Patent Publication Number: US-2007104947-A1

Title: Nanotube/matrix composites and methods of production and use

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is a continuation of U.S. Ser. No. 10/223,559, filed Aug. 16, 2002, now U.S. Pat. No. 7,001,556, which claims priority under 35 U.S.C. § 119( e ) from U.S. Provisional Application No. 60/312,980, filed Aug. 16, 2001, all of which are hereby incorporated herein by reference in their entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT  
      Not applicable.  
     BACKGROUND OF THE INVENTION  
      For sheets and films, a number of investigators have used nanotubes as the reinforcement in a nanotube/polymer composite. For example, Schadler et al. (1998) and Gong et al. (2000) produced composites of nanotubes in epoxy. Shaffer and Windle (1999) examined a nanotube/poly(vinyl alcohol) composite. Bower et al. (1999) fabricated a composite with nanotubes in a polyhydroxyaminoether. Unless sheets and films are unusually thin, they can also be reinforced with more normally-sized (e.g., diameters of 100 microns or more) reinforcement. However, nano-scale reinforcement is uniquely suited for strengthening polymer fibers, since the fibers themselves are typically only 10 to 100 microns in diameter. Since nanotubes are orders of magnitude smaller in diameter, a nanotube cannot occlude a high fraction of the fiber cross-section.  
      Presently, fibers produced from “commodity” polymers (e.g., polyester, polypropylene, and nylon) have tensile strengths from about 0.15 to 0.6 Gpa. More expensive “specialty” fibers (such as Kevlar® and PAN carbon fiber) have strengths of about 2 to 5 Gpa. The recently discovered carbon nanotubes have a theoretical strength of 200 Gpa (Schadler et al., 1998) —about 40 times higher than existing materials. However, capitalizing on this potential strength has thus far been problematic.  
      Several research teams have used single walled carbon nanotubes (SWNTs) to enhance the strength of neat fibers. Andrews et al. (1999) dispersed SWNTs in isotropic petroleum pitch. With a 5 wt % loading, the tensile strength and modulus were increased 90 and 150%, respectively. Haggenmueller et al. (2000) reinforced PMMA (polymethyl methacrylate) with SWNTs. They found a 54% increase in tensile strength and a 94% increase in modulus when an 8 wt % loading of nanotubes was used.  
      Most multiwalled carbon nanotubes (MWNTs) are believed to have the “Russian doll” structure where only weak van de Waal forces bond one tube to another (Harris, 1999). Hence, the outer layers of MWNTs could slide or telescope relative to each other (Schadler et al., 1998; Shaffer and Windle, 1999). However, kinks and defects could help prevent this sliding (Harris, 1999). Ruoff and Lorents (1996) believe that SWNTs are preferable to MWNTs because SWNTs are easier to bond than MWNTs. These researchers also feel that the tensile strength of the modified SWNTs might be affected by bonding. However, Garg and Sinnott (1998; also see Harris, 1999) showed in theoretical calculations that covalent attachments only decrease SWNT strength by about 15%.  
      Carbolex® AP Grade Nanotubes is a type of commercially available nanotube material. Carbolex® AP Grade Nanotubes is an “as prepared” nanotube material that contains about 70% SWNT and is produced by a carbon arc process. Previous investigators have used ultrasonic mixing (Schadler et al., 1998) or mechanical mixing and a surfactant (Gong et al., 2000) in attempts to disperse material of this type. However, both of these investigative teams reported that dispersion was not uniform, and that further work was needed. With the unique size range of nanotubes, the phase behavior of nanotubes in polymers will probably affect their dispersion. For submicron particles, phase separation processes occur which are not observed in macroscopic (micron-scale) systems. In particular, colloidal crystals are produced which depend on the form of interparticle forces (Calvert, 1997; Milling et al., 1991; Vincent, 1987).  
      Melt spinning involves polymers that are melt processable (thermoplastic). The polymer is melted, pressurized, and forced through a fine capillary. The fiber can be drawn down with either a mechanical roll (with speeds up to 10,000 m/min) or with air jets (with speeds to 30,000 m/min). If the air jets are placed in the melt die, this process is called melt blowing. The speeds possible with melt spinning are orders of magnitude higher than the speeds used in solution spinning. Hence, melt spinning is an inherently less expensive process for producing fibers.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic representation of a process for producing a composite mixture utilizing at least one solvent, in accordance with the present invention.  
