POSS-Induced Dynamic Cross-Links Produced Self-Healing and Shape Memory Physical Hydrogels When Copolymerized with N-Isopropyl acrylamide

The hydrogel morphology and viscoelasticity and the two-way shape memory behavior of copolymers of N-isopropyl acrylamide–polyhedral oligomeric silsesquioxane (PNIPAm-POSS) were investigated. NIPAm-POSS copolymers are unentangled and exhibit a thermoplastic and thermoresponsive hydrogel behavior (Romo-Uribe; ; Albanil, Eur. Polym. J. 2018, 99, 350−360). Here, we demonstrated by high-resolution transmission electron microscopy that POSS segregated into crystals to form physical cross-links. The hydrogels exhibited significant swelling, higher than chemical hydrogels at the same POSS content. The morphology of the hydrogels consisted of a honeycomb structure, and the pore size and swelling were a decreasing function of POSS. Moreover, the hydrogels were self-healing, exhibiting a liquid-like behavior under large amplitude shear and rubber-like behavior immediately after cessation of shear, that is, the POSS aggregates break and re-form, acting as dynamic cross-linkers. Strikingly, the physical networks behaved as perfect networks, thus matching chemically cross-linked networks and holding the scaling |η*| ∼ ω–1. Furthermore, the physical hydrogels exhibited a thermally activated shape memory (SM) behavior, without external stress and the lower critical solution temperature (LCST) being the only stimulus. The SM process involved swelling–deswelling, effectively opening opportunities for drug delivery. Furthermore, the SM was reversible, and no external stress was involved, therefore behaving as the two-way SM. Finally, POSS modulated the shear elastic modulus G′ and LCST; therefore, the PNIPAm-POSS copolymers open opportunities for reusable, thermoformable SM hydrogels with tuned mechanical modulus and activation temperature.