Tough superabsorbent poly(acrylic acid) nanocomposite physical hydrogels fabricated by a dually cross-linked single network strategy

In this work, we report a facile method for the preparation of tough and highly stretchable physical hydrogels by dual cross-linking composed of vinyl-hybrid silica nanoparticles（VSNPs） as multivalent covalent cross-linking and hydrogen bonding as physical cross-linking. Poly（acrylic acid） nanocomposite physical hydrogels（NCP gels） are obtained without adding any organic chemical cross-linkers. When the content of VSNPs is 0.7 wt%（relative to the monomer）, the NCP gels exhibit good mechanical properties（fracture strength = 370 k Pa, elongation at break = 2200%） and a high swelling capacity in both deionized water（2300 g/g） and saline（220 g/g）. Meanwhile, the NCP gels have good recovery ability.

Phase Transition Temperature Controllable Poly(acrylamide-co-acrylic acid) Nanocomposite Physical Hydrogels with High Strength

Poly（acrylamide-co-acrylic acid） nanocomposite physical （P（AAm-co-AAc）NCP） hydrogels have been prepared through the in situ free radical solution polymerization based on a ＂single network, dual cross-linkings＂ strategy. The P（AAm-co-AAc） NCP hydrogels are composed of nanobrushes of P（AAm-co-AAc） chains grafted on the surface of vinyl- hybrid silica nanoparticles （VSNPs）. In the hydrogel system, the VSNPs act as the ＂analogous chemical cross-linking points＂ once the hydrogen bonds formed between the P（AAm-co-AAc） chains of the nanobrushes, thus leading to the fabrication of high-strength P（AAm-co-AAc） NCP hydrogels. Compared with conventional thermosensitive P（AAm-co-AAc） hydrogels, the P（AAm-co-AAc） NCP hydrogels have a broader range of phase transition temperature, which can be adjusted by altering the monomer ratio, the VSNPs concentration, the addition of urea and N,N-dimethylacrylamide （DMAAm）. At the same time, the mechanical properties of the P（AAm-co-AAc） NCP hydrogels have been improved significantly by the introduction of VSNPs. Furthermore, both the phase transition and the tensile strength of the P（AAm-co-AAc） NCP hydrogels are largely influenced when Fe3＋ ions are introduced as the ionic crosslinkers into the hydrogel networks.

Toughening Mechanism of Nanocomposite Physical Hydrogels Fabricated by a Single Gel Network with Dual Crosslinking-- The Roles of the Dual Crosslinking Points

A facile method to fabricate tough and highly stretchable polyacrylamide （PAM） nanocomposite physical hydrogel （NCP gel） was proposed. The hydrogels are dually crosslinked single network with the PAM grafted vinyl hybrid silica nanoparticles （VSNPs） as the analogous covalent crosslinking points and the reversible hydrogen bonds among the PAM chains as the physical crosslinking points. In order to further elucidate the toughening mechanism of the PAM NCP gel, especially to understand the role of the dual crosslinking points, the PAM hybrid hydrogels （H gels） and a series of poly（acrylamide-co-dimethylacrylamide） （P（AM-co-DMAA）） NCP gels were designed and fabricated. Their mechanical properties were compared with those of the PAM NCP gels. The PAM H gels are prepared by simply mixing the PAM chains with bare silica nanoparticles （SNPs）. Relative to the poor mechanical properties of the PAM H gel, the PAM NCP gel is remarkably tough and stretchable and also generates large number of micro-cracks to stop notch propagation, indicating the important role of PAM grafted VSNPs in toughening the NCP gel. In the P（AM-co-DMAA） NCP gels, the P（AM-co- DMAA） chains are grafted on VSNPs and the polydimethylacrylamide （PDMAA） only forms very weak hydrogen bonds between themselves. It is found that mechanical properties of the PAM NCP gel, such as the tensile strength and the elongation at break, are enhanced significantly, but those of the P（AM-co-DMAA） NCP gels decreased rapidly with decreasing AM content. This result reveals the role of the hydrogen bonds among the grafted polymer chains as the physical crosslinking points in toughening the NCP gel.