Internally reinforced aerogel and uses thereof

Abstract

Internally reinforced aerogels, articles of manufacture and uses thereof are described. An internally reinforced aerogel includes an aerogel having a support at least partially penetrating the aerogel and having the aerogel penetrating the porous structure of the support.

Claims

1. A composite comprising: a non-fibrous organic polymer aerogel layer having a first surface and an opposing second surface; and a support layer having a first surface and an opposing second surface, wherein an interface is formed between a portion of the first surface of the aerogel layer and a portion of the second surface of the support layer such that the aerogel and support layers are attached to one another, wherein a majority of the volume of the aerogel layer does not include the support layer, wherein the aerogel layer has a thickness of at most 14 mils, wherein the support layer has a thickness of 0.5 mils to 2 mils; and wherein the composite has a thickness of 1.5 to 15 mils thick.

2. The composite of claim 1, wherein the support layer is integrated with the aerogel layer such that the support layer adheres to the aerogel layer without the use of an adhesive or binder.

3. The composite of claim 1, wherein the aerogel layer is a polyimide aerogel.

4. The composite of claim 1, wherein a ratio of the thickness of the support layer to the aerogel layer is 1:1 to 1:10.

5. The composite of claim 1, wherein a ratio of the thickness of the support layer to the aerogel layer is 1:10 to 1:50.

6. The composite of claim 1, wherein the composite has a flex fatigue of at least 100,000 cycles to failure.

7. The composite of claim 1, wherein the composite has a tensile strength of at least 15 MPa.

8. The composite of claim 1, wherein the support layer is an adhesive layer.

9. The composite of claim 1, wherein the support layer is a polymeric support layer.

10. The composite of claim 1, wherein the composite is comprised in an article of manufacture.

11. The composite of claim 10, wherein the article of manufacture comprises circuitry.

12. The composition of claim 11, wherein the composite provides thermal insulation to the circuitry.

13. The composite of claim 10, wherein the article of manufacture is a radiofrequency antenna, a radome, an apparel, a building, or an automobile.

14. The composition of claim 10, wherein the article of manufacture is an insulating material for an oil or gas pipeline or an aerospace application.

15. The composite of claim 10, wherein the article of manufacture comprises an RF substrate.

16. The composition of claim 15, wherein the RF substrate is transparent to RF radiation.

17. The composite of claim 10, wherein the article of manufacture comprises a substrate with a low dielectric constant.

18. The composite of claim 1, consisting of the non-fibrous aerogel layer and the support layer.

19. The composite of claim 1, wherein the entire interface is positioned within the volume of the non-fibrous aerogel layer.

20. The composite of claim 1, wherein the entire volume of the support layer is positioned in the volume of the non-fibrous aerogel layer.

21. The composite of claim 1, wherein a portion of the volume of the support layer is positioned within the volume of the non-fibrous aerogel layer and a second portion of the support layer is positioned outside the volume of the non-fibrous aerogel layer.

22. The composite of claim 1, wherein the composite is rolled-up such that the composite is in the form of a roll.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The following drawings form part of the present specification and are included to further demonstrate certain aspects of the present invention. The invention may be better understood by reference to one or more of these drawings in combination with the detailed description of the specification embodiments presented herein.

(2) FIG. 1 is an illustration of an embodiment of an internally reinforced aerogel having the support positioned at about the midline of the aerogel.

(3) FIG. 2 is an illustration of embodiment of an internally reinforced aerogel having the support positioned at internal offset position in the aerogel.

(4) FIG. 3 is an illustration of an embodiment of an internally reinforced aerogel having the support positioned at the edge of the aerogel.

(5) FIG. 4 is an illustration of an embodiment of an internally reinforced aerogel having the support positioned partially penetrating the aerogel.

(6) FIG. 5 is an illustration of an embodiment of an aerogel laminate comprising a plurality or reinforced aerogels formed into a multiplayer laminate structure.

(7) FIG. 6 is a schematic of system of an embodiment for filtering a fluid using a branched polyimide aerogel, the system having a separation zone, an inlet, and an outlet.

(8) FIG. 7 is a schematic of system of an embodiment for filtering a fluid using a branched polyimide aerogel, the system having a separation zone and an inlet.

(9) FIG. 8 is a schematic of system of an embodiment for filtering a fluid using a branched polyimide aerogel, the system having a separation zone and an outlet.

(10) While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and may herein be described in detail. The drawings may not be to scale.

DETAILED DESCRIPTION

(11) Aerogels are well-known for their low-density and effectiveness as thermal insulators. Aerogels are produced from a gel wherein the liquid component of the gel has been replaced with a gas. Aerogels consist of a highly porous network of micropores and mesopores—“micropores” being pores with diameters less than 2 nm, and “mesopores” being pores with diameters between 2 nm and 50 nm. The pores of an aerogel can frequently account for over 90% of the volume when the density of the aerogel about 0.05 g/cm.sup.3. Aerogels are generally prepared by a supercritical drying technique to remove the solvent from a gel (a solid network that encapsulates its solvent) such that no solvent evaporation can occur, and consequently no contraction of the gel can be brought by capillary forces at its surface. For polymer-based aerogels aerogel preparation typically proceeds as follows: (1) polymerization of the polymer gel; (2) formation of the gel; and (3) solvent removal by supercritical drying.

