Microfiber and graphene reinforced polymer matrix composites and methods for preparing same

Abstract

The present disclosure provides novel microfiber and graphene reinforced polymer matrix composites and methods of forming them. The disclosed microfiber and graphene reinforced polymer matrix composites exhibit improved mechanical properties such as stiffness, tensile strength, and impact energy absorption.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority under 35 U.S.C. § 119 to U.S. Provisional Patent Application No. 63 / 488,398, filed March 3, 2023, which is incorporated herein by reference in its entirety.

[0002] The present invention relates to a microfiber and graphene reinforced polymer matrix composite formed by reinforcing a graphene reinforced polymer matrix composite with microfibers. [Background technology]

[0003] Polymer compositions are increasingly being used in a wide range of applications traditionally reserved for other materials, such as metals. Polymers have many desirable physical properties and are lightweight and inexpensive. In addition, many polymeric materials can be formed into many different shapes and forms, exhibiting great flexibility in the form they assume, and can be used as coatings, dispersions, extrusion resins, pastes, powders, and the like.

[0004] There are various applications in which it may be desirable to use polymer compositions that require materials with mechanical strength properties comparable to those of metals. However, a significant number of polymer materials and composites do not inherently possess sufficient strength for many of these applications. For example, thermoplastic polymers inherently exhibit low tensile modulus, i.e., stiffness values ​​less than 5 GPa, thereby limiting the applicability of the polymer for any load-bearing application. Therefore, various modifications have been attempted to increase the mechanical properties of neat polymer compositions.

[0005] Fiber-reinforced polymers (FRPs), which contain polymers coated with micron-sized fibers on their surfaces, have been explored for some applications to improve the shear strength of composites. However, surface reinforcement alone between material interfaces limits the improvement of mechanical properties, thus preventing their use in commercial applications requiring more robust materials, such as aircraft and aerospace systems, automotive systems and vehicles, electronics, government defense / security, pressure vessels, and reaction chambers.

[0006] More recently, nanoscale graphene or fiber reinforcement has been utilized to increase the stiffness of composites. Nanoscale reinforcement requires distributing fiber materials or graphene into the molten polymer phase to achieve composite reinforcement that exhibits improved mechanical properties. The resulting fiber or graphene-thermoplastic composites have been shown to exhibit a 2- to 6-fold increase in stiffness compared to neat, unreinforced polymers. However, the stiffness of graphene-reinforced polymer-matrix composites remains significantly inferior to conventional materials such as aircraft-grade aluminum and other metals and alloys, due to the upper limit on composite stiffness according to the linear mixture law associated with composites.

[0007] Typical aircraft-grade aluminum used in automotive systems has a stiffness value of approximately 69 GPa. In comparison, the highest stiffness achieved by either microscale carbon fiber-reinforced polymers or nanoscale graphene-reinforced polymer composites is capped at 30 GPa. Therefore, there is a need to provide solutions that overcome this ultimate stiffness limit while simultaneously providing a lighter, more economically scalable material platform. One such proposed solution is a multiscale, dual-reinforced polymer matrix composite, i.e., a microfiber- and graphene-reinforced polymer matrix composite (referred to herein as MF-G-PMC), which includes a graphene-reinforced matrix composite and is further reinforced with a carbon-based microfiber reinforcement. Such multiscale reinforcement provides both nanoscale reinforcement between the graphene and polymer, and microscale reinforcement that provides additional reinforcement within the matrix composite.

[0008] Progress in developing low-cost methods for effectively producing microfiber- and graphene-reinforced polymer-matrix composites remains very slow. Some of the challenges currently affecting the development of MF-G-PMCs for practical applications include the high cost of materials and the impracticality of currently used chemical and / or mechanical manipulations for large-scale commercial production. Therefore, a low-cost method for producing MF-G-PMCs suitable for large-scale commercial production that offers numerous property advantages, such as lower material density, increased nonlinearity in mechanical properties, improved electrical and thermal conductivity, and transparency to X-rays and electromagnetic pulses (EMPs), would be desirable. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] U.S. Provisional Patent Application No. 63 / 488,398 [Patent Document 2] U.S. Patent No. 11,479,652 [Patent Document 3] U.S. Patent No. 11,098,175 [Patent Document 4] U.S. Patent No. 11,174,366 [Patent Document 5] U.S. Patent No. 6,962,431 [Patent Document 6] U.S. Patent No. 9,533,432 [Patent Document 7] U.S. Patent Publication No. 2022 / 0097259 [Patent Document 8] U.S. Patent No. 9,896,565 [Patent Document 9] U.S. Patent No. 11,059,945 [Patent Document 10] U.S. Patent No. 11,702,518 Summary of the Invention

[0010] The present disclosure relates to the discovery that microfiber and graphene reinforcement of thermoplastic polymer matrix composites can provide multiscale reinforcement, enabling very high stress transfer in the resulting composite. Accordingly, the present disclosure provides stiffer and stronger microfiber and graphene reinforced polymer matrix composites and methods of forming them.

[0011] One aspect of the present invention is a method for forming a microfiber and graphene reinforced polymer matrix composite, comprising: distributing graphite microparticles in a first molten thermoplastic polymer phase comprising at least one thermoplastic polymer; exfoliating the graphite microparticles in the first molten thermoplastic polymer phase to obtain a graphene-reinforced polymer matrix composite by applying a first series of shear strain events to the first molten thermoplastic polymer phase such that the first molten thermoplastic polymer phase at least partially exfoliates the graphite microparticles into single-layer and multi-layer graphene nanoparticles, the shear strain events being equal to or greater than an interlaminar shear strength (ISS) of the graphite microparticles; distributing the microfibers and additional thermoplastic polymer into the graphene-reinforced polymer matrix composite while continuing to apply a second series of shear strain events until graphene fractures in the exfoliated single-layer and / or multi-layer graphene nanoparticles form across the basal plane defined by the a-axis and b-axis of the exfoliated particles, the edges of the graphene fractures containing reactive free-radical graphene carbon bonding sites that react with one or more molten thermoplastic polymers to covalently bond the thermoplastic polymer chains directly to the single-layer and / or multi-layer graphene nanoparticles and provide a composite intermolecularly crosslinked by the single-layer and / or multi-layer graphene nanoparticles; The present invention covers a method including:

[0012] In various embodiments, the first series of shear strain events can be applied until about 20% to about 100% of the graphite microparticles are exfoliated to form a distribution of single- and multi-layer graphene nanoparticles in the molten thermoplastic polymer phase. In some embodiments, the series of shear strain events can be applied until about 50% to about 100% of the graphite microparticles are exfoliated to form a distribution of single- and multi-layer graphene nanoparticles in the molten thermoplastic polymer phase. In some embodiments, the first molten thermoplastic polymer phase can comprise about 40 wt.% to about 100 wt.% of the total polymer weight. In various embodiments, the second molten thermoplastic polymer phase can comprise about 0 wt.% to about 60 wt.% of the total polymer weight.

[0013] In various embodiments, a first series of shear strain events can be applied until graphene breaks in the exfoliated single-layer and / or multi-layer graphene nanoparticles form across the basal plane defined by the a- and b-axes of the exfoliated particles, where the edges of the graphene breaks contain reactive free-radical graphene carbon bonding sites that react with thermoplastic polymer chains to covalently bond the thermoplastic polymer chains directly to the single-layer and multi-layer graphene nanoparticles, providing a composite intermolecularly crosslinked by the single-layer and multi-layer graphene nanoparticles.

[0014] Another aspect of the invention is directed to a microfiber and graphene reinforced polymer matrix composite comprising an essentially uniform distribution in a thermoplastic polymer matrix of 0.01 wt.% to about 50 wt.% graphene nanoparticles and graphite microparticles based on the total composite weight, about 10 wt.% to about 50 wt.% graphene based on the total composite weight, and about 5 wt.% to about 55 wt.% microfibers based on the total composite weight. In some embodiments, the microfiber and graphene reinforced polymer matrix composite can comprise about 20 wt.% to about 60 wt.% graphene nanoparticles, graphite microparticles, and microfibers based on the total composite weight.

