Preparation Of Tough And Strong Hydrogels Based On Ionic Crosslinking

As a kind of soft and wet material,hydrogels have high water content,biocompatibility and permeability.Hydrogels have been extensively investigated for their potential applications in biomedical fields.However,traditional hydrogels are limited in practical application due to their fragile mechanical properties.In recent years,in order to improve the mechanical properties of hydrogels,researchers have synthesized a variety of tough hydrogels based on the introduction of effective energy dissipation mechanism or the overcoming the inherent structural heterogeneity.However,the synthesis of hydrogels with higher strength and better toughness,which can further expand the use of hydrogels,remains a huge challenge.Ionic cross-linking is a kind of reversible physical cross-linking interaction,and the hydrogels formed by ionic cross-linking network have the characteristics of recovery,which increase the fatigue resistance of hydrogels.Since the introduction of the interaction between sodium alginate and calcium ions as the cross-linking network of hydrogels,the properties of ionic cross-linked hydrogels have been extensively explored.Based on ionic cross-linking,a new hydrogel,namely poly(acrylamide-co-acrylic acid)/sodium alginate/Fe3+ hydrogel,was designed in this thesis by using the strong ionic interaction between ferric ions and carboxylic acid groups.The weak hydrogen bond interactions among amide and carboxyl groups replaced the chemical cross-linking network of traditional hydrogels,leading to the pure physical cross-linking network of hydrogels.The obtained poly(acrylamide-co-acrylic acid)/sodium alginate/Fe3+ hydrogels exhibited ultra-high elastic modulus(about 24.6MPa),tensile strength(about 10.4MPa),compression strength(about 44MPa)and fracture energy(about 4800J/m2).Meanwhile,hydrogels have high stability and excellent mechanical properties in 0.5-2 M NaCl solution.Because hydrogels are formed by complete physical interaction,they also exhibited healing properties.The treated healing hydrogels can withstand the fracture stress of 7.1 MPa,which was the maximum value of the healing hydrogels reported so far.Moreover,with its healing properties,we can use hydrogels as building blocks to construct hydrogels with various complex structures by healing between the original hydrogels.In order to improve the mechanical properties of hydrogels,it is an effective method to introduce anisotropic structure into hydrogels.We designed two simple strategies for the preparation of anisotropic hydrogels,namely the preparation of anisotropic hydrogels by ionic interaction under limited conditions.We first prepared a single ionic cross-linked acrylamide/acrylic acid copolymer/Fe3+ hydrogel without chemical cross-linking,and then we stretched and fixed the single ionic cross-linked hydrogel on the mold under the action of tension using the pre-stretching method.In order to fix the anisotropic structure under external force,the stretched hydrogels fixed on the mold were directly dried in vacuum or freeze-dried,respectively.During the process of drying,migration and rearrangement of Fe3+ within hydrogels will happen,and the anisotropy structures of hydrogels were fixed after drying.Finally,in order to obtain hydrogels with high water content,we placed the dry gels fixed on the mold in hot water.The excess ferric ions would migrate out of the hydrogels during the re-swelling.The anisotropic hydrogels with good mechanical properties were thus obtained.Under optimum experimental conditions,the mechanical properties of the obtained hydrogels have been greatly improved.The tensile strength and elastic modulus of the hydrogels can reach 20.6MPa and 30MPa,respectively,along the pre-stretching direction.The water content of the anisotropic hydrogels can maintain at 52.8wt%.