(12) During manufacture of a polyimide aerogel the inventors used a reinforcing support film as a carrier to support the gelled film during processing. During rewinding, the gelled film was unexpectedly and irreversibly pressed into the carrier film, providing a substantial durability improvement. The inventors have subsequently applied this observation to a solvent casting process where the aerogel is cast into a reinforcement or support to achieve maximum penetration. The substrate selection and direct casting have allowed the minimization of the thickness of the resulting reinforced aerogel material.

(13) The present invention also provides processes for the production of fiber reinforced polymer aerogels—internally reinforced polyimide aerogels are provided as an example. The process includes: (a) forming poly(amic acid) solution from a mixture of dianhydride and diamine monomers in a polar solvent such as dimethylsulfoxide (DMSO), N,N-dimethylacetamide (DMAc), N-methylpyrolidone (NMP), or N,N-dimethylformamide (DMF); (b) contacting the poly(amic acid) solution with chemical curing agents such as triethylamine and acetic anhydride to initiate chemical imidization; (c) casting the poly(amic acid) solution onto a fibrous support prior to gelation and allow it to permeate it; (d) allowing the catalyzed poly(amic acid) solution to gel around and into the fibrous support during chemical imidization; (e) optionally performing a solvent exchange, which can facilitate drying; and (f) removal of the transient liquid phase contained within the gel with supercritical, subcritical, or ambient drying to give an internally reinforced aerogel. The polyimide aerogels can be produced from aromatic dianhydride and diamine monomers, such as aromatic diamines or a mixture of at least one aromatic diamine monomer and at least one aliphatic diamine monomer. The resulting polyimide aerogel can be optimized to possess low density, meso-pores, narrow pore size distribution and good mechanical strength.

(14) The preparation of polyimide wet gels is a two-step procedure: (a) formation of the poly(amic acid) solution from a mixture of dianhydride and diamine in a polar solvent such as N,N-dimethylacetamide (DMAc), N-methylpyrolidone (NMP), N,N-dimethylformamide (DMF), or dimethylsulfoxide (DMSO); and (b) catalyzed cyclization with chemical catalyzing agents such as acetic anhydride and triethylamine to form polyimide. Previous work with synthesizing polyimide aerogels has shown that the first step typically requires at least 30 minutes mixing at room temperature allowing for significant formation of the poly(amic acid) polymer and yielding stable, robust wet gels. Gelation conditions depend on several factors, including the prepared density of the solution and the temperature of the heating oven. Higher density solutions will gel faster than lower density solutions. Once the system has reached the gelled state, the gels are rinsed repeatedly with acetone, ethanol, or the like. Rinsing occurs at least three times prior to drying, and serves to remove residual solvent and unreacted monomers. CO.sub.2 can then be used in techniques known to those in the art for wet solvent extraction to create the aerogel structure. Other techniques for preparing and optimizing polyimide aerogels can be used and are known in the art.

(15) A. Internally Reinforced Aerogels

(16) The internally reinforced aerogels can be any width or length. The internally reinforced aerogel can be in the form of defined geometry (e.g., a square or circular patch) or in the form of a sheet or roll. In some instances, the internally reinforced aerogels can have a width up to 6 meters and a length of up to 10 meters, or from 0.01 to 6 meters, 0.5 to 5 meters, 1 to 4 meters, or any range in between, and a length of 1 to 10,000 meters, 5 to 1,000 meters, 10 to 100 meters or any range there between. The width of the composite can be 0.01, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50, 0.55, 0.60, 0.65, 0.70, 0.75, 0.80, 0.85, 0.90, 0.95, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0 feet or meters, including any value there between. The length of the internally reinforced aerogels can be 1, 10, 100, 500, 1000, 1500, 2000, 2500, 3000, 3500, 4000, 4500, 5000, 5500, 6000, 6500, 7000, 7500, 8000, 8500, 9000, 9500, 10000 meters or feet and include any value there between. In certain aspect the length of the internally reinforced aerogel can be 1000 feet or meters, and 60 inches or 1.5 meters, respectively, in width. In a further embodiment the internally reinforced aerogel is 100 feet in length and 40 inches wide.

(17) In certain embodiments the internally reinforced aerogel includes a non-woven support at least partially or fully embedded or incorporated in a polymeric aerogel.

(18) The support can be comprised of a plurality of fibers. The fibers can be unidirectionally or omnidirectionally oriented. The support can comprise, by volume, at least 0.1 to 50% of the internally reinforced aerogel. The support can be in the form of a plurality of fibers, a film or layer of fibers, fiber containing films or layers, or a support film or layer comprising two or more fiber layers pressed together to form the support. The support can comprise cellulose fibers, glass fibers, carbon fibers, aramid fibers, polyethylene fibers, polyester fibers, polyamide fibers, ceramic fibers, basalt fibers, rock wool, or steel fibers, or mixtures thereof. The fibers can have an average filament cross sectional area of 7 μm.sup.2 to 800 μm.sup.2, which equates to an average diameter of 3 to 30 microns for circular fibers. Bundles of various kinds of fibers can be used depending on the use intended for the internally reinforced aerogel. For example, the bundles may be of carbon fibers or ceramic fibers, or of fibers that are precursors of carbon or ceramic, glass fibers, aramid fibers, or a mixture of different kinds of fiber. Bundles can include any number of fibers. For example, a bundle can include 400, 750, 800, 1375, 1000, 1500, 3000, 6000, 12000, 24000, 50000, or 60000 filaments. The fibers can have a filament diameter of 5 to 24 microns, 10 to 20 microns, or 12 to 15 microns or any range there between, or 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24 microns or any value there between. The fibers in a bundle of fibers can have an average filament cross sectional area of 7 μm.sup.2 to 800 μm.sup.2, which equates to an average diameter of 3 to 30 microns for circular fibers. Cellulose and paper supports can be obtained from Hirose Paper Mfg Co (Kochi, Japan) or Hirose Paper North America (Macon, Ga., USA).

