Preparation And Properties Of High Strength Hydrogels With Multiple Crosslinked Network Structures

Hydrogel is a kind of hydrophilic polymer with three-dimensional network structure.It can absorb large amounts of water but insoluble in water.Traditional hydrogels are chemically cross-linked.Hydrogel materials were caused catastrophic damage by small tensile stresses because conventional chemically crosslinked hydrogels always exhibit mechanically weak and brittle properties.It was largely limited the path for hydrogels to be commercialized.Therefore,preparation of hydrogels with remarkable mechanical properties is an important direction for the development of hydrogels.Double network hydrogels(DN),nanocomposite hydrogels(NC gels),hydrophobically associating hydrogels(HA gels)all exhibit excellent mechanical properties.The combination of different cross-linking effects to prepare good mechanical properties hydrogels is a way to effectively increase the mechanical strength of hydrogels.Polyethylenimine(PEI)is a common water-soluble polymer,also known as polyazacyclopropane.The amine groups on the molecular chain can exhibit ammonium cations(NH4+)in aqueous solution and can form cross-linked structures by ionic bonds which containing anionic group.This way is an effective way to improve the mechanical properties of hydrogels.Polyvinyl alcohol(PVA)has good biocompatibility for a large number of hydroxyl groups.In the aqueous solution state,PVA molecules can form a amount of hydrogen bonds.Studies have shown that after PVA freeze-thaw treatment,PVA molecules will crystallize to form crystalline domains.These crystalline domains have the role of cross-linking sites and can effectively increase the mechanical strength of hydrogel materials.In Chapter 2 of this paper,the first network was formed by ultraviolet initiated free-radical polymerization of polyethylenimine(PEI)and sodium acrylate(AAS).The ionic and chemical cross-linking was simultaneously formed during this process.Then,acrylamide(AM)is introduced into the second network to form a composite hydrogel with multiple crosslinked structures.The influence of the introduction of PEI on the molecular structure of the hydrogel was investigated by Fourier transform infrared spectroscopy(FT-IR).The internal morphology of the hydrogel was observed and analysed by scanning electron microscopy(SEM)and discussed by the tensile test.The introduction of PEI improves the mechanical properties of hydrogels.The results show that the introduction of PEI can effectively increase the strength and elongation hydrogels.With the increase of the PEI content,the network structure of the hydrogels is more compact and the tensile strength increased sequentially.The PEI with a higher molecular weight can more effectively increase the elongation.of different concentration and molecular weight PEI,fracture stress and elongation for hydrogels have been effectively improved.The hydrogels still has good swelling properties.The highest tensile modulus of the prepared hydrogel can reach 1.41MP,and the maximum elongation at break of the hydrogel is 260%.In Chapter 3 a hydrophobic associative poly(acrylamide-tridecafluoromethyl methacrylate)hydrogel(PAM-G06B)was first constructed by the micelle polymerization,and The influence of the content of hydrophobic monomer G06B on the mechanical strength of hydrogel was discussed.The results show that with the increase of G06B,the tensile modulus of PAM-G06B hydrogel increases sequentially,and the fracture strength first increases and then decreases,and the fracture productivity decreases in turn.When the amount of G06B added to the PAM is 1.5%,the breaking strength of the hydrogel is 117±2KPa,and the elongation at break can reach 691±18%.PVA was then introduced into the PAM-G06B hydrogel and through the freeze-thaw cycle to form a PVA crystalline microdomain to construct a cross-linked network structure.At the same time,hydrogen bonds can be formed between the molecules of the PVA polymer and between PVA and PAM.The high-strength multi-crosslinked composite hydrogels having hydrophobic association,a physical crosslinking point in the crystalline microdomain,and a hydrogen bond can be prepared by the above method.With the increase of the concentration of PVA solution,the tensile modulus and the breaking strength of the hydrogel have been significantly improved,and have shown an upward trend with the increase of the PVA concentration.