WO2006033678A2 - Integrated molecular synthesis and net shape manufacturing consolidation of polymer nanocomposites and nanocomposites formed therefrom - Google Patents

Abstract

A sequential multi-step integrated molecular synthesis, fiber functionalization and nancomposite manufacturing method for net shape consolidation of nancomposites based on nanofibers and at least one matrix material is described.

Description

INTEGRATED MOLECULAR SYNTHESIS AND

NET SHAPE MANUFACTURING CONSOLIDATION OF

POLYMER NANOCOMPOSΠΈS AND NANOCOMPOSΠΈS FORMED THEREFROM

This invention was made with Government support under ARO No. DAAD19-03-1- 0012 which is entitled, "MOLECULAR SYNTHESIS AND NET SHAPE FABRICATION AND MANUFACTURING OF POLYMER NANCOMPOSITES" awarded by the Army Research Office, Grant "Polymer Nanocomposites". The government has certain rights in this invention.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent Application Serial No. 60/571,574 filed May 14, 2004.

FIELD OF THE INVENTION

This invention relates to a novel, integrated framework and methodology for molecular synthesis and net-shape consolidation and manufacturing of nanocomposites. A sequential multi-step integrated molecular synthesis, fiber functionalization and nanocomposite manufacturing method for net shape consolidation of nanocomposites based on nanofibers and at least one matrix material is described.

BACKGROUND OF THE INVENTION

There is an increasing interest in developing composite materials that combine the desirable properties of inorganic materials (e.g., high strength, electrical conductivity, thermal stability) with the advantages of polymers (processing ease, corrosion resistance, flexibility). Nanocomposite materials based on polymeric matrices and inorganic fibers have tremendous potential for advances in material performance (strength, toughness, stiffness, reliability, conductivity) as well as manufacturing simplification and cost reduction. Of specific commercial interest are inorganic fillers that can serve multiple functions within the polymer matrix. For example, composite materials with combined high strength and thermal conductivity would be an attractive alternative to steel for use in automotive industry. While much progress has been made in development of novel fillers and polymers, nanocomposites have yet to achieve expected properties based on pure component properties. This has been attributed to a mismatch between the surface of fiber and polymer matrix that leads to poor adhesion at the matrix-filler interface, hi addition, poor distribution of nanofibers within the matrix results from a combination of structure mismatch and processing of composites using bulk mixing techniques. Considerable work over the past few years has focused on improving both the fiber distribution and interfacial properties of carbon nanofiber composites. For example, oxidation significantly improves compatibility of fibers within a polymer matrix with resulting improvements in mechanical properties. Several recent publications have demonstrated that the solubility of single wall carbon nanotubes (SWNT) can be enhanced in organic solvents by covalently binding functional groups on an oxidized SWNT surface using a wide variety of chemistries, hi addition, composites formed from by bulk mixing of functionalized SWNT within the polymer matrix exhibit improved properties relative to those which incorporate pristine materials. hi particular, nanocomposites based on polymeric, ceramic or metallic matrices and carbonaceous or inorganic reinforcements in the form of fibers have tremendous potential for advances in material performance (strength, stiffness, toughness, conductivity) as well as manufacturing simplification and cost reduction. Ideally, a high performance polymer nanocomposite material can be fabricated from any fiber of nanoscale dimensions, by mixing with either a thermoset or a thermoplastic material, either crystalline or amorphous. In practice, as illustrated in Prior Art Figure 1, there are two key challenges or hurdles to the practical realization of the full potential of polymer nanocomposites: 1) achieving uniform distribution of the nanofibers in the polymer matrix, and 2) achieving strong molecular bonding between the two phases.

Conventional polymer composites formed by fibers of micron dimensions also suffer from these problems. There, typically the solution adopted is to react or bond to the surface of the fiber (e.g., glass or carbon) a molecular moiety or coupling agent (sizing) with multiple functional groups which can react with the fiber surface on the one side and to the polymer matrix on the other side, to form an interface (see Figure 1) which is intended to facilitate dispersion of the fiber in the polymer matrix (Mittal, 1992; Mader and Pisanova, 2001; Nakai et al., 2001; Iwashita et al., 1998; Zhang et al, 2002) and promote load transfer between the fiber and the matrix material. In some cases, an extended polymeric region (i.e., interphase in Figure 1) is believed to form between the interface layer and the bulk matrix polymer material. The "science" of sizing (i.e., molecular coupling agents) applied to formation of an interphase between polymers and glass fibers has been described (Thomason and Adzima, 2001), but a clear nature of the interphase region was not indicated. Moreover, considerable ambiguity (Baillie et al., 2001) exists in the theory and experimental determination and interpretation of knowledge about the interface and interphase regions. Nevertheless, in empirical commercial applications, the approach usually adopted is to mix a fiber which is pre-treated with a dual function coupling agent with the matrix polymer material, which also may contain reactive groups to link the matrix to the coupling agent. This approach is intended to disperse fibers uniformly with high adherence between fiber and matrix (Hull and Clyne, 1996; Miracle and Donaldson, 2001). Practical technological limitations to achieving the desired result for conventional polymer composites include lack of strong chemical bonds coupling the interface or interphase to the fiber and inhomogeneous dispersion of the fiber in the matrix.

Carbon nanofiber, the earliest candidate for carbon based polymer nanocomposites was initially called filamentous carbon (Baker and Harris, 1978; Oberlin, Endo and Koyama, 1976; Baker, 1989; Yates and Baker, 1986; Tennent, 1987). This material, which predates the single wall and multiple wall hanotubes by many years, has been of interest since its discovery in synthesis and growth by decomposition of volatile hydrocarbons on small metal catalyst particles in the early 1970's (Tibbetts, 1984; Tibbetts and Beetz, 1986; Tibbetts, 1990; Tibbetts et al., 1993; Tibbetts et al., 1994; Endo, 2002). The potential of carbon nanofibers for automotive applications (Hughes, Burton and Glasgow, 1999) builds on the recognized need for advanced materials in the automotive industry (Narula, Allison, Bauer and Gandhi, 1996). A technical and marketing review article (Tarasen, 1999) has popularized the commercial potential of this material consisting of hollow carbon nanofibers of diameter in the lOOnm range, with lengths up to 100 micron. Carbon nanofiber is now available commercially for about $100/pound, with projections to reach levels of below $10/pound within five years. For the case of vapor grown carbon nanofiber, published results of nanocomposite processing emphasize conventional mixing and dispersion (Lozano, 200 IA; Lozano, 2001B)₃ resulting in problems in producing uniform polymer nanocomposites similar to those encountered with conventional composites. Some improvements were recently obtained by the processing of carbon nanofiber with molten polymer under low-shear conditions (Glasgow, 2002). In addition, some improvement of dispersion was reported by utilizing carbon nanofiber with synthetic surface treatment to provide higher surface energy (Glasgow, 2003), but these treatments do not appear to involve polymer functionalization of the carbon nanofiber surface. Therefore, the technical capability of providing a fiber with a molecularly bonded polymer coating suitable for pseudo-copolymer formation between the fiber and the matrix does not yet appear to have been obtained. The earlier work (Witzke and Kear, 1990A and 1990B) of infiltrating intergrown filamentous carbon with thermoset precursors, without consolidation and without introduction of a functionalization for a strong interface, did not receive further attention.

