Freeze-Resistant, Conductive, and Robust Eutectogels of Metal Salt-Based Deep Eutectic Solvents with Poly(vinyl alcohol)

Hydrogels have received extensive attention due to their flexibility and conductivity. However, hydrogels often cannot achieve high conductivity and high mechanical strength at the same time, and the working temperature is also limited due to water as the solvent. Deep eutectic solvents (DESs) are regarded as one kind of green solvent due to their good stability, biocompatibility, and conductivity. The eutectogel prepared with DESs as the solvent has also attracted attention. In this work, LiCl and ZnCl2 were used as the hydrogen bond acceptors, while ethylene glycol (EG) was the hydrogen bond donor to prepare DESs. Poly(vinyl alcohol) (PVA) was then dissolved in DES to form eutectogel via the multihydrogen bonds among the components and coordinate bond between the metal ion and PVA molecules. The as-prepared eutectogels showed high strength, high conductivity, and excellent frost resistance. Interestingly, by introducing lignin, the mechanical properties of the eutectogel have been greatly improved. The maximum mechanical strength was 4.8 MPa with the elongation at break of 550%, the conductivity of 19.7 mS cm–1 could be reached at room temperature, and it could be stable for one month at −20 °C, which contributed to the lignin enhancement due to its multifunctional groups and interactions with the other components. Similarly, glycerol can also enhanced the performance of the eutectogels by controlling the crystalline of PVA chains in domains. Moreover, the prepared gel proved a huge application potential in flexible sensing.