(19) Non-limiting examples of thermoplastic polymers include polyethylene terephthalate (PET), a polycarbonate (PC) family of polymers, polybutylene terephthalate (PBT), poly(1,4-cyclohexylidene cyclohexane-1,4-dicarboxylate) (PCCD), glycol modified polycyclohexyl terephthalate (PCTG), poly(phenylene oxide) (PPO), polypropylene (PP), polyethylene (PE), polyvinyl chloride (PVC), polystyrene (PS), polymethyl methacrylate (PMMA), polyethyleneimine or polyetherimide (PEI) and their derivatives, thermoplastic elastomer (TPE), terephthalic acid (TPA) elastomers, poly(cyclohexanedimethylene terephthalate) (PCT), polyethylene naphthalate (PEN), polyamide (PA), polysulfone sulfonate (PSS), sulfonates of polysulfones, polyether ether ketone (PEEK), polyether ketone ketone (PEKK), acrylonitrile butyldiene styrene (ABS), polyphenylene sulfide (PPS), co-polymers thereof, or blends thereof.

(20) Non-limiting examples of thermoset polymers include unsaturated polyester resins, polyurethanes, polyoxybenzylmethylenglycolanhydride (e.g., bakelite), urea-formaldehyde, diallyl-phthalate, epoxy resin, epoxy vinylesters, polyimides, cyanate esters of polycyanurates, dicyclopentadiene, phenolics, benzoxazines, co-polymers thereof, or blends thereof.

(21) In other aspects the internally reinforced aerogel can includes two or more layers of a support. In certain instances a support can include two unidirectional supports in contact with each other and arranged such that the unidirectional fibers are oriented in different directions to each other. In other instances the support can comprises two or more layers of a support having omnidirectional fibers.

(22) The support is positioned at least partially or fully inside a polymeric aerogel, forming an internal support and an external aerogel. As used herein any support that is at least partially permeated with aerogel material is can be partially internalized. The width and length of the aerogel is substantially similar to the width and length of the internal or partially internalized support.

(23) FIG. 1 illustrates an internally reinforced aerogel having internal support 112 positioned at about the midline of aerogel 110. Support 112 is approximately equidistant from top edge 114 and bottom edge 116.

(24) FIG. 2 illustrates and embodiment where internal support 212 is in an offset position within aerogel 210. Support 212 being closer to one edge (in the illustrated case top edge 214) than the other edge (bottom edge 216). In other embodiments support 212 can be positioned closer to the bottom edge.

(25) FIG. 3 illustrates an embodiment where the outer edge of support 312 is positioned along the top edge 314 of aerogel 310. In other embodiments support 312 can be positioned along bottom edge 316.

(26) FIG. 4 illustrates another embodiment where support 412 is partially incorporated into aerogel 410. In this embodiments a portion of the support is above or outside top edge 414. In other embodiments the position of the support can be at bottom edge 416.

(27) In certain embodiments a reinforced aerogel laminate can be constructed having 2, 3, 4, 5 or more reinforced aerogel layers (see FIG. 5). FIG. 5 shows a two-layer laminate. In this example each layer is configured as shown in FIG. 1; however, any number of reinforced aerogel configurations can be used and in any combination. Each of the reinforced aerogel layer depicted in FIG. 5 comprise aerogel 510 and support 512. The layers can be adhered to each through an aerogel/aerogel interface or by an adhesive 518. The laminate having top edge 514 and a bottom edge 516.

(28) The cross-sectional thickness of the internally reinforced aerogel measure from top most edge to bottom most edge can be between 3 and 16 mils, including all values and ranges there between. The support can be positioned in the aerogel so that about 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, or 15 mil of the aerogel is above the support. In certain instances the support is approximately within about 0.5 mils of the aerogel midline. In a further aspect about 0.1 to 0.5 mil of support extends beyond one of the aerogel edges with a portion of the support being embedded or incorporated in the aerogel.

(29) B. Polymeric Aerogels

(30) The aerogel matrix of the present invention may be organic, inorganic, or a mixture thereof. The aerogels or wet gels used to prepare the aerogels may be prepared by any known gel-forming techniques: examples include adjusting the pH and/or temperature of a dilute metal oxide sol to a point where gelation occurs. Organic aerogels can be made from polyacrylates, polystyrenes, polyacrylonitriles, polyurethanes, polyimides, polyamides, polyfurfural alcohol, phenol furfuryl alcohol, melamine formaldehydes, resorcinol formaldehydes, cresol formaldehyde, phenol formaldehyde, polyvinyl alcohol dialdehyde, polycyanurates, polyacrylamides, various epoxies, agar, agarose, and the like. In particular embodiments the aerogel is a polyimide aerogel.

(31) Polyimides are a type of polymer with many desirable properties. In general, polyimide polymers include a nitrogen atom in the polymer backbone, where the nitrogen atom is connected to two carbonyl carbons, such that the nitrogen atom is somewhat stabilized by the adjacent carbonyl groups. A carbonyl group includes a carbon, referred to as a carbonyl carbon, which is double bonded to an oxygen atom. Polyimides are usually considered an AA-BB type polymer because usually two different classes of monomers are used to produce the polyimide polymer. Polyimides can also be prepared from AB type monomers. For example, an aminodicarboxylic acid monomer can be polymerized to form an AB type polyimide. Monoamines and/or mono anhydrides can be used as end capping agents if desired.