Regarding chemical modification of surfaces of standard commercial carbon fibers with diameter approximately 6 microns, reaction of maleic anhydride or tetracychanoethylene to the C=C double bonds on the external surfaces (Severini, 2002) has been used to add functional groups to the fiber surface. Likewise, oxidation of commercial carbon fiber surfaces (Bismarck, Wuertz, and Springer, 1999) provided carboxylic acid groups on the surfaces which could be used as a basis for further reaction with polymeric monomers and oligomers for the formation of strong chemical bonds (i.e., functionalization of the surface).

Regarding polymer functionalization of fibers to obtain fiber surfaces compatible with polymer matrices, published literature has focused on the functionalization of single wall carbon nanotubes (Chen, 1998; Hatnon, 1999; Bahr, 2002; Hill, 2002; Lin, 2003A; Lin, 2003B; Liu, 2002; Dyke and Tour, 2003) and multi-wall carbon nanotubes (Lin, 2002), with negligible information on the functionalization behavior of carbon nanofibers. Regarding processing of carbon nanotube composites, the vision for these materials (Lau and Hui, 2002) and techniques for dispersing multi-walled carbon nanotubes in polymers (Andrews, 2002; Thostenson and Chou, 2002) have been published, stressing the use of conventional techniques for mixing and composite formation, without attention to forming strong molecular bonds between the fiber and the polymer matrix. As discussed above (Baillie et aL, 2001), a precise scientific definition of the relation between the fiber, interface, interphase and matrix in polymer composites is still a matter of scientific controversy.

Regarding processing of carbon nanofiber composites, carbon nanofiber reinforced poly (ether ether ketone) composites have been prepared (Sandler, 2002) using standard polymer processing techniques which neglect the requirement of fiber functionalization to enhance load transfer and provide for environmental stability. The inadequacy of achieving optimum composite performance without providing strong chemical bonding between fiber and matrix is illustrated by the occurrence of delaminated interfaces between carbon nanofiber and epoxy in a nanocomposite matrix in a structural graphite-epoxy composite (Tandon, Kim and Rice; 2003). Regarding preparation of polymer matrix nanocomposites for incorporation of nanoparticles of CaCO₃ in polymethylmethacrylate (PMMA)₃ in-situ polymerization of the PMMA in the presence of the CaCO₃ nanoparticles was described (Avella, 2001), but no method for forming strong molecular bonds to the nanoparticles was disclosed. Hence, this technique suffers from the same difficulties encountered with the various mixing techniques.

Miracle and Donaldson, 2001, provides a detailed set of descriptions of numerous aspects of composites science and technology dating back to the 1950's and before, which are important for commercial application of conventional composites. These include information on synthesis and processing of a range of reinforcement fillers and fibers, thermoset and thermoplastic polymers, coupling agents, and processing and fabrication equipment and techniques. Because of the newness of the nanofiber and nanocomposite field, no similar text exists describing the science and technology of nanofibers and nanocomposites. However, Liu, 2003, provides an overview of the scientific principles which must be considered in the nanocomposite fabrication in polymer matrices. The knowledge and technology required to combine the many separate steps of material synthesis and processing of conventional fibers and polymers, through intermediate steps of prepregs or semiprocessed composite materials, to final rough forms which must be cut and finished before joining into subassemblies are highly complicated and time consuming, expensive, wasteful and environmentally unfriendly. For optimum use and application of polymer nanocomposites, the established paradigm for conventional composites is inadequate to realize the full potential of polymer nanocomposite technology in performance, manufacturing simplification, and cost reduction.

The present invention of integrated molecular synthesis and net shape manufacturing consolidation of polymer nanocomposites, addresses the technology potential of these materials. By comparison with petrochemical refining of petroleum into a variety of products, the desired integrated factory for polymer nanocomposites resulting from the present invention can be considered a "composite refinery".

Therefore, one key aspect of this invention is an improved method to improve compatibility of nanofibers within polymer matrices by providing molecular bonding or polymer functionalization of the nanofiber surfaces.

Another key aspect is the use of these functionalized nanofibers to obtain improvements in miscibility in polymer processing and enhanced load transfer from matrix to fiber required for high mechanical performance of composites.

Another key aspect is the development of processing and manufacturing routes to prepare nanocomposites with varying ranges of fiber concentrations and orientations.

Yet another aspect is a method for the achievement of manufacturing simplification and the fabrication of net shape parts, subassemblies and products, by simultaneous or sequential consolidation of functionalized nanofibers under the influence of heat, catalyst, or irradiation to link and shape functionalized nanofibers to a final product. Yet another aspect of the present invention is an improved composite material comprising the nanofiber matrix formed by the methods described herein.

The present invention is an improvement over the known art to date by providing a , method for homogeneous fiber reinforced nanocomposite formation.

SUMMARY OF THE INVENTION

In one aspect, the present invention relates to a method for manufacturing a nanocomposite where at least a portion of outer surfaces of a plurality of nanofibers are reacted with at least one suitable reactive group to form surface-treated nanofibers. The surface-treated nanofibers are reacted, in situ, with at least one matrix material to form the nanocomposite. In certain embodiments, the in situ reaction of the surface-treated nanofibers with at least one matrix material causes the polymerization of the matrix. In another aspect, the suitable reactive group forms a chemically compatible outer surface on the nanofibers. The chemically compatible outer surface is capable of coupling to at least one intermediate molecular moiety. The surface-treated nanofibers is then reacted with at least one intermediate molecular moiety to form functionalized surface-treated nanofibers. Thereafter, the functionalized surface-treated nanofibers are reacted with the at least one matrix.

In another aspect, the intermediate molecular moiety is further added when the functionalized surface-treated nanofibers are reacted with the matrix material, the intermediate molecular moiety being capable of polymerizing with the matrix material. Also, the surface-treated nanofibers can have a plurality of chemical bridges on their outer surfaces which bridges are capable of coupling the surface-treated nanofibers to the matrix material.

In certain embodiments, at least one suitable reactive group comprises an oxide, amine, amide, vinyl or sulfur functionality. The matrix material can comprise at least one of a thermoset, thermoplastic, organic or inorganic elastomer polymer, ceramic, or metal matrix. In certain embodiments, the matrix comprises monomers and/or oligomers capable of being polymerized. The nanofibers can comprise carbon or ceramic materials.

In certain embodiments, the nanofibers are oxidized prior to be reacted with the at least one suitable reactive group. The oxidized nanofibers are functionalized with a monomer using a two-reaction process. In one embodiment, a first reaction comprises having carboxylic acid groups on the nanofiber surfaces react with a first reactive group to form surface-treated nanofibers. A second reaction comprises having the surface-treated nanofibers react with a second reactive group to form functionalized surface-treated nanofibers. ,

In one embodiment, a thermoplastic polyimide matrix is synthesized in the presence of amine-functionalized-nanofibers from a fluorine containing diahydride and a diamine. In another embodiment, a thermoset polymer matrix is synthesized in the presence of a mixture comprising at least one crosslinking group with two reactive end groups and at least one reactive diluent. The reactive group on the surface of the nanofibers provides at least one of the reactive end groups capable of undergoing further reaction by being synthesized into the thermosetting matrix. In one embodiment, the first reactive group comprises thionyl chloride which is reacted with the nanofibers to form chloride-surface-treated nanofibers, and the second reactive group comprises a diamine, whereby a reactive amine group is formed on the nanofiber surfaces.

In another aspect, the present invention relates to a method for forming an article comprising net shape consolidating the nanocomposite formed by the methods herein to a desired shape and scale. In certain embodiments, the consolidated nanocomposite is formed to full density under using at least one of a suitable catalyst, pressure, UV microwave energy, heat or other desirable energy source.

