Low-Density Self-Assembled Poly(N-Isopropyl Acrylamide) Sponges with Ultrahigh and Extremely Fast Water Uptake and Release.

Poly(N-isopropyl acrylamide) (PNIPAM) hydrogels are well known for their temperature-dependent water uptake and release. Hence, they are ideal candidates for water management applications. However, efficiency and rate of water uptake and release, respectively, have to be optimized. Here, highly stable 3D PNIPAM sponges that show a sufficiently low density and high specific pore volume, required for maximizing the amount and rate of water absorption-desorption, are presented. They are prepared by a top-down approach based on freeze-drying a dispersion of short crosslinked PNIPAM fibers coated with crosslinked PNIPAM. The sponges have low densities (4.10-21.04 mg cm-3 ), high porosities >98%, and high specific pore volumes in the range of 47-243 cm3 g-1 depending on the concentration of the dispersions. The sponges absorb high amounts of water (≈7000%) at temperatures below the lower critical solution temperature (LCST) of PNIPAM and can release more than 80% of the absorbed water above the LCST in less than 2 min. Moreover, the water-swollen sponges are reversibly foldable, can be confined to different shapes, and have compressive elastic modulus below 10 Pa. Hence, these spongy materials are of interest not only for water management but also for biomedical applications, smart textiles, and catalysis.