       FIG. 2  is a schematic representation of another process for producing a composite mixture utilizing at least one solvent, in accordance with the present invention.  
       FIG. 3  is a chart illustrating the increase in strength of fibers produced in accordance with the present invention compared to fibers produced by prior art methods. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention is a method for producing single walled carbon nanotube/matrix composite materials which preferably have an anisotropic structure and which can be commercially produced utilizing commercially available equipment. In general, effective amounts of nanotubes, preferably single walled carbon nanotubes, are mixed with a matrix material to form a composite mixture, generally containing less than about 5 weight percent of the nanotubes to about parts per billion of the nanotubes and wherein the nanotubes are preferably generally randomly oriented in the composite mixture. The composite mixture is then passed through an orifice or a capillary (e.g., extruded) to form an extrudate or composite material. Shear attenuation pressure is applied about the extrudate, either as the extrudate passes through the orifice or capillary, and/or after the extrudate passes through the orifice or capillary, wherein the nanotubes are preferably substantially aligned within the polymer with the longitudinal axis of the extrudate. The shear attenuation pressure is preferably applied through a melt blowing process, although, other processes could also be utilized to provide the shear attenuation pressure to the extrudate, such as a melt spinning process, or a gel spinning process. Examples of such methods are shown in U.S. Pat. No. 6,299,812, the entire specification of which is hereby expressly incorporated herein by reference.  
      The term “matrix material” as used herein, broadly refers to any composition capable of functioning as the matrix constituent of the materials produced by the present invention. Examples of suitable matrix materials which can be employed in the practice of the present invention, include, but are not limited to, metal, glass, metal alloy, metal oxide glass or alloy thereof, polymer, such as a thermoplastic resin, or any suitable blend thereof that can be prepared to have a viscosity suitable for extruding, stretching, shearing or otherwise deforming of the composite mixture containing the matrix material and the nanotubes preferably in a direction whereby an anisotropic structure, e.g., an enhanced orientation/alignment of the fibers within the composite mixture is created, in the direction the composite mixture has been pulled, stretched, extruded, sheared, and combinations thereof or otherwise deformed and which can be mixed with the nanotubes. Any polymer capable of achieving the above described results can be utilized as the matrix. Examples of matrix materials which will function as a “matrix material” as used herein, are polyethylene, poly(para-phenylenevinylene), polypyrrole, polypropylene, nylon-6, polystyrene, polytrifluorochloroethylene and Resin Epon-812 and combinations thereof. Further, the “matrix material” could be organic/inorganic hybrid based.  
      A suitable viscosity of the nanotube/matrix composite mixture can be adjusted so that the composite mixture can be pulled, stretched, extruded, sheared, or otherwise deformed by any suitable process, such as melting the composite mixture to form a composite material.  
      For example, the transformation of the composite mixture having the nanotubes into the nanotube/matrix composite material having an anisotropic structure can be enhanced by increasing the temperature of the composite mixture up to and above a temperature where the matrix phase is considered a melted phase. By further increasing the temperature of the composite mixture its viscosity will decrease thus enhancing its fluid or fluid-like properties thus further enhancing its ability to be stretched, pulled, extruded, or combinations thereof or otherwise deformed in a preferential direction.  
      As a further example, the transformation of the composite mixture into a composite material having an anisotropic structure may, in some instances, be enhanced by the addition of a solvent that dissolves portions of some or all of the components considered to compose the matrix material of the composite mixture as described further below. The solvent dissolution of the matrix material increases its fluid or fluid-like properties and by doing so allows its deformation with less force and thus increases its ability to be stretched, pulled, extruded, or otherwise deformed in a preferential direction. Only those small volumes of solvents are required that contribute to lower viscosity of the composite mixture while simultaneously maintaining the self adhesive quality of the matrix phase in such a way that it maintains its self continuity and may be still stretched, pulled, extruded or otherwise deformed.  
      Further, plastic deformation of materials including some metals and metal alloys may be accomplished by the application of significant pressures or forces. Likewise, the application of pressure to the composite mixture can improve, create and/or initiate the stretching, pulling, extruding, or deforming of the composite mixture. Therefore, depending on the particular properties of the composite mixture, low to extremely high pressures can contribute to formation of the preferentially aligned fibers within the composite mixture by aiding the composite mixture in being deformed, stretched, extruded, pulled or otherwise linearized into a fiber, tube, or rope-like form.  
