Salt-Induced Structural Changes in PNIPAm Hydrogels and their Water Transport Behavior

Poly(N-isopropyl acrylamide) (PNIPAm) hydrogels are soft thermoresponsive materials that find use in fields ranging from human to planetary health, and are promising for water purification. However, due to the closed-pore structure of the hydrogel, the water release rate of gels tends to be slow. Thus, we seek to tune the network structure of hydrogels by changing the polymerization medium, e.g., we investigated the effect of different salts on the structure of hydrogels. Our results show that the addition of different salts to aqueous solutions of PNIPAm decreases the lower critical solution temperature (LCST) and the LCST has an inverse relationship with salt concentration. Furthermore, salt concentration has a direct influence on the hydrogel structure (e.g., pore size, pore shape), and once increased past a critical salt concentration, the hydrogels exhibit open-pore structures. These open-pore structures have a wide range of impacts on the structure-property relationships. Most notably, we were able to achieve an over 20 times increase to the water release rate. With this open-pore structure and fast water release property, our materials can have great potential in facilitating the application of hydrogels in the field of water purification.