[0015] In some embodiments of the microfiber and graphene-reinforced polymer matrix composites described above, the microfibers can be carbon fibers. In various embodiments, the carbon fibers can include single-walled or multi-walled carbon nanotubes (SWCNTs and MWCNTs, respectively), carbon nanofibers, micron-sized carbon fibers, chopped carbon fibers, and combinations thereof. In some embodiments, the microfibers can have lengths ranging from about 3 mm to about 50 mm. In various embodiments, the microfibers can have lengths ranging from about 10 mm to about 30 mm.

[0016] In various embodiments, the thermoplastic polymer can be selected from the group consisting of polyethylene terephthalate (PET), polyaryletherketone (PAEK), polyphenylene sulfide (PPS), polyethylene sulfide (PES), polyetherimide (PEI), polyvinylidene fluoride (PVDF), polysulfone (PSU), polycarbonate (PC), polyphenylene ether, thermoplastic polyimide, liquid crystal polymer, thermoplastic elastomer, polyethylene (PE), polypropylene (PP), polystyrene (PS), acrylic resins such as polymethyl methacrylate (PMMA), polyacrylonitrile (PAN), acrylonitrile butadiene styrene (ABS), polytetrafluoroethylene (PTFE / Teflon®), polyamide (PA), such as nylon, polyphenylene oxide (PPO), polyoxymethylene plastics (POM / acetal), polyvinyl chloride (PVC), and mixtures thereof.

[0017] In various embodiments, the prepared microfiber and graphene-reinforced polymer matrix composites can include microfibers distributed in a molten polymer phase containing exfoliated graphene, where the polymer is directly covalently crosslinked to the exfoliated graphene in the polymer phase.

[0018] Another aspect of the present invention is directed to a polymer composition comprising a host thermoplastic polymer and the microfiber- and graphene-reinforced polymer matrix composite of the present disclosure. In some embodiments, an automotive, aircraft, marine, or aerospace part can be formed from the graphene-reinforced polymer matrix composite disclosed above. In one embodiment, the part can be an engine part.

[0019] Yet another aspect of the present invention is directed to a high strength microfiber and graphene reinforced polymer matrix composite prepared by a method comprising: (a) forming a microfiber and graphene reinforced polymer matrix composite as disclosed above; and (b) distributing said polymer matrix composite in a molten, uncrosslinked host thermoplastic polymer phase.

[0020] In various embodiments, the host thermoplastic polymer can be selected from the group consisting of polyethylene terephthalate (PET), polyaryletherketone (PAEK), polyphenylene sulfide (PPS), polyethylene sulfide (PES), polyetherimide (PEI), polyvinylidene fluoride (PVDF), polysulfone (PSU), polycarbonate (PC), polyphenylene ether, thermoplastic polyimide, liquid crystal polymer, thermoplastic elastomer, polyethylene (PE), polypropylene (PP), polystyrene (PS), acrylic resins such as polymethyl methacrylate (PMMA), polyacrylonitrile (PAN), acrylonitrile butadiene styrene (ABS), polytetrafluoroethylene (PTFE / Teflon®), polyamide (PA), such as nylon, polyphenylene oxide (PPO), polyoxymethylene plastics (POM / acetal), polyvinyl chloride (PVC), and mixtures thereof.

[0021] The details of one or more aspects of the present disclosure are set forth in the following description. Other features, objects, and advantages of the techniques described in this disclosure will become apparent from the specification and claims. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 1 is a graph showing the tensile modulus of polyethylene terephthalate (PET) as a function of increasing graphene concentration, prepared using high shear melt processing of graphite microparticles. [Figure 2]FIG. 1 is a graph showing flexural stress-strain curves of graphene-reinforced polyethylene terephthalate (PET) as a function of increasing graphene concentration, prepared using high shear melt processing of graphite microparticles. [Figure 3] FIG. 1 is a graph showing the tensile modulus of neat and 35 wt.% graphene-reinforced thermoplastic polymer matrix composites prepared using high shear melt processing. [Figure 4A] 1 is a graph showing the tensile modulus of neat and reinforced PEEK, PA6, and PP polymers with 12 wt.% graphene and 40 wt.% carbon fiber loading. [Figure 4B] 1 is a graph showing the tensile strength of neat and reinforced PEEK, PA6, and PP polymers with 12 wt.% graphene and 40 wt.% carbon fiber loading. [Figure 5A] 1 is a graph showing the tensile modulus of neat and reinforced PEEK and PA6 polymers with 12 wt.% graphene and 40 wt.% carbon fiber loading. [Figure 5B] 1 is a graph showing the tensile strength of neat and reinforced PEEK and PA6 polymers with 12 wt.% graphene and 40 wt.% carbon fiber loading. [Figure 6A] 1 is a graph showing the flexural modulus of carbon fiber only PA6 composite samples and PA6 composite samples reinforced with carbon fiber and graphene. [Figure 6B] 1 is a graph showing flexural modulus as a function of graphene loading for PA6 composite samples with various total carbon loadings. [Figure 6C] 1 is a graph showing flexural modulus as a function of carbon fiber loading for PA6 composite samples with various total carbon loadings. [Figure 6D] 1 is a graph showing flexural modulus as a function of graphene to carbon fiber weight ratio for PA6 composite samples with various total carbon loadings. [Figure 7A]1 is a graph showing the tensile modulus of carbon fiber only PA6 composite samples and PA6 composite samples reinforced with carbon fiber and graphene. [Figure 7B] 1 is a graph showing tensile modulus as a function of graphene loading for PA6 composite samples with various total carbon loadings. [Figure 7C] 1 is a graph showing tensile modulus as a function of carbon fiber loading for PA6 composite samples with various total carbon loadings. [Figure 7D] FIG. 1 is a graph showing tensile modulus as a function of graphene to carbon fiber weight ratio for PA6 composite samples with various total carbon loadings. [Figure 8A] 1 is a graph showing the flexural modulus of carbon fiber only PET composite samples and PET composite samples reinforced with carbon fiber and graphene. [Figure 8B] 1 is a graph showing flexural modulus as a function of graphene loading for PET composite samples with various total carbon loadings. [Figure 8C] 1 is a graph showing flexural modulus as a function of carbon fiber loading for PET samples with various total carbon loadings. [Figure 8D] 1 is a graph showing flexural modulus as a function of graphene to carbon fiber weight ratio for PET samples with various total carbon loadings. [Figure 9A] 1 is a graph showing the tensile modulus of carbon fiber only PET composite samples and carbon fiber and graphene reinforced PET composite samples. [Figure 9B] 1 is a graph showing tensile modulus as a function of graphene loading for PET composite samples with various total carbon loadings. [Figure 9C] 1 is a graph showing tensile modulus as a function of carbon fiber loading for PET composite samples with various total carbon loadings. [Figure 9D] 1 is a graph showing tensile modulus as a function of graphene to carbon fiber weight ratio for PET composite samples with various total carbon loadings. DETAILED DESCRIPTION OF THE INVENTION

[0023] The present disclosure provides novel polymer matrix composites reinforced with microfibers (e.g., carbon fibers) and graphene, as well as methods for their preparation. The disclosed method is a low-cost, highly efficient mixing process that further converts graphene-reinforced polymer matrix composites into stronger polymer composites by introducing microfibers into the composite. The disclosed microfiber- and graphene-reinforced polymer matrix composites exhibit nonlinear improvements in mechanical properties with respect to the weight percentage of nano- and micro-reinforcements added, thus enabling the achievement of much higher stiffness values ​​than previously thought possible. The disclosed microfiber- and graphene-reinforced polymer matrix composites offer numerous property advantages compared to unreinforced polymer composites, including increased stiffness and tensile strength, improved electrical and thermal conductivity, and transparency to X-rays and electromagnetic pulses. Furthermore, these properties can be tuned by modifying the process.

[0024] As used in this document, the singular forms "a," "an," and "the" include plural references unless the context clearly dictates otherwise. All technical and scientific terms used herein unless otherwise defined.

[0025] As used herein, the term "and / or" means any one of the items, any combination of the items, or all of the items with which this term is associated.

[0026] The compositions of the present invention may comprise, consist essentially of, or consist of the claimed components. The words "comprising" (and all forms of comprising such as "comprise" and "comprises"), "having" (and all forms of having such as "have" and "has"), "including" (and all forms of including such as "includes" and "include"), or "containing" (and all forms of containing such as "contains" and "contain") are inclusive or open-ended and do not exclude additional, unrecited elements or method steps.

