Modified carrageenan. 2. Hydrolyzed crosslinked k-carrageenan-g-PAAm as a novel smart superabsorbent hydrogel with low salt sensitivity

A novel pH-responsive superabsorbing hydrogel based on K-carrageenan (kC) was prepared through polyacrylamide crosslinking grafting followed by alkaline hydrolysis. The hydrogel structure was confirmed using FF-IR spectroscopy. The hydrolysis conditions were systematically optimized to obtain a hydrogel with maximum swelling capacity. Thus, the reaction variables, including the hydrolysis time and temperature, concentration of sodium hydroxide, amount of hydrogel hydrolyzed and post-neutralization pH, were optimized. The swelling measurements of the hydrogels were conducted in 0.15 M aqueous solutions of LiCl, NaCl, KG, CaCl2 and AlCl3. As observed for the hydrolyzed hydrogel (H-carragPAM), it was found that a 'charge screening' action of small cations and carboxylate anions affected the swelling in univalent salt solutions. In the case of the non-hydrolyzed hydrogel (carragPAM), however, a converse trend was observed. As a result, carragPAM and H-carragPAM superabsorbent hydrogels showed a maximum swelling of 45 and 135 g/g in LiCl and KCl solutions, respectively. Due to the high swelling capacity in salt solutions, the hydrogels may be referred to as anti-salt superabsrbents. The swelling of superabsorbing hydrogels, was examined in buffer solutions with pH values ranging between 1 and 13. The H-carragPAM hydrogel exhibited a pH-responsie character so that a swelling-deswelling pulsatile behavior was recorded at pH 4 and 9. The swelling kinetics of H-carragPAM were preliminary investigated.