(32) One class of polyimide monomer is usually a diamine, or a diamine monomer. The diamine monomer can also be a diisocyanate, and it is to be understood that an isocyanate could be substituted for an amine in this description, as appropriate. There are other types of monomers that can be used in place of the diamine monomer, as known to those skilled in the art. The other type of monomer is called an acid monomer, and is usually in the form of a dianhydride. In this description, the term “di-acid monomer” is defined to include a dianhydride, a tetraester, a diester acid, a tetracarboxylic acid, or a trimethylsilyl ester, all of which can react with a diamine to produce a polyimide polymer. Dianhydrides are to be understood as tetraesters, diester acids, tetracarboxylic acids, or trimethylsilyl esters that can be substituted, as appropriate. There are also other types of monomers that can be used in place of the di-acid monomer, as known to those skilled in the art.

(33) Because one di-acid monomer has two anhydride groups, different diamino monomers can react with each anhydride group so the di-acid monomer may become located between two different diamino monomers. The diamine monomer contains two amine functional groups; therefore, after the first amine functional group attaches to one di-acid monomer, the second amine functional group is still available to attach to another di-acid monomer, which then attaches to another diamine monomer, and so on. In this manner, the polymer backbone is formed. The resulting polycondensation reaction forms a poly(amic acid).

(34) The polyimide polymer is usually formed from two different types of monomers, and it is possible to mix different varieties of each type of monomer. Therefore, one, two, or more di-acid monomers can be included in the reaction vessel, as well as one, two or more diamino monomers. The total molar quantity of di-acid monomers is kept about the same as the total molar quantity of diamino monomers if a long polymer chain is desired. Because more than one type of diamine or di-acid can be used, the various monomer constituents of each polymer chain can be varied to produce polyimides with different properties. For example, a single diamine monomer AA can be reacted with two di-acid co monomers, B.sub.1B.sub.1 and B.sub.2B.sub.2, to form a polymer chain of the general form of (AA-B.sub.1B.sub.1).sub.x-(AA-B.sub.2B.sub.2).sub.y in which x and y are determined by the relative incorporations of B.sub.1B.sub.1 and B.sub.2B.sub.2 into the polymer backbone. Alternatively, diamine co-monomers A.sub.1A.sub.1 and A.sub.2A.sub.2 can be reacted with a single di-acid monomer BB to form a polymer chain of the general form of (A.sub.1A.sub.1-BB).sub.x-(A.sub.2A.sub.2-BB).sub.y. Additionally, two diamine co-monomers A.sub.1A.sub.1 and A.sub.2A.sub.2 can be reacted with two di-acid co-monomers B.sub.1B.sub.1 and B.sub.2B.sub.2 to form a polymer chain of the general form (A.sub.1A.sub.1-B.sub.1B.sub.1).sub.w-(A.sub.1A.sub.1-B.sub.2B.sub.2),(A.sub.2A.sub.2-B.sub.1B.sub.1).sub.y-(A.sub.2A.sub.2-B.sub.2B.sub.2).sub.z, where w, x, y, and z are determined by the relative incorporation of A.sub.1A.sub.1-B.sub.1B.sub.1, A.sub.1A.sub.1-B.sub.2B.sub.2, A.sub.2A.sub.2-B.sub.1B.sub.1, and A.sub.2A.sub.2-B.sub.2B.sub.2 into the polymer backbone. More than two di-acid co-monomers and/or more than two diamine co-monomers can also be used. Therefore, one or more diamine monomers can be polymerized with one or more di-acids, and the general form of the polymer is determined by varying the amount and types of monomers used.

(35) There are many examples of monomers that can be used to make polyimide polymers. A non-limiting list of possible diamine monomers comprises 4,4′-oxydianiline, 3,4′-oxydianiline, 3,3′-oxydianiline, p-phenylenediamine, m-phenylenediamine, o-phenylenediamine, diaminobenzanilide, 3,5-diaminobenzoic acid, 3,3′-diaminodiphenylsulfone, 4,4′-diaminodiphenyl sulfones, 1,3-bis-(4-aminophenoxy)benzene, 1,3-bis-(3-aminophenoxy)benzene, 1,4-bis-(4-aminophenoxy)benzene, 1,4-bis-(3-aminophenoxy)benzene, 2,2-Bis[4-(4-aminophenoxy)phenyl]-hexafluoropropane, 2,2-bis(3-aminophenyl)-1,1,1,3,3,3-hexafluoropropane, 4,4′-isopropylidenedianiline, 1-(4-aminophenoxy)-3-(3-aminophenoxy)benzene, 1-(4-aminophenoxy)-4-(3-aminophenoxy)benzene, bis-[4-(4-aminophenoxy)phenyl]sulfones, 2,2-bis[4-(3-aminophenoxy)phenyl]sulfones, bis(4-[4-aminophenoxy]phenyl)ether, 2,2′-bis-(4-aminophenyl)-hexafluoropropane, (6F-diamine), 2,2′-bis-(4-phenoxyaniline)isopropylidene, meta-phenylenediamine, para-phenylenediamine, 1,2-diaminobenzene, 4,4′-diaminodiphenylmethane, 2,2-bis(4-aminophenyl)propane, 4,4′diaminodiphenyl propane, 4,4′-diaminodiphenyl sulfide, 4,4′-diaminodiphenylsulfone, 3,4′diaminodiphenyl ether, 4,4′-diaminodiphenyl ether, 2,6-diaminopyridine, bis(3-aminophenyl)diethyl silane, 4,4′-diaminodiphenyl diethyl silane, benzidine, dichlorobenzidine, 3,3′-dimethoxybenzidine, 4,4′-diaminobenzophenone, N,N-bis(4-aminophenyl)-n-butylamine, N,N-bis(4-aminophenyl)methylamine, 1,5-diaminonaphthalene, 3,3′-dimethyl-4,4′-diaminobiphenyl, 4-aminophenyl-3-aminobenzoate, N,N-bis(4-aminophenyl)aniline, bis(p-beta-amino-t-butylphenyl)ether, p-bis-2-(2-methyl-4-aminopentyl)benzene, p-bis(1,1-dimethyl-5-aminopentyl)benzene, 1,3-bis(4-aminophenoxy)benzene, m-xylenediamine, p-xylenediamine, 4,4′-diaminodiphenyl ether phosphine oxide, 4,4′-diaminodiphenyl N-methyl amine, 4,4′-diaminodiphenyl N-phenyl amine, amino-terminal polydimethylsiloxanes, amino-terminal polypropyleneoxides, amino-terminal polybutyleneoxides, 4,4′-Methylenebis(2-methylcyclohexylamine), 1,2-diaminoethane, 1,3-diaminopropane, 1,4-diaminobutane, 1,5-diaminopentane, 1,6-diaminohexane, 1,7-diaminoheptane, 1,8-diaminooctane, 1,9-diaminononane, 1,10-diaminodecane, and 4,4′-methylenebisbenzeneamine.