In yet another aspect, the present invention relates to nanocomposite materials comprising nanofϊbers and at least one matrix material formed by the methods herein. Li certain embodiments, the polymer, ceramic or metal matrix is reinforced with a carbonaceous or ceramic reinforcement constituents that have nanometer dimensions.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic illustration of a prior art fiber-interface or fiber-interphase region linking the fiber and matrix in a conventional polymer composite.

Fig. 2 is schematic illustration of a sequential procedure for an integrated molecular synthesis and nanocomposite manufacturing process.

Fig. 2a is schematic illustration of a sequential procedure for an integrated molecular synthesis and nanocomposite manufacturing process, showing method for thermoplastic and thermosetting polymers.

Fig. 3 is a diagram showing a nanofiber functionalization reaction (step 3) illustrating a two-step functionalization of carbon nanofibers (PR24-oxidized) with diamine (PDA) using intermediate reaction with thionyl chloride (SOCl₂).

Fig. 4 is a diagram showing a composite synthesis reaction (step 4) incorporating in- situ polymerization reaction of dianhydride (6FD A) with diamine (PDA) in presence of functionalized carbon nanofiber (PR24-PDA).

Fig. 5 is a diagram showing a composite synthesis reaction for an epoxy functionalization.

Fig. 6 is a diagram showing a composite synthesis reaction for a vinylester functionalization. Fig. 7 is a graph showing Raman spectra of pristine (Pris), oxidized (Oxi), chlorinated (Cl) and amine fonctionalized (PDA) fiber.

Fig. 8 is a graph showing Tan delta of blank 6FD A-PDA (o), 6FDA-PDA synthesized in the presence of oxidized carbon nanofiber (π), and 6FD A-PDA synthesized in the presence of functionalized fiber ( A ) .

Fig. 9 is a graph showing glassy state transitions of blank 6FD A-PDA (o), 6FDA- PDA synthesized in the presence of oxidized carbon nanofiber (D), and 6FD A-PDA synthesized in the presence of functionalized fiber (A).

Fig. 10 is a graph showing the storage modulus of 6FD A-PDA for materials that contain 1.5 wt. % CNF with different surface chemistries.

Fig.l 1 is a graph the storage modulus of 6FD A-PD and comparing the % difference relative to base PI.

Fig. 12 is a graph showing the glass transition of 6FD A-PDA for the materials with different surface chemistries. Fig. 13 shows Raman results of pristine fibers (PR 24), oxidized fibers (Fiber-

COOH)₅ and functionalized fibers (Fiber-COCl, Fiber-CONH-, respectively).

Figs. 14a and 14b are diagrams showing chemical structures.

Fig. 14c is a diagram showing an overall polymerization reaction to form 6FD A-PD A polyimide. Fig. 14d is a diagram showing the in-situ polymerization reaction to make 6FDA-PDA polyimide based composite

Fig. 15 is a graph showing the Tg and modulus comparison between PI and a composite with 1.5 wt% fibers.

Fig. 16 is a graph showing the Tg results for polyimide and composites with 1.5 wt% fibers.

Fig.17 is a graph showing the Moduli (room temperature) for polyimide and composites with 1.5 wt% fibers.

Figure 18 is a graph showing glassy state transitions: Tan delta of blank 6FD A-PDA (o), 6FD A-PDA synthesized in presence of oxidized carbon nanofiber (D), and 6FD A-PDA synthesized in presence of functionalized fiber (A). Figure 19 is a graph showing the Glassy state transitions of blank 6FD A-PDA (o), 6FD A-PDA synthesized in presence of oxidized carbon nanofϊber (ϋ), and 6FD A-PDA synthesized in presence of functionalized fiber (A).

Figure 20 shows a TEM of fibers in composite (1.5% loading of fiber-pPDA) where the bubble background was carbon film, part of the grid support.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) Due to global competition based on technology push and market pull considerations, companies are continually striving to improve and optimize material properties, improve product designability, reduce product complexity though simplification of subassemblies, and reduce manufacturing throughput time and cost.

According to one aspect, the present invention provides a method for the synthesis of nanoconstituent reinforcements, and in another aspect provide for a method to functionalize the nanoconstituent reinforcements with an appropriate polymer interface or interphase. In yet another aspect, the present invention provides for the formation of strong molecular bonding between reinforcements and functionalizing agents. Still further, the present invention provides a method for a sequential or in-situ net shape consolidation, which (in certain embodiments) may also include participation of additional monomer, oligomer or polymer moieties in the fabrication and manufacturing operation to achieve the desired shape and form. The present invention advantageously provides additional features such as manufacturing simplification, cost optimization, and performance maximization, as compared with current conventional fiber-reinforced composite materials.

One aspect of the present invention relates to a general method for achieving carbon and inorganic or ceramic nanofiber-reinforced nanocomposites based on polymer, ceramic and metal matrices which meet these advanced requirements. Therefore, one feature of this invention is a method to improve compatibility of nanofibers within polymer matrices, including miscibility in polymer processing and enhanced load transfer from matrix to fiber required for high mechanical performance.

Other key aspects include methods to develop processing and manufacturing routes to prepare nanocomposites with varying ranges of fiber concentrations and orientations to achieve tailored materials performance. The method of the present invention achieves manufacturing simplification and the fabrication of net shape parts, subassemblies and products, at affordable cost.

The present invention provides a method for molecular synthesis and net shape manufacturing consolidation of nanocomposites which comprise nanofibers and a matrix material. The method includes: reacting at least a portion of an outer surface of a plurality of the nanofibers with at least one suitable reactive groups reacting the surface treated nanofibers with at least one or more intermediate molecular structures or moieties, including thermoplastic, thermoset and elastomeric monomers or oligomers, to form functionalized nanofibers; and, forming, by in situ polymerization, a desired composite comprising the functionalized nanofibers and matrix material.

In certain embodiments, the suitable reactive group forms a desired chemically compatible outer surface on the nanofiber such that the outer surface of the nanofibers is capable of coupling to the at least one intermediate molecular moiety.

Also, in certain embodiments, the functionalized nanofibers have a plurality of chemical "bridges" or linkages on their outer surfaces. These bridges are capable of coupling the functionalized nanofiber both chemically and structurally to a suitable matrix material. In one aspect, the present invention related to the development of synthetic procedures to functionalize carbon nanofiber surfaces with organic functional groups that mimic reactive monomers. In-situ polymerization is used to synthesize polymers in the presence of these organically functionalized fibers. The present invention improves carbon nanofiber dispersion and enhances interfacial bonding of the fiber to the polymer matrix. In particular, the present invention is especially useful in thermosetting, thermoplastic, and elastomeric resins.

The present invention thus relates, in part, to a multiple-step, integrated method for molecular synthesis and net shape manufacturing consolidation of nanocomposites and to the composites formed thereby. The composites preferably comprise nanofibers and a polymer, ceramic, or metallic matrix material. In one aspect, the method includes: treating at least portions of the outer surface of the nanofibers with at least one suitable reactive group (for example, inorganic or organic moiety) to form a desired chemically compatible outer surface on the nanofiber, the outer surface of the nanofibers being capable of coupling to at least one intermediate molecular moiety; reacting the surface treated nanofibers with at least one or more of such intermediate molecular moieties (for example, suitable inorganic or organic groups) to form functionalized nanofibers, the functionalized nanofibers having a chemical bridge (for example, interface or interphase) on their outer surfaces which chemical bridge is capable of coupling the functionalized nanofiber both chemically and structurally to a suitable matrix material; and forming, in situ, a desired composite comprising the functionalized nanofibers and the suitable matrix material.

