For example, a method in which a compound having a carbon-carbon double bond and a functional group such as a carboxyl group, acid anhydride group, acidamide group and imide group is added as a third component (Japanese Provisional Patent Publication No. 26913/1981), and a method in which a copolymer of styrene type compound and an ??, ??-unsaturated dicarboxylic acid is blended (Japanese Patent P