(36) A non-limiting list of possible dianhydride (“diacid”) monomers includes hydroquinone dianhydride, 3,3′,4,4′-biphenyl tetracarboxylic dianhydride, pyromellitic dianhydride, 3,3′,4,4′-benzophenone tetracarboxylic dianhydride, 4,4′-oxydiphthalic anhydride, 3,3′,4,4′-diphenylsulfone tetracarboxylic dianhydride, 4,4′-(4,4′-isopropylidenediphenoxy)bis(phthalic anhydride), 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride, 4,4′-(hexafluoroisopropylidene)diphthalic anhydride, bis(3,4-dicarboxyphenyl) sulfoxide dianhydride, polysiloxane-containing dianhydride, 2,2′,3,3′-biphenyltetracarboxylic dianhydride, 2,3,2′,3′-benzophenonetetraearboxylic dianhydride, 3,3′,4,4′-benzophenonetetraearboxylic dianhydride, naphthalene-2,3,6,7-tetracarboxylic dianhydride, naphthalene-1,4,5,8-tetracarboxylic dianhydride, 4,4′-oxydiphthalic dianhydride, 3,3′,4,4′-biphenylsulfone tetracarboxylic dianhydride, 3,4,9,10-perylene tetracarboxylic dianhydride, bis(3,4-dicarboxyphenyl)sulfide dianhydride, bis(3,4-dicarboxyphenyl)methane dianhydride, 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride, 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane, 2,6-dichloronaphthalene-1,4,5,8-tetracarboxylic dianhydride, 2,7-dichloronapthalene-1,4,5,8-tetracarboxylic dianhydride, 2,3,6,7-tetrachloronaphthalene-1,4,5,8-tetracarboxylic dianhydride, phenanthrene-, 8,9,10-tetracarboxylie dianhydride, pyrazine-2,3,5,6-tetracarboxylic dianhydride, benzene-1,2,3,4-tetracarboxylic dianhydride, and thiophene-2,3,4,5-tetracarboxylic dianhydride.

(37) A poly(amic acid) is soluble in the reaction solvent and, thus, the solution may be cast into a film on a suitable substrate, support, or support on a substrate such as by spin casting, gravure coating, three roll coating, knife over roll coating, slot die extrusion, dip coating, Meyer rod coating, or other techniques. The cast film can then be heated in stages to elevated temperatures to remove solvent and convert the amic acid functional groups in the poly(amic acid) to imides with a cyclodehydration reaction, also called imidization. “Imidization” is defined as the conversion of a polyimide precursor into an imide. Alternatively, some poly(amic acid)s may be converted in solution to polyimides by using a chemical dehydrating agent, catalyst, and/or heat.

(38) Many polyimide polymers are produced by preparing a poly(amic acid) polymer in the reaction vessel. The poly(amic acid) is then formed into a sheet or a film and subsequently processed with heat (often temperatures higher than 250 degrees Celsius) or both heat and catalysts to convert the poly(amic acid) to a polyimide.

(39) The characteristics or properties of the final polymer are significantly impacted by the choice of monomers which are used to produce the polymer. Factors to be considered when selecting monomers include the properties of the final polymer, such as the flexibility, thermal stability, coefficient of thermal expansion (CTE), coefficient of hydroscopic expansion (CHE) and any other properties specifically desired, as well as cost. Often, certain important properties of a polymer for a particular use can be identified. Other properties of the polymer may be less significant, or may have a wide range of acceptable values; so many different monomer combinations could be used.

(40) In some instances, the backbone of the polymer can include substituents. The substituents (e.g., oligomers, functional groups, etc.) can be directly bonded to the backbone or linked to the backbone through a linking group (e.g., a tether or a flexible tether). In other embodiments, a compound or particles can be incorporated (e.g., blended and/or encapsulated) into the polyimide structure without being covalently bound to the polyimide structure. In some instances, the incorporation of the compound or particles can be performed during the polyamic reaction process. In some instances, particles can aggregate, thereby producing polyimides having domains with different concentrations of the non-covalently bound compounds or particles.

