Incorporation of polyzwitterions in superabsorbent network membranes for enhanced saltwater absorption and retention

Background

Superabsorbent polymers (SAPs) have a remarkable ability to absorb significant quantities of water. However, their absorption capacity is significantly reduced when exposed to saline solutions, such as urine, due to the polyelectrolyte effect and charge screening.

Methods

In this study, we demonstrate a zwitterionic superabsorbent polymer (ZSAP) with excellent salt-water absorption and retention capacities. ZSAP was synthesized by grafting a copolymer of p(sulfobetaine methacrylate-co-2-hydroxyethyl methacrylate) (p(SBMA-co-HEMA)) onto an acrylic acid (AA)-based hydrogel via free-radical polymerization. The introduction of zwitterionic SBMA significantly enhances the hydrophilicity of the polymer, particularly in a saline solution due to the anti-polyelectrolyte effect, thereby accelerating the rate of salt absorption. Additionally, the hydroxyl groups from HEMA facilitate the formation of covalent bonds with the AA network membrane through esterification, effectively mitigating polymer leaching. The hydration/dehydration behaviors of linear polymers were measured using the dynamic vapor sorption (DVS) method. Moreover, the salt-water absorption capacity, centrifuge retention capacity (CRC), and absorbency under load (AUL) of ZSAP with various SBMA moieties and copolymer dosages were comprehensively evaluated in a 0.9 wt% sodium chloride solution. Additionally, the water retention under different temperatures and polymer leaching of ZSAP were investigated.

Significant Findings

The copolymer p(SBMA-co-HEMA) not only demonstrates a high salt-water absorption rate at 90% RH in a 0.9 wt% NaCl solution but also exhibits superior water retention at 0% RH compared to the AA polymer. Moreover, the ZSAP exhibits superior salt-water absorption capacity and AUL in a 0.9 wt% NaCl solution compared to conventional AA-based SAP. Additionally, the introduction of the hydroxyl moiety from the p(SBMA-co-HEMA) copolymer reduces free polymer leaching from ZSAP. This work presents an approach for the development of new SAP with high salt-water absorption and retention.