As swollen polymer networks in water, hydrogels are usually brittle. However, hydrogels with high toughness play critical roles in many plant and animal tissues as well as in diverse engineering applications. Here we review the intrinsic mechanisms of a wide variety of tough hydrogels developed over the past few decades. We show that tough hydrogels generally possess mechanisms to dissipate substantial mechanical energy but still maintain high elasticity under deformation. The integrations and interactions of different mechanisms for dissipating energy and maintaining elasticity are essential to the design of tough hydrogels. A matrix that combines various mechanisms is constructed for the first time to guide the design of next-generation tough hydrogels. We further highlight that a particularly promising strategy for the design is to implement multiple mechanisms across multiple length scales into nano-, micro-, meso-, and macro-structures of hydrogels.

Multi-scale multi-mechanism design of tough hydrogels: building dissipation into stretchy networks

Large hydrophobic monomers stearyl methacrylate (C18) and dococyl acrylate (C22) could be copolymerized with the hydrophilic monomer acrylamide in a micellar solution of sodium dodecyl sulfate (SDS). This was achieved by the addition of salt (NaCl) into the reaction solution. Salt leads to micellar growth and, hence, solubilization of the hydrophobes within the SDS micelles. The hydrogels thus obtained without a chemical cross-linker exhibit unique properties due to the strong hydrophobic interactions. They can only be dissolved in SDS solutions demonstrating the physical nature of cross-links. Results of dynamic light scattering, rheological and mechanical measurements show that the hydrophobic associations between the blocks of C18 or C22 units prevent water solubility and flow, while the dynamic nature of the junction zones provides homogeneity and self-healing properties together with a high degree of toughness. When fractured, the hydrogels formed using C18 associations can be repaired by bringing to...

Tough and Self-Healing Hydrogels Formed via Hydrophobic Interactions

The technological need for new and better soft materials as well as the drive for new knowledge and fundamental understanding has led to significant advances in the field of nanocomposite gels. A variety of complex gel structures with unique chemical, physical, and biological properties have been engineered or discovered at the nanoscale. The possibility to form self-assembled and supramolecular morphologies makes organic polymers and inorganic nanoparticles desirable building blocks for the design of water based gels. In this review, we highlight the most recent (2004–2008) accomplishments and trends in the field of nanocomposite polymer hydrogels with a focus on creative approaches to generating structures, properties, and function within mostly biotechnological applications. We examine the impact of published work and conclude with an outline on future directions and challenges that come with the design and engineering of new nanocomposite gels.

Nanocomposite polymer hydrogels

The remarkable progress in efforts to prepare conductive self-healing hydrogels mimicking human skin’s functions has been witnessed in recent years. However, it remains a great challenge to develop an integrated conductive gel combining excellent self-healing and mechanical properties, which is derived from their inherent compromise between the dynamic cross-links for healing and steady cross-links for mechanical strength. In this work, we design a tough, self-healing, and self-adhesive ionic gel by constructing synergistic multiple coordination bonds among tannic acid-coated cellulose nanocrystals (TA@CNCs), poly(acrylic acid) chains, and metal ions in a covalent polymer network. The incorporated TA@CNC acts as a dynamic connected bridge in the hierarchically porous network mediated by multiple coordination bonds, endowing the ionic gels the superior mechanical performance. Reversible nature of dynamic coordination interactions leads to excellent recovery property as well as reliable mechanical and elect...

Mussel-Inspired Cellulose Nanocomposite Tough Hydrogels with Synergistic Self-Healing, Adhesive, and Strain-Sensitive Properties

Hydrogels were prepared with physical cross-links comprising 2-ureido-4[1H]-pyrimidinone (UPy) hydrogen-bonding units within the backbone of segmented amphiphilic macromolecules having hydrophilic poly(ethylene glycol) (PEG). The bulk materials adopt nanoscopic physical cross-links composed of UPy–UPy dimers embedded in segregated hydrophobic domains dispersed within the PEG matrix as comfirmed by cryo-electron microscopy. The amphiphilic network was swollen with high weight fractions of water (wH2O ≈ 0.8) owing to the high PEG weight fraction within the pristine polymers (wPEG ≈ 0.9). Two different PEG chain lengths were investigated and illustrate the corresponding consequences of cross-link density on mechanical properties. The resulting hydrogels exhibited high strength and resilience upon deformation, consistent with a microphase separated network, in which the UPy–UPy interactions were adequately shielded within hydrophobic nanoscale pockets that maintain the network despite extensive water content....

