Swelling characterization of ionic responsive superabsorbent resin containing carboxylate sodium groups

Superabsorbent resins (SARs) are classified as the insoluble polymers. Unlike other linear molecules, all hydrophilic groups are linked in network polymer chains. To determine the systematic laws of SAR, we observed the swelling characteristics of the resin in various aqueous solutions. The focus of comprehensive immersion experiments was to investigate the effects of electrolyte species and concentration on water absorbency of resin. This research aimed to explore the response behavior of carboxylate sodium groups (-COONa) in resin to different cations and anions. Results indicated that the contribution of the carboxyl groups (-COO−) to enhance water absorbency of resin was higher than the hydroxyl (-OH) or acylamino (-CONH2) species. Time-dependent swelling process and alternative immersion experiments of the resin were dependent on the cation types of the external solutions. SARs containing -COO− groups expanded first and then contracted in Ca2+, Mg2+, Fe3+, and H+ solutions. However, in other monovalent cationic solutions, including NaCl, KCl, NH4Cl, and NaOH solutions, the resin exhibited the continuous expansion, contraction, and re-expansion between these solutions and distilled water. The swelling process of resin containing -COONa in electrolyte solutions was accompanied with the diffusion of Na+ into the outer solutions. The migrating process of positive ions from the outer solution into the inner part of the resin was also observed. The continuous elution experiments to SARs containing -COONa in distilled water indicated that Na+ was partly dispersed into the external solution, which was quite different from the linear polymers. These results demonstrated that SARs with -COONa are a solid polyelectrolyte material and ionic responsive polymer.