Polyoxometalate-based self-adhesive hydrogels with both proton conductive and photochromic functions†

With the rapid development of flexible conductive materials, the demand for ionic conductive hydrogels with excellent mechanical properties, high conductivity, and multifunctionality under various environmental conditions is growing at an unprecedented rate. However, hydrogels with both proton conductivity and photochromic properties have hardly been reported. Herein, we introduce a straightforward and environmentally friendly one-pot method for the synthesis of multifunctional POM-based hydrogels denoted as PAM-HPC-Gly-Mo₇-x% (PHGMo-x%) by encapsulating [(NH₄)₆Mo₇O₂₄·4H₂O] (Mo₇) and glycerol in hydrogels with a semi-interpenetrating polymer network (Semi-IPN) made of polyacrylamide (PAM) and hydroxypropyl cellulose (HPC). As an exemplary representative, PHGMo-15% exhibits a very high proton conductivity (0.142 S cm⁻¹ at room temperature and 4.57 × 10⁻² S cm⁻¹ at −10 °C), a fast UV response time (20 s), and good color change reversibility. Moreover, PHGMo-15% also has excellent self-adhesion capability as well as high stretchability (606%). This design idea based on the combination of a semi-interpenetrating network and a multifunctional component (Mo₇) realizes the integration of adhesion, photochromic properties, conductivity, and excellent mechanical properties of the hydrogels, which enables these multifunctional hydrogels to show new potential applications in the fields of flexible strain sensors, information storage devices, and inkless printing.