Mechanically Robust Polyurethane Elastomers Based On Dynamic Cross-linking

Polyurethane（PU）elastomers have gained significant interest in both the traditional automobile and aerospace industry,as well as emerging fields such as electronic skins,soft robots.With the continuous development of production,the ideal PU elastomers are often required with high mechanical strength.For example,when used as ship bearings,gaskets and other engineerging components,high strength has become one of the necessary properties of PU elastomers.While PU promotes the development of modern society,conventional PU elastomers cannot be healed and recycled,which leads to serious waste of resources and environmental pollution.Therefore,it is urgent to endow PU elastomers with healable and recyclable abilities.Dynamic cross-linking is an effective method to fabricate healable/recyclable PU elastomers.However,most dynamically cross-linked PU polymer networks are mechanically weak,which is not conducive to achieving high strength of PU elastomers.Moreover,from the perspective of molecular mechanism,healable/recyclable properties require the good mobility of polymer chains,which is also contradictory to the high strength.Therefore,it is important as well as challenging to fabricate healable/recyclable PU elastomers with high mechanical strength.To solve the above-mentioned contradictions,we designed and synthesized three different kinds of mechanically robust healable/recyclable PU elastomers based on dynamic cross-linking in this thesis.By adjusting the dynamic interaction and phase-separated nanostructure in the PU networks,the fabricated PU elastomers show high mechanical strength,excellent healable,recyclable abilities and different additional functions（such as scratch resistance,erasability,degradability,etc.）.The mechanical properties and thermo-mechanical properties of the PU elastomers are investigated in detail.The research contents are as follows:1.Room-temperature healable,recyclable and mechanically strong poly（urea-urethane）s cross-linked with nitrogen-coordinated boroxines.In this work,the nitrogen-coordinated boroxines（NCBs）cross-linked poly（urea-urethane）s（denoted as NCB-PUU）with a tensile strength of～47 MPa were fabricated by using poly（tetramethylene ether glycol）（PTMEG）as the main polymer chain.The unique tripodal molecular architecture of NCBs endows the polymer networks with high cross-linking density.The hydrogen bonds and phase-separated nanostructures in the NCB-PUU polymer networks can act as cross-linkers to further enhance NCB-PUU.Due to the dynamic nature of NCBs and hydrogen bonds,NCB-PUU can be healed and recycled at room temperature.Mechanical damage can be healed at room temperature by dipping the fractured ends of broken sheets in water/ethanol mixture for 2 min.NCB-PUU can also be recycled by dissolving in ethanol at room temperature and casting on a clean glass plate.NCB-PUU is suitable for using as a protective coating because of the excellent transparency and high adhesion.The strategy of combing high-density crosslinking and phase-separated nanostructure for fabricating high strength materials can also be applied to similar materials.2.Engineering of chain rigidity and hydrogen bond cross-linking toward ultra-strong,healable and recyclable polyurethane elastomers.By copolymerizing flexible poly（urea-urethane）（PUU）segments and rigid polyimide（PI）segments,we fabricated ultra-strong and ultra-tough polyurethane elastomers（PI-PUU）.PI-PUU exhibits a tensile strength of up to 142 MPa and a toughness of 527 MJ m^-3,which is even comparable to the engineering plastics.Rigid PI segments can self-assemble to form phase-separated nanostructure to strengthen PI-PUU elastomers.The PUU segments contains a large number of hydrogen bonds,which can realize effective energy dissipation during the stretching process,further enhancing the mechanical properties.Meanwhile,due to the properties of PI segments,PI-PUU shows excellent thermal stability and water stability.The thermal decomposition temperature of PI-PUU is up to 320℃.The mechanical strength can still reach 90 MPa at 120℃.After soaking PI-PUU in water,aqueous hydrochloric acid solution,and saturated aqueous sodium chloride solution for 7 days,followed by drying,the mechanical properties of the PI-PUU were almost unchanged.Thanks to the excellent mechanical properties,PI-PUU also exhibits excellent scratch and puncture resistance,greatly improving the stability and durability of PI-PUU.Moreover,PI-PUU elastomers can be efficiently healed and recycled.With the assistance of solvents,mechanical properties of PI-PUU can recover to the original value.These properties can greatly extend service life of the elastomers,which can reduce the consumption of raw materials and reduce the pollution of environment.We believe that our design strategy,which employs rigid nanostructure and non-covalently cross-linked soft polymer matrices to strengthen elastomers,can provide efficient method for fabricating elastomers with high mechanical strength and toughness.3.Phase-separated nanostructures-reinforced healable,recyclable and degradable polyurethane elastomers.In this work,non-covalently cross-linked polyurethane elastomers were fabricated from polylactic acid（PLA）and polycarbonate（PC）.The obtained materials were denoted as PLA-PC.The PC-PC polymer networks contain carbamate and acylsemicarbazide（ASC）moieties which can form hydrogen bonds with varied intensities.Due to the thermodynamic incompatibility between PLA and PC segments,PLA-PC shows a more obvious phase-separated nanostructure.Thus,it shows a higher mechanical strength and better elasticity/recovery than the comparison samples synthesized only by PLA or PC.The tensile strength and toughness of PLA-PC reached 35 MPa and 115 MJ m^-3,respectively.The multiple hydrogen bonds in PLA-PC networks can not only serve as cross-linker to achieve high strength,but also provide energy dissipation in the tensile process,which can endow PLA-PC with high toughness and excellent damage tolerance.The PLA-PC elastomer with a 1 mm-long notch on one side could still be stretched to～790%strain.The calculated fracture energy is 68.78 kJ m^-2 according to the Greensmith method.Based on the dynamic cross-linking strategy,PLA-PC can be healed at 80℃ and be recycled by re-dissolving in N,N-dimethylacetamide.In addition,the complete degradation of PLA-PC was achieved after burying in soil for 9 months.We believe that the concept,which the phase-separated nanostructure improves mechanical strength and elasticity can be regarded as a reference for the design of polyurethane elastomers.Furthermore,we hope that the non-covalently dynamically cross-linking strategy for the preparation of healable/recyclable/degradable materials can be helpful for the sustainable development of polymer materials.