(41) Specific properties of a polyimide can be influenced by incorporating certain compounds into the polyimide. The selection of monomers is one way to influence specific properties. Another way to influence properties is to add a compound or property modifying moiety to the polyimide.

(42) C. Synthesis of Aerogels

(43) The first stage in the synthesis of an aerogel is the synthesis of a polymerized gel. For example, if a polyimide aerogel is desired, at least one acid monomer can be reacted with at least one diamino monomer in a reaction solvent to form a poly(amic acid). As discussed above, numerous acid monomers and diamino monomers may be used to synthesize the poly(amic acid). In one aspect, the poly(amic acid) is contacted with an imidization catalyst in the presence of a chemical dehydrating agent to form a polymerized polyimide gel via an imidization reaction. Any imidization catalyst suitable for driving the conversion of polyimide precursor to the polyimide state is suitable. Preferred chemical imidization catalysts comprise at least one compound selected from the group consisting of pyridine, methylpyridines, quinoline, isoquinoline, 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU), triethylenediamine, lutidine, N-methylmorpholine, triethylamine, tripropylamine, tributylamine, and other trialkylamines. Any dehydrating agent suitable for use in formation of an imide ring from an amic acid precursor is suitable for use in the methods of the present invention. Preferred dehydrating agents comprise at least one compound selected from the group consisting of acetic anhydride, propionic anhydride, n-butyric anhydride, benzoic, anhydride, trifluoroacetic anhydride, phosphorus trichloride, and dicyclohexylcarbodiimide.

(44) The reaction solvent may be selected from the group consisting of dimethylsulfoxide, diethylsulfoxide, N,N-dimethylformamide, N,N-diethylformamide, N,N-dimethylacetamide, N,N-diethylacetamide, N-methyl-2-pyrrolidone, 1-methyl-2-pyrrolidinone, N-cyclohexyl-2-pyrrolidone, 1,13-dimethyl-2-imidazolidinone, diethyleneglycoldimethoxyether, o-dichlorobenzene, phenols, cresols, xylenol, catechol, butyrolactones, hexamethylphosphoramide, and mixtures thereof. The reaction solvent and other reactants will be selected based on the compatibility with the support material.

(45) The polyimide solution may optionally be cast onto a casting sheet covered by a support film for a period of time. In one embodiment, the casting sheet is a polyethylene terephthalate (PET) casting sheet. After a passage of time, the polymerized reinforced gel is removed from the casting sheet and prepared for the solvent exchange process.

(46) 1. Solvent Exchange

(47) After the polymer gel is synthesized and a support film incorporated, it is desirable to conduct a solvent exchange wherein the reaction solvent is exchanged for a more desirable second solvent. Accordingly, in one embodiment, a solvent exchange can be conducted wherein the polymerized gel is placed inside of a pressure vessel and submerged in a mixture comprising the reaction solvent and the second solvent. Then, a high pressure atmosphere is created inside of the pressure vessel thereby forcing the second solvent into the polymerized gel and displacing a portion of the reaction solvent. Alternatively, the pressure exchange step may be conducted without the use of a high pressure environment. It may be necessary to conduct a plurality of rounds of solvent exchange.

(48) The time necessary to conduct the solvent exchange will vary depending upon the type of polymer undergoing the exchange as well as the reaction solvent and second solvent being used. In one embodiment, each solvent exchange can range from 1 to 168 hours or any period time there between including 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, or 23, 24, 25, 50, 75, 100, 125, 150, 155, 160, 165, 166, 167, or 168 hours. In another embodiment, each solvent exchange can take approximately 15 to 60 minutes, or about 30 minutes. Exemplary second solvents include methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, isobutanol, tert-butanol, 3-methyl-2-butanol, 3,3-dimethyl-2-butanol, 2-pentanol, 3-pentanol, 2,2-dimethylpropan-1-ol, cyclohexanol, diethylene glycol, cyclohexanone, acetone, acetyl acetone, 1,4-dioxane, diethyl ether, dichloromethane, trichloroethylene, chloroform, carbon tetrachloride, water, and mixtures thereof. Each second solvent has a freezing point. For example tert-butyl alcohol has a freezing point of 25.5 degrees Celsius and water has a freezing point of 0 degrees Celsius under one atmosphere of pressure.

(49) The temperature and pressure used in the solvent exchange process may be varied. The duration of the solvent exchange process can be adjusted by performing the solvent exchange at a varying temperatures or atmospheric pressures, or both, provided that the pressure and temperature inside the pressure vessel does not cause either the first solvent or the second solvent to leave the liquid phase and become gaseous phase, vapor phase, solid phase, or supercritical fluid. Generally, higher pressures and/or temperatures decrease the amount of time required to perform the solvent exchange, and lower temperatures and/or pressures increase the amount of time required to perform the solvent exchange.

(50) 2. Cooling and Drying

(51) In one embodiment after solvent exchange, the polymerized reinforced gel is exposed to supercritical drying. In this instance the solvent in the gel is removed by supercritical CO.sub.2 extraction.

(52) In another embodiment after solvent exchange, the polymerized reinforced gel is exposed to subcritical drying. In this instance the gel is cooled below the freezing point of the second solvent and subjected to a freeze drying or lyophilization process to produce the aerogel. For example, if the second solvent is water, then the polymerized gel is cooled to below 0° C. After cooling, the polymerized gel is subjected to a vacuum for a period of time wherein the second solvent is allowed to sublime.

