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Recently, many tough and self-healing hydrogels have been developed based on physical bonds as reversible sacrificial bonds. As breaking and re-forming of physical bonds are time-dependent, these hydrogels are viscoelastic and the deformation rate and temperature pronouncedly influence their fracture behavior. Using a polyampholyte hydrogel as a model system, we observed that the time-temperature superposition principle is obeyed not only for the small strain rheology but also for the large strain hysteresis energy dissipation and the fracture energy below a certain temperature. The three processes possess the same shift factors that obey the equation of Williams, Landel, and Ferry (WLF) time-temperature equivalence. The fracture energy Γ scales with the crack velocity Vc over a wide velocity range as Γ ~ Vc α (α = 0.21). The exponent α of the power law is well-related to the exponent κ of the relaxation modulus G (t) ~ t-κ (κ = 0.26), obeying the prediction α = κ/(1 + κ) from classic viscoelasticity theory. These results show that the fracture energy of the polyampholyte gel is dominated by the bulk viscoelastic energy dissipated around the crack tip. This investigation gives an insight into designing tough and self-healing hydrogels and predicting their fracture behaviors from their dynamic mechanical spectrum. [ABSTRACT FROM AUTHOR]

Polyampholyte (PA) hydrogels are a new class of tough and self-healing supramolecular hydrogels that have a potential as load-bearing soft materials. Studying on the creep behavior of these hydrogels and understanding the molecular mechanism are important for prediction of lifetime of the materials. In the present work, we study the creep rupture dynamics of the PA hydrogels with and without chemical cross-linking, in a certain observation time window. We have found that above some critical loading stress both physical and lightly chemically cross-linked hydrogels undergo creep rupture while moderately chemically cross-linked hydrogel resists creep flow. To elucidate the molecular mechanism, we have further compared the creep behaviors of the physical and lightly chemically cross-linked samples. The creep rate of the samples decreases with the creep time, following a power law relation, regardless of the loading stress variation. The fracture time of both of these hydrogels exponentially decreases with the increase of the loading stress, following the same master curve at high loading stress region, while the behavior of the two samples becomes different in the low loading stress region. We have explained the delayed fracture dynamics at high loading stress region in terms of a relatively weak strong bond rupture mechanism. [ABSTRACT FROM AUTHOR]

Recently, polyampolytes have been discovered to form hydrogels that possess high toughness, full resilience, and self-healing between two cut surfaces. The self-healing of this class of hydrogels is based on the re-forming of the multiple ionic bonds at the fractured surfaces, in which the mobility of the polymer segments and strength of the ionic bonds play an important role. In this work, we study the effects of healing temperature and chemistry of the polyampholyte hydrogels (chemical cross-linker density and chemical structure of the monomers) on the healing kinetics and healing efficiency. The high healing temperature substantially accelerates the self-healing kinetics. Chemical cross-linking reduces the self-healing efficiency. Monomers with more hydrophobic feature give a low self-healing efficiency. For polyampholyte physical hydrogels with a softening temperature below the room temperature, excellent healing efficiency (∼84% on average and maximum 99%) was observed without any external stimuli. We found a correlation between the self-healing efficiency and the fraction of dynamic bonds in the total bonds for relatively soft samples, which is an evidence that the self-healing is due to the re-forming of dynamic bonds. [ABSTRACT FROM AUTHOR]

