A block
copolymer comprises three block segments of a first segment, a second segment, and a third segment, and containing a polyalkenyl
ether main chain, wherein the first segment is hydrophilic or hydrophobic, the second segment is hydrophobic, and the third segment is ionic; and at least one of the block segments has a structure represented by General Formula (1) below: —(CRaRb-CRc(OR1))— (1) wherein Ra, Rb, and Rc are respectively H or CH3; R1 is selected from the group consisting of linear, branched, or cyclic
alkyl groups of 1 to 18 carbon atoms, —(CH(R2)—CH(R3)—O)l—R4, and —(CH2)m—(O)n—R4; l and m are an integer from 1 to 12; n is 0 or 1; R2 and R3 are H or CH3; R4 is selected from the group consisting of H, linear, branched or cyclic
alkyl groups of 1 to 6 carbon atoms, -Ph, -Pyr, -Ph-Ph, -Ph-Pyr, —CHO, —CH2CHO, —CO—CH═CH2, —CO—C(CH3)═CH2, —CH2COOR5, -PhCOOR5, —CH2SO3R5, and -PhSO3R5; the
hydrogen on the carbon atoms constituting R4 except R5
moiety may be replaced by a linear or branched
alkyl group of 1 to 4 carbon atoms, and the carbon in the aromatic ring may be replaced by
nitrogen; and R5 is H, an alkyl group of 1 to 5 carbon atoms, or mono- or higher-valent cation.