[0027] The publications disclosed herein are provided solely for their disclosure prior to the filing date of the present application. Nothing herein should be construed as an admission that the present invention is not entitled to antedate such publication by virtue of prior invention. Further, the publication dates provided may be different from the actual publication dates, which may need to be independently confirmed.

[0028] In one aspect, the present disclosure provides a method for forming a microfiber and graphene reinforced polymer matrix composite. In some embodiments, the method includes: distributing graphite microparticles in a first molten thermoplastic polymer phase comprising at least one thermoplastic polymer; exfoliating the graphite microparticles in the first molten thermoplastic polymer phase to obtain a graphene-reinforced polymer matrix composite by applying a first series of shear strain events to the first molten thermoplastic polymer phase such that the first molten thermoplastic polymer phase at least partially exfoliates the graphite microparticles into single-layer and multi-layer graphene nanoparticles, the shear strain events being equal to or greater than an interlaminar shear strength (ISS) of the graphite microparticles; distributing the microfibers and additional thermoplastic polymer into the graphene-reinforced polymer matrix composite while continuing to apply a second series of shear strain events until graphene fractures in the exfoliated single-layer and / or multi-layer graphene nanoparticles form across the basal plane defined by the a-axis and b-axis of the exfoliated particles, the edges of the graphene fractures containing reactive free-radical graphene carbon bonding sites that react with one or more molten thermoplastic polymers to covalently bond the thermoplastic polymer chains directly to the single-layer and / or multi-layer graphene nanoparticles and provide a composite intermolecularly crosslinked by the single-layer and / or multi-layer graphene nanoparticles; may include:

[0029] As defined herein, "essentially uniform distribution or dispersion" (and all forms of distribution and dispersion, such as "distributed" and "dispersed") indicates that the graphene particles are thoroughly mixed throughout the molten thermoplastic polymer phase, such that each individual aliquot of the composite contains the same amount of graphene within about 10 wt.% of the average value, preferably within about 5 wt.% of the average value, and more preferably within about 1 wt.% of the average value.

[0030] Graphite, the starting material for graphene, is composed of a layered, planar structure in which the carbon atoms in each layer are arranged in a hexagonal lattice. The planar layers have an "a" axis and a "b" axis, with the "c" axis defined as perpendicular to the plane defined by the "a" and "b" axes. Graphene particles produced by the methods of the present invention have an aspect ratio defined by the distance along the "a" or "b" axis divided by the distance along the "c" axis. The aspect ratio values ​​of the nanoparticles of the present invention are greater than 25:1, and typically range from 50:1 to 1000:1.

[0031] As used herein, the terms "graphite" or "graphite microparticles" refer to graphite in which at least 50% of the graphite consists of multilayer graphite crystals ranging in thickness from 1.0 to 1000 microns along the c-axis of the lattice structure. Typically, 75% of the graphite consists of crystals ranging in thickness from 100 to 750 microns. Expanded graphite can also be used. Expanded graphite is made by forcing the crystal lattice planes of natural flake graphite to separate, thus expanding the graphite, for example, by immersing the flake graphite in an acid bath of chromic acid followed by immersion in concentrated sulfuric acid. Expanded graphite suitable for use in the present invention includes expanded graphite with open ends at the bilayer level, such as MESOGRAF.

[0032] As used herein, the term "graphene" or "graphene nanoparticle" refers to the name given to a single layer of carbon atoms densely packed in a benzene ring structure. Graphene, when used alone, can refer to pure, uncontaminated forms of multilayer graphene, graphene flakes, graphene platelets, multilayer graphene, few-layer graphene, or single-layer graphene.

[0033] In various exemplary embodiments, the microfiber and graphene-reinforced composite matrix comprises carbon fiber. As used herein, the term "microfiber" refers to any high aspect ratio fibrous structure that can be mixed with a graphene-reinforced polymer matrix composite to further strengthen the composite. When used alone, microfiber can refer to glass fiber, carbon fiber, polymer fiber, metal fiber, or fiber produced from natural materials.

[0034] As used herein, the term "carbon fiber" refers to at least 92 wt.% elemental carbon bonded together in long chains. Carbon fiber, when used alone, can include one or more single-walled carbon nanotubes, multi-walled carbon nanotubes, carbon nanofibers, or micron-sized carbon fibers, in both chopped and unchopped form.

[0035] Mechanical functionalization of graphene nanoparticles within a polymer matrix can be achieved through polymer processing techniques that impart repetitive high shear strain events to exfoliate graphite microparticles into graphene nanoparticles within the polymer matrix. As used herein, the phrase "series of shear strain events" is defined as subjecting a molten polymer to an alternating series of higher and lower shear strain rates at essentially equal time intervals, such that the graphite particles in the molten polymer are subjected to a pulsating series of higher and lower shear forces related to the shear strain rate. "Shear strain" is defined as the application of opposing forces along or perpendicular to one or more planes to break van der Waals interactions within the graphite microparticles. Higher and lower shear strain rates are defined as a first higher shear strain rate being at least twice as large as a second lower shear strain rate. The first shear strain rate is typically between 100 and 10,000 seconds. -1The shear strain ranges from at least 1,000 to over 10,000,000 alternating pulses of higher and lower shear strain are applied to the molten polymer to form exfoliated graphene nanoparticles. The number of alternating pulses required to exfoliate graphite particles or graphite microparticles into graphene nanoparticles may depend on the size of the original graphite particles at the start of the process; i.e., smaller original graphite particles may require fewer alternating pulses to achieve graphene than larger original graphite particles. This can be readily determined without undue experimentation by one of ordinary skill in the art guided by this specification. After high-shear mixing, the graphite microparticles are exfoliated into multilayer and single-layer graphene nanoparticle sheets, which are uniformly dispersed in the molten polymer. Further discussion of methods for forming covalent conjugates of graphene and polymer chains is found, for example, in U.S. Pat. No. 11,479,652, the entire disclosure of which is incorporated herein by reference.

[0036] "Mechanical exfoliation," as used herein, refers to an in-situ exfoliation process that is distinct from other exfoliation methods, such as thermal treatment, chemical exfoliation, microwave treatment, ultrasonic treatment, and the like. An advantage of mechanical exfoliation is that a contaminant-free graphene-polymer interface is formed during high-shear mixing because the newly formed graphene interface or fracture is not exposed to air or other chemicals. This ensures strong interfacial adhesion or bonding. Other advantages of in-situ mechanical exfoliation of graphite microparticles include: (a) it is not necessary to preform graphene by other methods (e.g., chemical exfoliation, thermal exfoliation, microwave exfoliation, or ultrasonic exfoliation); (b) graphite is much cheaper than graphene; and (c) graphene is much easier to handle than graphene because graphene is flexible and difficult to distribute uniformly in a molten polymer phase. Further discussion of in-situ mechanical exfoliation is provided, for example, in U.S. Pat. Nos. 11,098,175 and 11,174,366, the entire disclosures of each of which are incorporated herein by reference.

[0037] To mechanically exfoliate graphite microparticles into single- and / or multi-layer graphene nanoparticles, the shear strain rate generated within the polymer during processing must produce a shear stress, or interlaminar shear strength (ISS), in the graphite microparticles greater than the critical stress required to separate the two layers of the graphite microparticles. The shear strain rate within the polymer is controlled by the type of polymer and processing parameters such as mixer geometry, processing temperature, and revolutions per minute (RPM).

[0038] The processing temperature and speed (RPM) required for a particular polymer can be determined from polymer rheology data, assuming that the shear strain rate (γ' (gamma dot)) is linearly dependent on the RPM at constant temperature, as shown by Equation 1. The mixer geometry is expressed as the rotor radius r and the rotor-to-barrel spacing Δr.

[0039]

number

[0040] The ISS of graphite ranges from 0.2 MPa to 7 GPa, but a new method quantified the ISS at 0.14 GPa. Therefore, the processing temperature, shear strain rate, and RPM required to mechanically exfoliate graphite within a polymer matrix during processing can be determined for a particular polymer from a logarithmic shear stress versus logarithmic shear strain rate graph collected for the polymer at a constant temperature, such that the shear stress within the polymer is equal to or greater than the ISS of the graphite. Under typical processing conditions, the polymer possesses sufficient surface energy to behave like the sticky side of adhesive tape, thus allowing for shared shear stress between the polymer melt and the graphite particles.