      Examples of processes that transform bulk mixtures into composite materials such as fibers, tubes or rope-like structures are those processes known in the art as “wet spinning,” “gel spinning,” “melt spinning,” “melt blowing,” or “extrusion.” These processes are well known in the art and a detailed discussion of each of these processes is not deemed necessary to teach one of ordinary skill in the relevant art to make and use the present invention.  
      Referring now to the Figures, shown in  FIG. 1  and designated therein by the reference numeral  100 , is a schematic representation of a process for producing a nanotube/matrix composite mixture  102  utilizing at least one solvent, in accordance with the present invention. The nanotube/matrix composite mixture  102  includes a matrix material  104  having a uniform dispersion of nanotubes  106  contained therein. Preferably the nanotubes comprise 70-90% single walled carbon nanotubes, more preferably 90-95% single walled carbon nanotubes, and most preferably at least 98% single walled carbon nanotubes.  
      In an example which will be discussed in more detail below, the nanotube/matrix composite mixture  102  contains about 1% by weight of nanotubes. The nanotube/matrix composite mixture  102  can then be used to form a composite material such as a drawn fiber having an increase in strength in excess of 50% over a drawn fiber formed from the matrix material alone. This substantial and unexpected increase in strength is believed to be due to the method of mixing the nanotubes  106  and the matrix material  104 , wherein the nanotubes  106  are maintained in a uniformly dispersed state prior to and during mixing of the nanotubes with the matrix material.  
      To produce the nanotube/matrix composite mixture  102 , an amount of the nanotubes is first combined with a solvent material  108  to form a nanotube/solvent mixture  110 . The nanotubes  106  and the solvent material  108  are mixed together in the nanotube/solvent mixture  110  such that the nanotubes  106  are uniformly dispersed in the nanotube/solvent mixture  110 .  
      Where used herein, the term “uniformly dispersed” means, in one embodiment of the invention, that in a sampling of ten one-milliliter samples randomly selected from the nanotube/solvent mixture, or from the nanotube/solvent/matrix mixture, at least nine of the ten one-ml samples differ from each other in the amount of nanotubes contained therein by no more than 10% by weight of the nanotubes. In a preferred version of the invention, the sampling comprises ten 100-microliter samples. In a more preferred version of the invention, the sampling comprises ten 10-microliter samples. In an especially preferred version of the invention, the sampling comprises ten 1-microliter samples.  
      The solvent material  108  is preferably a solvent material which dissolves the matrix material  104  that is being used (e.g., decalin and toluene are known solvents capable of dissolving a matrix material, such as polypropylene).  
      Other solvents contemplated for use in the present invention include, but are not limited to tetralin and tetrachlorethane e.g., for polypropylene; acetone, e.g., for polystyrene; cyclohexanone, methyl cyclohexanone, dimethyl foramide, nitrobenzene, tetrahydrofuran, isophonone, mesityl oxide, dipropyl ketone, methyl amyl ketone, methyl isobutyl ketone, acetonyl acetone, methyl ethyl ketone, dioxone, and methylene chloride, e.g., for polyvinyl chloride; and trichlorobenzene, e.g., for polyethylene. Other examples of solvents for the thermoplastic resins used as matrix materials herein will readily come to mind to persons of ordinary skill in the art.  
      The nanotubes  106  and the solvent material  108  can be mixed by any suitable method for any suitable period of time so as to uniformly disperse the nanotubes  106  in the nanotube/solvent mixture  110 . For example, the nanotubes  106  and the solvent material  108  can be mixed by mechanical mixing, shaking, sonification, stirring, or the like. The most preferredly method is sonication. Optionally, a surfactant  112  can be added to the nanotube/solvent mixture  110  to enhance dispersion of the nanotubes  106 .  
      Where used herein in reference to nanotubes or single walled carbon nanotubes, the term “substantially unbroken” means that the average length of a random sampling of the nanotubes in the mixture after the predetermined mixing period is at least about 80% of the average length of a random sampling of the nanotubes before the mixing step was initiated.  
      In addition, the temperature of the nanotube/solvent mixture  110  can be increased or varied to enhance the uniform dispersion of the nanotubes  106  in the nanotube/solvent mixture  110  as further described below.  
