Co-Ion Specific Effect Aided Phase Separation in Polyelectrolyte Hydrogels toward Extreme Strengthening and Toughening

Polyelectrolyte (PE) hydrogels are ubiquitous in nature. Tailoring the interactions between PE gels and salt provides versatile implements to overcome their critical drawbacks such as weak and brittle nature. Adding salts to PE gels introduces both counterions and co-ions, and the variations of PE chain conformations and their macroscopic properties have been mainly attributed to the interplay between PE charged groups and their counterions, yet the role of co-ions is generally ignored. This work demonstrates that by changing co-ion species with larger atomic sizes, monovalent haloids are capable of triggering the phase separation of common poly(acrylic acid) single network (PAAc-SN) hydrogels into polymer-sparse and polymer-rich regions, accompanied by remarkable enhancement in strength and toughness. The capabilities of co-ions inducing phase separation of PAAc-SN gels follow a reverse Hofmeister series: Cl– < Br– < I–. Due to the enhanced polymer–polymer interactions and viscoelastic energy dissipation, the phase-separated PAAc-SN gels exhibit exceptional mechanical properties, with fracture stress and tearing energy of 96.7-folds and 1636-folds larger than those of the as-prepared gels, respectively. The largest fracture stress and tearing energy reach 8.8 ± 1.3 MPa and 29.8 ± 2.4 kJ/m2, exceeding most state-of-the-art PE hydrogels. A combination of FTIR, 1H NMR, and all-atom molecular dynamics simulations reveals that the reverse Hofmeister series-aided phase separation originates from changes of network chain conformation through the following influences rooted in the larger size of co-ions: (i) enhancing hydrophobic polymer–polymer interactions; (ii) accelerating the ion pair formation between carboxyl groups and their sodium counterions. These results not only provide a new method for hydrogel strengthening and toughening but also emphasize the significant role of co-ions in tuning the properties of PE gels.