Oppositely charged homopolyelectrolytes were found to form strong, tough, and self-healing polyion-complex (PIC) hydrogels, similar to polyampholytes (PA) which have opposite charges randomly distributed on the same polymer chains. The excellent mechanical performances of these two novel hydrogels are the results of dynamic ionic bonds formation between entangled polymer chains. For the PIC system, only interchain bonding occurs, while for the PA system both inter- and intrachain bonding exist. In addition, the ion pairs are expected to form stronger bonding in the PIC system than those in the PA system. In this work, we performed a comparative study of PIC hydrogels with the PA hydrogels. The PIC hydrogels are synthesized by sequential homopolymerization of cationic and anionic monomers at varied formulation, and their swelling and mechanical properties are systematically studied in comparison to the PA hydrogels that were synthesized from random copolymerization of anionic monomers and cationic monomers of the similar formulation. Different from the PA system which only forms tough hydrogels around zero net charge composition without chemical cross-linking, the PIC system forms tough physical hydrogels even at weakly off-balanced charge composition. At the charge-balanced composition, the low entanglement concentration of homocharged polyelectrolyte chains leads to tough PIC hydrogels formation at much lower concentrations than that of PA hydrogels. As a result, the PIC hydrogels are much tougher than the PA hydrogels prepared at the same monomer composition. In similar to PA hydrogels, the PIC hydrogels also exhibit broad dynamic mechanical spectra, indicating the formation of ion complexes with widely ranged bond strength. The PIC hydrogels have strong viscoelasticity in comparison with PA hydrogels. However, the two systems show the similar activation energies of the dynamic mechanical spectra. The SEM microstructural observation shows that the PIC hydrogels have segregated structure while PA hydrogels are more homogeneous. [ABSTRACT FROM AUTHOR]

Recently, we have reported that polyampholytes,synthesized fromfree radical copolymerization of anionic monomer and cationic monomer,form physical hydrogels of high toughness and self-healing. The randomdistribution of the opposite charges forms ionic bonds of a wide distributionof strength. The strong bonds serve as permanent cross-links, impartingelasticity, whereas the weak bonds serves as reversible sacrificialbonds by breaking and reforming to dissipate energy. In this work,we focus on the rupture behaviors of the polyampholyte physical hydrogel,P(NaSS-co-MPTC), copolymerized from sodium p-styrenesulfonate (NaSS) and 3-(methacryloylamino)propyltrimethylammoniumchloride (MPTC). Tensile test and pure shear test were performed atvarious stretch rates in the viscoelastic responses region of thematerial. Tensile test showed yielding, strain softening, and strainhardening, revealing the dually cross-linked feature of the gel. Pureshear test showed crack blunting at the notched tip and a large yieldingzone with butterfly shaped birefringence pattern ahead of the cracktip. After blunting, crack advanced at steady-state velocity witha constant angle. The conditions for the occurrence of crack bluntingand variables governing the crack advancing angle are discussed. Wefound that even for these highly stretchable samples, significantblunting only occurs when the tensile fracture stress σfis larger than modulus Eby a factor of about 2, in consistent with Hui’s theoreticalprediction for elastic materials. The crack advancing angle θwas found to be proportional to σy/Eover a wide stretch rate range, where σyis the yielding stress. In addition, thefracture energy was correlated to small strain modulus by a powerlaw in the viscoelastic response region. This systematic study willmerit revealing the fracture mechanism of tough viscoelastic materialsincluding biological tissues and recently developed tough and highlystretchable hydrogels. [ABSTRACT FROM AUTHOR]

Double-network hydrogels (DN gels)have aroused considerable interestbecause of their excellent mechanical strength and toughness, lowsliding friction, good biocompatibility, as well as wide tunabilityin components. By revisiting DN gels, we provide an ingenious wayto fabricate a kind of strong and tough microgel-reinforced hydrogels(MR gels), that densely cross-linked polyelectrolyte microgels ofpoly(2-acrylamido-2-methylpropanesulfonic sodium) (PNaAMPS) (replacingthe densely cross-linked PNaAMPS macro-network for conventional DNgels) are incorporated into sparsely cross-linked neutral polyacrylamide(PAAm) matrix. The structure of MR gels can be considered as a two-phasecomposite, where the disperse phase is the rigidDNmicrogels, and the continuous phase is the softPAAmmatrix. Similar to DN gels, MR gels show the irreversible energy dissipationin the hysteresis measurement, demonstrating the permanent fractureof the brittle PNaAMPS phase. Thus, the discontinuous brittle phasealso serves as sacrificial bonds. Through quantitativecomparison of the hysteresis curves with DN gels and monitoring themorphology change of the embedded microgels in MR gels during thereal-time stretching process, we conclude that the DN microgels inMR gels show four times higher in fracture efficiency of the sacrificialbonds than bulk DN gels at the same strain, as a result of the stressconcentration around the microgels. [ABSTRACT FROM AUTHOR]