(53) In still another embodiment after solvent exchange, the polymerized reinforced gel is exposed to subcritical drying with optional heating after the majority of the second solvent has been removed through sublimation. In this instance the partially dried gel material is heated to a temperature near or above the boiling point of the second solvent for a period of time. The period of time can range from a few hours to several days, although a typical period of time is approximately 4 hours. During the sublimation process, a portion of the second solvent present in the polymerized gel has been removed, leaving the mesoporous and microporous gel. After the sublimation process is complete, or nearly complete, the aerogel has been formed.

(54) In yet another embodiment after solvent exchange, the polymerized reinforced gel can be dried under ambient conditions, for example by removing the solvent under a stream of air or anhydrous gas.

(55) D. Articles of Manufacture

(56) In some aspects, an article of manufacture is presented including the reinforced aerogel described above. In some embodiments, the article of manufacture is a thin film, monolith, wafer, blanket, core composite material, substrate for radiofrequency antenna, a sunscreen, a sunshield, a radome, insulating material for oil and/or gas pipeline, insulating material for liquefied natural gas pipeline, insulating material for cryogenic fluid transfer pipeline, insulating material for apparel, insulating material for aerospace applications, insulating material for buildings, cars, and other human habitats, insulating material for automotive applications, insulation for radiators, insulation for ducting and ventilation, insulation for air conditioning, insulation for heating and refrigeration and mobile air conditioning units, insulation for coolers, insulation for packaging, insulation for consumer goods, vibration dampening, wire and cable insulation, insulation for medical devices, support for catalysts, support for drugs, pharmaceuticals, and/or drug delivery systems, aqueous filtration applications, oil-based filtration applications, and solvent-based filtration applications.

(57) 1. Fluid Filtration Applications

(58) In some embodiments, the reinforced aerogel described above can be used in fluid filtration systems and apparatus. In such applications, the support film is permeable to the fluid being filtered. A feed fluid can be contacted with the reinforced aerogel such that all or, substantially all, of the impurities and/or desired substances are removed from the feed fluid to produce a filtrate essentially devoid of the impurities and/or desired substances. The filtrate, impurities, and/or desired substances can be collected, stored, transported, recycled, or further processed. The highly branched polyimide aerogel can be further processed to release the impurities and/or desired substances from the aerogel.

(59) The reinforced aerogel described herein can be used in or with filtration apparatuses known in the art. Non-limiting examples of filtration apparatuses and applications include gas filters such as, but not limited to, building air filters, automotive cabin air filters, combustion engine air filters, aircraft air filters, satellite air filters, face mask filters, diesel particulate filters, in-line gas filters, cylinder gas filters, soot filters, pressure swing absorption apparatus, etc. Additional non-limiting examples of filtration apparatuses and applications include solvent filtration systems, column filtration, chromatography filtration, vacuum flask filtration, microfiltration, ultrafiltration, reverse osmosis filtration, nanofiltration, centrifugal filtration, gravity filtration, cross flow filtration, dialysis, hemofiltration, hydraulic oil filtration, automotive oil filtration, etc. Further, non-limiting examples of the purpose of filtration includes sterilization, separation, purification, isolation, etc.

(60) A fluid for filtration (“feed”) and a filtrate can be any fluid. The fluid can be a liquid, gas, supercritical fluid, or mixture thereof. In some instances, the fluid can be aqueous, organic, non-organic, biological in origin, or a mixture thereof. In some instances, the fluid can contain solids and/or other fluids. As non-limiting examples, the fluid can be or can be partially water, blood, an oil, a solvent, air, or mixtures thereof. Water can include water, any form of steam and supercritical water.

(61) In some instances, the fluid can contain impurities. Non-limiting examples of impurities include solids, liquids, gases, supercritical fluids, objects, compounds, and/or chemicals, etc. What is defined as an impurity may be different for the same feed fluid depending on the filtrate desired. In some embodiments, one or more aerogels can be used to remove impurities. Non-limiting examples of impurities in water can include ionic substances such as sodium, potassium, magnesium, calcium, fluoride, chloride, bromide, sulfate, sulfite, nitrate, nitrites, cationic surfactants, and anionic surfactants, metals, heavy metals, suspended, partially dissolved, or dissolved oils, organic solvents, nonionic surfactants, defoamants, chelating agents, microorganisms, particulate matter, etc. Non-limiting examples of impurities in blood can include red blood cells, white blood cells, antibodies, microorganisms, water, urea, potassium, phosphorus, gases, particulate matter, etc. Non-limiting examples of impurities in oil can include water, particulate matter, heavy and/or light weight hydrocarbons, metals, sulfur, defoamants, etc. Non-limiting examples of impurities in solvents can include water, particulate matter, metals, gases, etc. Non-limiting impurities in air can include water, particulate matter, microorganisms, liquids, carbon monoxide, sulfur dioxide, etc.

(62) In some instances, the feed fluid can contain desired substances. Desired substances can be, but are not limited to, solids, liquids, gases, supercritical fluids, objects, compounds, and/or chemicals, etc. In some embodiments, one or more aerogels can be used to concentrate or capture a desired substance, or remove a fluid from a desired substance. Non-limiting examples of desired substances in water can include ionic substances such as sodium, potassium, magnesium, calcium, fluoride, chloride, bromide, sulfate, sulfite, nitrate, nitrites, cationic surfactants, and anionic surfactants, metals, heavy metals, suspended, partially dissolved, or dissolved oils, organic solvents, nonionic surfactants, chelating agents, defoamants, etc. Non-limiting examples of desired substances in blood can include red blood cells, white blood cells, antibodies, lipids, proteins, etc. Non-limiting examples of desired substances in oil can include hydrocarbons of a range of molecular weights, gases, metals, etc. Non-limiting examples of desired substances in solvents can include particulate matter, fluids, gases, proteins, lipids, etc. Non-limiting examples of desired substances in air can include water, fluids, gases, particulate matter, etc.

