Double network hydrogels (DN gels) are considered as one of the toughest soft materials. However, conventional chemically linked DN gels often lack high self-recovery and fatigue resistance properties due to permanent damage of covalent bonds upon deformation. Current strategies to improve selfrecovery and fatigue resistance properties of tough DN gels mainly focus on the manipulation of the first network structure. In this work, we proposed a new design strategy to synthesize a new type of Agar/PAMAAc-Fe 3+ DN gels, consisting of an agar gel as the first physical network and a PAMAAc-Fe 3+ gel as the second chemical−physical network. By introducing Fe 3+ ions into the second network to form strong coordination interactions, at optimal conditions, Agar/PAMAAc-Fe 3+ DN gels can achieve extremely high mechanical properties (σ f of ∼8 MPa, E of ∼8.8 MPa, and W of ∼16.7 MJ/m 3 ), fast self-recovery (∼50% toughness recovery after 1 min of resting), and good fatigue resistance compared to properties of cyclic loadings by simply controlling acrylic acid (AAc) content in the second network. The high toughness and fast recovery of Agar/PAMAAc-Fe 3+ DN gel is mainly attributed to energy dissipation through reversible noncovalent bonds in both networks (i.e., hydrogen bonds in the agar network and Fe 3+ coordination interactions in the PAMAAc network). The time-dependent recovery of Agar/PAMAAc-Fe 3+ gels at room temperature and the absence of recovery in Agar/PAMAAc gels also confirm the important role of Fe 3+ coordination interactions in mechanical strength, self-recovery, and fatigue resistance of DN gels. Different mechanistic models were proposed to elucidate the mechanical behaviors of different agar-based DN gels. Our results offer a new design strategy to improve strength, selfrecovery, and fatigue resistance of DN gels by controlling the structures and interactions in the second network. We hope that this work will provide an alterative view for the design of tough hydrogels with desirable properties.
Gelatin/polycrylamide double-network (DN) hydrogels composed of two different polymer networks with strong asymmetry are excellent structural platforms to integrate different mechanical properties into a single material.
Combining both chemical and physical
cross-links in a double-network hydrogel (DN gel) has emerged as a
promising design strategy to obtain highly mechanically strong hydrogels.
Unlike chemically cross-linked DN gels, little is known about the
fracture process and toughening mechanisms of hybrid chemically physically
linked DN gels. In this work, we engineered tough hybrid DN gels of
agar/polyacrylamide (Agar/PAAm) by combining two types of cross-linked
polymer networks: a physically linked, first agar network and a chemical-linked,
second PAAm network. The resulting Agar/PAAm exhibited high stiffness
of 313 kPa and high toughness of 1089 J/m2. We then specifically
examined the effect of the first agar network on the mechanical properties
of hybrid Agar/PAAm gels. We found that by controlling agar concentrations
above a critical value, the physically linked agar network can simultaneously
enhance both stiffness and toughness of Agar/PAAm DN gels, as evidenced
by a linear relationship of elastic modulus and tearing energies of
the gels as the increase of agar concentration. This toughening behavior
is different from that of chemically linked DN gels. Complement to
chemically linked DN gels, this work provides a different view for
the design of new stiff and tough hydrogels using hybrid physical
and chemical networks.
Hydrophobically associated hydrogels (HA gels) are one of most extensively investigated high strength hydrogels. Semicrystalline HA gels, prepared by micellar copolymerization, show high strength and notable functionalities of self-healing and shape-memory. However, the hydrophobic comonomers in these semicrystalline HA gels are usually limited to the long alkyl length monomers (18-alkyl(meth)acrylates). In the present work, N-acryloyl 11-aminoundecanoic acid (A11AUA), consisting of 10 -CH groups and a -COOH group at the end of alkyl chain, was used as hydrophobic comonomer to prepare physical A11AUA-based HA gels in the presence of high concentration cetyltrimethylammonium bromide (CTAB) or sodium dodecyl sulfate. Differential scanning calorimetry, wide-angle X-ray scattering, and small-angle X-ray scattering experiments had identified that the A11AUA-based HA gels possessed crystalline domains and clusters of crystalline domains, while lauryl methacrylate (C12M)-based HA gels were amorphous. As a result, A11AUA-based HA gels displayed much better tensile properties than those of C12M-based HA gels. At the optimal condition, the A11AUA-CTAB HA gel demonstrated integrated high performances, including high stiffness (E of 1016 kPa), high strength (σ of 0.75 MPa), high toughness (T of 7540 J/m), rapid self-recovery (94% recovery after heat treatment at 60 °C for 2 min), outstanding shape memory (fully recovered to the permanent shape only 2-14 s), and excellent self-healing properties (as healed at 60 °C for 2 h; stress and strain healing efficiency reached to 64% and 85%, respectively). We believe this work provides a new insight for HA gels, which is beneficial to design new hydrogels with integrated high performances, such as high strength, high toughness, large extensibility, and shape-memory and self-healing properties.
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