      An amount of matrix material  104  is then combined with the nanotube/solvent mixture  110  to form a nanotube/solvent/matrix mixture  114 . The amount of matrix material  104  combined with the nanotube/solvent mixture  110  can vary depending on the particular matrix material  104  selected, and the particular types of nanotubes  106  and solvent material  108  contained in the nanotube/solvent mixture  110  and the desired properties of the composite material formed from the nanotube/matrix composite mixture  102 . For example, the nanotube/solvent mixture  110  and the matrix material  104  in the nanotube/solvent/matrix mixture  114  are mixed such that the nanotubes  106  are maintained in a uniformly dispersed state in the nanotube/solvent/matrix mixture  114 , for example by continued sonication after the nanotubes  106  and matrix material  104  are combined. Optionally, a surfactant  116  can be added to the nanotube/solvent/matrix mixture  114 .  
      In addition, the temperature of the nanotube/solvent/matrix mixture  114  can be increased or varied to enhance the uniform dispersion of the nanotubes  106  and the matrix material  104  in the nanotube/solvent/matrix mixture  114 .  
      The solvent material  108  is then removed from the nanotube/solvent/matrix mixture  114  to form the nanotube/matrix composite mixture  102  preferably containing from about 0% by weight to about 1% by weight to about 2% by weight to about 5% by weight of the solvent material  108 . The solvent material  108  can be removed from the nanotube/solvent/matrix mixture  114  via any suitable process known by persons of ordinary skill in the art. For example, the solvent material  108  can be removed by evaporating the solvent material  108  at room temperature and/or elevated temperatures. Also, removal can be effected at atmospheric pressure and/or reduced pressure (vacuum) in a manner well known in the art.  
      For example, the nanotube/solvent/matrix mixture  114  can be disposed in one or more drying surfaces  118 . The drying surface  118  containing the nanotube/solvent/matrix mixture  114  can be placed in a hood  120  to evaporate the solvent material  108  for a suitable period of time of from hours to days.  
      The drying surface  118  can then be removed from the hood and positioned in a vacuum oven  122  to further evaporate or remove the solvent material  108  for a suitable period of time, and at a suitable temperature and pressure to form the nanotube/matrix composite mixture  102  containing preferably about 0% by weight to about 1% by weight to about 2% by weight to about 5% by weight of the solvent material  108 .  
      In a preferred embodiment, the nanotube/matrix composite mixture  102  will contain roughly 1% by weight of residual solvent material  108  or less; the residual solvent material  108  thereby acting as a plasticizer. Preferably all removed solvent material  108  will be recycled. Since removing and recycling solvent material  108  is expensive, the amount of solvent material  108  will be minimized whenever possible.  
      After the solvent material  108  has been removed, the nanotube/matrix composite mixture  102  can be crushed into a coarse material (e.g., chunks having a diameter of 0.5 cm or less). The crushed nanotube/matrix composite mixture  102  can then be further processed into a composite material such as fibers, drawn fibers or other products utilizing any suitable process, such as “wet spinning,” “gel spinning,” “melt spinning,” “melt blowing,” or “extrusion” as discussed above. These processes are well known in the art and a detailed discussion of each of these processes is not deemed necessary to teach one of ordinary skill in the relevant art to make and use the present invention.  
      Referring now to  FIG. 2 , shown therein and designated by the reference numeral  130  is a schematic representation of an alternate process for producing the nanotube/matrix composite mixture  102  utilizing at least one solvent, in accordance with the present invention. To produce the nanotube/matrix composite mixture  102 , an amount of the nanotubes  106  is combined with a first solvent material  132  to form the nanotube/solvent mixture  110 . The nanotubes  106  and the first solvent material  132  are continuously mixed in the nanotube/solvent mixture  110  wherein the nanotubes  106  are in a uniformly dispersed condition in the nanotube/solvent mixture  110 . The first solvent material  132  can be decalin, or any other solvent in which nanotubes can be generally mixed and uniformly dispersed.  
      The nanotubes  106  and the first solvent material  132  can be mixed by any suitable method so as to uniformly disperse the nanotubes  106  in the nanotube/solvent mixture  110 . For example, the nanotubes  106  and the first solvent material  132  can be mixed by mechanical mixing, shaking, stirring, sonification, or the like. In addition, to enhance the mixing and dispersion of the nanotube/solvent mixture  110 , a surfactant material  134 , such as is commonly known in the art and which is soluble in the first solvent material  132  can be added to the nanotube/solvent mixture  110 . In a preferred embodiment, the nanotube/solvent mixture  110  is mixed for about 1 hour prior to being mixed with the matrix material  104 , the nanotubes maintained in a uniformly dispersed condition before being mixed with the matrix material.  
