Polymer Hydrogel Sheets With Perpendicular Cross-Linking Gradient: Non-monotonic Actuation And Ion-specific Effects on The Actuation Kinetics.

Non-monotonous actuation, i.e., different kinds of motion in response to a single stimulus, is observed in some natural materials but difficult to implement in synthetic systems. Here, we report polymer hydrogel sheets made from polyacrylamide (PAAm) or poly(dimethylacrylamide) (PDMAA) with a cross-linking gradient along the sheet thickness. 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 Na 2 SO 4 is only moderate as the gel did not swell in that solvent. Bilayer hydrogels made from PAAm and PDMAA (without gradient) showed monotonic actuation, closing in NaCl solution and re-opening in Na 2 SO 4 . This article is protected by copyright. All rights reserved.