The Preparation And Properties Of Non-covalent Bonds Modified Covalently Reversible Crosslinked Networks

Intrinsic self-healing materials that based on reversible cross-linking reactions have become a hot research field because of their ability to spontaneously repair damages at molecular level,which could extend the service life,broaden the application range and reduce the cost of materials.Covalent bonds have higher bond energy than non-covalent ones,so covalently crosslinked polymers are more suitable for applications in structural materials because of their better mechanical properties as compared to non-covalently crosslinked systems.The introduction of reversible cross-linking reactions into covalently crosslinked polymers thus leads to a new class of crosslinked polymers with self-healing properties.However,because of the lower bond energy of reversible covalent bonds in comparison to traditional covalent bonds,reversible crosslinked polymers often fail to achieve the desired strength.Therefore,improving the strength of reversible crosslinked polymers without affecting their self-healing abilities is a difficult problem to be solved at present.In order to improve the mechanical properties,a common strategy used in organisms is the introduction of non-covalent bonds into as sacrificial bonds.When subjected to external force,the non-covalent bonds broke first to consume part of energy and preserved the integrity of the whole material.By adopting this bionic strategy in synthetic materials,that is,incorporating non-covalent bonds into reversibly crosslinked polymers,it is promising to improve the mechanical properties while maintain the self-healing ability due to the reversible characteristics of non-covalent bonds.Based on disulfide bonds and Zn²⁺-imidazole coordination bonds,co-crosslinked polymer networks and interpenetrating polymer networks were designed and synthesized in this thesis to investigate the effect of non-covalent bonds and their present forms on the properties of reversible covalent crosslinked networks.Nuclear magnetic resonance?NMR?,Fourier transform infrared Spectrum?FT-IR?,Thermogravimetric analysis?TGA?,Dynamic mechanical analysis?DMA?,Polarizing microscope,Scanning electron microscopy?SEM?,Electro-mechanical universal testing machine and so on were employed to investigate the structure,thermodynamic properties,surface morphology,mechanical properties and self-healing properties of the polymers.It is expected to provide new ideas for the development of crosslinked polymers with excellent mechanical properties and healing efficiency.The specific research contents are as follows:Polypropylene glycol glycidyl ether?PPGDGE?and 1-?3-aminopropyl?imidazole?API?were first reacted at a molar ratio of 2:1 to obtain dimers with middle imidazole group and terminal epoxy groups,the dimers were then non-covalent crosslinked by Zn²⁺-imidazole coordination bonds,followed by further covalent crosslinking via amine-epoxy ring-opening reaction between the amine groups in 4'4-diaminodiphenyl disulfide?AFD?and the epoxy groups in dimers.Reversible covalent crosslinked network without Zn²⁺-imidazole coordinations was also synthesized as a comparison.The chemical structure of the co-crosslinked network was proved by means of NMR and FTIR.The thermodynamic and mechanical properties of the co-crosslinked network were studied by TGA,DMA and universal testing machine.The experimental results showed that introduction of non-covalent bond greatly improved the mechanical properties of the polymer.Cyclic tensile tests showed that the improvement of mechanical properties was due to the energy dissipation of non-covalent bonds upon external forces.The self-healing properties of the co-crosslinked network were tested by polarizing microscope and universal testing machine.The experimental results showed that the co-crosslinked network and the pure covalent crosslinked network all exhibited good healing performance due to the exchange reactions of disulfide bonds and metal coordination bonds at high temperature.At low temperature,the covalent cross-linked network could not be repaired due to the sluggish of disulfide exchange reactions,but the co-crosslinked network restored part of mechanical properties duo to the dissociation/association of non-covalent bonds,demonstrating the benefit of non-covalent bonds on the healing efficiency of polymers.The reprocessing experiments showed that the co-crosslinked network and the covalent crosslinked network could be re-processed after complete destruction to reduce the whole usage cost of materials.Secondly,in order to fully exploit the advantages of reversible covalent network?RCN?and reversible non-covalent network?Rn CN?,an interpenetrating polymer network?IPN?was prepared and the influence of both components in interpenetrating network on the performance of IPN was studied.The structure of IPN was characterized by SAXS and SEM.The thermodynamic properties and self-healing properties of IPN were tested by TGA,DMA and universal testing machine.The results showed that Tg of IPN was between RCN and Rn CN,indicating them had good compatibility.SAXS tests showed that IPN had the same regularity with RCN and Rn CN,indicating that IPN didn't undergo micro-phase separation.SEM also showed the homogeneous distribution of RCN and Rn CN in IPN.Cyclic tensile tests showed that non-covalent crosslinked networks were destroyed first by external force to ensure the integrity of the whole materials,which toughened the IPN.The elongation at break of RCN was 44.1%,the elongation at break of IPN reached 200% as the content of Rn CN increased.The self-healing efficiency of RCN was only 80% after 48 h,while IPN reached more than 90% after repairing 12 h in the same conditions.The research in this chapter showed that the combination of reversible covalent and reversible non-covalent crosslinked networks in the form of interpenetrating network could also enhance the mechanical properties and improve the self-healing efficiency.