A thin film is provided, which includes a polymer formed by reacting (a) p-hydroxybenzoic acid, (b) 6-hydroxy 2-naphthoic acid and (c) branched-monomer. (c) branched-monomer is or a combination thereof, wherein R is aryl group, heteroaryl group, or cycloalkyl group, and each of R1 is independently —OH, —NH2, or —COOH. The molar ratio of (a) p-hydroxybenzoic acid over (b) 6-hydroxy 2-naphthoic acid is between 50:50 and 90:10. The molar ratio of (c) branched-monomer over the sum of (a) p-hydroxybenzoic acid and (b) 6-hydroxy 2-naphthoic acid is between 0.25:100 and 0.5:100. The polymer has an intrinsic viscosity of 4 dL/g to 6 dL/g.