In certain embodiments, the suitable reactive group can include at least one oxide functionality such as a carboxylic acid group, -OH, or other such suitable group, including, for example, amines or amide, or sulphur moiety.

According to certain embodiments, the polymer functionalization of the nanofiber and the matrix is based on at least one of the chemistries of thermoset polymers, thermoplastic polymers, and/or organic or inorganic elastomer polymers.

In certain embodiments, the method for molecular synthesis and net-shape consolidation and manufacturing of nanocomposites is based on at least one polymer, ceramic or metal matrix that is reinforced with carbonaceous or ceramic reinforcement constituents that have nanometer dimensions.

In certain embodiments, the present invention can further include a step of net shape consolidation of the composite to a desired shape and scale. It should be understood that the net shape consolidation step can still further include forming the consolidated article to full density under the influence of at least one of a suitable catalyst, pressure, and/or heat or other desirable energy source including microwave energy.

Referring now to a specific embodiment shown in Figure 2, the method includes: 1) forming nanofibers by a suitable chemical synthesis; 2) surface treating, by for example, chemically reacting a outer surface of the nanofiber with at least one suitable inorganic or organic moiety to form a desired chemically compatible outer surface which couples to at least one intermediate molecular moiety, the intermediate moiety, in turn, being capable of bonding the outer surface of the nanofiber to a matrix material; 3) reacting the chemically treated nanofibers with at least one or more inorganic or organic groups to build a chemical bridge or interface or interphase suitable to couple the nanofiber both chemically and structurally to the matrix material; that is, polymer chains are grown from the fiber surface using in-situ polymerization to form a molecular level composite material; and

4) forming a desired concentration of a nanofiber/polymer composite material. In certain embodiments, the present invention further includes: 5) net shape consolidating the composite to a desired shape and scale.

It should be understood that it is within the contemplated scope of the present invention that the method can be practiced using commercially available nanofibers (as described in step 1, and further that such fibers can be surface treated, as described in step 2). Step 4 can be considered to form a "nano-prepreg" which is then consolidated in Step

5 to full density under the influence of catalyst, pressure, and heat or other desirable energy source including microwave energy. The generic sequence of steps is illustrated conceptually in Figure 2, for the case of polymer nanocomposites. Fig. 2a illustrates a method including

1) and 2) forming and oxidizing nanofibers by a suitable chemical synthesis; 3) surface treating, by for example, chemically reacting a outer surface of the nanofiber with at least one suitable inorganic or organic moiety to form a desired chemically compatible outer surface which couples to at least one intermediate molecular moiety, the intermediate moiety, in turn, being capable of bonding the outer surface of the nanofiber to a matrix material; 4a) reacting the chemically treated nanofibers with at least one or more inorganic or organic groups to build a chemical bridge or interface or interphase suitable to couple the nanofiber both chemically and structurally to a thermoplastic matrix material; that is, thermoplastic polymer chains are grown from the fiber surface using in-situ polymerization to form a molecular level composite material; and 5a) forming a desired concentration of a nanofiber/polymer composite material.

Alternatively, the method includes steps 1) - 3), then

4b) reacting the chemically treated nanofibers with at least one or more oligomer groups to couple the nanofiber both chemically and structurally to a thermosetting matrix material; that is, thermosetting polymer chains are formed on the fiber surface using in-situ polymerization to form a molecular level composite material; and

5b) forming a desired concentration of a nanofiber/polymer composite material. It should be understood that it is within the contemplated scope of the present invention that the method can be practiced using various suitable polymer, ceramic and/or metal matrix materials.

Several embodiments of the present invention are described herein with illustrations of several different functionalization systems for thermoplastic and thermoset polymer functionalization and polymer matrices.

EXAMPLES

Materials : All monomers and solvents were purchased from Aldrich Chemical and purified prior to synthesis. Specific monomer and solvents used for different reactions will be described in subsequent examples. The oxidized carbon nanofibers (PR-24-PS-OX) were supplied by Applied Sciences with approximately 12 % surface oxidation, as measured by XPS as an O/C atomic ratio of 12%. It is to be understood that a useful range is approximately 1 to about 25%, as measured by XPS analysis as an O/C atomic ratio. In all cases, the nanofibers were used without further purification.

Examples IA and IB - Thermoplastic Polyimide- Carbon Nanofiber Composite Polyimide Functionalization

Consistent with the method of this invention schematically illustrated in Figure 2, carbon nanofibers used were synthesized and oxidized to provide oxidation on the surfaces characterized by XPS to have O/C ratio in the range 1.5%-12.5%. Formation of nanocomposites by in-situ polymerization of functionalized fibers is illustrated using the polyimide system. Polyimides are synthesized using a solution phase condensation reaction of a diamine and dianhydride. In Step 3, the oxidized carbon nanofibers were functionalized with a diamine monomer using a two-reaction process (see the reaction sequence in Figure 3). The carboxylic acid (CO₂FI) groups on the nanofiber surface were initially reacted with thionyl chloride (SOCl₂) followed by reaction with 1,3' phenylyene diamine (mPDA) to place a reactive amine group on the nanofiber surface. Raman spectroscopy was used to confirm each step of the functionalization chemistry. Following the functionalization step 3, the nanofibers (designated PR24-PDA have a free amine group on their surface that is utilized in the in-situ polymerization step 4). It is to be understood that, while a similar approach is useful with nanofibers for the vinyl ester and PMMA systems, the monomer used for the final reaction step was selected to be compatible with the matrix material. Example IA:

Prior to any reactions, dimethyl acetimide (DMAC) was refluxed under argon in the presence OfCaH₂ for three to four hours followed by distillation. The triethylamine and acetic acid were also distilled to remove any impurities. The diahydride, 4,4'- (Hexafluoroisopropylidene diphthalic anhydride) (6FD A) and diamines, 1,3' phenylyene diamine (mPDA) and 1,4' phenylyene diamine (pPDA) were purified using vacuum sublimation.

For the first reaction of the functionalization step 3, 0.60 g PR24-OX oxidized nanofibers were stirred in 25 ml of thionyl chloride at 80 ⁰C under an argon atmosphere for 24 hours. The thionyl chloride was evaporated and nanofibers were dried in a vacuum oven at 80 ⁰C for 12 hours to remove residual reactant and any byproducts. The final mass of powder was 0.60 g. Nanofibers following treatment with thionyl chloride are designated as PR24-C1. For the second reaction of the functionalization step 3, PR24-C1 nanofibers were reacted with an excess of the diamine mPDA. Specifically, 3.1 g mPDA was mixed with 0.42 g PR24-C1 under argon atmosphere and heated to 90 ⁰C where the diamine melted. The heated mixture was stirred continuously for one to six days to allow for reaction of one amine group of the diamine with the chloride groups on the nanofiber surface. The nanofibers were subsequently dried under vacuum at 100 ⁰C to evaporate any unreacted diamine and designated PR24-PDA. Characterization of Fibers and Composites:

The carbon nanofibers were analyzed following each reaction step using both Raman spectroscopy and thermal gravimetric analysis. The effect of functionalization on the disorder and graphite peak for the carbon nanofibers was monitored. A Jobin Yvon Horiba LabRam confocal Raman microscope with He/Ne laser at 632.8 nm wavelength. Raman analysis of four samples of each fiber batch was done. In addition, a TA Instruments thermal gravimetric analyzer Q50 was used to determine weight loss of the each type of carbon nanofiber. Dynamic mechanical analysis was performed on polymer films using a TA Instruments DMA Q800 series.