(63) FIGS. 6 through 8 are non-limiting schematics of a system 600 used to carry out a filtration of a fluid using an aerogel. System 600 can include a separation zone 602. The materials, size, and shape of the separation zone 602 can be determined using standard engineering practice to achieve the desired flow rates and contact time. The separation zone 602 is capable of holding or may be made of one or more aerogels and includes a feed fluid inlet 604 (inlet) and/or a filtrate outlet 606 (outlet). In some instances, the separation zone is made entirely of one or more branched polyimide aerogels or one or more branched polyimide aerogels in or around a supporting structure. The feed fluid 608 can be introduced to the separation zone 602 through the inlet 104 (See, FIGS. 6 and 7) or through direct contact with the separation zone 602 (FIG. 8). In some embodiments, the feed fluid 608 can be received under greater or reduced pressure than ambient pressure. Introduction of the feed fluid 608 into separation zone 602 can be at a rate sufficient to allow optimum contact of the feed fluid with the one or more aerogels. Contact of the feed fluid 608 with the aerogel can allow the feed fluid to be filtered by the aerogel, which results in the filtrate 610. The filtrate 610 can have less impurity and/or desired substance when compared with the feed fluid 608. In certain aspects, the filtrate 610 can be essentially free of the impurity and/or the desired substance. The filtrate 610 can exit the separation zone 602 via the outlet 606 (See, FIGS. 6 and 8) or through directly exiting the separation zone 602 (See, FIG. 7). In some instances, the filtrate can be recycled back to a separation zone, collected, stored in a storage unit, etc. In some instances, one or more aerogels can be removed and/or replaced from the separation zone. In some instances, the filtrate 610 can be collected and/or removed from the separation zone 602 without the filtrate 610 flowing through an outlet 606. In some instances, the impurities and/or desired substance can be removed from the separation zone 602. As one non-limiting example, the impurities and/or desired substances can be removed from the separation zone by flowing a fluid through the separation zone in the reverse direction from the flow of the feed fluid through the separation zone.

(64) The filtration conditions in the separation zone 602 can be varied to achieve a desired result (e.g., removal of substantially all of the impurities and/or desired substance from the feed fluid). The filtration conditions include temperature, pressure, fluid feed flow, filtrate flow, or any combination thereof. Filtration conditions are controlled, in some instances, to produce streams with specific properties. The separation zone 602 can also include valves, thermocouples, controllers (automated or manual controllers), computers or any other equipment deemed necessary to control or operate the separation zone. The flow of the feed fluid 604 can be adjusted and controlled to maintain optimum contact of the feed fluid with the one or more aerogel. In some embodiments, computer simulations can be used to determine flow rates for separation zones of various dimensions and various aerogels.

(65) The compatibility of an aerogel with a fluid and/or filtration application can be determined by methods known in the art. Some properties of an aerogel that may be determined to assess the compatibility of the aerogel may include, but is not limited to: the temperature and/or pressures that the aerogel melts, dissolves, oxidizes, reacts, degrades, or breaks; the solubility of the aerogel in the material that will contact the aerogel; the flow rate of the fluid through the aerogel; the retention rate of the impurity and/or desired product form the feed fluid; etc.

(66) 2. Radiofrequency (RF) Applications

(67) Due to their low density, mechanical robustness, light weight, and low dielectric properties, the branched polyimide aerogels can be used in radiofrequency (RF) applications. The use of branched polyimide aerogels in RF applications enables the design of thinner substrates, lighter weight substrates and smaller substrates. Non-limiting examples of radiofrequency applications include a substrate for a RF antenna, a sunshield for a RF antenna, a radome, or the like. Antennas can include flexible and/or rigid antennas, broadband planar circuited antennas (e.g. a patch antennas, an e-shaped wideband patch antenna, an elliptically polarized circular patch antenna, a monopole antenna, a planar antenna with circular slots, a bow-tie antenna, an inverted-F antenna and the like). In the antenna design, the circuitry can be attached to a substrate that includes the branched polyimide aerogel and/or a mixture of the branched polyimide aerogel and other components such as other polymeric materials including adhesives or polymer films, organic and inorganic fibers (e.g. polyester, polyamide, polyimide, carbon, glass fibers), other organic and inorganic materials including silica aerogels, polymer powder, glass reinforcement, etc. The use of branched polyimide aerogels in antennas enables the design substrates with higher throughput. In addition, the branched polyimide aerogels have coefficient of linear thermal expansion (CTE) similar to aluminum and copper (e.g., CTE of 23/K and 17 ppm/K), and is tunable through choice of monomer to match CTE of other desirable materials. In some embodiments, the aerogel can be used in sunshields and/or sunscreens used to protect RF antennas from thermal cycles due to their temperature insensitivity and RF transparency. In certain embodiments, the aerogel can be used as a material in a radome application. A radome is a structural, weatherproof enclosure that protects a microwave (e.g., radar) antenna. Branched polyimide aerogels can minimize signal loss due to their low dielectric constant and also provide structural integrity due to their stiffness.