Tough, anti-freezing and conductive ionic hydrogels

The increasing demand for soft conductive materials in wearable electronics, including soft aqueous batteries, supercapacitors, wearable sensors, soft robots, ionic skins, and ionic touch panels, drives the search for materials with superior mechanical properties, high conductivity, and functionality under diverse environmental conditions. Hydrogels are attractive candidates due to their softness and tunability, but their freezing at subzero temperatures poses a significant challenge, leading to material failure and device malfunctions. While antifreezing agents can be added, this often compromises conductivity or mechanical properties. Existing strategies, such as using organic solvents with high salt content, suffer from drawbacks like salt precipitation at low temperatures, reduced conductivity due to suppressed salt dissociation, and environmental concerns. High salt concentrations, while offering improved conductivity and antifreezing properties, can negatively impact mechanical properties through a salting-in effect. The Hofmeister effect, which describes the ion-specific effects on solute solubility, provides a potential solution. Salting-out salts, like Na2SO4 and Na2CO3, promote polymer aggregation and enhance hydrogel toughness, but their limited solubility restricts their use. This research focuses on identifying an optimal salting-out salt with high solubility to create tough, conductive, and antifreezing hydrogels. The study specifically investigates potassium acetate (KAc) due to its high solubility and potential salting-out effect on PVA, offering a promising avenue for overcoming the limitations of existing antifreezing strategies.

Extensive research has been dedicated to improving the conductivity and mechanical properties of soft conductive materials, including exploring liquid-free solid-state conductors. Conductive ionic hydrogels, with their solid-like mechanical performances and liquid-like transport properties, have shown great potential for various soft devices. However, their freezing at sub-zero temperatures significantly limits their applicability. Previous attempts to address this issue by incorporating organic solvents and high salt contents have encountered limitations. Organic solvents, while providing anti-freezing capacity, reduce conductivity due to lower salt solubility and suppressed salt dissociation. They also present environmental and safety hazards. High salt concentrations, though improving conductivity and antifreezing performance, often compromise mechanical properties through the salting-in effect. The Hofmeister effect, which governs the influence of ions on solute solubility, has been utilized to tune hydrogel mechanical properties. Salting-out salts enhance toughness, but their limited solubility is a constraint. This study aims to leverage the Hofmeister effect with a highly soluble salting-out salt to create mechanically robust, highly conductive, and antifreezing hydrogels.

The study employed a freeze-soak method to fabricate the hydrogels. First, the salting-out effect of KAc on PVA was investigated by observing the precipitation of PVA in a 50 wt% KAc solution. For hydrogel fabrication, PVA solutions were poured into molds, frozen at -20 °C, and then immersed in KAc solutions of varying concentrations (10 wt%, 30 wt%, 50 wt%). The freezing process pre-packs polymer chains, facilitating aggregation during the subsequent salting-out process. The effect of KAc concentration and PVA molecular weight (27 kDa, 89 kDa, 195 kDa) on the mechanical properties was systematically studied using tensile testing. The freezing temperatures of the hydrogels were determined using differential scanning calorimetry (DSC). Conductivity measurements were performed using an electrochemical workstation at various temperatures (20 °C to -60 °C). Scanning electron microscopy (SEM) was used to characterize the microstructure of the hydrogels. The universality of the method was tested by applying the same procedure to poly(acrylamide) (PAAm) and poly(2-hydroxyethyl acrylate) (PHEA) hydrogels. The anti-dehydration capacity of the hydrogels was also assessed.

The study found that KAc induced a significant salting-out effect on PVA, resulting in a substantial improvement in hydrogel mechanical properties. Increasing KAc concentration from 10 wt% to 50 wt% increased the tensile strength from 0.1 MPa to 4.0 MPa and toughness from 0.1 MJ/m³ to 7.8 MJ/m³ for 10 wt% PVA (89 kDa). Increasing PVA molecular weight from 27 kDa to 195 kDa further improved the mechanical properties, reaching a tensile strength of 8.2 MPa and toughness of 25.8 MJ/m³ in 50 wt% KAc. DSC analysis showed that the freezing temperature of the hydrogels decreased significantly with increasing KAc concentration, reaching below -70 °C for 50 wt% KAc. Conductivity measurements revealed a high conductivity of 8.0 S/m at room temperature and 1.2 S/m at -60 °C for the hydrogel made from 195 kDa PVA in 50 wt% KAc. SEM images showed a decrease in pore size and denser fiber structures with increasing KAc concentration. The method's universality was demonstrated by applying it to PAAm and PHEA hydrogels, resulting in a two-order-of-magnitude increase in toughness for PHEA. All KAc-containing hydrogels showed excellent anti-dehydration properties. The hydrogels showed superior mechanical properties and conductivity compared to many previously reported tough and conductive ionic hydrogels.

The results demonstrate that the combination of the freeze-soak method and the salting-out effect of KAc provides a simple and effective strategy for creating tough, anti-freezing, and conductive ionic hydrogels. The high solubility of KAc allows for the incorporation of a high concentration of ions, leading to both high conductivity and excellent antifreezing properties. The salting-out effect enhances the mechanical properties by promoting the aggregation of polymer chains. The synergistic effect of the polymer matrix confinement and the colligative properties of KAc results in freezing temperatures even lower than those of the KAc solution alone. The success of this method with different polymers highlights its versatility and potential for broader applications. The superior performance of these hydrogels, particularly in terms of their mechanical strength, conductivity, and freeze tolerance, positions them as promising candidates for use in various soft electronic devices.

This study successfully demonstrated a simple and scalable method for fabricating tough, anti-freezing, and conductive ionic hydrogels using potassium acetate. The method leverages the salting-out effect of KAc to enhance mechanical properties and its high solubility to achieve high conductivity and excellent antifreezing capacity. The resulting hydrogels exhibit superior performance compared to previously reported materials and showcase the versatility of this approach. Future research could explore other high-solubility salting-out salts, optimize the hydrogel compositions for specific applications, and investigate long-term stability and durability under various conditions.

While this study demonstrates the efficacy of the freeze-soak method using KAc, further investigations are needed to thoroughly assess the long-term stability of these hydrogels under diverse environmental conditions, including prolonged exposure to low temperatures and varying humidity levels. Additionally, a comprehensive analysis of the biocompatibility of these materials would be essential for biomedical applications. The study focused on a limited set of polymers, and further research is warranted to explore the applicability of this technique to a broader range of polymers and to understand the specific interactions between the selected salts and polymers.