The oxidized carbon nanofibers (PR24-OX) were fϊinctionalized with acyl chloride to provide a very reactive intermediate for reaction with the diamine, 1,3 PDA. The efficiency of functionalization following thionyl chloride and diamine reactions was monitored using Raman spectroscopy. The Raman spectra of the pristine nanofibers exhibits a broad disorder peak at 1326 cm^"1 and tangential mode at 1574 cm^"1 with a very small shoulder peak at 1611 cm^"1 (Figure 7). The relative peak locations are given in Table 1. There is a significant increase in relative intensity of the disorder peak following oxidation and a shift of this peak to higher wavelengths (1335 cm^"1) that is similar to result for oxidized nanotubes reported in the literature, hi addition, the shoulder peak is shifted down to 1598 cm^"1 and increased in relative intensity to the peak at 1577 cm^"1. Functionalization with the acyl chloride results in further increase in relative intensity of the disorder peak and a large shift in metallic peak to 1585 cm^"1. Finally, functionalization with the diamine, PDA, results in a shift of the metallic peak down to 1575 cm^"1. The impact of functionalization on relative peak intensities and location are consistent with those for SWNT reported in the literature. Also, the results for thermal gravimetric analysis of these fibers confirm Raman results and show increased weight loss for fibers functionalized with diamine moiety relative to oxidized samples.

Table 1: Location of Raman Peaks in the Functionalized Fibers

Peak Location (cm^"1)

Fiber Type

Peak l Peak 2 Peak 3

PR 24 1327 ± 2.5 1574 ± 3.0 1612 ± 0.8

PR24-OX 1335 ± 1.8 1578 ±2.2 1598 ± 1.1

PR24-C1 1329 ± 2.7 1585 ± 1.3 ^" 1598 ± 1.1

PR24-PDA 1330 ± 2.3 1575 ± 0.4 1599 ± 0.1

In-Situ Polyimide Polymerization

The polyimide was synthesized in the presence of PR24-PDA nanofibers from a fluorine containing diahydride (6FDA) and diamine (PDA) using well established methods. The procedure used for polyimide synthesis was modified slightly to account for the presence of reactive groups on the nanofiber surface. A polyimide composite (designated 6FD A-PD A- PR24) of high molecular weight was successfully formed in the presence of the PR24-PDA fibers (see the reaction sequence in Figure 4). It is within the contemplated scope of the present invention that the method of the present invention includes a further modified synthesis procedure to increase the polymer molecular weight and increase the loading of nanofibers within the polymer matrix.

Similar results have been obtained for the vinyl ester system, where samples with oxidized fiber loadings of up to 4 wt% have been made. Example IB: Initially 0.055 g functionalized nanocarbon fiber was suspended in 65 ml DMAC and shaken overnight. To this solution, 0.84 g 1,4-PDA was added and stirred until the diamine dissolved completely. Finally, an equimolar quantity of the 6FDA was added to the reaction mixture and stirred for 12 hours at room temperature to produce the polyamic acid. A mixture consisting of 25 ml DMAC, 6 ml TEA and 4 ml acetic anhydride was added to the polymer solution and stirred for 1 hour. The temperature was increased to between 50 and 70 C for another hour. The solution was poured into methanol to precipitate the polyimide-fiber composite. The black precipitate was filtered and washed thoroughly with methanol to remove any unused reactants. Polymer was dried in vacuum oven at 50 ⁰C followed by heating at 200 ⁰C for 24 hours. The weight percentage of functionalized nanocarbon fiber was estimated to be 1%.

Polvimide Nanocomposite Characterization

Raman spectroscopy has been used to confirm that the fibers were modified by diamine functional groups. A TA Dynamic Mechanical Analysis (DMA) was used to assess the mechanical properties of the composite materials formed using in-situ polymerization. The results are compared to samples of blank polymer that have been mixed with functionalized fiber. In addition, DMA is used to monitor the impact of the nanofiber on the glassy state mobility of the polymer chains. Preliminary DMA results indicate that the 6FDA-PDA-PR24 improved mechanical properties relative to the blank polymer, 6FDA- PDA. In addition, a new high temperature sub-t_g transition appears in the 6FDA-PDA-PR24 that may indicate interaction between polymer chain and fiber. Polymerization and Composite Formation

The following three batches of the polyimide 6FD A-PDA were synthesized to determine impact of fiber functionalization on synthesis: (i) blank 6FD A-PDA with no fiber, (ii) 6FDA-PDA in presence of oxidized fiber (PR24-OX), and (iii) 6FDA-PDA in presence of functionalized fiber (PR24-PDA). The diamine ratio in the polyimide was 95 % pPDA and 5 % mPDA. The batch with the oxidized fibers was synthesized to determine whether the carboxylic acid groups on the fiber surface would participate in the polyimide reaction (Figure 4) and to provide comparison with the functionalized fiber. Note that the batch synthesized with the PR24-OX was approximately 3 wt % fiber and the batch with PR24- PDA was approximately 1 wt % fiber.

In all three cases, the polymer was of sufficient molecular weight to form films for mechanical testing. Each batch was tested to determine impact of fibers on molecular weight of the polyimide. If a reaction occurred at the fiber surface, then there would be two populations of polymer within the polymerization batch, unbound polyimide and polyimide that is covalently bound to the fiber surface. Since the local environment will affect the molecular motions of polymer chains, dynamic mechanical analysis was used to monitor impact of fiber the glassy state transition temperatures of the polyimide. The tan delta as a function of temperature in each batch polymer is shown in Figure 8. The glassy state transition as a function of temperature is shown in Figure 9. The three polymers exhibit very similar data.

Raman analysis and thermal gravimetric analysis indicate that the carbon nanofibers were functionalized with the diamine, PDA, via intermediate reaction with thionyl chloride. In-situ polymerization of the 6FD A-PD A in the presence of either oxidized or functionalized fiber resulted in formation of high molecular weight polymer. Example II - Thermoset Polymer - Carbon Nanofiber- Composite

While the above example discussed a polyimide system, similar approaches are useful for nanocomposites from other classes of polymers, such as, vinyl ester resin and polymethyl methacrylate (PMMA) matrices. These very different polymer systems represent a wide spectrum of matrix materials and applications. For example, the polyimide is a high temperature thermoplastic that is synthesized using step growth or condensation polymerization and the PMMA represents a commodity polymer that is synthesized using free radical chemistry. The vinyl ester-styrene resin represents a thermosetting material of considerable commercial interest for high temperature applications.

Thermoset Polymer — Carbon Nanofiber Composite Typically, thermosetting materials such as vinyl ester or epoxy resins are polymerized using a mixture crosslinking group (DGEBA) with two reactive groups (epoxy or vinyl end groups) and a reactive diluent (i.e., styrene) using free radical polymerization. Traditional approaches to forming composites using thermosetting resins as matrix material can be separated into to primary categories: (i) mixing of fiber with resin followed by a curing process and (ii) vacuum assisted resin transfer molding (VARTM) processing. Consistent with the organic chemistry of thermoset polymer precursor molecules, numerous types of thermoset functionalities can be bonded to the carbon nanofibers via the reaction of the thermoset precursor with carboxylic acid, amide or vinyl groups on the carbon nanofibers. Examples of reaction paths to achieve functionalization and nanocomposite formation by epoxide and vinylester polymers are shown below.

