A copolymer is provided, which includes 1 part by mole of a repeating unit represented by Formula 1, 3 to 10 parts by mole of a repeating unit represented by Formula 2, and 5 to 15 parts by mole of a repeating unit represented by Formula 3. R1 is hydrogen or methyl; R2 is a single bond or C1-3 alkylene group; R3 is a polysiloxy group with vinyl and aromatic group; and each of R4 and R5 is independently hydrogen, C1-3 alkyl or a polysiloxy group with vinyl and aromatic group.