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Provided are low-melt polyimides and poly(amic acids) (PAAs) for use as adhesives, and methods of using the materials for attaching two substrates. The methods typically form an adhesive bond that is hermetically sealed to both substrates. Additionally, the method typically forms a cross-linked bond

Provided are low-melt polyimides and poly(amic acids) (PAAs) for use as adhesives, and methods of using the materials for attaching two substrates. The methods typically form an adhesive bond that is hermetically sealed to both substrates. Additionally, the method typically forms a cross-linked bonding material that is flexible.

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1. A method of attaching two substrates comprising: (a) contacting two substrates with a low-melt polymer; and(b) heating the low-melt polymer to a bonding temperature effective to melt or cross-link the low-melt polymer and form an adhesive bond between the two substrates;wherein the low-melt polym

1. A method of attaching two substrates comprising: (a) contacting two substrates with a low-melt polymer; and(b) heating the low-melt polymer to a bonding temperature effective to melt or cross-link the low-melt polymer and form an adhesive bond between the two substrates;wherein the low-melt polymer comprises a poly(amic acid) or a polyimide. 2. The method of claim 1 wherein the method bonds the two substrates effective to pass a lap shear test with a shear load of at least 400 psi. 3. The method of claim 1 wherein the melted or cross-linked low-melt polymer passes a flexibility test at a material thickness of 0.0050 inches with a mandrel diameter of 0.048 inches. 4. The method of claim 1 wherein both substrates are polyimides. 5. The method of claim 1 wherein at least one of the substrates is a fabric. 6. The method of claim 1 wherein the low-melt polymer comprises a polyimide with reactive end groups, wherein step (b) comprises heating the polyimide with reactive end groups to a bonding temperature at which the polyimide reacts through its reactive end groups with itself to cross-link and form an adhesive bond between the two substrates. 7. The method of claim 1 wherein the low-melt polymer layer comprises a mixture of at least two poly(amic acids). 8. The method of claim 7 wherein the mixture of poly(amic acids) melts at a lower temperature than at least two of the at least two poly(amic acids). 9. The method of claim 1 wherein the low-melt polymer is formed from a reaction mixture of monomers, wherein at least 15% by weight of the monomers are fully aliphatic. 10. The method of claim 1 wherein at least 15% of carbon atoms in the low-melt polymer are aliphatic carbon atoms. 11. The method of claim 1 wherein 15% to 60% of the carbon atoms in the low-melt polymer are aliphatic. 12. The method of claim 1 wherein the bonding temperature is 80° C. to 250° C. 13. The method of claim 1 wherein the step of heating the low-melt polymer to the bonding temperature is effective to cross-link the low-melt polymer and form a polyimide with no melt temperature or a melt temperature higher than its cure temperature. 14. The method of claim 1 wherein the method comprises: (a) contacting two substrates with a mixture comprising a low-melt polymer and a cross-linker; and(b) heating the mixture to a bonding temperature effective to cross-link the low-melt polymer and form an adhesive bond between the two substrates. 15. The method of claim 14 wherein the cross-linker is acrylic acid, methacrylic acid, allyl alcohol, allyl amine, maleic anhydride, or vinyl amine or a residue of acrylic acid, methacrylic acid, allyl alcohol, allyl amine, maleic anhydride, or vinyl amine end capped on the low-melt polymer. 16. The method of claim 6 wherein the reactive end groups are selected from the group consisting of anhydrides, carboxylic acids, esters, amines, alcohols, isocyanates, epoxides, olefins, and dienes. 17. The method of claim 1 wherein the low-melt polymer comprises a polyimide comprising polyether or polysiloxane blocks. 18. The method of claim 17 wherein the polyether block is a polyether block of the formula —(C2-C6)alkyl-[O—(C2-C6)alkyl-]nO—(C2-C6)alkyl- and the polysiloxane block is a polysiloxane of the formula —(C2-C6)alkyl-O—Si(CH3)2—[O—Si(CH3)2—]nO—Si(CH3)2—O—(C2-C6)alkyl-. 19. The method of claim 17 wherein 15% to 60% of the carbon atoms of the low-melt polymer are in polyether or polysiloxane blocks. 20. The method of claim 1 wherein the low-melt polymer is formed by a process comprising: reacting a dianhydride and a diamine in a polymerization reaction mixture to form a poly(amic acid);wherein the dianhydride is a dianhydride of formula 1, wherein each R1 is independently H or (C1-C4)alkyl, and each R2 is independently H or (C1-C4)alkyl or CF3;or wherein the diamine is a diamine of formula 2 or 3: H2N—(C2-C6)alkyl-[O—(C2-C6)alkyl-]nO—(C2-C6)alkyl-NH2  (2)H2N—(C2-C6)alkyl-O—Si(CH3)2—[O—Si(CH3)2—]nO—Si(CH3)2—O—(C2-C6)alkyl-NH2  (3)wherein at least 50% by weight of reactants in the polymerization reaction mixture are a compound of formula 1, 2, or 3 or a combination thereof. 21. The method of claim 20 wherein the low-melt polymer is a polyimide and is formed by a process further comprising: heating the poly(amic acid) to form a polyimide. 22. The method of claim 1 wherein the method comprises: (a) contacting two substrates with a mixture comprising a low-melt polymer, which does not include a cross-linker; and(b) heating the mixture to a bonding temperature effective to melt the low-melt polymer and form an adhesive bond between the two substrates.