FILLER PARTICLES FOR POLYMERS

Abstract

A method of forming a composite material includes photo-initiating a polymerization of a monomer in a pattern of interconnected units to form a polymer microlattice. Unpolymerized monomer is removed from the polymer microlattice. The polymer microlattice is coated with a metal. The metal-coated polymer microlattice is dispersed in a polymer matrix.

Claims

1. A method of forming a composite material, the method comprising: photo-initiating a polymerization of a monomer in a pattern of interconnected units to form a polymer microlattice; removing unpolymerized monomer from the polymer microlattice; coating the polymer microlattice with a metal; removing the polymer microlattice to form a metal microlattice; depositing graphitic carbon on the metal microlattice; converting the graphitic carbon to graphene; removing the metal microlattice to form a graphene microstructure; and dispersing the graphene microstructure in a polymer matrix.

2. The method of claim 1, wherein photo-initiating the polymerization of the monomer comprises passing collimated light through a photomask.

3. The method of claim 1, wherein photo-initiating the polymerization of the monomer comprises multi-photon lithography.

4. The method of claim 1, wherein coating the polymer microlattice with the metal comprises an electroless deposition of copper.

5. The method of claim 1, wherein coating the polymer microlattice with the metal comprises an electroless deposition of nickel.

6. The method of claim 1, wherein the polymer microlattice comprises polystyrene.

7. The method of claim 1, wherein the polymer microlattice comprises poly(methyl methacrylate).

8. A method of forming a composite material, the method comprising: photo-initiating a polymerization of a monomer in a pattern of interconnected units to form a polymer microlattice; removing unpolymerized monomer from the polymer microlattice; coating the polymer microlattice with a metal; and dispersing the metal-coated polymer microlattice in a polymer matrix.

9. The method of claim 8, wherein photo-initiating the polymerization of the monomer comprises passing collimated light through a photomask.

10. The method of claim 8, wherein photo-initiating the polymerization of the monomer comprises multi-photon lithography.

11. The method of claim 8, wherein coating the polymer microlattice with the metal comprises an electroless deposition of copper.

12. The method of claim 8, wherein coating the polymer microlattice with the metal comprises an electroless deposition of nickel.

13. The method of claim 8, wherein the polymer microlattice comprises polystyrene.

14. The method of claim 8, wherein the polymer microlattice comprises poly(methyl methacrylate).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] FIG. 1A is a schematic drawing of a fabrication process for a metal-based microlattice template in accordance with an example.

[0018] FIG. 1B is a flowchart for the fabrication process depicted schematically in FIG. 1A.

[0019] FIG. 2 is a schematic diagram of the fabrication of an electrically, thermally, or both thermally and electrically conductive composite by embedding a fabricated metal-based or graphene-based microlattice within an organic polymer matrix.

[0020] FIG. 3 is a schematic diagram of a surface functionalization methodology for functionalizing a fabricated metal-based microlattice.

[0021] FIG. 4 is a schematic diagram of example superlattice fillers being incorporated within an organic polymeric matrix such as polyvinylidene difluoride (PVDF), polyethylene, polybutadiene, polytetrafluoroethylene (PTFE), and the like.

DETAILED DESCRIPTION

[0022] An organic/inorganic superstructure may be used as a template for the formation of a 3D metal superstructure that may then be used to grow e.g. graphitic carbon on the surface of the metal. The template may be fabricated through a self-propagating photopolymer waveguide technique (see, e.g., Xiaoyu Zheng et. al., Ultralight, Ultrastiff Mechanical Metamaterials; Science 344 (2014) 1373-1377 and T. A. Schaedler, et al., Ultralight Metallic Microlattices; Science 334 (2011) 962-965). As illustrated schematically in FIG. 1A, an interconnected 3D photopolymer lattice may be produced upon exposure of an appropriate liquid photomonomer 16 to collimated UV light 12 through a specifically designed (e.g. using a computer-aided design model 10) digital mask 14 that contains openings with certain spacing and size. The fabricated organic polymer template microlattice 18 may then be then coated by electroless copper or other suitable metal (e.g. Ni, Co, Au, Ag, Cu, and alloys thereof) followed by etching away the organic polymeric matrix (scaffold). The resulting metal-based microlattice may be then used as a template to grow the graphitic carbon. The thickness of the electroless plated metal may be controlled in the range of nanometer to micrometer by adjusting the plating time, temperature, and/or plating chemistry.

