Polymer Hydrogel Sheets with Perpendicular Cross-Linking Gradient: Non-Monotonic Actuation and Ion-Specific Effects...

Non-monotonous actuation, that is, different kinds of motion in response to a single stimulus, is observed in some natural materials but difficult to implement in synthetic systems. Herein, polymer hydrogel sheets made from polyacrylamide (PAAm) or poly(dimethylacrylamide) (PDMAA) with a cross-linking gradient along the sheet thickness are reported. These are obtained by thermally initiated free radical polymerization using a specially designed Teflon mold with a glass lid. The resulting PAAm hydrogels undergo non-monotonous actuation (rolling into a tube and re-opening) when exposed to aqueous media as a single external stimulus. Their actuation kinetics is tuned with anions that have specific ion effects in their interaction with the surrounding solvent and the polymer itself: structure-breaking chloride enhances the hydration of the polymer backbone, structure-making sulfate decreases it, and is thus slowing down the actuation kinetics of the PAAm hydrogels. The PDMAA gel rolls up instantaneously in aqueous NaCl and only re-opens after 24 h. PDMAA actuation in aqueous Na₂ SO₄ is only moderate as the gel did not swell in that solvent. Bilayer hydrogels made from PAAm and PDMAA (without gradient) show monotonic actuation, closing in NaCl solution and re-opening in Na₂ SO₄ .