Tough, anti-freezing and conductive ionic hydrogels

• Published in: NPG Asia materials 2022-12, Vol.14 (1), p.65, Article 65
• Main Authors: Wu, Shuwang, Wang, Ta-Wei, Du, Yingjie, Yao, Bowen, Duan, Sidi, Yan, Yichen, Hua, Mutian, Alsaid, Yousif, Zhu, Xinyuan, He, Ximin
• Format: Article
• Language: English
• Subjects: 639/301/923/1027; 639/638/298/923/1027; Acrylamide; Biomaterials; Chains (polymeric); Chemistry and Materials Science; Cold tolerance; Dehydration; Electronic devices; Energy Systems; Freezing; Hydrogels; Hydroxyethyl acrylate; Low temperature resistance; Malfunctions; Materials failure; Materials Science; Mechanical properties; Optical and Electronic Materials; Polymers; Polyvinyl alcohol; Potassium; Salting; Softness; Structural Materials; Structural stability; Surface and Interface Science; Tensile strength; Thin Films; Toughness
• ISSN: 1884-4049; 1884-4057
• DOI: 10.1038/s41427-022-00410-7

With rapid advances in soft electronic devices, the demand for soft conductive materials, including hydrogels, with superior mechanical properties, high conductivity and functionality under extreme environmental conditions are increasing at an unprecedented rate. Although hydrogels have favorable properties such as softness and broad tunability ranges, they freeze at subzero temperatures, leading to materials failure and device malfunctions, and the introduction of antifreezing agents into hydrogels often severely compromises their conductive or mechanical properties. The tradeoff between simultaneously endowing antifreezing hydrogels with excellent mechanical properties and high conductivity severely limits their practical applicability over a broad range of conditions. Herein, we discovered that potassium acetate (KAc) induces a salting-out effect on polyvinyl alcohol (PVA), promoting aggregation of the polymer chains and significantly improving the mechanical properties of the hydrogels. Moreover, concentrated KAc exhibits excellent anti-freezing capacity and high conductivity. The hydrogels produced by soaking frozen PVA in KAc solutions show superior mechanical properties, with a tensile strength of 8.2 MPa, conductivity of 8.0 S/m and outstanding freeze tolerance to a temperature of −60 °C. This strategy also works for other polymers, such as poly(acrylamide) and poly(2-hydroxyethyl acrylate). Additionally, the as-prepared hydrogels possess excellent anti-dehydration capacity, which is another important feature that is desirable for further enhancing the applicability and durability of hydrogel-based devices. Hydrogels: Adding salt improves cold tolerance and toughness Adding salts to water-filled polymers known as hydrogels can improve their cold-weather resistance and structural stability. Soft conductive hydrogels have found increasing use in wearable electronics, but these materials are naturally unstable in subzero conditions. Ximin He from the University of California, Los Angeles, USA, and colleagues report that a common salt called potassium acetate can give hydrogels freeze tolerance down to −60 °C. When frozen samples of polyvinyl alcohol hydrogels were soaked in potassium acetate solutions, the team saw that the salts expelled water from between polymer chains. This ‘salting out’ effect caused the polymer chains to agglomerate into more resilient structures while exhibiting high conductivity. Optimizing potassium acetate additions produced