[0041] In one embodiment, the extrusion compounding element comprises a compounding section known as an axially grooved extensional mixing element or a helically grooved extensional mixing element, as described in U.S. Pat. No. 6,962,431, the disclosure of which is incorporated herein by reference. The compounding section acts to elongate the polymer and graphite microparticle stream, subsequently folding and stretching the material repeatedly. This results in excellent distributive mixing, which then induces the gradual fragmentation of the graphite microparticles. Batch mixers can also be equipped with equivalent mixing elements. In another embodiment, a standard injection molding machine is modified to replace the standard screw with a compounding screw for the purpose of compounding materials when injection molding the composition. Such a device is disclosed in U.S. Pat. No. 9,533,432 and U.S. Patent Publication No. 2022 / 0097259, the entire disclosures of each of which are incorporated herein by reference.

[0042] Automated extrusion systems equipped with mixing elements as described in U.S. Patent No. 6,962,431 and equipped with a recirculation stream to return the flow to the extruder throat can be used to make as many passes of the composite as desired. Because the processing of graphene-reinforced polymers is straightforward and minimizes material handling, fabrication costs are low.

[0043] In various embodiments, graphite microparticles are added to a molten polymer and mechanically exfoliated into graphene through a series of shear strain events. The graphite microparticles are typically 1,000 microns or less in size, and the degree of exfoliation of the graphite microparticles can typically be 1-100%, with a graphene-to-graphite weight ratio ranging from 1:99 to 100:0. Such exfoliation methods are disclosed in U.S. Patent No. 9,896,565, the entire disclosure of which is incorporated herein by reference.

[0044] It should be understood that essentially any polymer that is inert to graphite, graphite microparticles, or graphene nanoparticles and that is capable of imparting sufficient shear strain to exfoliate graphene from the graphite microparticles can be used in the methods of the present invention. Examples of such polymers include, but are not limited to, polyetheretherketone (PEEK), polyethylene terephthalate (PET), polyaryletherketone (PAEK), polyphenylene sulfide (PPS), polyethylene sulfide (PES), polyetherimide (PEI), polyvinylidene fluoride (PVDF), polysulfone (PSU), polycarbonate (PC), polyphenylene ether, thermoplastic polyimide, liquid crystal polymer, thermoplastic elastomer, polyethylene (PE), polypropylene (PP), polystyrene (PS), acrylic resins such as polymethyl methacrylate (PMMA), polyacrylonitrile (PAN), acrylonitrile butadiene styrene (ABS), polytetrafluoroethylene (PTFE / Teflon®), polyamides (PA) such as nylon, polyphenylene oxide (PPO), polyoxymethylene plastics (POM / acetal), polyvinyl chloride (PVC), and mixtures and the like thereof. Polymers that can wet the graphene surface and high melting point amorphous polymers can also be used in accordance with the methods of the present invention.

[0045] In various embodiments of the present invention, "polyaryletherketone" (PAEK) can include polymers characterized by a molecular backbone with alternating ketone and ether functional groups. The very rigid backbone gives such polymers very high glass transition temperatures and melting points compared to other plastics. The most common of these high-temperature resistant materials is polyetheretherketone (PEEK). Other representative examples of polyaryletherketones include PEKK (poly(etherketoneketone)), PEEEK (poly(etheretheretherketone)), PEEKK (poly(ether-etherketoneketone)), and PEKEKK (poly(etherketone-etherketoneketone)).

[0046] In various embodiments, the microfiber and graphene reinforced polymer matrix composite can include a distribution of particles selected from two or more of graphite microparticles, single-layer graphene nanoparticles, and multi-layer graphene nanoparticles in a thermoplastic polymer matrix at about 0.01 wt.% to about 50 wt.%, about 5 wt.% to about 40 wt.%, about 10 wt.% to about 30%, or about 15 wt.% to about 20 wt.% of the total composite weight, wherein at least 50 wt.% of the particles consist of single-layer and / or multi-layer graphene nanoparticles less than 50 nanometers thick along the c-axis direction. Non-limiting examples include particles selected from two or more of graphite microparticles, single-layer graphene nanoparticles, and multi-layer graphene nanoparticles that are about 0.01 wt.% to about 5 wt.%, about 5 wt.%, about 10 wt.%, about 15 wt.%, about 20 wt.%, about 25 wt.%, about 30 wt.%, about 35 wt.%, about 40 wt.%, about 45 wt.%, or about 50 wt.% of the total composite weight, wherein at least 50 wt.% of the particles consist of single-layer and / or multi-layer graphene nanoparticles less than 50 nanometers thick along the c-axis direction.

[0047] In some embodiments, the microfiber and graphene reinforced polymer matrix composite can include graphene in an amount between about 0.01 wt.% and about 50 wt.%, between about 5 wt.% and about 40 wt.%, between about 10 wt.% and about 30 wt.%, or between about 15 wt.% and about 20 wt.% of the total composite weight. Non-limiting examples include graphene in an amount between about 5 wt.%, about 10 wt.%, about 15 wt.%, about 20 wt.%, about 25 wt.%, about 30 wt.%, about 35 wt.%, about 40 wt.%, about 45 wt.%, or about 50 wt.% of the total composite weight.

[0048] In some embodiments, graphene-reinforced polymer matrix composites (G-PMCs) are further reinforced with microfibers by distributing one or more carbon-based microfiber materials into the graphene-reinforced polymer matrix composite under conditions suitable for mixing the microfibers with the G-PMC to obtain microfiber- and graphene-reinforced polymer matrix composites (MF-G-PMCs). Microfiber reinforcement can be applied to graphene-reinforced polymer matrix composites by conventional methods, such as twin-screw extrusion. As used herein, the terms "reinforced" or "reinforced" refer to the use of graphene and microfiber additives to reinforce a neat polymer or polymer composite to increase mechanical properties, such as stiffness and tensile strength, compared to the base material. Further discussion of the reinforcement of carbon-based nanomaterials with carbon fibers can be found, for example, in U.S. Pat. Nos. 11,059,945 and 11,702,518, the entire disclosures of each of which are incorporated herein by reference.

[0049] In various embodiments, the microfiber reinforcement can be mixed with the graphene-reinforced polymer matrix composite by conventional methods, such as twin-screw extrusion. The duration of the shear strain event to uniformly distribute the microfibers throughout the graphene-reinforced polymer matrix composite may depend on various parameters, including, but not limited to, fiber dimensions, mixing speed, tool radius, and extrusion system clearance. This can be readily determined without undue experimentation by those skilled in the art guided by this specification. After low-shear mixing, the microfiber reinforcement is uniformly mixed with the graphene-reinforced polymer matrix composite to produce a microfiber- and graphene-reinforced polymer matrix composite.

[0050] In some embodiments, various carbon-based microfibers can be utilized, such as, but not limited to, single-walled or multi-walled carbon nanotubes (SWCNTs and MWCNTs, respectively), carbon nanofibers, micron-sized carbon fibers, or combinations thereof. In some embodiments, the carbon fibers can include a coating to prevent clumping and fraying of the carbon fibers and to provide optimal mixing and compatibility with various types of polymers. In various embodiments, the coating can include bisphenol A glycidyl ether epoxy, glycidyl amine epoxy, unsaturated polyester, polyether copolymer, acrylic polymer, polyurethane, acrylonitrile butadiene styrene, or mixtures thereof.

[0051] In various embodiments, the microfiber and graphene reinforced polymer matrix composite can include about 5 wt.% to about 55 wt.%, about 10 wt.% to about 50 wt.%, about 20 wt.% to about 45 wt.%, or about 25 wt.% to about 40 wt.% of the total composite weight of microfiber. In various embodiments, the microfiber and graphene reinforced polymer matrix composite can include about 5 wt.% to about 15 wt.%, about 15 wt.% to about 25 wt.%, about 25 wt.% to about 35 wt.%, about 35 wt.% to about 45 wt.%, or about 45 wt.% to about 55 wt.% of the total composite weight of microfiber. Non-limiting examples include about 5 wt.%, about 10 wt.%, about 15 wt.%, about 20 wt.%, about 25 wt.%, about 30 wt.%, about 35 wt.%, about 40 wt.%, about 45 wt.%, about 50 wt.%, or about 55 wt.% of the total composite weight being microfibers.

