A poly(amide-imide) copolymer comprising a reaction product of at least a tetracarboxylic acid dianhydride, at least a diamine, and at least a dicarboxylic acid derivative, wherein the at least a tetracarboxylic acid dianhydride comprises a tetracarboxylic acid dianhydride represented by Chemical Formula 1, the at least a diamine comprises a diamine represented by Chemical Formula 2, and the at least a dicarboxylic acid derivative comprises a dicarboxylic acid derivative represented by Chemical Formula 3: wherein, in Chemical Formula 1, R 1 is a substituted or unsubstituted tetravalent C4 to C12 saturated or unsaturated aliphatic hydrocarbon ring group; Chemical Formula 2 NH 2 -R 2 -NH 2 wherein, in Chemical Formula 2, R 2 comprises two or more substituted or unsubstituted C6 to C30 aromatic hydrocarbon ring groups, wherein the two or more substituted or unsubstituted C6 to C30 aromatic hydrocarbon ring groups comprise two or more aromatic hydrocarbon rings linked through a functional group selected from -O-, -S-, -C(=O)-, -CH -, - S(=O) 2 -, -(CH 2) p - wherein 1≤p≤10, -(CF 2) q - wherein 1≤q≤10, -Si(C n H 2n+1) 2 -, - C(C n H 2n+1) 2 -, -C(C n F 2n+1) 2 -, wherein 1≤n≤10, -C(=O)NH-, and a combination thereof; wherein, in Chemical Formula 3, R 3 comprises a substituted or unsubstituted C6 to C30 aromatic hydrocarbon ring group, wherein the substituted or unsubstituted C6 to C30 aromatic hydrocarbon ring group comprises one aromatic hydrocarbon ring, two or more aromatic hydrocarbon rings fused together to provide a condensed ring system, or two or more moieties independently selected from the foregoing linked through a single bond or through a functional group selected from -O-, -S-, -C(=O)-, -CH -, - S(=O) 2 -, -(CH 2) p - wherein 1≤p≤10, -(CF 2) q - wherein 1≤q≤10, -Si(C n H 2n+1) 2 -, - C(C n H 2n+1) 2 -, -C(C n F 2n+1) 2 -, wherein 1≤n≤10, -C(=O)NH-, and a combination thereof; and X 1 and X 2 are the same or different, and are each independently selected from a halogen, hydroxy group, or a C1 to C4 alkoxy group, and wherein the diamine represented by Chemical Formula 2 is comprised in an amount of less than or equal to 30 mole% based on the total amount of the at least a diamine. wherein the at least a tetracarboxylic acid dianhydride comprises a tetracarboxylic acid dianhydride represented by Chemical Formula 1, the at least a diamine comprises a diamine represented by Chemical Formula 2, and the at least a dicarboxylic acid derivative comprises a dicarboxylic acid derivative represented by Chemical Formula 3: wherein, in Chemical Formula 1, R 1 is a substituted or unsubstituted tetravalent C4 to C12 saturated or unsaturated aliphatic hydrocarbon ring group; Chemical Formula 2 NH 2 -R 2 -NH 2 wherein, in Chemical Formula 2, R 2 comprises two or more substituted or unsubstituted C6 to C30 aromatic hydrocarbon ring groups, wherein the two or more substituted or unsubstituted C6 to C30 aromatic hydrocarbon ring groups comprise two or more aromatic hydrocarbon rings linked through a functional group selected from -O-, -S-, -C(=O)-, -CH -, - S(=O) 2 -, -(CH 2) p - wherein 1≤p≤10, -(CF 2) q - wherein 1≤q≤10, -Si(C n H 2n+1) 2 -, - C(C n H 2n+1) 2 -, -C(C n F 2n+1) 2 -, wherein 1≤n≤10, -C(=O)NH-, and a combination thereof; wherein, in Chemical Formula 3, R 3 comprises a substituted or unsubstituted C6 to C30 aromatic hydrocarbon ring group, wherein the substituted or unsubstituted C6 to C30 aromatic hydrocarbon ring group comprises one aromatic hydrocarbon ring, two or more aromatic hydrocarbon rings fused together to provide a condensed ring system, or two or more moieties independently selected from the foregoing linked through a single bond or through a functional group selected from -O-, -S-, -C(=O)-, -CH -, - S(=O) 2 -, -(CH 2) p - wherein 1≤p≤10, -(CF 2) q - wherein 1≤q≤10, -Si(C n H 2n+1) 2 -, - C(C n H 2n+1) 2 -, -C(C n F 2n+1) 2 -, wherein 1≤n≤10, -C(=O)NH-, and a combination thereof; and X 1 and X 2 are the same or different, and are each independently selected from a halogen, hydroxy group, or a C1 to C4 alkoxy group, and wherein the diamine represented by Chemical Formula 2 is comprised in an amount of less than or equal to 30 mole% based on the total amount of the at least a diamine.