Incorporation of the functionalized fibers within the thermosetting matrix using traditional processing techniques is demonstrated for the vinyl ester system. Specifically, the functionalized nanofibers were incorporated within a vinyl ester matrix consisting of a commercially available vinyl ester (DGEBA) crosslinked with styrene as the matrix material. Specifically, the matrix material consisted of the DGEBA based vinyl ester moiety diluted with styrene to 35 % styrene which was purchased from Ashland Chemical Company. The final matrix material was further diluted with styrene to form a mixture that was 50 % by weight styrene and 50 % vinyl ester. The fibers were functionalized with a DGEBA group that mimicked the structure of the repeat unit of the vinyl ester resin. Note that the group on the fiber surface not only provides a reactive end group but also improves fiber-matrix compatibility and dispersion in the resin prior to crosslinking. Epoxide Functionalization

In the top reaction of the schematic reaction path shown in Figure 5, carboxylic acid functionalized carbon nanofibers as a slurry in acetone are reacted with a soluble diepoxide molecule in the presence of a catalyst. One end of the diepoxide (e.g. Bisphenol A) bonds by addition to the -OH group of the carboxylic acid, yielding an epoxide functionalized carbon nanofiber with an exposed reactive epoxide group capable of undergoing further reaction. Note that the R group can be varied to control diepoxide linkage properties (e.g., Bisphenol A). Also, note that there is no by-product formation.

To produce fibers with epoxide functionalities, two grams of the diepoxide (DGEBA) were added to 30 ml of NMP solvent and stirred under argon. Two grams of the functionalized fiber were suspended in a 30 ml of hexane and sonicated for several minutes to improve fiber dispersion. The fiber solution was slowly added to the DGEBA mixture and stirred at room temperature. To this mixture 0.60 g of catalyst (Imidazole) and 0.2 g of hydroquinone were added and the temperature was increased to 100⁰C. The solution was stirred under reflux at 100 ⁰C for two hours. Fibers were filtered, thoroughly washed with acetone to remove unused reactant and dried in vacuum oven. Following this step the fibers have a DGEBA bound to the surface with a free reactive epoxide end group and are designated PS-24-DGEBA. Note that with the reactive epoxide groups that these fibers are useful in the formation of epoxy based thermosetting resins. In addition, this is the base material for production of vinyl ester functionalized fibers as described in subsequent example. Vinylester Functionalization

In the schematic reaction path shown in Figure 6, the epoxide functionalized carbon nanofiber from the epoxy functionalization described above is first converted to a slurry in acetone. To this slurry is added a solution of methacrylic acid in acetone, thereby converting the reactive epoxide group to a reactive vinylester group. The vinylester functionalized carbon nanofiber formed has increased miscibility with vinylester reagents. This provides the basis for synthesis of vinylester-carbon-nanofiber composites by incorporating the functionalized fiber in a vinylester-styrene mixture, with polymerization initiated by addition of a catalyst.

To place a reactive vinyl ester group on surface, the 1.5 g of PS-24-DGEBA fibers were suspended in 50 ml of NMP to which 2 gram of methyl methacrylate (MMA) were added. Reaction was performed under reflux in an argon environment. The solution was mixed at room temperature for several minutes and 0.6 g of imidazole and 0.2 g of hydroquinone were added. The temperature was increased to 100 ⁰C and reaction proceeded for two hours. The fibers were filtered, washed with acetone and dried in vacuum oven. These fibers have reactive vinyl ester groups were covalently bound to the surface and are designated PS-24-VE.

Functionalized Fiber Reinforced Vinyl Ester Matrix

The vinyl ester resin purchased from Ashland Chemical Co. was 65 % DGEBA (crosslinking group) and 35 % styrene by weight. To further reduce the matrix viscosity, styrene was added to the dilute the vinyl ester so that the final mixture was 45 % DGEBA and 55 % styrene by weight. Methyl ethyl ketone peroxide (MEKP) was used as the catalyst and cobalt naphthenate (CoNap) was used as promoter. A 35.5 g plaque of composite that is 5 wt % fiber comprises 1.84 g functionalized fiber, 24.2 g neat resin, and 10.8 g of styrene. Initially, 1.84 g of PV-24-VE was mixed in styrene and sonicated to improve dispersion. The 24.2 g of neat vinyl ester resin was then added to the styrene solution and mixed carefully to avoid air bubbles. To this mixture, 0.052 g of the CoNap promoter was added and mixed until the promoter was completely dissolved. Then 0.007 g of dimethylaniline and 0.42 g of methyl ethyl ketone peroxide initiator were added and mixed slowly. The solution was poured into a mold and allowed to cure at room temperature for several hours followed by a cure in oven at 8O⁰C for to ensure complete crosslinking.

Example HI -Impact of Incorporating Carbon Nanofibers in Polyimide on Thermal- Mechanical Properties

The samples contain 1.5 wt % CNF with different surface chemistries within the polyimide, 6FD A-PD A, and methods of production. Figure 10 includes an average value for several samples of the storage modulus with a standard deviation of about. There is little change in the modulus for composites which contain the fibers with either oxidized or pristine surface. However, there is approximately 15 % increase for blending with monomer on surface and a 34 % increase for blending with oligomer. In-situ polymerization leads to approximately 27 % increases in modulus. Adding polymer chains to the fiber surface improves adhesion. The Figure 11 shows the percent difference in storage modulus for samples relative to the base PI.

The Figure 12 shows the glass transition temperature of the base polymer and each of the composites as measured by DMA and indicated by peak in the tan delta. The Tg of all the composite samples except one are close in value and are about 20 C higher than the value for the base polyimide. The exception is the polymer synthesized in the presence of the oxidized fiber which was lower in Tg value and was a bit brittle in general. This sample also exhibited lower modulus in general. This may be due to a relatively low molecular weight for this polymer, or due to low molecular weight polymer which is formed near fiber but not attached to the fiber that acts as a plasticizer. Example TV - Functionalization of Carbon Nanofibers

Since carbon nanofibers have very big specific surface area, they tend to agglomerate due to electrostatic and van der Waals forces, posing difficulty in the dispersion. Chemical modification of the surfaces by covalently bonding functional groups, namely, functionalization, is an effective way to address the dispersion problem. Meanwhile, functionalized carbon nanofibers (FCNFs) may become more compatible and have stronger interfacial adhesion to a matrix. Chemistry of Functionalization

Carbon nanofibers were supplied by Applied Science Inc. (OH, USA) and were oxidized with some oxygen atoms on the surfaces mainly in the form of carboxylic groups (- COOH). In this example, functionalization started from oxidized carbon nanofibers (OCNFs). Two-step chemistry was applied.

