Preparation Of High-performance Hydrogels Based On Solvent Interaction And The Corresponding Structural Mechanism

Traditional hydrogels have poor mechanical properties due to low polymer chains density and sparse cross-linked networks,which severely limit their applications in biomedical,tissue engineering,soft robotics,and other fields.In recent years,researchers have proposed a series of mechanical modification methods,such as the construction of anisotropic hydrogels,the construction of double network hydrogels,and the preparation of strong and tough hydrogels by nanofillers.Although the above modification methods can effectively improve the mechanical parameters of hydrogels,other polymer components are often required,and the processing steps are complicated.The mechanical modification method based on solvent interaction provides the possibility to overcome the above problems.By introducing organic solvent or inorganic salt solution into hydrogel matrix,the microstructure and mechanical properties of hydrogel can be effectively improved.However,this method greatly reduces the inherent water content of hydrogel,and even part of them becomes organogel,which loses the unique advantages of hydrogels,such as high water content,ductility and transparency.Therefore,how to prepare hydrogels with excellent comprehensive properties by means of solvent modification method becomes particularly important.For this purpose,the specific research contents of this paper are listed as follows:1.In order to effectively improve the mechanical properties of hydrogels without sacrificing the inherent water content,the“confined-chain-aggregation”method was proposed in this study.In order to overcome the severe loss of water content caused by molecular chain aggregation,a strategy of selective phase separation of polymer chains was proposed.When the first cross-linked network was formed and the second alginate network was uncross-linked,the selective phase separation was carried out on the precursor of alginate double network hydrogel by non-solvent reaction.That is,using calcium alginate/polyacrylamide（Ca-Alg/PAAm）double network hydrogel,in the second stage of Ca-Alg/PAAm preparation,and ethanol/water binary solutions with different ethanol concentrations were used for solvent exchange.Under the interaction of alcohol,a poor solvent,the polymer chains of alginate accumulated and forms a large physical cross-linking region.It should be pointed out that,at this time,the pre-crosslinked polyacrylamide covalent network has been formed,and the aggregation of alginate chains was limited by the polyacrylamide network.In this case,the aggregation of alginate chains did not affect the distribution of interpenetrating network.At the same time,under the interaction of subsequent metal ion crosslinking and deionized water,the polyacrylamide chains return to the initial state of uniform distribution.Therefore,the characteristics of interpenetrating distribution of the double network and high water content were ensured,and the density of the alginate chains was increased,inducing the formation of locally physical cross-linked region of the alginate chains with micron scale.The new large-size physical cross-linked region was accompanied by the formation of hydrogen bonds and metal ion bonds,making a great contribution to mechanical parameters.The optimal mechanical parameters(σ_f=1.26 MPa,E=0.14 MPa,ε_f=16.31 mm/mm,W=11.34 MJ m^-3)of alginate-based double network hydrogels were obtained by this method,which were 3.2,2.0,1.5 and 4.1 times of the traditional standard alginate-based double-network hydrogels(E=0.07 MPa,ε_f=11.06mm/mm,W=2.77 MJ m^-3),respectively.The hydrogels showed high strength and toughness.This study provides a new strategy for the development of high strength,high toughness and high water content double network hydrogels,which was conducive to the expansion of their application in structural materials.2.Taking typical polyvinyl alcohol（PVA）as the model polymer of frozen hydrogel,this study proposed a novel physically cross-linked polyvinyl alcohol（PVA）hydrogel prepared by suppressing freeze-thaw（SFT）method using calcium chloride aqueous solution.In this strategy,inorganic salts play the following roles in the forming process of frozen hydrogel:（1）Calcium chloride,as an antifreeze salt,can effectively reduce the freezing temperature of water and change the ice crystal size of water molecules at low temperature after incorporating with water.By changing the concentration of inorganic salts,a wide range of ice crystal size can be adjusted;（2）Based on the inherent characteristics of calcium chloride,its addition will endow hydrogel with some advantages,such as water retention ability,frost resistance and ionic conductivity.Based on the above effects,the size of ice crystal is significantly suppressed during the freezing process of PVA precursor solution,which induces the PVA chains to approach homogeneously in a small level,thus forming an amorphous multi-scale structure in the PVA hydrogel and releasing some free hydroxyl groups.According to the systematic results from different concentrations of inorganic salts,multiple properties of PVA hydrogel were stimulated by suppressed freezing strategy when the concentration of calcium chloride was in the medium range（e.g.,2 mol/L,3 mol/L,3.5 mol/L）.Firstly,the tensile strength and Young’s modulus of the hydrogel were significantly reduced,while the fracture strain was significantly increased.The mechanical transformation of“hard-flexible”took place,and the hydrogel could bend freely with elbow and deform together.Secondly,the hydrogel exhibited high self-adhesiveness,which could be tightly attached to substrates made of different materials,such as glass,plastic,steel and pig skin.At the same time,the hydrogel showed high transparency,and its transmittance at 600 nm was up to 91.6%.Finally,the suppressing freezing strategy also endowed hydrogel with excellent instantaneous self-repairing property,water retention ability and low temperature stability.Through the structural analysis from large scale to small scale,it was proved that the multiple properties above were caused by the new multi-scale structure of PVA hydrogels given by the suppressing freezing strategy.The suppressing freezing PVA hydrogels present completely amorphous state,realizing the coexistence of hydrogen bonded hydroxyl groups and free hydroxyl groups.It overcomes the defects of traditional frozen hydrogels such as hardness,non-adhesion and opacity caused by crystalline crosslinking structure.This study provides theoretical basis and technical reference for the preparation of amorphous and multi-performance novel frozen hydrogels.