Tough and Responsive Oppositely Charged Nanocomposite Hydrogels for Assembled Bilayer Actuators Through Interfacial Electrostatic Attactions 

Journal of Materials Chemistry B, 2016, DOI: 10.1039/C6TB00583G

Responsive nanocomposite (NC) hydrogels are widely recognized as strong and tough soft materials for smart devices, but yet with challenges in the synthesis of such hydrogels and convenient and versatile fabrication of devices based on such hydrogels. This work reports on a versatile synthesis of nanocomposite polyelectrolyte hydrogels with high strength and toughness by using either cationic or anionic monomers, and demonstrates a simple and versatile fabrication of bilayer actuators by assembling the oppositely charged hydrogels through electrostatic attraction. Herein, exfoliated sodium montmorillonite (NaMMT) nanosheets were used as crosslinkers through adsorption of monomers and initiators prior to in situ free radical polymerization. In order to prevent precipitation of clay nanosheets, tetrasodium pyrophosphate decahydrate (TSPP) was added. NC hydrogels with negative charges were obtained by copolymerization of acrylamide (AAm) and 2-acrylamido-2-methylpropanesulfonic acid (AMPS), while positively charged NC gels were obtained by using AAm and dimethylaminoethyl methacrylate methylchloride (DMC). These NC polyelectrolyte hydrogels were responsive to pH, and ion strength of buffer solutions, and showed outstanding fatigue resistance against cyclic compression loadings, and high tensile strength and toughness. After tensile tests, the gels were able to recover at room temperature. Moreover, the oppositely charged hydrogels were assembled with robust interface based on electrostatic attractions between the opposite charges. These hydrogels were actuated under electric field. Besides, the oppositely charged hydrogels were assembled into bilayers, which were reversibly actuated due to the contrasting responsiveness of each gel to ion strength .