A copolymer including a structural unit represented by General Formula (1) below, and a structural unit represented by General Formula (2) below, where, in General Formula (1), R1 is a hydrogen atom or a methyl group, L1 is an alkylene group having 2 or greater carbon atoms but 5 or less carbon atoms, and X is a hydrogen atom or a cation, where, in General Formula (2), R2 is a hydrogen atom or a methyl group and L2 is an alkylene group having 2 or greater carbon atoms but 18 or less carbon atoms.