and b. at least one side chain moiety attached to at least one repeat unit in the polymer backbone, wherein the side chain moiety has a molecular structure containing a plurality of carbon-carbon double bonds and wherein the pi- electrons present in the carbon-carbon double bonds are sufficiently conjugated to form an aromatic species.

2) The polymer composition of claim 1 further comprising a spacer group attached in- between the side chain moiety and the repeat unit.

3) The polymer composition of claim 1 wherein at least one side chain moiety is attached to a repeat unit located terminally in the polymer backbone to form an end group.

4) The polymer composition of claim 1 wherein the at least two monomers deriving the plurality of repeat units are capable of undergoing radical reversible addition- fragmentation chain transfer (RAFT) polymerization to form the repeat units.

5) The polymer composition of claim 1 wherein the polymer backbone further comprises amine functional groups.

6) A synthetic polymer composition for dispersing single walled carbon nanotubes (SWNTs) comprising a polymer backbone having the structure:

or the structure:

7) The polymer composition of claim 6; wherein the polymer backbone further comprises at least one side chain moiety attached to the polymer backbone, wherein the side chain moiety has a molecular structure containing a plurality of carbon-carbon double bonds and wherein the pi-electrons present in the carbon-carbon double bonds are sufficiently conjugated to form an aromatic species.

8) The polymer composition of any one of claims 4-7 wherein the polymer backbone has a cationic charge.

9) The polymer composition of any one of claims 1-5 or 7 wherein the side chain moiety is derived from an organic dye.

10) The polymer composition of any one of claims 1-5 or 7 wherein the side chain moiety is derived from an aromatic hydrocarbon.

11) The polymer composition of claim 10 wherein the aromatic hydrocarbon is selected from the group consisting of benzene, naphthalene, anthracene, tetracene, acenaphthylene, bezoanthracene, benzopyrene, benzofluoranthene, benzofluoranthene, benzophenanthrene, fluoranthene, fluorine, phenanthrene, acenapthene, pyrene, perylene and mixtures thereof.

12) A synthetic polymer and salts thereof for dispersing SWNTs in a medium comprising a repeat unit having the formula:

-([X-YW[X-Y(Z)W- wherein the ratio of (n-m):m is from about 9:1 to about 100:1; and a. X is derived from a monomer capable of undergoing polymerization, b. Y is derived from a monomer capable of undergoing polymerization and contains electrophilic functionalities capable of reacting with nucleophilic functionalities present on Z, c. Z is a side chain or a terminal moiety and contains nucleophilic functionalities capable of reacting with electrophilic functionalities present on Y, and wherein the [X-Y] component of the repeat unit formula provides the polymer with solubility with the medium and Z comprises side group moieties having extended aromatic structures that exhibit graphene-like chemical properties resembling the chemical properties of SWNTs, and wherein Y is attached to Z through interaction of the electrophilic functionalities and the nucleophilic functionalities.

13) The polymer of claim 12 wherein X and Y are derived from monomers capable of undergoing RAFT polymerization. 14) The polymer of claim 12 wherein Y contains acid or acid anhydride functionalities and Z is derived from an aromatic hydrocarbon and contains amine functionalities.

15) The polymer of claim 14 wherein the aromatic hydrocarbon is selected from the group consisting of benzene, naphthalene, anthracene, tetracene, acenaphthylene, bezoanthracene, benzopyrene, benzofluoranthene, benzofluoranthene, benzophenanthrene, fluoranthene, fluorine, phenanthrene acenapthene, pyrene, perylene and mixtures thereof.

16) A method for dispersing SWNTs in a medium comprising the steps of: a. providing at least one SWNT; b. providing a medium; c. providing a synthetic polymer wherein the polymer comprises, i. a polymer backbone comprising a plurality of repeat units having chemical compatibility with the medium, and ii. at least one side chain moiety attached to at least one repeat unit in the polymer backbone, wherein the side chain moiety has chemical compatibility with the SWNT; and d. contacting the at least one SWNT with the polymer for a sufficient time to disperse the SWNT in the medium.

17) The method of claim 16 wherein at least one side chain moiety is terminally attached to the polymer backbone to form an end group.

18) The method of claim 16 wherein the medium is selected from the group consisting of water, an organic solvent, a polymer matrix and mixtures thereof.

19) The method of claim 16 wherein the repeat unit is derived from at least two monomers capable of undergoing polymerization to form the repeat unit.

20) The method of claim 19 wherein one monomer is styrene or vinyl ether.

21) The method of claim 20 wherein one monomer is selected from the group consisting of acrylic acid, acrylic anhydride, acrylamide, acrylate, methacrylate, methacrylamide, methyl-methacrylate, fumaric acid, fumaric anhydride, maleic acid, and maleic anhydride and mixtures thereof.

22) The method of claim 21 wherein the repeat unit is derived from at least two monomers capable of undergoing RAFT polymerization to form the repeat unit.

23) The method of claim 16 wherein the polymer backbone further comprises amine functional groups.

24) The method of either claim 16 or 23 wherein the polymer backbone has the structure:

or the structure:

25) The method of claim 16 wherein the polymer is selected from poly(lysine), poly(N- isopropylacrylamide), poly(N-(3-ethoxypropyl)acrylamide) or derivatives thereof.

26) The method of any one of claims 16-17 or 19-23 wherein the side chain moiety is derived from an organic dye.

27) The method of any one of claims 16-17 or 19-23 wherein the side chain moiety is derived from an aromatic hydrocarbon.

28) The method of claim 31 wherein the aromatic hydrocarbon is selected from the group consisting of benzene, naphthalene, anthracene, tetracene, acenaphthylene, bezoanthracene, benzopyrene, benzofluoranthene, benzofluoranthene, benzophenanthrene, fluoranthene, fluorine, phenanthrene acenapthene, pyrene, perylene and mixtures thereof.

29) The method of claim 16 wherein the synthetic polymer is an underivatized styrene- maleic acid copolymer.