A modified copolymer is provided. The modified copolymer includes a random copolymer of a repeat unit of formula (I) and a repeat unit of formula (II) wherein M is derived from monomers with double bonds, R is a direct bond or an aliphatic hydrocarbon chain group having 1 to 12 carbon atoms, a cyclic aliphatic hydrocarbon group having 3 to 16 carbon atoms, an alkylcarbonyloxy group having 2 to 6 carbon atoms, a carbonyl group, an ether group, an ester group, an amide group, an aromatic group having 6 to 16 carbon atoms, or a divalent group having any of the above groups, wherein m and x are positive integers less than 50. The weight average molecular weight of the modified copolymer is between 3000 and 30000.