[0023] FIG. 1A schematically illustrates an example fabrication process of organic polymeric microlattices (scaffolds) 18 before coating with electroless plating.

[0024] This description includes a “periodically structured” carbon nanostructure. Conventional carbon nanostructures are irregular and have much larger dimensions than those which may be achieved using the methodology described herein.

[0025] This process may be used to create a regular array, and the superstructure dimensions (unit) and structure may be optimized for strength, thermal and other fundamental properties.

[0026] Aspects of this procedure include: [0027] it provides a regular structure with defined dimensions; [0028] it can form very thin metal (e.g. Ni, Co, Cu, Ag, Au) microlattices; [0029] it enables the formation of graphitic carbon on very thin metals by a surface-limited process for very thin metal wires or tubes.

[0030] This process uses a polymeric structure as a template for such fabrication with the subsequent formation of a metal superstructure that may then be exposed to a hydrocarbon (e.g. methane, ethylene, acetylene, benzene) to form graphitic carbon, followed by etching of the metal from under the graphitic carbon using appropriate etchants such as, for example, FeCl.sub.3 or potassium permanganate.

[0031] Collimated UV light 12 through a photomask 14 or multi-photon lithography may be used in a photo-initiated polymerization to produce a polymer microlattice 18 comprising a plurality of interconnected units. Example polymers include polystyrene and poly(methyl methacrylate) (PMMA). After polymerization in the desired pattern, the remaining un-polymerized monomer may be removed.

[0032] The polymer structure (polymer scaffold) may then be plated with a suitable metal using an electroless plating process.

[0033] Electroless nickel plating (EN) is an auto-catalytic chemical technique that may be used to deposit a layer of nickel-phosphorus or nickel-boron alloy on a solid workpiece, such as metal, plastic, or ceramic. The process relies on the presence of a reducing agent, for example hydrated sodium hypophosphite (NaPO.sub.2H.sub.2.H.sub.2O) which reacts with the metal ions to deposit metal. Alloys with different percentages of phosphorus, ranging from 2-5 (low phosphorus) to up to 11-14 (high phosphorus) are possible. The metallurgical properties of the alloys depend on the percentage of phosphorus.

[0034] Electroless plating has several advantages over electroplating. Free from flux-density and power supply issues, it provides an even deposit regardless of workpiece geometry, and with the proper pre-plate catalyst, may deposit on non-conductive surfaces. In contradistinction, electroplating can only be performed on electrically conductive substrates.

[0035] Before performing electroless plating, the material to be plated must be cleaned by a series of chemicals; this is known as the pre-treatment process. Failure to remove unwanted “soils” from the part's surface results in poor plating. Each pre-treatment chemical must be followed by water rinsing (normally two to three times) to remove chemicals that may adhere to the surface. De-greasing removes oils from surfaces, whereas acid cleaning removes scaling.

[0036] Activation may be done with an immersion into a sensitizer/activator solution—for example, a mixture of palladium chloride, tin chloride, and hydrochloric acid. In the case of non-metallic substrates, a proprietary solution is often used.

[0037] The pre-treatment required for the deposition of metals on a non-conductive surface usually consists of an initial surface preparation to render the substrate hydrophilic. Following this initial step, the surface may be activated by a solution of a noble metal, e.g., palladium chloride. Electroless bath formation varies with the activator. The substrate is then ready for electroless deposition.

[0038] The reaction is accomplished when hydrogen is released by a reducing agent, normally sodium hypophosphite (with the hydrogen leaving as a hydride ion) or thiourea, and oxidized, thus producing a negative charge on the surface of the part. The most common electroless plating method is electroless nickel plating, although silver, gold and copper layers can also be applied in this manner.

