Studies On Multi-dynamic Bond Crosslinking Synergies To Improve The Mechanical And Self-healing Properties Of Polyurethane

Self-healing polymeric materials are usually applied to flexible and intelligent electronic devices,and they can automatically recover functions from physical damage,which greatly improves their functional stability,reliability and service life.In order to adapt to complicated deformation and maintain robustness,reliability and efficient recovery ability,the material should have outstanding comprehensive properties of high tensile strength,high toughness,high elongation,and high self-healing efficiency,which is called"one material with four highs".However,it is an enormous challenge to simultaneously enhance the above four properties,since the dependence of mechanical properties and self-healing ability of materials on segment motion is competitive.Therefore,From the perspective of molecular structure design to balance the competitive dependence of the two on the movement of chain segments,it is possible to fundamentally achieve the synergistic improvement of mechanical properties and self-healing ability simultaneously.Based on the above standpoints,three kinds of polyurethane elastomers were synthesized in this paper.utilizing the reversible exchange reaction of disulfide bonds in the main chain to provide the basic self-healing ability and the non-covalent interaction of coordination bonds to construct dynamic cross-linking point between neighboring chains.As a result,a synergistic improvement of mechanical properties and self-healing efficiency was achieved simultaneously.The details were as follows:?1?Polyurethane elastomers with high self-healing efficiency based on disulfide exchange reaction.Poly?urethane urea?elastomers of PUU-Cy series were synthesized with polypropylene glycol?PPG?and 4,4'-diphenylmethane diisocyanate?MDI?as starting materials and cystamine?Cy?as the chain extender.The role of hydrogen bonding in improving mechanical properties and self-healing properties were analyzed by cyclic tensile test.The results showed that the introduction of disulfide bonds significantly enhances the self-healing ability of poly?urethane urea?elastomers,and the self-healing efficiency of synthetic elastomers reaches 66.10±0.83%,while the self-healing efficiency of PUU-Hex elastomers without disulfide bond is only 20.01±0.37%.This suggests that the introduction of disulfide bonds is crucial to obtain high self-healing efficiency in this system,and self-healing performance is hard to be improved only in the presence of hydrogen bonds between urethane and urea groups.?2?High toughness polyurethane elastomer based on coordination bond and hydrogen bond dynamic cross-linkingPolypropylene glycol?PPG2000?and MDI were used as raw materials,and 2,6-diaminopyridine?Py?was used as a chain extender to synthesize PUU-Py series poly?urethane urea?elastomer.The pyridine ligand in the main chain of the polymer is complexed with Zn²⁺,and the non-covalent bonding force of the coordination bond is used to build a dynamic cross-linking point between the molecular chains to achieve the synergistic improvement of mechanical properties and self-healing efficiency simultaneously.Research showed that a Zn²⁺/Py molar ratio of 1:4 is the most effective for improving robustness.Specifically,tensile strength,elongation,and toughness could be remarkably increased to 16.00±2.41 MPa,1286±30%,and 89.3±12.07MJ/m³ with 226%,29%,and 185%increments compared to uncomplexed PUU,respectively.Analysis from UV-vis,Fourier transform infrared spectroscopy reveals that half of vacant ligands enable dynamic complexation during stretching,which consequently ensures constant noncovalent cross-linkages for constraining mutual chain sliding,contributing to simultaneous improvement of tensile strength,elongation,and toughness.?3?Polyurethane elastomers based on disulfide bond,coordination bond and hydrogen bond dynamic cross-linking synergy to simultaneously improve the mechanical properties and self-healing ability.In this part of work,a novel kind of poly?urethane urea?elastomers?PUU-Py-Cy-Zn?was synthesized by using Py and Cy as chain extenders and subsequently complexed with Zn²⁺ions.It turned out that a Py/Cy molar ratio of 1:2 is optimal to remarkably improve the toughness and self-healing ability.Specifically,its tensile strength,toughness and self-healing efficiency simultaneously improve to 9.40±0.10MPa,64.49±1.75 MJ/m³,and 96.64±1.52%with 92%,139%and 29%increment compared to PUU-Cy,respectively.It is shown that the enhancement of the synergistic effect is due to the interference of the coordination bond with the aggregation state of the hard segment phase.Firstly,the decrease of the size of the hard segment leads to the distribution of the hard segment in the soft segment,which is helpful for the exchange reaction of disulfide bond.Secondly,the coordination bond used as the dynamic crosslinking point can limit the relative slip of the polymer chain.