1,4-phenylenediamine (1,4-PDA or pPDA) (Aldrich, USA), one of the monomers for the synthesis of base PI, was used in the functionalizing agent. In the first step, approximately 0.1 g of purified oxidized carbon nanofibers and 5 ml thionyl chloride (SOCl₂) (Aldrich, USA) was put together to reflux for 24 hrs to produce fiber-COCl. Most of the excessive SOCl₂ was distilled and collected. Resulting fibers were dried in vacuum oven at 170 ⁰C overnight. Chemistry in the first step is shown in Figure 3. hi the second step, shown in Fig. 4, fiber-COCl, excessive pPDA and dimethyl acetimide (DMAc) (Aldrich, USA) were refluxed for 3 to 6 days. Before use of DMAc, purification of DMAc was refluxed under the protection of argon in the presence Of CaH₂ for three to four hours followed by distillation. After reaction, DMAc and excessive pPDA were removed by filtration, then by wash with acetone for several times. Functionalized fibers were obtained by dry in vacuum oven at 250 ⁰C for 24 hrs. Chemistry of the second step is shown in Figure 4. Functionalized carbon nanofibers were also further functionalized by polyimide oligomer with anhydride groups on the molecular chain ends. Oligomer was synthesized with the ratio of 6FDA to PDA of 3:2, in which pPDA over 1,3-phenylenediamine (1,3-PDA or rnPDA) (Aldrich, USA) was 80%:20%. Prepared oligomer was then reacted with fiber- pPDA in DMAc for 6 days. Fiber-oligomer was separated by filtration of the reacted mixture diluted by acetone. Separated fiber-oligomer was finally dried in vacuum oven in the same 5 way for fiber-pPDA. The functionalization of fibers by diamine or oligomer improves compatibility and interaction with PI.

A joint Yvon Horiba LabRam confocal Raman microscope with He/Ne laser at 632.8 nm wavelength was used to characterize functionalized fibers. Raman microscopy works as: When light is directed onto specimen, part of it is absorbed and re-emitted at a different

10 wavelength from incident light. The emitted light is collected through microscope and fed to diffraction spectrometer where a spectrum is recorded. The emitted light is unique to a particular material. The shift of peak in the spectra can indicate the impact of functional group attached on the surface of fibers.

In Raman spectra, there are primarily two peaks for carbon nanofibers. Disorder band

15. (D-band) peak centers around 1320 cm^"1 comes from disorder portion of fibers. Graphite-like band (G-band) is caused by carbon-carbon bond stretching tangentially and is between 1550 cm^"1 and 1590 cm^"1. After the surfaces of carbon nanofibers are attached with more oxidizing functional groups, D-band peak becomes larger and up-shifted, while G-band peak gets up- shifted. G-band peak is often split into two peaks indicating metallic property and semi-

20 conducting property, at lower and higher positions, respectively. Pristine fibers of PR-24-PS and oxidized fibers of PR-24-PS-OX were used in the experiment as references. Oxidized fibers were purified by nitric acid before use.

Fig. 13 shows Raman results for pristine fibers, oxidized fibers and functionalized fibers in different steps. There are differences for these fibers, which indicate the degrees of oxidation: 5 1) relative to the corresponding G-band main peak, height of the shoulder, which is caused by semiconducting property, varies with functional group attached. Relative heights are: Pristine fibers < Fiber-CONH- < Fiber-COOH < Fiber -COCl;

2) oxidizabilities of groups of -NH-, -OH and -Cl increase in this order as well, which means these group may have been attached on the surfaces of fibers; and 3) shifts of G-band main peaks which are caused by oxidation, can also be observed.

Peak positions from low to high are: Pristine fibers < Fiber-CONH- < Fiber-COOH < Fiber -

COCl.

The higher position the main peak, the more oxidized the fibers. This ranking is in agreement with the conclusion made above. The Raman results qualitatively indicate that desirable functional group in each step of functionalization may have been attached on the surfaces of fibers.

Example V- Nanocomposites

Composites were made by different ways, which were two categories, blending and in-situ polymerization. DMA results show improvements in modulus and T_g as well as possible interaction between the functionalized carbon nanofibers and the polyimide matrix.

TEM and SEM images indicate the distribution of fibers in polyimide

Synthesis of Polvimide

Three monomers, 2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FD A) (Clariant LSM Inc, USA), pPDA and mPD A were used in the synthesis of 6FDA-

PDA polyimide. All monomers were purified by sublimation. Catalysts, Triethylamine

(TEA) (Aldrich, USA) and acetic acid (AA) (Aldrich, USA) were distilled to remove impurities prior to use. The chemical structures of these monomers and synthesized polyimide are shown in Figures 14a and 14b, respectively. Synthesis reactions were carried out under the protection of argon. Typically, 50 ml

DMAc were added into a flask containing 6FDA, pPDA (80%) and mPDA (20%) with equimolar dianhydride and diamine of 5-15 mmol. Then, the solution was stirred in the temperature range of 50° to 58⁰C for 3 hrs to form a polyamic acid solution. Approximately,

40 mmol acetic anhydride and 40 mmol triethylamine were added into the reaction mixture. Subsequent imidization reaction to form polyimide was carried out at 50⁰C for 1 hr. Finally, the temperature will be increased to 100⁰C for 20 min. The chemistry of synthesis of polyimide is shown in Figure 14c.

The resulting solution was cooled down slightly and poured into 100 ml methanol to immediately form precipitates that were solid polyimide. The solids were separated from the solvents by filtration using coarse filter paper and dumped into a blender containing another

50 ml methanol to break into small particles, which were dried in hood for 12 hrs. The dried particles were put in blender to further break into fine powder. The powders were dried in vacuum oven at about 8O⁰C and 25O⁰C for 24 hrs, respectively. Dried powders were dissolved in methylene chloride and were filtered to remove impurities. Methylene chloride solution of polyimide was cast into a glass dish with cover placed in plastic bag for slow evaporation of methylene chloride to make film for further testing. Example VI -Composite formation

In the blending approach, oxidized carbon nanofibers or functionalized carbon nanofϊbers were dispersed in DMAc assisted by 15 to 30 minutes sonication. The suspension of oxidized carbon nanofibers or functionalized carbon nanofibers was mixed with the solution of base polyimide to make composite solution, which were then poured into methanol to obtain composite precipitates. The remaining procedure to produce composite film is the same as that will be used to make a polyimide film.

Figure 14d shows the in-situ polymerization reaction to make 6FD A-PDA PI based composite. In the in-situ polymerization method, diamine or oligomer-functionalized fibers (FCNFs)were added along with monomers in the polymerization reaction. All the other procedures were essentially the same as those for synthesis of the base polyimide. Example VTf -Composite Characterization - Composite of 1.5 % fibers

In the synthesis of control polyimide, the ratio of pPDA over mPDA was 80%:20%, which was a change a the ratio of 95%:5% in order to improve the solubility of the polyimide or the composites in methylene chloride.

Seven composites containing about 1.5 wt% fibers were made, as shown in Table 2 below. Ih the blending, for example, 0.024 g ground fibers were added into 10 ml DMAc and sonicated for about 30 min. The fiber suspension was dumped into hot DMAc solution of polyimide to form composite solution. After being vigorously stirred for about 5 min; the composite solution was transferred into a beaker containing about 100 ml methanol. Black composite precipitates were immediately formed. In the in-situ polymerization, for instance, 0.024 g ground functionalized carbon nanofibers were added into 10 ml DMAc followed by 10 min sonication before being transferred into reactor for in-situ polymerization. Table 2 Composites formed in different systems.

DMA analysis was carried out by heating from room temperature to 400⁰C at the rate of 10⁰C. DMA data show that, in most cases, there are very good improvements in both Tg and modulus. Figure 15 is an example for the comparison between composite and base polyimide. Figure 16 and Figure 17 show the improvements in Tg and modulus in all cases, respectively.

