Block copolymer, method for producing same, and absorber and absorbent article using same

A block copolymer (A), comprising:

• a vinyl alcohol-based polymer block (b);
• and
• an ionic polymer block (c) comprising (c-i) a first monomer unit comprising an ionic group forming a salt and (c-ii) a vinyl alcohol-based monomer unit,
• wherein the ionic group is a carboxylic acid group, a sulfonic acid group, or an ammonium group,
• wherein the vinyl alcohol-based polymer block (b) has a number-average molecular weight (Mnb) in a range of from 15,000 to 220,000,
• wherein the ionic polymer block (c) comprises the vinyl alcohol-based monomer unit (c-ii) in a range of from 5 to 95 mol. %, based on total monomer units,
• wherein the block copolymer (A) has a number-average molecular weight (MnA) in a range of from 20,000 to 440,000, and
• wherein an Mnb/MnA ratio of the number-average molecular weight (Mnb) to the number-average molecular weight (MnA) is in a range of from 0.1 to 0.9.