| Inspired by the fact that biological tissues can spontaneously repair their mechanical damages,self-healing materials with the introduction of dynamic covalent bonds and non-covalent interactions can repair the damages under external stimuli,which can improve the safety,stability,and service time of materials.However,due to the incorporation of weak bonds,most self-healing materials suffer from the poor mechanical performances,which severely limits their industrial application in the real world.By controlling the morphology and ratio of hard and soft segments,the mechanical properties of polyurethane can be widely adjustable.Thus,by introducing reversible interactions into the chain segments of polyurethane,it has a great possibility to solve the key problem of how to balance the mechanical strength and self-healing performance.In order to solve the above problem,the purpose of this thesis is to fabricate a self-healing polyurethane elastomer with high mechanical strength.Based on different self-healing systems,taking the strategies of supramolecular crosslinking with dangling quadruple hydrogen bonds,adjusting hard segment contents,dynamic covalent crosslinking and mixed chain extension with reversible bond,the mechanical strength was greatly improved without sacrificing the self-healing efficiency.The main research contents are as follows.（1）For improving the mechanical strength of self-healing polyurethane with weak dynamic bonds（disulfide bond）,on the basis of chain extending with 2-hydroxyl disulfide（SS-diol）,dangling UPy（2-ureido-4[1H]-pyrimidinone）group was introduced by additional chain extending with UPy-diol.UPy group can form complimentary quadruple hydrogen bonds as supramolecular crosslinking to strengthen the obtained colorless transparent self-healing polyurethane elastomer.The tensile strength of the elastomer increases first and then decreases with the increasing of UPy content.The optimal ratio of UPy-diol is between 20-30%,and in this range,the tensile strength can be increased twice as before modification,reaching 25 MPa.The elongation at break and toughness are 1460%and 101 MJ m-3,respectively.At the same time,the self-healing ability did not significantly decrease with the increase of UPy content.Combining the characterization of TGA、DMA and rheology test,it is convinced that this design ensures the existence of dangling side chain which can promote the chain diffusion after UPy quadruple bonds being broken at elevated temperatures,so that the flow transition temperature and relaxation time of the material are only slightly increased.Therefore,samples with increased UPy content can display similar self-healing performances.PU-U2S8 sample with the best mechanical properties also shows excellent self-healing performance.The surface scratches completely disappeared after 30 minutes at 100°C,and the cut sample can be healed at 100°C for 2 h with a healing efficiency of more than 90%.（2）Imine-diol,a diol chain extender with dynamic imine bonds was synthesized from terephthaladehyde and ethanolamine.By changing the prepolymerization ratio of HMDI/Imine-diol,the hard segment contents were regulated to prepare an optimized transparent high-strength self-healing polyurethane elastomer PUI.Mechanical tests show that the tensile strength was improved with the increase of hard segment content,when below the critical ratio of 60 wt%.After exceeding this critical content,the polyurethane will display plastic-like mechanical behavior and the mechanical strength will be reduced with continuing increased hard segment content.Self-healing tests show that an increase of the hard segment content will constantly increase the effective healing temperature and lower the self-healing ability.The best-performing PUI-3 sample has a tensile strength up to 41 MPa,with elongation at break and toughness are 836%and 166 MJ m-3,respectively.Meanwhile,it exhibited excellent self-healing performances.Surface scratches with a width up to 50μm completely diminished in 5mins at 80°C.The cut sample can be fully repaired at 80°C with a healing efficiency of 95%.By comparing the planar rigid C=N double bond in Imine-diol chain extender with the rotable C-S single bond of its counterpart chain extender,it is found that the sample with C=N bond was much stronger.The results indicate that the coexistence of structural rigidity and highly dynamic reversibility in C=N bond is the main reason why PUI elastomers can simultaneously have high mechanical strength and excellent self-healing ability.（3）Using tetrabromobisphenol A（TBBPA）as reversible chain extender and propyl gallate（PG）as dynamic crosslinker,a transparent robust self-healing polyurethane elastomer（PPU）based on reversible phenolic carbamate was successfully prepared,through optimizing the crosslink density and hard segment content.The feed ratios of raw materials were uniquely determined by the specified crosslinking density and hard segment content.Based on this,the independent effects of crosslinking density and hard segment content on mechanical properties and self-healing abilities were studied separately.After optimization,the best performing sample PPU-0.5-60 elastomer has a tensile strength up to 46 MPa,and its elongation at break and toughness are 615%and 89 MJ m-3,respectively.Meanwhile,it has excellent self-healing ability.The mechanical strength of cut sample can be recovered after 2 hours of healing at100°C,reaching a healing efficiency of 95%.In addition,the nearly irreversible bisphenol A（BPA）and trihydroxylmethylpropane（TMP）were chosen to replace as chain extender or crosslinker separately to prepare control samples.The comparison results indicate that the combination of TBBPA and PG is responsible for the remarkable mechanical properties and excellent self-healing performances of PPU elastomers.And the crosslinker PG with structure rigidity plays a key role in improve the mechanical strength.（4）The reversible chain extender DA-diol with reversible DA adducts was synthesized and was combined with 1,4-butanediol（BDO）as complexed chain extender.By adjusting the ratio of the two chain extenders,a transparent self-healing polyurethane elastomer DAPU with high mechanical strength was prepared.BDO chain extended hard segments ensures the high strength at room temperature,while the DA-diol endows it with self-healing ability at elevated temperatures.Taking advantage of the high temperature sensitivity of DA reaction equilibrium,only a small amount of DA adducts,resulting from a low DA-diol chain extending ratio（0.2-0.3）,will make the DAPU elastomer highly self-healable without sacrificing its mechanical strength.The optimal DAPU-0.2 sample has an ultra-high tensile strength of 65 MPa,and its elongation at break and toughness are 1050%and 216 MJ m-3,respectively.Meanwhile,the elastomer can be fully healed at 110°C for 2 h,achieving a healing efficiency of 95%. |
PDF Full Text Request