      In addition, the temperature of the nanotube/solvent mixture  110  can be increased or varied to enhance the uniform dispersion of the nanotubes  106  in the nanotube/solvent mixture  110 .  
      An amount of matrix material  104  is combined with a second solvent material  136  (which may be the same or different from first solvent material  32 ) to form a matrix/solvent mixture  138 . The matrix material  104  and the second solvent material  136  are mixed in the matrix/solvent mixture  138  such that the matrix material  104  is dissolved in the matrix/solvent mixture  138 . To enhance the mixing of the matrix/solvent mixture  138 , a surfactant material  139 , such as a non-polar surfactant, can be added to the matrix/solvent mixture  138 . The second solvent material  136  is preferably miscible with the first solvent material  132  to optimize mixing. The second solvent material  136  can be any solvent able to dissolve the matrix material  104  as discussed elsewhere herein and which is miscible with the first solvent material  132 .  
      An amount of the matrix/solvent mixture  138  is then combined with the nanotube/solvent mixture  110  to form the nanotube/solvent/matrix mixture  114 . The amount and ratio of matrix/solvent mixture  138  combined with the nanotube/solvent mixture  110  can vary depending on the particular matrix material  104  selected, and the particular type of nanotubes  106 , and the first and second solvent materials  132  and  136 . The nanotube/solvent mixture  110  and the matrix/solvent mixture  138  in the nanotube/solvent/matrix mixture  114  are mixed such that the nanotubes  106  are uniformly dispersed in the nanotube/solvent/matrix mixture  114 .  
      The nanotube/solvent mixture  110  and the matrix material  104  can be mixed by any suitable method for any suitable period of time so as to about uniformly disperse the nanotubes  106  and the matrix material  104  in the nanotube/solvent/matrix mixture  114 , as discussed previously above. Optionally, a surfactant  140  can be added to improve mixing.  
      In addition, the temperature of the nanotube/solvent/matrix mixture  114  can be increased or varied to enhance the dispersion of the nanotubes  106  and the matrix material  104  in the nanotube/solvent/matrix mixture  114 .  
      The first and second solvent materials  132  and  136  are then removed from the nanotube/solvent/matrix mixture  114  to form the nanotube/matrix composite mixture  102  preferably containing about 0% by weight to about 1% by weight to about 2% by weight to about 5% by weight of the first and second solvent materials  132  and  136 . The first and second solvent materials  132  and  136  can be removed from the nanotube/solvent/matrix mixture  114  via any suitable process as previously described. For example, the first and second solvent materials  132  and  136  can be removed by evaporating the first and second solvent materials  132  and  136  at room temperature and/or elevated temperatures. Also, removal can be effected at atmospheric pressure and/or reduced pressure (vacuum) as previously discussed.  
      Preferably, the nanotube/matrix composite mixture  102  will contain roughly 0% to 1% to 2% to 5% by weight of the first and second solvent materials  132  and  136  and preferably less than 1% by weight. The residual first and second solvent materials  132  and  136  can act as a plasticizer. Preferably all removed first and second solvent materials  132  and  136  will be recycled.  
      While the invention will now be described in connection with certain preferred embodiments in the following examples so that aspects thereof may be more fully understood and appreciated, it is not intended to limit the invention to these particular embodiments. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the scope of the invention as defined by the appended claims. Thus, the following examples, which include preferred embodiments will serve to illustrate the practice of this invention, it being understood that the particulars shown are by way of example and for purposes of illustrative discussion of preferred embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of formulation procedures as well as of the principles and conceptual aspects of the invention.  
     EXAMPLE 1  
      In this example, commercial carbon nanotubes (nanotubes  106 ) were combined with commercially available polypropylene (matrix material  104 ). More specifically, 150 ml of decalin (solvent material  108 ) was placed in a 400-ml container. Next, 0.200 g of AP grade CARBOLEX carbon nanotubes were added to the decalin forming a nanotube/solvent mixture. Then, the mixture was sonicated with a Fisher Scientific Model 550 Ultrasonic Generator. The sonification was conducted for 60 minutes at a power setting of “4”.  
