Freeze–Thaw-Induced, Metal Ion Cross-Linked, Mechanically Robust, and Highly Stretchable Composite Poly(vinyl alcohol) Hydrogels for Flexible Electronic Applications

Abstract

The soft-wet nature of hydrogels makes them resemble biological tissues, but lack of robust mechanical properties limits the application of traditional synthetic hydrogels in various fields like the biomedical field, flexible devices, drug delivery, etc. Incorporating hydrogen-bonding interaction in combination with metal–ligand interaction along with a small number of chemical cross-linkers, we synthesized mechanically robust composite hydrogel materials. Free radical copolymerization of acrylamide (AM) and vinyl imidazole (VI) in the presence of poly(vinyl alcohol) (PVA) chains and Ni²⁺ ions followed by freeze–thaw cycles to allow self-assembly of the PVA network furnished hydrogels with imidazole–Ni²⁺ cross-links and multiple hydrogen-bonding interactions (in the PVA microcrystalline domains as well as interchain interactions between PVA hydroxyls and acrylamide). In the optimized condition, the hydrogel achieved a tensile strength of ∼3.1 MPa without compromising fracture strain (∼1260%) in addition to a high work of fracture (∼22 MJ m^–3) and fracture energy (∼8.7 kJ m^–2, ∼9 times higher than the fracture energy of the natural load-bearing collagen). This gel also showed a high compressive strength of ∼18 MPa, good self-recovery (recovery of ∼93% of its dissipated energy in 15 min), and robust antifatigue properties. The hydrogel exhibited good puncture resistance behavior, as well as high tearing energy (17 kJ m^–2). The potential applications of this hydrogel material in resistive sensing and as an electrolyte in a flexible supercapacitor device were demonstrated.