[0052] In some embodiments, the amount of graphite and graphene added to the molten polymer can be up to and including the amount of microfiber component added (i.e., total carbon loading), provided that the total content of microfiber and the resulting graphene or graphite and graphene blend does not exceed 60 wt.%. Non-limiting examples include about 20 wt.%, about 25 wt.%, about 30 wt.%, about 35 wt.%, about 40 wt.%, about 45 wt.%, about 50 wt.%, about 55 wt.%, or about 60 wt.% of the total composite weight of graphite, graphene, and microfiber in the molten polymer. In some embodiments, the weight ratio of graphene, or a mixture of graphite and graphene, to microfibers (G:MF), when both graphene and microfibers are present, ranges from about 0.10 to about 10, from about 0.10 to about 2, from about 2 to about 4, from about 4 to about 6, from about 6 to about 8, or from about 8 to about 10. Non-limiting examples of graphite / graphene to microfiber ratios, when both graphene and microfibers are present, include about 0.5, about 1, about 1.5, about 2, about 2.5, about 3, about 3.5, about 4, about 4.5, about 5, about 5.5, about 6, about 6.5, about 7, about 7.5, about 8, about 8.5, about 9, about 9.5, or about 10.

[0053] Typically, the microfibers have a diameter of about 5 to about 10 microns. In various embodiments, the carbon fibers can be pre-cut or chopped before incorporation into the G-PMC. In some embodiments, the carbon fibers can have a length of about 3 mm to about 50 mm. In various embodiments, the carbon fibers can have a length of about 3 mm to about 5 mm, about 5 mm to about 10 mm, about 10 mm to about 20 mm, about 20 mm to about 30 mm, about 30 mm to about 40 mm, or about 40 mm to about 50 mm.

[0054] In some embodiments, exfoliation of graphite and graphene in a molten polymer can be accomplished in two steps. First, in a pre-exfoliation or first exfoliation step, graphite and graphene are introduced into a first molten polymer phase and subjected to a first series of shear strain events, resulting in exfoliation. In various embodiments, the first series of shear strain events can be applied until graphene fractures in the exfoliated single-layer and / or multi-layer graphene nanoparticles form across the basal plane defined by the a- and b-axes of the exfoliated particles, where the edges of the graphene fractures contain reactive free-radical graphene carbon-bonding sites that react with thermoplastic polymer chains to directly and covalently bond the thermoplastic polymer chains to the single-layer and multi-layer graphene nanoparticles, providing a composite intermolecularly crosslinked by the single-layer and multi-layer graphene nanoparticles. In various embodiments, the pre-exfoliation step may include a pre-exfoliation step in which the polymer is present in the molten polymer phase at a concentration of about 0.01 wt.% to about 50 wt.%, about 5 wt.% to about 45 wt.%, about 10 wt.% to about 40%, about 15 wt.% to about 35 wt.%, about 20 wt.% to about 30 wt.%, about 25 wt.% to about 35 wt.%, about 0.01 wt.% to about 10 wt.%, about 10 wt.% to about 20 wt.%, about 20 wt.% to about 30 wt.%, about 25 wt.% to about 35 wt.%, about 0.01 wt.% to about 10 wt.%, about 10 wt.%, or about 20 wt.% of the total composite weight. The composition may contain from about 15 wt.%, from about 15 wt.%, from about 20 wt.%, from about 20 wt.% to about 25 wt.%, from about 25 wt.% to about 30 wt.%, from about 30 wt.% to about 35 wt.%, from about 35 wt.% to about 40 wt.%, from about 40 wt.% to about 45 wt.%, or from about 45 wt.% to about 50 wt.% of graphite and graphene.

[0055] In some embodiments, the pre-stripping step can comprise about 40 wt.% to about 100 wt.%, about 50 wt.% to about 90 wt.%, about 60 wt.% to about 80 wt.%, about 40 wt.% to about 50 wt.%, about 50 wt.% to about 60 wt.%, about 60 wt.% to about 70 wt.%, about 70 wt.% to about 80 wt.%, about 80 wt.% to about 90 wt.%, or about 90 wt.% to about 100 wt.% of the total polymer weight of the first molten thermoplastic polymer. Non-limiting examples include the first molten thermoplastic polymer being up to about 100 wt.%, up to about 90 wt.%, up to about 80 wt.%, up to about 70 wt.%, up to about 60 wt.%, up to about 50 wt.%, or up to about 40 wt.% of the total polymer weight. In various embodiments, pre-exfoliation of graphite in the molten polymer as a result of repeated shear strain events can result in exfoliation of graphite and graphene from about 20% to about 100%, about 30% to about 90%, about 40% to about 80%, about 50% to about 70%, about 20% to about 30%, about 30% to about 40%, about 40% to about 50%, about 50% to about 60%, about 60% to about 70%, about 70% to about 80%, or about 80% to about 90%, or about 90% to about 100%. In various embodiments, partial or complete exfoliation of graphite into graphene can generate dangling bonds with vacant valences, i.e., free radicals, which provide opportunities for various chemical reactions and crosslinking with the molten polymer.

[0056] After the pre-exfoliation step, the pre-exfoliated product, which now contains the resulting partially or fully exfoliated graphite and graphene in the molten polymer, can be mixed with microfibers and an additional second molten polymer, and the microfibers and molten polymer can then be mixed with the pre-exfoliated product (i.e., the microfiber mixing step). In various embodiments, the microfiber mixing step can include microfibers in an amount of about 5 wt.% to about 55 wt.%, about 10 wt.% to about 50 wt.%, about 15 wt.% to about 45 wt.%, about 20 wt.% to about 40 wt.%, about 25 wt.% to about 35 wt.%, about 5 wt.% to about 10 wt.%, about 10 wt.% to about 15 wt.%, about 15 wt.% to about 20 wt.%, about 20 wt.% to about 25 wt.%, about 25 wt.% to about 30 wt.%, about 30 wt.% to about 35 wt.%, about 35 wt.% to about 40 wt.%, about 40 wt.% to about 45 wt.%, or about 45 wt.% to about 50 wt.% of the total composite weight.

[0057] In various embodiments, the incorporation of a second batch of molten polymer and subsequent mixing with the pre-exfoliated product through a second shear strain event can result in further exfoliation of the graphite particles in the molten polymer and tearing of the graphene flakes, thereby opening additional free radicals and providing further cross-linking with the molten polymer. In various embodiments, the second series of shear events can be applied until graphene breaks in the exfoliated monolayer and / or multilayer graphene nanoparticles form across the basal plane defined by the a- and b-axes of the exfoliated particles, where the edges of the graphene breaks contain reactive free radical graphene carbon bonding sites that react with one or more molten thermoplastic polymer chains to covalently bond the thermoplastic polymer chains directly to the monolayer and / or multilayer graphene nanoparticles and provide a composite intermolecularly cross-linked by the monolayer and / or multilayer graphene nanoparticles. In various embodiments, the mixing step can include about 0 wt.% to about 60 wt.%, about 10 wt.% to about 50 wt.%, about 20 wt.% to about 40 wt.%, about 0 wt.% to about 10 wt.%, about 10 wt.% to about 20 wt.%, about 20 wt.% to about 30 wt.%, about 30 wt.% to about 40 wt.%, about 40 wt.% to about 50 wt.%, or about 50 wt.% to about 60 wt.% of the total polymer weight of the second molten thermoplastic polymer. Non-limiting examples include up to 60 wt.%, up to about 50 wt.%, up to about 40 wt.%, up to about 30 wt.%, up to about 20 wt.%, up to about 10 wt.%, or up to about 0 wt.% of the total polymer weight of the second molten thermoplastic polymer.