      During the sonification, the temperature of the nanotube/solvent mixture gradually rose due to the effects of sonification. After 60 minutes, the temperature of the nanotube/solvent mixture reached about 100° C. and the nanotubes were uniformly dispersed within the decalin. Then, 20 g of polypropylene pellets (matrix material  106 ) were added to the nanotube/solvent mixture having the nanotubes uniformly dispersed therein. Heat was applied to bring the temperature to 135° C. After another 60 minutes of sonification at power level “4”, the polypropylene was totally dissolved to create a nanotube/solvent/matrix mixture with the nanotubes uniformly dispersed therein.  
      The container of the nanotube/solvent/matrix mixture was placed in a vacuum oven at a temperature of 70° C. and an absolute pressure of 10.1 kPa for two days. The resultant dried nanotube/matrix composite mixture was a coarse powder (the chunks were all 0.5 cm size and less). The final weight of the nanotube/matrix composite mixture was 20.13 g, which indicated that about 0.4 ml (about 0.65% by weight) of the composite mixture was decalin and about 1% was nanotubes.  
      For forming into a composite material, the nanotube/matrix composite mixture was placed in the barrel of a ram extruder and was then heated to 190° C. The nanotube/matrix composite mixture was extruded out at a mass rate of 0.50 g/min. A 15.2 cm diameter mechanical roll was placed at a distance of 1.35 meters below the spinneret. The roll was run at a surface speed of 519 m/min forming a fiber (the composite material) which was collected on the roll.  
      The collected nanotube/matrix composite material was then post drawn by passing the fiber through an oven set at 118° C. The oven length was 38 cm, the feed roll (6.5 cm diameter) was run at a surface speed of 1.5 m/min, and the takeup roll (also 6.5 cm diameter) was run at a surface speed of 9 m/min (for a draw ratio of 6x).  
      The drawn fiber was tested with an Instron tensile test machine at a strain rate of 2.54 cm/min. The results of four replicate tests are given in  FIG. 6 . Also shown in the graph of  FIG. 3  are four replicate tests of the stress-strain behavior of a blank (polypropylene matrix with no nanotubes) subjected to the same processing conditions. As will be understood in the art, the drawn fibers produced in accordance with the present invention had a common stress-strain behavior of 13.1 grams/denier. This surprising and unexpected result represents over a 50% increase of the stress-strain behavior of blank polypropylene matrix (without nanotubes).  
      The nanotubes utilized in the example discussed above are provided as an “as prepared” nanotube material (“CARBOLEX”) that contains about 70% single wall nanotubes produced commercially by a carbon arc process. The nanotubes were not treated or cleaned before being mixed with the decalin solvent. The strength of the drawn fibers produced in accordance with the present invention can be increased by using more highly purified or “clean” nanotubes before the nanotubes are added to the solvent material  108 .  
      For example, in one method, single walled carbon nanotubes may be provided in combination with catalyst particles and silica support due to the process used for producing the nanotubes. To purify the nanotubes, a caustic treatment could be used to remove any silica and then an acid treatment could be used to remove any catalyst material for example, as shown in U.S. Pat. No. 6,413,487, the entire specification of which is hereby expressly incorporated herein by reference.  
     EXAMPLE 2  
      In an alternative embodiment the nanotubes were dispersed in toluene, rather than decalin, before addition of polypropylene and the process was followed as previously described in Example 1. Notably, it was found that sonicating carbon nanotubes in toluene for a relatively short time (e.g., 30 minutes) provided a uniformly dispersed suspension of the carbon nanotube material that was stable for several days, unlike carbon nanotubes mixed in decalin, which settled out of suspension soon after ultrasonic mixing was ceased. Additionally, mixtures of solvents such as toluene and decalin, for example at concentrations of 20, 40, 50, 60, and 80 vol. % (prior to mixing) were used, as described below, and in Example 1.  
      In the present embodiment of the method, carbon nanotube material was combined with toluene to form a nanotube/solvent mixture which was sonicated for 30 minutes. In a separate container, polypropylene was heated and dissolved in decalin to form a matrix/solvent mixture. After ultrasonic mixing, the nanotube/solvent mixture was combined with the matrix/solvent mixture which was further mixed by sonication, forming a nanotube/solvent/matrix mixture. The nanotube/solvent/matrix mixture was sonicated for additional 35 minutes, after which the mixture was removed from the heat and allowed to dry as before to form the nanotube/matrix composite mixture for further processing.  
      Changes may be made in the combinations, operations, and arrangements of the various components, elements and processes described herein without departing from the spirit and scope of the invention as defined in the following claims.  
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