| Hydrogels are widely used in biomedical,industrial and agricultural applications due to their high water content,high flexibility,low toxicity and good bio-affinity.In recent years,with the rise of research fields include tissue engineering,electronic skin,etc.ultra-tough and anti-fatigue hydrogels has shown an increasing trend.Due to the simple topological design,conventional hydrogels are unable to combine high strength and high toughness.Therefore,the new design of topological compositions and the construction of a"structure-effect"relationship between topology and mechanical properties are of great importance for the design and preparation of strong-tough anti-fatigue hydrogels.The"double network（DN）"structure is one of the most commonly used theories in the design and preparation of strong-tough hydrogels,and has attracted widespread attention.Inspired by the"double network"theory,this project reported two original topological models for the design and preparation of strong-tough hydrogels,and experimental validation of the proposed structural models is carried out.A new strong-tough/high-tenacity poly（AAm-co-AA）/PVA-Fe（Ⅲ）hydrogel was designed and prepared inspired by the DN topological model,which has two modes of energy dissipation,i.e.unzipping of the Fe（Ⅲ）-COO-ligand structure and decomposition of the PVA crystalline domain structure.This structural model has been shown to impart high fracture stresses and high fracture energies to the hydrogels.However,this structural design also has certain drawbacks,namely:it compromises the tensile properties,toughness recovery,shape recovery and fatigue resistance of the hydrogel.It was thus recognized that an elastic and ductile matrix was essential to the design of a strong hydrogel.Therefore,a modified energy dissipation model has been developed for the design of strong hydrogels based on the theoretical model of a"binary-like DN"hydrogel,namely:poly（AAm-co-AA）/PVA/GEL-Fe（Ⅲ）.This structural model consists of three interpenetrating polymer networks:a covalent/ionic double cross-linked contracting polymer network（rigid,first network）;a highly functional cross-linked,semi-crystalline hidden chain polymer network（subrigid,second network）and an elastic as well as ductile polymer network（soft,third network）.In addition,there are different energy dissipation structures for different magnitudes of external forces.The first network acts as the main sacrificial phase at low stresses,dissipating energy through the unzipping of ionic crosslinks,while the second and third networks together act as an elastic matrix network;the second network:as the stress increases,the ionic crosslinks are gradually disrupted,at which point the second network becomes the main sacrificial phase,disintegrating/dissipating energy through the formation of a polymer network with a semi-crystalline domain structure.At the same time,the third network acts as an elastic matrix and maintains the overall structure of the hydrogel.Due to the special binary-like DN structure,the prepared hydrogels should,in principle,show enhanced energy dissipation,toughness/shape recovery and fatigue/tear resistance.Finally,through a series of characterizations,the binary-like DN hydrogel can be found to have a maximum fracture stress of 2.08 MJ·m-2,the fracture energy of 219.06 KJ·m-2 and the energy dissipation capacity of 10.10 MJ·m-3,while even at high strains（at a maximum tensile length of 7）its toughness/shape recover in only 30 min and40 min respectively.The unique binary-like DN structure was demonstrated to be in good agreement with the initial theoretical assumptions.Furthermore,this structural design and toughness recovery shows significant advantages in terms of overall performance compared to other energy dissipation models.Therefore,it is suggested that the"binary-like DN"design concept will provide a new design idea for the improvement of the overall performance of strong hydrogels,thus expanding the application area of hydrogels. |
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