[0058] In some embodiments, the microfiber and graphene-reinforced polymer matrix composite can be ground into particles and blended with a non-crosslinked host polymer to function as a toughening agent for the host polymer. The non-crosslinked polymer acquires the properties of the microfiber and graphene-reinforced polymer matrix composite due to chain entanglement between the two polymer species. Therefore, the present invention also includes crosslinked polymers of the present invention in particle form that can be blended with other polymers to form high-strength composites. In one embodiment, the microfiber and graphene-reinforced nylon-6 (PA6) and polyethylene terephthalate (PET) particles of the present invention can be used as a toughening agent for the host polymer. Compositions according to the present invention can include from about 1 wt.% to about 75 wt.%, from about 10 wt.% to about 60 wt.%, from about 20 wt.% to about 50 wt.%, or from about 30 wt.% to about 40 wt.% of a host thermoplastic polymer reinforced with the microfiber and graphene-reinforced polymer matrix composite particles of the present invention.

[0059] In some embodiments, examples of host polymers include, but are not limited to, polyethylene terephthalate (PET), polyaryletherketone (PAEK), polyphenylene sulfide (PPS), polyethylene sulfide (PES), polyetherimide (PEI), polyvinylidene fluoride (PVDF), polysulfone (PSU), polycarbonate (PC), polyphenylene ether, thermoplastic polyimide, liquid crystal polymer, thermoplastic elastomer, polyethylene (PE), polypropylene (PP), polystyrene (PS), acrylic resins such as polymethyl methacrylate (PMMA), polyacrylonitrile (PAN), acrylonitrile butadiene styrene (ABS), polytetrafluoroethylene (PTFE / Teflon®), polyamides (PA), such as nylon, polyphenylene oxide (PPO), polyoxymethylene plastics (POM / acetal), polyvinyl chloride (PVC), mixtures thereof, and the like. When the host polymer and the crosslinked polymer are the same polymeric species, the crosslinked polymer particles are essentially a concentrated masterbatch of the degree of crosslinked species desired to be incorporated into the polymer blend.

[0060] The microfiber- and graphene-reinforced polymer matrix composites of the present disclosure differ from conventional neat polymer or graphene-reinforced polymer matrix composites in that there is multiscale reinforcement, first at the nanolevel between the graphene and polymer matrix, and then at the microlevel between the graphene-polymer matrix composite and the microfiber reinforcement. It has been proposed that the introduction of a uniform distribution of microfiber reinforcement into a graphene-polymer matrix composite should nonlinearly enhance mechanical properties, such as the stiffness of the material, far beyond the linear enhancement observed in conventional graphene- or carbon fiber-reinforced polymer composites, which are limited by the rules of mixtures associated with composites.

[0061] This obstacle has now been overcome by a novel processing method in which graphite and graphene in a first molten polymer phase are mixed and exfoliated at high shear to produce a graphene-reinforced polymer matrix composite, and then a second molten thermoplastic polymer phase and microfiber reinforcement are co-distributed with the graphene-reinforced polymer matrix composite to continue exfoliating the graphite and graphene in the second molten thermoplastic polymer phase, resulting in a microfiber- and graphene-reinforced polymer matrix composite that exhibits superior mechanical properties compared to conventionally reinforced polymers, metals, and alloys.

[0062] The disclosed polymer compositions, comprising a host polymer and a polymer composite matrix reinforced with microfibers and graphene, have very high strength-to-weight ratios, making them suitable for automotive, aircraft, and aerospace applications. Therefore, the present invention also includes automotive, aircraft, and aerospace parts fabricated from the polymer compositions, which can replace heavier metal parts without sacrificing mechanical or thermal properties. For example, the polymer compositions have high melting points and creep resistance, allowing them to be used in engine components such as pistons, valves, camshafts, turbochargers, and the like. Forming the rotating portions of turbocharger turbine and compressor parts (including their respective blades) from the microfiber- and graphene-reinforced polymer matrix composites of the present invention reduces turbocharger lag while improving fuel economy through the resulting weight savings. In other embodiments, the polymer compositions can be used in automotive and aircraft interior and exterior components, such as door and floor panels, seat frames, consoles, bumpers, trunks and hoods, fender liners, suspension components, transmissions, and electronics enclosures.

[0063] Thus, the resulting microfiber and graphene reinforced polymer matrix composites (MF-G-PMCs) produced by various embodiments of the present invention exhibit higher mechanical properties, such as flexural and tensile moduli, compared to conventional materials such as neat polymers, fiber reinforced polymer composites, graphene reinforced polymer composites, aircraft grade aluminum, other metals and alloys, and subsequently, the fabrication costs of this multi-scale microfiber and graphene reinforced polymer matrix composite are significantly reduced. [Example]

[0064] This invention is further illustrated by the following examples, which should not be construed as limiting the invention. [Example]

[0065] Flexural properties of graphene-reinforced PET composites (G-PET) Graphite microparticles were fed directly into the hopper of a homogeneous high-shear injection molding machine along with polyethylene terephthalate (PET) using the method described in U.S. Pat. No. 9,896,565 (the entire disclosure of which is incorporated herein by reference) to produce PET matrix composites reinforced with 0, 5, 10, 15, 20, 25, 30, and 35 wt.% graphene. Using the high-shear processing method described above, the graphite microparticles were exfoliated into single- and multi-layer graphene nanoparticles and bonded with the molten polymer phase. The interactions in the graphene-reinforced polymer matrix result in efficient load transfer and increased mechanical properties. Stiffness and flexural properties were determined with increasing graphene wt.% reinforcement (see Figures 1 and 2). [Example]

[0066] Mechanical properties of graphene-reinforced polymer matrix composites Figure 3 shows the tensile modulus of various neat thermoplastic polymers and the same polymers reinforced with 35 wt.% graphene according to the embodiment described in Example 1. The thermoplastic polymers include high-density polyethylene (HDPE), polystyrene (PS), nylon 6,6 (PA66), polysulfone (PSU), polyphenylene sulfide (PPS), and polyetheretherketone (PEEK). Up to a six-fold improvement in stiffness was observed in all graphene-reinforced polymer matrix composites when 35 wt.% graphene was loaded into the thermoplastic polymers by a homogeneous high-shear process. [Example]

[0067] Mechanical properties of carbon fiber and graphene reinforced polymer matrix composites (CF-G-PMC) Table 1 shows the mechanical properties (e.g., stiffness and strength) of various materials, including neat polymer, polymer matrix composites reinforced with microfiber and graphene, and aircraft-grade aluminum (6061-T6) alloy. The table shows that the polymer (40CF-12G) reinforced with 40 wt.% carbon fiber and 12 wt.% graphene unexpectedly exhibited 25-fold and 3.6-fold increases in both stiffness and strength, respectively, compared to neat, unreinforced polyetheretherketone (PEEK), nylon 6 (PA6), and polypropylene (PP) polymers (Figures 4A and 4B).

[0068] [Table 1]

[0069] 5A and 5B show the stiffness and strength per density of 40CF-12G-PEEK and 30CF-12G-PA6 according to embodiments of the present invention. While aluminum (6061-T6) has higher stiffness and strength than 40CF-12G-PEEK and 40CF-12G-PA6 polymers, surprisingly, both 40CF-12G-PEEK and 40CF-12G-PA6 exhibit greater stiffness and tensile strength per density (i.e., specific stiffness and specific strength) compared to aluminum (6061-T6), making microfiber and graphene reinforced polymer matrix composites an ideal alternative to aircraft-grade aluminum, allowing them to support heavier payloads while reducing aircraft weight, thereby lowering fuel costs. [Example]

[0070] Carbon fiber and graphene reinforced polyamide 6 (CF-G-PA6) In this example, for the method described in U.S. Pat. No. 9,896,565 (the entire disclosure of which is incorporated herein by reference), carbon fiber and graphene reinforced PA6 were produced with increasing graphene / graphite to carbon fiber weight ratios (G:CF) of 20, 30, 35, 40, 45, and 50% total carbon loadings (graphene / graphite + carbon microfiber), respectively. CF-G-PA6 flexural properties Flexural tests were performed on an MTS QTest / 25 Universal Testing System at a spacing of 53.8 mm and a test speed of 1.38 mm / min until failure or 0.55% strain, whichever occurred first. Figure 6A shows the extrapolated modulus of a sample containing only carbon fiber (CF-PA6), i.e., 0 wt.% graphene loading, compared to a sample loaded with carbon fiber and graphene (CF-G-PA6). As shown in Table 2, flexural tests demonstrate that increasing the total carbon loading, i.e., the amount of graphene / graphite and carbon fiber present in the polymer matrix, increases the flexural modulus by nearly three-fold, from an average flexural modulus of 8.52 GPa at a total carbon loading of 20 wt.% to a maximum average flexural modulus of 23.36 GPa at a total carbon loading of 50 wt.% (see Figures 6B-6D). Additionally, within each total carbon loading band (i.e., 20, 30, 35, 50, 45, and 50%), the data show that the highest flexural modulus was observed at 5% graphene loading at total carbon loadings of 20%, 40%, and 50%. In comparison, the highest flexural modulus was observed at G:CF weight ratios of 0.3 and 0.75 at total carbon loadings of 30% and 35%, respectively, demonstrating the dual reinforcement effect of carbon fiber and graphene on flexural properties (see Figures 6B-6C).