Composites except the one of In-Oxi-F, exhibit higher T_g, which is believed due to the impedance of fibers to the movement of polyimide chain in the glass transition region. Composites containing functionalized fibers have substantial improvements in both modulus and T_g, indicating that functionalized fibers have the best compatibility with the polyimide resin. Example VIII -Interaction between fiber and polvimide chains

DMA data show the interactions between functionalized fibers and polyimide chains. Blank polyimide, oxidized, carbon nanofibers in-situ polymerized composite and functionalized carbon nanofibers in-situ polymerized composite were tested by DMA. Fibers were functionalized by mPDA. In all three cases, the polymer was of sufficient molecular weight to form films for mechanical testing. While not wishing to be bound by theory it is believed that reactions occur at the fiber surfaces such that there are two populations of polymer within the polymerization batch: unbound polymer and polymer covalently bound to the fiber surface. Since the local environment affects the molecular motions of polymer chains, dynamic mechanical analysis was used to monitor impact of the fiber on the glassy state transition temperatures of the polyimide.

The Tan δ as a function of temperature in each batch of polymer is shown in Figure 18. The three polymers exhibit very similar glass transition temperatures in range between 360⁰C and 37O⁰C. The glass transition temperature of the polyimide synthesized in the presence of the functionalized fiber was slightly lower than the T_g for the other two polymers. In addition, this polymer had a much broader peak for the glass transition region.

The beta transition in aromatic polyimide has been attributed to motions about the imide linkage and is dependent upon the relative concentration of meta (mPDA = 1,3-PD A) and para (pPDA = 1,4PDA ) diamines. While the para connected polyimide will exhibit a broad peak at temperatures between 100⁰C to 15O⁰C, meta connected polyimides exhibit a beta transition that is at much higher temperatures and often shows up as a shoulder to the glass transition temperature. An expanded plot of tan delta for the polyimide composites in the region of the beta transition is given in Figure 19. The blank polyimide and polyimide formed in the presence of the oxidized fiber exhibit very similar beta transition which is expected for polymers with very high pPDA concentration.

The polyimide formed in presence of functionalized fiber exhibits two transitions, a diffuse peak similar to that of the blank polyimide and a shoulder peak centered near 25O⁰C. A polyimide that is covalently bound to the fiber surface has regions of decreased molecular mobility near fiber surface that could result in onset of new a sub-T_g transition temperature. In addition, the fibers were functionalized with the mPDA, such that when reacted with the dianhydrides, there was an increase the effective concentration of the more rigid meta connected group in polymer mixture. Therefore, the results from dynamic mechanical analysis indicate that polymer was formed on surface of functionalized fiber but not on the surface of the oxidized fiber. There was no shoulder peak in polyimide synthesized in presence of the oxidized fiber so little reaction appears to have occurred on surface of these fibers. Example IX - TEM Image

Specimens in the TEM test were examined by passing the electron beam through them, revealing information of the internal structure of specimens. A trial TEM test was done to form image of fibers in composite. 2.5 mg composite of B-F-PDA was dissolved in 10 ml methylene chloride. A TEM grid support was placed on the bottom of glass dish with diameter about 9 cm. The composite solution was gently dumped into the dish, which was then covered. Methylene chloride slowly evaporated away in hood. The grid support covered with one thin composite layer was carefully picked three hours later. The TEM image is shown in Figure 20. Figure 20 shows that there are both aggregated and individual fibers distributed in the composite. The aggregates may partially explain why this composite has lower modulus compared with the best; however, it has modulus higher than that of the base polyimide, it is believed, because of some fibers have been distributed individually. While the present invention has particularly been described with reference to the embodiments described herein, it should be readily understood to those of ordinary skill in the art that changes and modifications in form and detail can be made without departing from the spirit and scope of the invention.

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Claims

CLAIMSWe claim:

1. A method for manufacturing a nanocomposite comprising: reacting at least a portion of outer surfaces of a plurality of nanofibers with at least one suitable reactive group to form surface-treated nanofibers; and, reacting the surface-treated nanofibers, in situ, with at least one matrix material to form the nanocomposite.

2. The method of claim 1, wherein the in situ reaction of the surface-treated nanofibers with at least one matrix material comprises polymerization of the matrix.

3. The method of claim 1, wherein the at least one suitable reactive group forms a chemically compatible outer surface on the nanofibers capable of coupling to at least one intermediate molecular moiety, and further including reacting the surface-treated nanofibers with the at least one intermediate molecular moiety to form functionalized surface-treated nanofibers; and, thereafter, reacting the functionalized surface-treated nanofibers with the at least one matrix.

4. The method of claim 2, wherein the intermediate molecular moiety is further added when the functionalized surface-treated nanofibers are reacted with the matrix material, the intermediate molecular moiety being capable of polymerizing with the matrix material.

5. The method of claim 1, wherein the at least one suitable reactive group comprises an oxide, amine, amide, vinyl or sulfur functionality.

6. The method of claim 1, wherein the surface-treated nanofibers have a plurality of chemical bridges on their outer surfaces which bridges are capable of coupling the surface- treated nanofibers to the matrix material.

7. The method of claim 1, wherein the matrix material comprises at least one of a thermoset, thermoplastic, organic or inorganic elastomer polymer, ceramic, or metal matrix.

8. The method of claim 1, wherein the nanofibers comprise carbon or ceramic materials.

9. The method of claim 1, the nanofibers are oxidized prior to be reacted with the at least one suitable reactive group.

10. The method of claim 9, wherein the oxidized nanofibers are functionalized with a monomer using a two-reaction process.

11. The method of claim 10, wherein a first reaction comprises having carboxylic acid groups on the nanofiber surfaces react with a first reactive group to form surface-treated nanofibers, and wherein a second reaction comprises having the surface-treated nanofibers react with a second reactive group to form functionalized surface-treated nanofibers.

12. ^" The method of claim 10, wherein the matrix comprises monomers and/or oligomers capable of being polymerized.

13. The method of claim 12, wherein the matrix comprises thermoplastic polymers.

14. The method of claim 13, wherein the matrix comprises polyimide thermoplastic polymers.

15. The method of claim 14, wherein the polyimide matrix is synthesized in the presence of amine-functionalized-nanofibers from a fluorine containing diahydride and a diamine.

16. The method of claim 12, wherein the matrix comprises thermoset polymers.

17. The method of claim 16, wherein the thermoset polymer matrix is synthesized in the presence of a mixture comprising at least one crosslinking group with two reactive end groups and at least one reactive diluent.

18. The method of claim 17, wherein the reactive group on the surface of the nanofibers provides at least one of the reactive end groups capable of undergoing further reaction by being synthesized into the thermosetting matrix.

19. The method of claim 10, wherein the matrix comprises organic or inorganic elastomer polymers.

20. The method of claim 11, wherein the first reactive group comprising thionyl chloride to form chloride-surface-treated nanofibers, and wherein the second reactive group comprising a diamine, whereby a reactive amine group is formed on the nanofiber surfaces.

21. A method for forming an article comprising net shape consolidating the nanocomposite of claim 1 to a desired shape and scale.

22. The method of claim 21, wherein the consolidated nanocomposite is formed to full density under using at least one of a suitable catalyst, pressure, UV microwave energy, heat or other desirable energy source.

23. A nanocomposite material comprising nanofibers and at least one matrix material formed claim 1, based on at least one polymer, ceramic or metal matrix reinforced with a carbonaceous or ceramic reinforcement constituents of nanometer dimensions.