Water vapor absorption and aqueous retention of hydrolyzed starch polyacrylonitrile graft copolymers — “superabsorbent starch”

Summary

Starch copolymers with polyacrylonitrile grafts were prepared using the Mn³⁺ initiation method and different amounts of acrylonitrile. The product was hydrolyzed in aqueous alkali (nitrile groups to amide and carboxyl groups) to enhance the hydrophilic character of the copolymer and achieve “superabsorbent properties”. At relative humidities (RH) up to 95–97 % the modified starch shows “normal” water vapor absorption like cellulose and other polysaccharides. When in contact with liquid distilled water, the modified starch forms a “superabsorbent” gel which retains very large amounts of water (up to 800 g/g). At low and high pH and in aqueous salt solutions (NaCl, CaCl₂ and synthetic urine) the water retention is strongly reduced. This is interpreted as due to the ionic strength (salt effect) and for Ca²⁺ ions probably also crosslinking by salt formation with carboxyl groups on adjacent chains or chain segments.