[0071] [Table 2]

[0072] Tensile properties of CF-G-PA6 Tensile mechanical properties were measured on an MTS Qtest / 25 Universal Testing System at a crosshead speed of 5.08 mm / min until failure or the load frame limit. Figure 7A shows the extrapolated modulus of a sample containing only carbon fiber (CF-PA6), i.e., a sample with 0 wt.% graphene loading, compared to a PA6 sample reinforced with carbon fiber and graphene (CF-G-PA6). As shown in Table 3, tensile tests demonstrate that with increasing total carbon loading, the tensile modulus increases nearly threefold, from an average flexural modulus of 11.28 GPa at a total carbon loading of 20 wt.% to a maximum average flexural modulus of 31.80 GPa at a total carbon loading of 50 wt.% (see Figures 7B-7D). Additionally, within each total carbon loading band (i.e., 20, 30, 35, 50, 45, and 50%), the tensile modulus shows a general trend of increasing (decreasing) with increasing (decreasing) carbon fiber content (G:CF ratio) (see Figures 7B-7C).

[0073] [Table 3] [Example]

[0074] Carbon fiber and graphene reinforced polyethylene terephthalate (CF-G-PET) In this example, carbon fiber and graphene reinforced PET were produced with increasing ratios of graphene / graphite to carbon fiber (G:CF) at total carbon loadings of 35, 45, and 55 wt.%, respectively (graphene / graphite + carbon microfiber). Flexural properties of CF-G-PET Flexural tests were performed on an MTS QTest / 25 Universal Testing System at a spacing of 53.8 mm and a test speed of 1.38 mm / min until failure or 0.55% strain, whichever occurred first. Figure 8A shows the flexural modulus by total carbon loading (CF-G-PET) compared to samples loaded with carbon fiber only (CF-PET). On average, the CF-G-PET samples outperformed the CF-only samples, demonstrating that replacing some of the carbon fiber with graphene improved the flexural properties of the composites. As shown in Table 4, flexural tests further demonstrate that with increasing total carbon loading, the flexural modulus increases from an average modulus of 18.11 GPa at a total carbon loading of 35 wt.% to a maximum average modulus of 31.99 GPa at a total carbon loading of 55 wt.% (see Figures 8B-8D).

[0075] Bending tests show that the highest flexural modulus was observed when PET samples were loaded with both carbon fiber and graphene at each loading band (i.e., 35, 45, and 55%). For samples with a total carbon loading of 35 wt.%, the modulus increased by 23%, from 16.45 GPa for the CF-only sample (35CF-0G-PET) to a maximum modulus of 20.28 GPa for the 15CF-20G-PET sample. Additionally, for the 15CF-30G-PET sample, a 15% increase in modulus was observed at a total carbon loading of 45 wt.%, from 21.89 GPa for the CF-only sample to 25.24 GPa. Finally, for a total carbon loading of 55 wt.%, an 8%–11% increase was observed for all samples with a G:CF weight ratio between 0.1 and 1.2 compared to the baseline CF-only sample (55CF-0G-PET). The highest modulus was found at 27.79 GPa for the 25CF-30G-PET sample. A general trend of decreasing yield stress and yield strain with increasing G:CF ratio was observed at all total carbon loadings.

[0076] [Table 4]

[0077] Tensile properties of CF-G-PET Tensile properties were performed on an MTS Qtest / 25 Universal Testing System at a crosshead speed of 5.08 mm / min to failure or the limit of the loading frame. Figure 9A shows the tensile modulus of the carbon fiber and graphene-loaded sample (CF-G-PET) compared to the carbon fiber-loaded sample (CF-PET). The trend line showing the linear modulus of the CF-only sample indicates that the CF-G-PET sample outperformed the expected value compared to the CF-only sample. As the tensile data in Table 5 show, the 10CF-25G-PET sample exhibited an outlier in the tensile modulus data, indicating a flaw in the sample's manufacturing process.

[0078] Similar to the flexural properties, the highest flexural tensile modulus was observed when the PET samples were loaded with carbon fiber and graphene at each carbon loading band (i.e., 35, 45, and 55%). Tensile tests show that at a total carbon loading of 35 wt.%, the maximum tensile modulus of the 25CF-10G-PET sample was measured to be 23.70 GPa, an 18% increase compared to the CF-only sample (35CF-0G-PET). At a total carbon loading of 45 wt.%, the maximum tensile modulus of the 40CF-5G-PET sample was measured to be 31.20 GPa, an 16% increase compared to the CF-only sample (45CF-0G-PET). Finally, at a total carbon loading of 55%, the maximum tensile modulus of the 45CF-10G-PET sample was measured to be 40.79 GPa, a 7% increase compared to the CF-only sample (55CF-0G-PET). (See Figures 9B-9C.)

[0079] [Table 5]

[0080] Carbon fibers have the highest strength along the axial direction due to the high aspect ratio of the material, so replacing carbon fibers with more graphene removes strength along the axial direction. Therefore, the greatest increase in modulus from bending to tension is seen in samples containing the highest CF loadings, where the carbon fibers are more likely to align along the axial direction, i.e., parallel to the Type I tensile bar. Similar to the bending data, the yield stress, yield strain, and energy to break all decrease with increasing graphene content.

[0081] This disclosure is not limited to the particular systems, methodologies, or protocols described, as these may vary, and the terminology used in the description is for the purpose of describing particular versions or embodiments only, not limiting the scope.

[0082] The foregoing examples and description of the preferred embodiment should be construed as illustrative rather than limiting of the invention as defined by the claims. As will be readily appreciated, numerous variations and combinations of the features described above can be utilized without departing from the invention as set forth in the claims. Such variations are not to be considered as a departure from the spirit and scope of the invention, and all such variations are intended to be included within the scope of the following claims.

Claims

1. 1. A method of forming a microfiber and graphene reinforced polymer matrix composite, comprising: distributing graphite microparticles in a first molten thermoplastic polymer phase comprising at least one thermoplastic polymer; exfoliating the graphite microparticles in the first molten thermoplastic polymer phase by applying a first series of shear strain events to the first molten thermoplastic polymer phase such that the first molten thermoplastic polymer phase at least partially exfoliates the graphite microparticles into single-layer and multi-layer graphene nanoparticles to obtain a graphene-reinforced polymer matrix composite, wherein the shear strain events are equal to or greater than an interlaminar shear strength (ISS) of the graphite microparticles; distributing microfibers and additional thermoplastic polymer into the graphene-reinforced polymer matrix composite while continuing to apply a second series of shear strain events until graphene breaks in the exfoliated single-layer and / or multi-layer graphene nanoparticles form across the basal plane defined by the a-axis and b-axis of the exfoliated particles, the ends of the graphene breaks containing reactive free radical graphene carbon bonding sites that react with one or more of the molten thermoplastic polymer to covalently bond thermoplastic polymer chains directly to the single-layer and / or multi-layer graphene nanoparticles and provide a composite intermolecularly crosslinked by the single-layer and / or multi-layer graphene nanoparticles; A method comprising:

2. 10. The method of claim 1, wherein the first series of shear strain events can be applied until about 20% to about 100% of the graphite microparticles exfoliate to form a distribution of single- and multi-layer graphene nanoparticles in the first molten thermoplastic polymer phase.