Tough Stimuli-Responsive Supramolecular Hydrogels with Hydrogen-Bonding Network Junctions

Design of high-toughness polyacrylamide hydrogels by hydrophobic modification

We present here a synthetic strategy for the preparation of melt-processable shape-memory hydrogels with self-healing ability. The supramolecular hydrogel with a water content of 60–80 wt % consists of poly(acrylic acid) chains containing 20–50 mol % crystallizable n-octadecyl acrylate (C18A) segments together with surfactant micelles. The key of our approach to render the hydrogel melt-processable is the absence of chemical cross-links and the presence of surfactant micelles. At temperatures above the melting temperature Tm of the crystalline domains of alkyl side chains, the hydrogel liquefies due to the presence of surfactant micelles effective for solubilizing the hydrophobic C18A segments. At this stage, it can easily be shaped into any desired form by pouring into molds. Cooling below Tm and removing the surfactant from the gel network results in a hydrogel of any permanent shape with a particularly high compressive strength of 90 MPa and a Young’s modulus of 26 MPa. If the hydrogel was damaged on p...

Melt-Processable Shape-Memory Hydrogels with Self-Healing Ability of High Mechanical Strength

Understanding the nanoscale structure and dynamics of supramolecular hydrogels is essential for exploiting their self-healing mechanisms. We describe here nanostructural evolution and self-healing mechanism of hydrogels formed from in situ generated hydrophobically modified hydrophilic polymers and wormlike sodium dodecyl sulfate (SDS) micelles. We observe a conformational transition in wormlike SDS micelles upon addition of hydrophobic as well as hydrophilic monomers. Several hundred nanometer long SDS micelles completely disappear after the monomer addition, in favor of spherical micelles with a radius of 2.4 nm. After conversion of the monomers to hydrophobically modified polymer chains via micellar copolymerization, the spherical shape of the micelles remains intact but the radius increases to 2.8 nm. The interconnected spherical mixed micelles consisting of SDS and hydrophobic blocks of the polymer self-assemble to form a layered hydrogel structure. Self-healing response of the damaged hydrogel sampl...

Nanostructural Evolution and Self-Healing Mechanism of Micellar Hydrogels

Among the hydrogels prepared in recent years, double network (DN) hydrogels exhibit the highest compression strength, toughness, and fracture energies. However, synthesis of DN hydrogels with extraordinary mechanical properties is limited to polyelectrolyte networks, which hinders their widespread applications. Herein, we prepared nonionic DN and triple network (TN) hydrogels based on polyacrylamide (PAAm) and poly(N,N-dimethylacrylamide) (PDMA) with a high mechanical strength by sequential polymerization reactions. The TN approach is based on the decrease of the translational entropy of the second monomer upon its polymerization in the first network, so that additional solvent (third monomer) can enter into DN hydrogel to assume its new thermodynamic equilibrium. The first network of TN hydrogels comprises chemically cross-linked PAAm or PDMA while the second and third networks are linear polymers. To increase the degree of inhomogeneity of the first network hydrogel, an oligomeric ethylene glycol dimeth...

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Nonionic Double and Triple Network Hydrogels of High Mechanical Strength

Nanocomposite hydrogels were prepared by free-radical polymerization of the monomers acrylam- ide (AAm), N,N-dimethylacrylamide (DMA), and N-iso- propylacrylamide (NIPA) in aqueous clay dispersions at 218C. Laponite XLS was used as clay nanoparticles in the hydrogel preparation. The hydrogels based on DMA or NIPA monomers exhibit much larger moduli of elasticity compared with the hydrogels based on AAm monomer. Calculations using the theory of rubber elasticity reveal that, in DMA-clay or NIPA-clay nanocomposites, both the effective crosslink density of the hydrogels and the func- tionality of the clay particles rapidly increase with increas- ing amount of Laponite up to 10% (w/v). The results sug- gest that DMA-clay and NIPA-clay attractive interactions are stronger than AAm-clay interactions due to the forma- tion of multiple layers on the nanoparticles through hydro- phobic associations. It was also shown that, although the nanocomposite hydrogels do not dissolve in good solvents such as water, they dissolve in dilute aqueous solutions of acetone or poly(ethylene oxide) of molecular weight 10,000 g/mol, demonstrating the physical nature of the crosslink points. 2008 Wiley Periodicals, Inc. J Appl Polym Sci 109: 3714-3724, 2008

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Volkan Can

Papers

Design of high-toughness polyacrylamide hydrogels by hydrophobic modification

Melt-Processable Shape-Memory Hydrogels with Self-Healing Ability of High Mechanical Strength

Nanostructural Evolution and Self-Healing Mechanism of Micellar Hydrogels

Nonionic Double and Triple Network Hydrogels of High Mechanical Strength

Equilibrium swelling behavior and elastic properties of polymer–clay nanocomposite hydrogels