Poly(amide-imide) copolymer, composition and polymer film comprising thereof

A polymer film comprising a cured product of a poly(amide-imide) copolymer comprising imide repeating units and amide repeating units,

• wherein at least one of the imide repeating units is substituted with a functional group including Chemical Formula 1 and at least one of the amide repeating units is substituted with a functional group including Chemical Formula 1,
• wherein the imide and amide repeating units each substituted with a functional group including Chemical Formula 1 are contained in an amount of 5 to 40 parts by mol based on 100 parts by mol of the whole repeating units of the poly(amide-imide) copolymer, and
• wherein the polymer film has a yellow index of 1.61 to 1.72 measured according to ASTM D1925 based on a thickness of 50±2 μm, a glass transition temperature of 185° C. to 193° C. measured using a TMA IC600 device, and a modulus of 4.08 GPa to 4.21 GPa measured using DMA 9800 based on a thickness of 50±2 μm:
• wherein,
• R1 is hydrogen or an alkyl group having 1 to 10 carbon atoms, and
• L is a single bond or an alkylene having 1 to 10 carbon atoms.