Study On Self-healing Polyurethane Based On Thermo-reversible Diels-Alder Reaction

Polyurethanes have been widely used as foams,elastomers,coatings,adhesives,and sealants because of its excellent mechanical property.However,micro-cracks may be generated in polyurethanes when suffered from heat,mechanical attack,and chemical erosion,etc.These cracks are difficult to detect and repair while the mechanical property can be reduced and service lifetime may be shortened.Inspired by the self-healing phenomenon of organism,scientists have developed a series of polymers with self-healing property by imitating the self-healing mechanism of organism.There are many methods to prepare self-healing polymer,and it is an effective method for the preparation of self-healing polymers by incorporating the thermo-reversible Diels-Alder?DA?bonds.DA reaction is a thermo-induced [4+2] cycloaddition that furan group and maleimide group are generally used as typical diene and dienophile,respectively.The resulting DA adduct undergoes a cleavage reaction?r-DA reaction?at high temperature,while the corresponding diene and dienophile re-generate and DA adduct forms once again when cooled down to low temperature.DA reaction has many advantages,such as low price and easily obtained raw materials,mild reaction condition,minimal side reactions,and no metal catalyst is needed.In addition,the thermo-reversible DA reaction has high self-healing efficiency and multiple self-healing property.In this dissertation,self-healing polyurethane?PU-DA?is designed and synthesized based on Diels-Alder?DA?reaction.The thermal property,thermal reversibility,self-healing behavior,and self-healing mechanism of resultant PU-DA are investigated in depth.Meanwhile,the effect of damage mode and nano-SiO₂ on self-healing behavior of PU-DA is investigated.The main contents and results are as follows:?1?Preparation and properties of self-healing polyurethane based on Diels-Alder reaction: Furan-terminated polyurethane prepolymer?MPF?is synthetized firstly by molecular designing,and then the self-healing polyurethane?PU-DA?is synthesized based on Diels-Alder?DA?reaction between MPF and bismaleimides?BMI?.The chemical structure and thermal property of resultant PU-DA are investigated by FT-IR and DSC,and results show that the thermo-reversible DA bonds are introduced into the polyurethane successfully.The gel-solution-gel experiment,viscosity test,and GPC analysis further confirm that self-healing polyurethane exhibits excellent thermal reversibility.Meanwhile,results of tensile test manifest that the mechanical property of polyurethane is reinforced by incorporating DA bonds.?2?Self-healing behavior and mechanism of self-healing polyurethane: The self-healing behavior of PU-DA is investigated by polarized optical microscope observation?POM?and tensile test.Results reveal that both polyurethane containing DA bonds?PU-DA?and polyurethane without DA bonds?PU-BDO?have some self-healing capability.When films with cracks of 0.5 mm in depth are heat-treated at 120 oC for 15 min,the crack in PU-DA films disappear completely and the healing efficiency can reach 71%.By contrast,crack in polyurethane without DA bonds?PU-BDO?only becomes narrow and shallow until 60 min at 120 oC,while and the healing efficiency can only arrive 19%.The self-healing behavior of PU-BDO is grounded on the thermal movement of the polymer chains,while the self-healing behavior of PU-DA is based on both the thermal movement of molecular chains and thermo-reversible DA reaction.Finally,the self-healing behavior of PU-DA with different-NCO/-OH ratios and soft segmens with various molecular weights is investigated,and results confirm that the self-healing property of PU-DA results from both thermal reversible DA reaction and thermal movement of molecular chains.?3?Effect of damage mode on healing behavior and multiple healing capability and reprocessing behavior of self-healing polyurethane: The self-healing behavior of PU-DA after different forms of damage,such as different cracks in depth,multiple damages,and tear damage are investigated by polarized optical microscope observation and tensile test.Results reveal that the healing effect decreases with the increase of crack depth.Under the same heat treatment,the healing efficiency decreases one by one,and it can reach 71%,49%,and 35%,respectively.Then the multiple healing effect of PU-DA with different cracks in depth is investigated by tensile test,and results show that the healing efficiency decreases gradually with the increase of cut-heat treatment numbers.The healing efficiency of PU-DA with cracks of 0.5 mm,1.0 mm,and 1.5 mm in depth can reach 38%,21%,and 13%,respectively when the polyurethanes are cut-heat treated for three times.Additionally,the broken PU-DA sections can recombine together as a whole when the samples are heat-treated under 120oC/40 min and 60oC/24 h,respectively.Meanwhile,a whole polyurethane sample can be obtained when the torn fragments are heat-treated with the similar procedure.These results indicate that PU-DA exhibits excellent thermal reprocessing performance,and which makes it possible of recycling waste polyurethanes.?4?Self-healing behavior of nano-SiO₂ reinforced self-healing polyurethanes: The chemical structure and thermal property of polyurethanes containing nano-SiO₂ are characterized by FT-IR and DSC,and results show that nano-SiO₂ is incorporated in self-healing polyurethanes successfully.Tensile test results show that the tensile strength enhances with the increase of nano-SiO₂ content,and the tensile strength can reach the highest value of 8.55 MPa for PU-DA with 4 wt% of nano-SiO₂.The self-healing behavior of PU-DA with different contents of nano-SiO₂ under heat treatment at 120 oC is investigated by observing the evolution of cracks with POM.Results reveal that the healing effect of cracks decreases with the content of nano-SiO₂ increase.The healing efficiency of 0.5 mm depth of crack for PU-DA with 4 wt% of nano-SiO₂ can reach 50% after heat-treated at 120 oC for 15 min.These results indicate that the tensile strength is enhanced while the self-healing capability is affected by introducing nano-SiO₂ in self-healing polyurethanes.