How can multi-bond network hydrogels dissipate energy more effectively: an investigation on the relationship between network structure and properties.

Constructing a multi-bond network (MBN), which involves hierarchical dynamic bonds with different bond association energies, is an effective method for achieving super tough hydrogels. In this work, a small amount of poly(vinyl alcohol) (PVA) is introduced into a loosely chemically crosslinked poly(acrylic acid) (PAA) network. The hydrophilic PVA chains can physically interact and form hydrogen bonds with the PAA chains. After a freeze-thaw process, PVA could partially crystallize and the generated microcrystals could become new crosslinking points of the hydrogels. Meanwhile, the hydrogen bonds between PAA and PVA, which connect to the microcrystal "core" through PVA chains, could also become new crosslinking points of the hydrogels. The obtained ternary-crosslinked hydrogels (T-gel 10%) exhibit toughness as high as 8 times that in pure PAA hydrogels. When the PVA content exceeds 15 wt%, PVA chains will run through the whole PAA network. Thus the PVA chains will be crosslinked by microcrystals through freeze-thaw treatment, leading to a double network structure, resulting in a brittle hydrogel. The step-increased modulus of the hydrogels with different PVA contents clearly demonstrates the change in the network structure of the hydrogels. Successively, Fe3+ is introduced into the MBN hydrogels as a third cross-linking point. The obtained quaternary-crosslinked hydrogels (Q-gel 10%-Fe5) (50 wt% water content) exhibit significantly improved mechanical properties: tensile strength as high as 6.83 MPa with a fracture energy of 29.9 MJ m-3. This work provides clear insight into the relationship between network structure and mechanical properties in super tough MBN hydrogels.

Dually cross-linked single network poly(acrylic acid) hydrogels with superior mechanical properties and water absorbency.

Poly(acrylic acid) (PAA) hydrogels with superior mechanical properties, based on a single network structure with dual cross-linking, are prepared by one-pot free radical polymerization. The network structure of the PAA hydrogels is composed of dual cross-linking: a dynamic and reversible ionic cross-linking among the PAA chains enabled by Fe(3+) ions, and a sparse covalent cross-linking enabled by a covalent cross-linker (Bis). Under deformation, the covalently cross-linked PAA chains remain intact to maintain their original configuration, while the Fe(3+)-enabled ionic cross-linking among the PAA chains is broken to dissipate energy and then recombined. It is found that the mechanical properties of the PAA hydrogels are significantly influenced by the contents of covalent cross-linkers, Fe(3+) ions and water, which can be adjusted within a substantial range and thus broaden the applications of the hydrogels. Meanwhile, the PAA hydrogels have excellent recoverability based on the dynamic and reversible ionic cross-linking enabled by Fe(3+) ions. Moreover, the swelling capacity of the PAA hydrogels is as high as 1800 times in deionized water due to the synergistic effects of ionic and covalent cross-linkings. The combination of balanced mechanical properties, efficient recoverability, high swelling capacity and facile preparation provides a new method to obtain high-performance hydrogels.

Robust and self-healable nanocomposite physical hydrogel facilitated by the synergy of ternary crosslinking points in a single network.

Acrylamide (AM) and a small amount of stearyl methacrylate (C18) hydrophobic monomer copolymerize to graft on the surface of vinyl hybrid silica nanoparticles (VSNPs), forming nanobrush gelators, thereby constructing ternarily crosslinked nanocomposite physical hydrogels (TC-NCP gels). The TC-NCP gel is composed of a single network ternarily crosslinked by hydrogen bonds and hydrophobic interactions among the grafting polymer chains as physical cross-linking points and thus the polymer grafted VSNPs as analogous covalent crosslinking points. Under stretching, the physical crosslinking points successively break to gradually dissipate energy and then recombine to homogenize the network. During the stretching process, the polymer chains grafted VSNPs can homogenize the stress distribution as transferring centers. The synergy of the ternary crosslinking points leads the TC-NCP gels to dissipate more energy and redistribute the stress more effectively when compared with hydrogels dually crosslinked by both hydrogen bonds and VSNPs as analogous covalent crosslinking points (without hydrophobic interactions) and by both hydrogen bonds and hydrophobic interactions (without VSNPs). As a result, the TC-NCP gels demonstrate remarkably improved mechanical properties, including tensile strength of 256 kPa, stretch ratio at break of 28.23 and toughness of 1.92 MJ m-3 at a water content of 90%. Pure shear test shows that the TC-NCP gel is able to resist notch propagation by micro-crack development from the notch tip to the whole gel network and has a high tearing energy of 1.21 × 104 J m-2. The dynamic nature of the network endows the TC-NCP gels with excellent self-healing ability. The results evidently indicate that constructing a single gel network with hierarchical crosslinking points is a versatile method to fabricate robust hydrogels.