3. 3. The method of claim 1 or 2, wherein the first series of shear strain events can be applied until about 50% to about 100% of the graphite microparticles exfoliate to form a distribution of single- and multi-layer graphene nanoparticles in the first molten thermoplastic polymer phase.

4. 4. The method of any one of claims 1 to 3, wherein the first molten thermoplastic polymer phase comprises from about 40 wt.% to about 100 wt.% of the total polymer weight.

5. 5. The method of any one of claims 1 to 4, wherein the second molten thermoplastic polymer phase comprises from about 0 wt. % to about 60 wt. % of the total polymer weight.

6. 6. The method of any one of claims 1 to 5, wherein the first series of shear strain events is applied until graphene breaks in the exfoliated single-layer and / or multi-layer graphene nanoparticles form across a basal plane defined by the a-axis and b-axis of the exfoliated particles, and the edges of the graphene breaks contain reactive free-radical graphene carbon bonding sites that react with thermoplastic polymer chains to covalently bond the thermoplastic polymer chains directly to the single-layer and multi-layer graphene nanoparticles and provide a composite intermolecularly crosslinked by the single-layer and multi-layer graphene nanoparticles.

7. 10. The method of claim 1 or 6, wherein the first and second thermoplastic polymer phases comprise the same thermoplastic polymer.

8. 8. The method of any one of claims 1 to 7, wherein the microfiber and graphene reinforced polymer matrix composite comprises about 0.01 wt. % to about 50 wt. % of the graphene nanoparticles and graphite microparticles, based on the total composite weight.

9. 9. The method of claim 1, wherein the microfiber and graphene reinforced polymer matrix composite comprises about 10 wt. % to about 50 wt. % of the graphene nanoparticles, based on the total composite weight.

10. 10. The method of claim 9, wherein the microfiber and graphene reinforced polymer matrix composite comprises about 30 wt. % to about 45 wt. % of the graphene nanoparticles based on the total composite weight.

11. 11. The method of any one of claims 1 to 10, wherein the microfiber and graphene reinforced polymer matrix composite comprises about 5 wt. % to about 55 wt. % of the microfiber, based on the total composite weight.

12. 12. The method of claim 11, wherein the microfiber and graphene reinforced polymer matrix composite comprises about 25 wt. % to about 45 wt. % of the microfiber, based on the total composite weight.

13. 13. The method of any one of claims 1 to 12, wherein the microfiber and graphene reinforced polymer matrix composite comprises about 20 wt.% to about 60 wt.% of graphene nanoparticles, graphite microparticles, and microfibers by total composite weight.

14. 14. The method of any one of claims 1 to 13, wherein the microfibers are selected from the group consisting of single-walled carbon nanotubes, multi-walled carbon nanotubes, carbon nanofibers, micron-sized carbon fibers, chopped carbon fibers, and combinations thereof.

15. The method of claim 14, wherein the microfibers have a length ranging from about 3 mm to about 50 mm.

16. The method of claim 15, wherein the microfibers have a length in the range of about 10 mm to about 30 mm.

17. 17. The method of any one of claims 1 to 16, wherein the thermoplastic polymer is selected from the group consisting of polyethylene terephthalate (PET), polyaryletherketone (PAEK), polyphenylene sulfide (PPS), polyethylene sulfide (PES), polyetherimide (PEI), polyvinylidene fluoride (PVDF), polysulfone (PSU), polycarbonate (PC), polyphenylene ether, thermoplastic polyimide, liquid crystal polymer, thermoplastic elastomer, polyethylene (PE), polypropylene (PP), polystyrene (PS), acrylic resins such as polymethyl methacrylate (PMMA), polyacrylonitrile (PAN), acrylonitrile butadiene styrene (ABS), polytetrafluoroethylene (PTFE / Teflon®), polyamide (PA), such as nylon, polyphenylene oxide (PPO), polyoxymethylene plastics (POM / acetal), polyvinyl chloride (PVC), and mixtures thereof.

18. 10. A microfiber and graphene reinforced polymer matrix composite prepared according to the method of claim 1, comprising microfibers distributed in a molten polymer phase containing exfoliated graphene, the polymer being directly covalently crosslinked to the exfoliated graphene in the polymer phase.

19. 20. The microfiber and graphene reinforced polymer matrix composite of claim 18, wherein the molten polymer is selected from the group consisting of polyamide 6, polyethylene terephthalate, polypropylene, polyether ether ketone, and combinations thereof.

20. 20. The microfiber and graphene reinforced polymer matrix composite of claim 18 or 19, wherein the microfibers are selected from the group consisting of single-walled carbon nanotubes, multi-walled carbon nanotubes, carbon nanofibers, micron-sized carbon fibers, chopped carbon fibers, and combinations thereof.

21. 21. The microfiber and graphene reinforced polymer matrix composite of any one of claims 18 to 20, comprising about 0.01 wt. % to about 50 wt. % of the graphene nanoparticles and graphite microparticles, based on the total composite weight.

22. 22. The microfiber and graphene reinforced polymer matrix composite of claim 21, comprising about 10 wt. % to about 50 wt. % of the graphene nanoparticles based on the total composite weight.

23. 23. The microfiber and graphene reinforced polymer matrix composite of any one of claims 18 to 22, comprising about 5 wt.% to about 55 wt.% of the microfibers, based on the total composite weight.

24. 24. The microfiber and graphene reinforced polymer matrix composite of claim 23, comprising about 20 wt.% to about 60 wt.% of the graphene nanoparticles, graphite microparticles, and microfibers by total composite weight.

25. 25. The microfiber and graphene reinforced polymer matrix composite of any one of claims 18 to 24, wherein the microfibers have a length ranging from about 3 mm to about 50 mm.

26. 25. The microfiber and graphene reinforced polymer matrix composite of any one of claims 18 to 24, wherein the microfibers have a length in the range of about 10 mm to about 30 mm.

27. 27. The microfiber and graphene reinforced polymer matrix composite of any one of claims 18 to 26, wherein the thermoplastic polymer is selected from the group consisting of polyethylene terephthalate (PET), polyaryletherketone (PAEK), polyphenylene sulfide (PPS), polyethylene sulfide (PES), polyetherimide (PEI), polyvinylidene fluoride (PVDF), polysulfone (PSU), polycarbonate (PC), polyphenylene ether, thermoplastic polyimide, liquid crystal polymer, thermoplastic elastomer, polyethylene (PE), polypropylene (PP), polystyrene (PS), acrylic resins such as polymethyl methacrylate (PMMA), polyacrylonitrile (PAN), acrylonitrile butadiene styrene (ABS), polytetrafluoroethylene (PTFE / Teflon®), polyamide (PA), such as nylon, polyphenylene oxide (PPO), polyoxymethylene plastics (POM / acetal), polyvinyl chloride (PVC), and mixtures thereof.

28. 28. A polymer composition comprising a host thermoplastic polymer and the microfiber and graphene reinforced polymer matrix composite of any one of claims 18 to 27 dispersed in said polymer.

29. 30. An automotive, aircraft, or aerospace part formed from the polymer composition of claim 28.

30. 30. The component of claim 29, which is an engine component.

31. 1. A method for forming a high strength polymer matrix composite reinforced with microfibers and graphene, comprising: forming a microfiber and graphene reinforced polymer matrix composite according to claim 14; distributing said polymer matrix composite in a non-crosslinked molten host thermoplastic polymer phase; A method comprising:

32. 32. The method of claim 31 , wherein the host thermoplastic polymer is selected from the group consisting of polyethylene terephthalate (PET), polyaryletherketone (PAEK), polyphenylene sulfide (PPS), polyethylene sulfide (PES), polyetherimide (PEI), polyvinylidene fluoride (PVDF), polysulfone (PSU), polycarbonate (PC), polyphenylene ether, thermoplastic polyimide, liquid crystal polymer, thermoplastic elastomer, polyethylene (PE), polypropylene (PP), polystyrene (PS), acrylic resins such as polymethyl methacrylate (PMMA), polyacrylonitrile (PAN), acrylonitrile butadiene styrene (ABS), polytetrafluoroethylene (PTFE / Teflon®), polyamides (PA) such as nylon, polyphenylene oxide (PPO), polyoxymethylene plastics (POM / acetal), polyvinyl chloride (PVC), and mixtures thereof.

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