High-strength nanocomposite self-regenerating hydrogels reinforced by additional crosslinking with trivalent metal cations

In this work, doubly physically crosslinked hydrogels were synthesized, based on poly(N-isopropylacrylamide) containing 10 mol% of methacrylate co-monomer, in which nano-platelets of hectorite clay acted as the primary strong physical crosslinker, and trivalent cations La³⁺, Fe³⁺, and Al³⁺ as an additional dynamic one. Due to their excellent mechanical and tensile properties, such gels might be of interest as advanced structural materials for soft robotics. Of especial scientific interest was the comparison of the seemingly similar trivalent cations embedded in analogous gels, where their different bonding to carboxylate made dramatic differences concerning the material properties: La³⁺—purely electrostatic and highly dynamic bonding, Fe³⁺—with tendency to coordination-covalent-, and Al³⁺—with a rather covalent bonding. The cations incorporated into the hydrogels caused a marked, or even very strong improvement of tensile toughness, which in case of La³⁺ occurred without reducing the extensibility. All the cations expectedly raised the modulus: the effect was most pronounced with Al³⁺, which tends to covalent bonding. Most importantly, all the cations improved the kinetics of internal self-healing (self-recovery) of the gels, which was fastest with the dynamically (purely electrostatically) bonding La³⁺ (complete recovery of properties in 1 h). All the cations were incorporated via impregnation of precursor gels. An original synthesis aspect was, that the impregnation was conducted for a brief period of time, which was found necessary to achieve maximum modulus (the modulus dropped again at longer times). In this way, a non-stoichiometric (usually sub-stoichiometric) amount was incorporated, which made a subsequent extraction of cation excess unnecessary. Additionally, the non-stoichiometry helped the self-healing (network reorganization), which was most striking in case of the rather covalently bonding Al-impregnated gel.