Self-healable, tough and highly stretchable ionic nanocomposite physical hydrogels.

We present a facile strategy to synthesize self-healable tough and highly stretchable hydrogels. Our design rationale for the creation of ionic cross-linked hydrogels is to graft an acrylic acid monomer on the surface of vinyl hybrid silica nanoparticles (VSNPs) for the growth of poly(acrylic) acid (PAA), and the obtained VSNP-PAA nanobrush can be used as a gelator. Physical cross-linking through hydrogen bonding and ferric ion-mediated ionic interactions between PAA polymer chains of the gelators yielded ionic nanocomposite physical hydrogels with excellent and balanced mechanical properties (tensile strength 860 kPa, elongation at break ∼2300%), and the ability to self-repair (tensile strength ∼560 kPa, elongation at break ∼1800%). The toughness and stretchability arise from the reversible cross-linking interactions between the polymer chains that help dissipate energy through stress (deformation) triggered dynamic processes. These unique properties will enable greater application of these hydrogel materials, especially in tissue engineering.

Highly stretchable and super tough nanocomposite physical hydrogels facilitated by the coupling of intermolecular hydrogen bonds and analogous chemical crosslinking of nanoparticles.

Highly stretchable and super tough nanocomposite physical hydrogels (NCP gels) were fabricated by a facile and one-pot process. NCP gels show superior mechanical properties with tensile strength of 73 kPa-313 kPa and elongation at break of 1210-3420%. This is due to the effective strengthening mechanism: under stretching, the intermolecular hydrogen bonds can dynamically break and recombine to dissipate energy and homogenize the gel network. In addition, vinyl hybrid silica nanoparticles (VSNPs) can work as stress transfer centres to transfer stress to the grafted polymer chains.

High strength of physical hydrogels based on poly(acrylic acid)-g-poly(ethylene glycol) methyl ether: role of chain architecture on hydrogel properties.

This investigation was to study the connections between polymer branch architecture of physical hydrogels and their properties. The bottle-brush-like polymer chains of poly(acrylic acid)-g-poly(ethylene glycol) methyl ether (PAA-g-mPEG) with PAA as backbones and mPEG as branch architecture were synthesized and in situ grafted from silica nanoparticles (SNs) to construct hydrogels cross-linked networks in aqueous solutions. The structural variables to be discussed included molecular weight and molar ratio of branch chains, and new aspects of the formation mechanism of physical hydrogels with branch structure in the absence of organic cross-links were present. The results indicated that the differences of polymer chain architecture could be distinguished via their different interactions that are present by gelation process and mature gel properties, such as gel strength and swelling ratio. The gelation occurred at the critical polymer concentration and molecular weight, respectively, and the inorganic/organic (SNs/PAA-g-mPEG) nanoparticles began to entangle and construct the cross-linking networks afterward. The gel-to-sol transition temperature (T(g-s)) and radii of SNs that were encapsulated by polymer chains as a function of time for chains' disentanglement were monitored according to the observation of the dissolution process, and the molecular weight between two consecutive entanglements (M(e)) was calculated thereafter. This study showed that the introduction of branch chain onto the linear backbone significantly promoted the chain interactions and increased entanglement density, which contributed to the hydrogels' network integrity and rigidity, thus illustrating greater elongation at break and tensile strength than the hydrogels formulated with linear polymer chains.

Dual cross-linked networks hydrogels with unique swelling behavior and high mechanical strength: based on silica nanoparticle and hydrophobic association.

A series of physically cross-linked hydrogels composed poly(acrylic acid) and octylphenol polyoxyethylene acrylate with high mechanical strength are reported here with dual cross-linked networks that formed by silica nanoparticles (SNs) and hydrophobic association micro-domains (HAMDs). Acrylic acid (AA) and octylphenol polyoxyethylene acrylate with 10 ethoxyl units (OP-10-AC) as basic monomers in situ graft from the SNs surface to build poly(acrylic acid) hydrophilic backbone chains with randomly distributed OP-10-AC hydrophobic side chains. The entanglements among grafted backbone polymer chains and hydrophobic branch architecture lead to the SNs and HAMDs play the role of physical cross-links for the hydrogels network structure. The rheological behavior and polymer concentration for gelation process are measured to examine the critical gelation conditions. The correlation of the polymer dual cross-linked networks with hydrogels swelling behavior, gel-to-sol phase transition, and mechanical strength are addressed, and the results imply that the unique dual cross-linking networks contribute the hydrogels distinctive swelling behavior and excellent tensile strength. The effects of SNs content, molecular weight of polymer backbone, and temperature on hydrogels properties are studied, and the results indicate that the physical hydrogel network integrity is depended on the SNs and HAMDs concentration.