Metal-ion-mediated hydrogels with thermo-responsiveness for smart windows

The poly(N-isopropylacrylamide-co-N-maleylgelatin) [P(NIPAAm-co-N-MAGEL)] hydrogel was prepared by the free radical polymerization of N-isopropylacrylamide (NIPAAm) and N-maleylgelatin (N-MAGEL) with N,N'-methylene bis(acrylamide) (BIS) as the cross linker in the complete aqueous environment. The hydrogel becomes opaque when the temperature exceeds lower critical solution temperature (LCST) and goes back to its original transparent state when it is cooled below the LCST which can be finely tuned as it shows a linear dependence on the concentration of Na+, Mg2+ and Al3+. Interestingly, different from Na+ and Mg2+, Al3+ could mediate the transparency of the hydrogel even when the temperature is below LCST and induce the formation of double network (DN) hydrogel. Metal-ion-carboxylate supramolecular interactions are proposed to dominate this property through ionic cross-linking. The hydrogel could be used as the temperature and metal ions controlled smart window. Effects of metal ions on swelling characteristics, mechanical property and microstructures of the hydrogel were further investigated.