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

Polyelectrolyte (PE) hydrogels are ubiquitous in nature.Tailoringthe interactions between PE gels and salt provides versatile implementsto overcome their critical drawbacks such as weak and brittle nature.Adding salts to PE gels introduces both counterions and co-ions, andthe variations of PE chain conformations and their macroscopic propertieshave been mainly attributed to the interplay between PE charged groupsand their counterions, yet the role of co-ions is generally ignored.This work demonstrates that by changing co-ion species with largeratomic sizes, monovalent haloids are capable of triggering the phaseseparation of common poly(acrylic acid) single network (PAAc-SN) hydrogelsinto polymer-sparse and polymer-rich regions, accompanied by remarkableenhancement in strength and toughness. The capabilities of co-ionsinducing phase separation of PAAc-SN gels follow a reverse Hofmeisterseries: Cl- < Br- < I-. Due to the enhanced polymer-polymer interactionsand viscoelastic energy dissipation, the phase-separated PAAc-SN gelsexhibit exceptional mechanical properties, with fracture stress andtearing energy of 96.7-folds and 1636-folds larger than those of theas-prepared gels, respectively. The largest fracture stress and tearingenergy reach 8.8 & PLUSMN; 1.3 MPa and 29.8 & PLUSMN; 2.4 kJ/m(2), exceeding most state-of-the-art PE hydrogels. A combination ofFTIR, H-1 NMR, and all-atom molecular dynamics simulationsreveals that the reverse Hofmeister series-aided phase separationoriginates from changes of network chain conformation through thefollowing influences rooted in the larger size of co-ions: (i) enhancinghydrophobic polymer-polymer interactions; (ii) acceleratingthe ion pair formation between carboxyl groups and their sodium counterions.These results not only provide a new method for hydrogel strengtheningand toughening but also emphasize the significant role of co-ionsin tuning the properties of PE gels.