A polymer is formed by a reaction of phenolic epoxy resin or bisphenol epoxy resin and carboxylic acid, wherein the phenolic epoxy resin has a chemical structure of




wherein W is H, alkyl group, or halogen. R1 is methylene, methylene diphenyl, dimethylene benzene, tetrahydrodicyclopentadiene, or




n=1 to 8. The bisphenol epoxy resin has a chemical structure of




wherein Z is H or alkyl group; R4 is methylene, methylmethylene, dimethylmethylene, ethylmethylmethylene, bi(trifluoromethyl)methylene, fluorenylidene, or sulfonyl group; and p=1 to 10. The carboxylic acid has a chemical structure of HOOC—Ar—(—X)m, HOOC—R2, or a combination thereof, wherein Ar is benzene or naphthalene; X is hydroxy group, alkoxy group, or alkyl group, and at least one X is hydroxy group; m=1 to 3, wherein R2 is C3-7 alkyl group.