Hydrophobic Association Hydrogel Fragments Form Macroscopic Gels Via Self-organization

Self-healing steming from biology concept, is one of the important properties of biological systems. Inspired by the bionic simulation, scientists designed and synthesized the intelligent polymer materials with self-healing.In the past two years, our group synthesized a kind of physical hydrogels, named hydrophobic association hydrogels (HA-gels), and found that they possessed of the capability of self-healing. In this paper,the HA-gels were prepared via a micelle copolymerization and acrylamide was adopted as main monomer, a small amount of octylphenol polyoxyethylene ether acrylate with10ethoxyl unites (OP10-AC)was used as hydrophobic monomer, and sodium dodecylsulfate (SDS) was acted as surfactant. On the basis of our previous results, we crushed the dry gels into powder(gel fragment), and made the gel fragments form macroscopic gels via self-organization. By tensile tests, we analyzed the tensile stress-strain curves of the self-healing gels under different conditions, and tried to explain the self-healing mechanism of gel fragments.Firstly, we proved that hydrophobic association hydrogels were able to self-organize to form a macro healing gel after being dried and crushed into gel fragments. Micelles being destroyed during crushing presented at the surface of gel fragments, when under circumstance of water, could be re-associated. And with the help of the surfactant micelles, chains and molecules dispelled and cut off by force crossed the cross-section, and finally re-associated or wound with other molecular chains and fragments. In the meanwhile, the original hydrophobic association crosslink in gel fragments is also in the dynamic process of association, re-association, and ultimately gel fragments were able to self-organize to form a macro healing gel with a certain tensile strength.The tensile tests showed that the particle size of the gel powder whether too large or too small, would affect the uniformity of the gel self-healing network, reducing the tensile strength of self healing gel. Increasing self-healing temperature would accelerate the self-healing rate of the gel fragments, contributing to perfect self-healing gel network. The increase of gel powder solids led to the increase of self healing gel network chain density, and improved tensile strength of the self healing gel.The low content of SDS in original gels not only affected the healing rate of gel fragments, but also affected the network structure of healing gels. The low polymer concentration helped molecular chain segments motion, but reduced the association density of network. So the elongation at break increased but the strength reduced. On the contrary, because of the high polymer concentration, not only limited molecular chain segments motion, but also made it easy to exsite stress concentration area. Therefore,the elongation at break and tensile strength reduced.