[0039] In principle, any hydrogen-based reducing agent can be used although the redox potential of the reducing half-cell must be high enough to overcome the energy barriers inherent in liquid chemistry. Electroless nickel plating most often employs hypophosphite as the reducer while plating of other metals like silver, gold and copper typically makes use of low-molecular-weight aldehydes.

[0040] A benefit of this approach is that the technique can be used to plate diverse shapes and types of surfaces.

[0041] As illustrated in FIG. 1B, the organic polymeric microlattice may be electrolessly plated 20 with metal followed by dissolving out 22 the organic polymer scaffold. The resulting metal-based microlattice may be used in several applications 24—e.g. it may then be coated with a thin layer of immersion tin in order to prevent the metal from oxidizing during the subsequent process which may include a heat treatment. Alternatively, the surface of the metal-based microlattice may be functionalized with appropriate functional groups 26 in order to provide anchoring or reactions sites for subsequent interaction with a polymer matrix. A copper/nickel super-lattice may be used. The fabricated and surface-treated metallic network 30 may be embedded within an organic polymeric matrix 32 to fabricate an electrically or thermally or both an electrically and thermally conductive composite 34 (see FIG. 2). Alternatively, the fabricated metal-based microlattice may be used as a template 28 to synthesize a graphitic carbon superstructure. The metal may then be etched away to produce a graphene microstructure comprising a plurality of interconnected units wherein the units are formed of interconnected graphene tubes that are interconnected by chemical electronic bonds (as distinguished from van der Waals forces which may cause carbon nanotubes to agglomerate).

[0042] In another example, metal-coated organic polymeric microlattices may be used as fillers in a polymer matrix. A method of forming such a composite material may comprise: photo-initiating the polymerization of a monomer in a pattern of interconnected units to form polymer microlattices; removing unpolymerized monomer; coating the polymer microlattices with a metal; and dispersing the metal-coated polymer microlattices in a polymer matrix. As described above, the metal coating may be applied by an electroless process.

[0043] In yet another example, hexagonal boron nitride (h-BN) may be grown on the metal-based microlattice (in the place of graphene) to produce a ceramic microstructure that is an electrical insulator with high thermal conductivity. A process for growing h-BN on a metal-based microlattice is described in co-owned U.S. patent application Ser. No. 16/230,070 filed Dec. 21, 2018.

[0044] Other ceramic materials such as alumina (Al.sub.2O.sub.3), zirconia (ZrO.sub.2), titania (TiO.sub.2), multilayers of alumina-titania, multilayers of alumina-zirconia, etc. may be coated on the fabricated metal-based microlattice template using Atomic Layer Deposition (ALD), or chemical vapor deposition (CVD), or the like. Subsequent removal of the metal-based template (by e.g. etching) leaves a ceramic microlattice.

[0045] FIG. 3 schematically illustrates a surface treatment methodology for functionalizing the fabricated metal-based microlattice. As shown, the surface of the metal microlattice 36 may be exposed to a mercaptan-based compound. The mercaptan-based compound may be a hydroxylated alkyl mercaptan such as 3-mercapto-1,2 dihydroxy propane or it may be based on an isocyanate functional group such as isocyanate alkyl trialkoxy silane. The mercaptan-based compound bonds to the metal (such as e.g. copper, silver or gold) through the sulfur atom of the mercapto moiety resulting in a hydroxylated metal surface 38. The hydroxyl functional groups implanted on the metal surface may then be reacted with a reactive functional group from the pre-polymer matrix (which may comprise a mixture of oligomers). In the case of isocyanate-based mercaptan compounds, the free isocyanate functional groups may subsequently be reacted with certain functional groups of the organic polymeric matrix such as —OH, —NH, etc. resulting in the formation of chemical bonds at the metal-organic polymer interface. The polymer 42 for the composite 44 may be selected for its mechanical properties and/or electronic properties. Example polymers include fluorocarbons (such as polytetrafluoroethylene) and polybutadiene.

[0046] Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims.