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  Carbon nanotube reinforced thermoplastic polymer composites achieved through benzoyl peroxide initiated interfacial bonding to polymer matricesUSPTO Application #: 20070099792Title: Carbon nanotube reinforced thermoplastic polymer composites achieved through benzoyl peroxide initiated interfacial bonding to polymer matrices Abstract: In some embodiments, the present invention is directed to methods of fully integrating CNTs and the surrounding polymer matrix in CNT/polymer composites. In some such embodiments, such integration comprises interfacial covalent bonding between the CNTs and the polymer matrix. In some such embodiments, such interfacial covalent bonding is provided by a free radical reaction initiated during processing. In some such embodiments, such free radical initiation can be provided by benzoyl peroxide. In some or other embodiments, the present invention is directed to CNT/polymer composite systems, wherein the CNTs within such systems are covalently integrated with the polymer. In some or other embodiments, the present invention is directed to articles of manufacture made from such CNT/polymer composite systems. (end of abstract) Agent: Winstead Sechrest & Minick P.C. - Dallas, TX, US Inventors: Valery N. Khabashesku, Enrique V. Barrera, Daneesh McIntosh, Laura Para-Pena USPTO Applicaton #: 20070099792 - Class: 501095300 (USPTO) Related Patent Categories: Compositions: Ceramic, Ceramic Compositions, Refractory, Fiber Or Fiber Containing, Composites (continuous Matrix With Dispersed Fiber Phase), Whisker Containing The Patent Description & Claims data below is from USPTO Patent Application 20070099792. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This Application for Patent claims priority to U.S. Provisional Patent Application Ser. No. 60/675,383, filed Apr. 27, 2005. FIELD OF THE INVENTION [0003] The present invention relates generally to carbon nanotube/polymer composites, and specifically to fully integrating carbon nanotubes into thermoplastic matrices via interfacial covalent bonding. BACKGROUND OF THE INVENTION [0004] Carbon nanotubes (CNTs), comprising multiple concentric shells and termed multi-walled carbon nanotubes (MWNTs), were discovered by Iijima in 1991 (Iijima, Nature, 1991, 354, 56). Subsequent to this discovery, single-walled carbon nanotubes (SWNTs), comprising a single graphene sheet rolled up on itself, were synthesized in an arc-discharge process using carbon electrodes doped with transition metals (Iijima et al., Nature, 1993, 363, 603; and Bethune et al., Nature, 1993, 363, 605). [0005] SWNTs have highly anisotropic mechanical properties, however, by processing fully integrated single-walled carbon nanotube composites into nanotube continuous fibers (NCFs), their highly directional properties can be more effectively exploited (Barrera, J. of Mater. 2000, 52, 38). Manipulating these nanoscopic materials into an aligned configuration can be accomplished more easily by processing the composites into fibers, allowing for better macroscopic handling of these nano-sized materials. In some cases, the SWNTs are used as nanoscale reinforcements in a polymer matrix in order to take advantage of their high elastic modulus (approaching 1 TPa) and tensile strengths (in the range 20-200 GPa for individual nanotubes) (Krishnan et al., Phys. Rev. B. 1998, 58, 14013). SWNTs are, however, more likely to be incorporated in the matrix as ropes or bundles of nanotubes, as a result of van der Waals forces that hold many entangled ropes together. These ropes or bundles are reported as having tensile strengths in the range of 15-52 GPa (Shenderova et al., Critical Revs Solid State Mater. Sci. 2002, 27, 227; Treacy et al., Nature 1996, 381, 678; Lourie et al., Phys. Rev. Lett. 1998, 81, 1638). [0006] Polypropylene is a thermoplastic material that has excellent chemical resistance, and good mechanical properties with tensile strengths in the range of 30-38 MPa and tensile modulii ranging from 1.1-1.6 GPa for the bulk material (Hertzberg, R. W. Deformation and Fracture Mechanics of Engineering Materials. 4.sup.th Ed. Publ. John Wiley and Sons, 1996). A number of researchers, such as Kearns and Shambaugh (Kearns et al., J. Appl. Polym. Sci. 2002, 86, 2079), and Moore et al. (Moore et al., J. Appl. Polym. Sci. 2004, 93, 2926), have incorporated SWNTs into polypropylene matrices. Kearns and Shambaugh reported a 40% increase in fiber tensile strength for composites containing a 1 wt. % loading of SWNTs, while Moore et al. did not find any significant improvements in mechanical properties. These studies seem to indicate that efficient load transfer between the polymer matrix and the stronger, reinforcing SWNTs was not necessarily achieved. [0007] In processing carbon nanotubes and a thermoplastic matrix into a fully integrated composite system, the chemically inert nature of each of these materials must be overcome in order to facilitate good interfacial adhesion, which in turn allows for better load transfer when a tensile load is applied to the system. Ineffective interfacial bonding, and sliding of individual nanotubes within nanotube ropes, will hamper load transfer from the matrix to the fiber, thereby limiting the amount of mechanical reinforcement that can be achieved in the composite (Ajayan et al., Adv. Mater. 2000, 12, 750). [0008] As a result of the foregoing, a method for enhancing interfacial adhesion between the carbon nanotubes and the surrounding polymer matrix in such above-described composites, would be quite beneficial. BRIEF DESCRIPTION OF THE INVENTION [0009] In some embodiments, the present invention remedies the limitations of the aforementioned carbon nanotube (CNT)/polymer composites of the prior art by improving the interfacial adhesion between the CNTs and the polymer. Generally, such improvement is accomplished by fully integrating the CNTs into the polymer matrix. Accordingly, in some embodiments, the present invention is directed to methods of fully integrating CNTs and the surrounding polymer matrix in CNT/polymer composites. In some such embodiments, such integration comprises interfacial covalent bonding between the CNTs and the polymer matrix. In some such embodiments, such interfacial covalent bonding is provided by a free radical reaction initiated during processing. In other embodiments, the present invention is directed to fully integrated CNT/polymer composite systems made by such methods, and to articles of manufacture made with such CNT/polymer composite systems. [0010] In some embodiments, the present invention is directed to a method comprising the steps of: (a) dispersing CNTs, thermoplastic polymer, and a free radical precursor species in a solvent; (b) removing the solvent to form polymer-overcoated CNTs comprising free radical precursor; and (c) compounding the polymer-overcoated CNTs comprising free radical precursor to form a fully-integrated CNT/polymer composite comprising interfacial covalent bonding between the CNTs and the polymer, the polymer serving as a matrix. In some such embodiments, there is an additional step of processing the fully-integrated CNT/polymer composite in an extruder to form a nanotube continuous fiber (NCF) product. In some cases, the nanotubes of which the NCF is comprised are substantially aligned in the NCF product by virtue of shear forces associated with extrusion. [0011] In some embodiments, the present invention is directed to a CNT/polymer composite system comprising CNTs dispersed in a thermoplastic matrix, wherein the CNTs serve as reinforcement elements, and wherein the CNTs are covalently bonded to the thermoplastic matrix. In some or other embodiments, the present invention is directed to articles of manufacture made with the aforementioned CNT/polymer composite system. [0012] Particularly, in some embodiments, single-walled carbon nanotubes (SWNTs) can be more effectively exploited as reinforcement elements in a thermoplastic matrix via the formation of a fully integrated composite system in which the SWNTs are covalently linked to the surrounding polymer matrix, as is facilitated by the incorporation of benzoyl peroxide in the processing. SWNTs are ideal for incorporation into composite systems as they are nanoscopic and have excellent mechanical properties. Additionally, in some embodiments, a more effective load transfer can be accomplished by the formation of a more substantial interface between the thermoplastic matrix (e.g., polypropylene) and the SWNTs, as well as the alignment of the incorporated SWNTs in the axial direction, via, e.g., fiber spinning. [0013] In some such above-described embodiments, the introduction of benzoyl peroxide in the processing stages of the composites provides an initiator for a reaction in which radical sites are generated along the polypropylene chain, thereby creating an opportunity for the otherwise inert polymer to interact with the similarly inert SWNT, resulting in a covalent bond being formed between the polymer chains and the surface of the SWNTs. Further processing via fiber spinning results in a system in which the SWNTs are then aligned in the axial direction. The high crystallinity of isotactic polypropylene, depicted in FIG. 1, is exploited as the SWNTs, which are covalently linked to the molecular chains, are forced to order themselves along with the chains in the small diameter NCFs, as shown in FIG. 2. [0014] The foregoing has outlined rather broadly the features of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. BRIEF DESCRIPTION OF THE DRAWINGS [0015] For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: [0016] FIG. 1 depicts crystalline regions of isotactic polypropylene showing alignment; [0017] FIG. 2 depicts a nanotube continuous fiber (NCF) showing nanotubes covalently bonded to polypropylene chains and aligned in the axial direction; [0018] FIG. 3 (Scheme 1a) depicts benzoyl peroxide being decomposed to form phenyl radicals with the loss of carbon dioxide gas, and (Scheme 1b) the proposed reaction scheme for benzoyl peroxide initiated functionalization of SWNT (BP-f-SWNT), wherein the polypropylene polymer chains are covalently linked directly with the SWNT; [0019] FIG. 4 depicts comparative spectra for the 10 wt. % BP-f-SWNT and 10 wt. % P-SWNT in polypropylene; [0020] FIG. 5 depicts the differences in the intensities of the D and G peaks for a 10 wt. % sample of P-SWNT fiber; Continue reading... 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