Tough, anti-freezing and conductive ionic hydrogels

Abstract

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.