Super Bulk and Interfacial Toughness of Physically Crosslinked Double‐Network Hydrogels

Abstract

Conventional design wisdom prevents both bulk and interfacial toughness to be presented in the same hydrogel, because the bulk properties of hydrogels are usually different from the interfacial properties of the same hydrogels on solid surfaces. Here, we design and synthesize a fully physically-linked Agar (the 1 st network)/poly(N-hydroxyethyl acrylamide) (pHEAA, the 2 nd network) where both networks are physically crosslinked via hydrogen bonds. Bulk Agar/pHEAA hydrogels exhibit high mechanical properties (~2.6 MPa tensile stress, 8.0 tensile strain, ~8000 J/m 2 tearing energy, ~1.62 MJ/m 3 energy dissipation), high self-recovery without any external stimuli (~62%/30% toughness/stiffness recovery), and self-healing property. More impressively, without any surface modification, Agar/pHEAA hydrogels can be easily and physically anchored onto different non-porous solid substrates of glass, titanium, aluminum, and ceramics to produce super adhesive hydrogel-solid interfaces (i.e. high interfacial toughness of ~2000-7000 J/m 2 ). Comparison of as-prepared and swollen gels in water and hydrogen-bond-breaking solvents reveals that strong bulk toughness provides a structural basis for strong interfacial toughness, and both high toughness mainly stem from cooperative hydrogen bonds between and within two networks and between two networks and solid substrates. This work demonstrates a new gel system to achieve super high bulk and interfacial toughness on non-porous solid surfaces.

Nanoclay Reinforced Self‐Cross‐Linking Poly(N‐Hydroxyethyl Acrylamide) Hydrogels with Integrated High Performances

“…As shown in Figure , PHEAA/Laponite NC gels were synthesized by in situ polymerization of the HEAA in the presence of Laponite nanoplatelets. Physical PHEAA gel could be formed by self‐cross‐linking due to the hydrogen bonding interactions among PHEAA chains . Meanwhile, additional cross‐linkings in the NC gels were also provided by adsorption of the PHEAA chains onto the surface of Laponite nanoplatelets.…”

“…HEAA, a commercially available monomer, contains two hydrophilic groups of hydroxyl and amide separated by two methylene groups. PHEAA can serve as promising biomaterials with long‐term antifouling and durability suitable for applications in complex biological media . Recently, Chen et al .…”

“…PHEAA can serve as promising biomaterials with long-term antifouling and durability suitable for applications in complex biological media. [41][42][43] Recently, Chen et al [43] found PHEAA could self-cross-link to form a physical gel without any chemical cross-linkers during polymerization. The critical gelation concentration to form a physical PHEAA gel was as low as 3 wt% of HEAA.…”

Spontaneously Regenerative Tough Hydrogels

“…Hydrogels are regenerated from their powder form, by addition of water, with preservation of the original appearance and mechanical properties. [5, 10-12, 15, 22-26] For instance, self-healing strategies based on supramolecular interactions are inefficient for healing a large area with severe damage, [12,15,16,21,27,28] for self-healing of physical bonds when the damage is no longer fresh because the effect of healing becomes weak and even disappears, [25,26,[29][30][31] and for repair of dynamic covalent bonds because external conditions need be applied. These DN hydrogels can be conveniently stored and easily shaped upon regeneration.…”

“…In addition, these powders can be stored for a long time and still undergo regeneration upon hydration. [25,26,[29][30][31] In contrast, for the presented strategy of drying and grinding, the powder can be stored for a long time and still reform a hydrogel having a smooth and controlled morphology. This regeneration/grinding cycle can be repeated several times without loss in the ability of the obtained powder to regenerate into the hydrogel, as illustrated in Figure 1 a,b and Figure S1a.…”