Light- and heat-responsive interpenetrating polymer networks for solar-driven water desalination

Abstract

Interpenetrating polymer networks (IPNs) consist of two interconnected networks that provide distinct physical and mechanical properties. When one of these networks is thermo-responsive, the volume phase transition temperature (VPTT) remains largely unaffected by the hydrophilicity of the second network. This makes IPNs an ideal candidate for modifying properties of hydrogels without altering VPTT. In this study, we explore the use of IPNs, based on poly(N-isopropyl acrylamide) (PNiPAAm) and sodium alginate (SAlg), as draw agents in forward osmosis (FO) desalination. The charged repeating units of SAlg increase the osmotic pressure, enabling water draw, and form crosslinks in the presence of calcium ions. Moreover, PNiPAAm is thermo-responsive and shrinks above VPTT, releasing the absorbed water. Additionally, incorporating graphene oxide (GO) as a light-absorbing agent further enhances the process efficiency by increasing temperature upon exposure to light. The results show that the swelling capacity, shrinkage behavior, and mechanical properties of the hydrogels are influenced by the content of SAlg and GO. However, the VPTT of the IPN hydrogels remains similar to that of the single PNiPAAm network. In FO desalination tests, the IPN hydrogel with 10 wt% SAlg and 0.2 wt% GO demonstrates the best balance between water flux and thermo-responsivity.