Preparation And Performance Of Recyclable Dynamic Covalent Cross-linked Polyurethane-based Materials

Thermoset polyurethane is widely used in aerospace,automotive,coating fields and so on due to high mechanical strength and adjustable properties.However,thermoset polyurethanes cannot be effectively reprocessed and recycled due to the presence of cross-linking networks,resulting in environmental pollution.Also,thermoset polyurethane is difficult to obtain complex shapes and cannot be reshaped after molding due to limitations of mold and demold.The recyclable cross-linked polyurethane with dynamic covalent bonds has been developed,but systematic studies on cross-linked polyurethane with both excellent mechanical properties and reprocessing performance are lacking.To solve this problem,a series of recyclable dynamic covalent cross-linked polyurethane and polythiourethane containing different dynamic covalent bonds were prepared,which could realize the efficient recycling of cross-linked polymers and the flexible editing of complex shapes.Meanwhile,recyclable cross-linked polythiurethane/carbon nanotube composites with dual photothermal responsiveness were prepared by in-situ polymerization.The precise regulation of local shape change was achieved by using its photo-induced solid-state plasticity.The research contents and conclusions of this paper are as follows:(1)Structure and properties of recyclable crosslinked polyurethaneThe colorless and transparent cross-linked polyurethane(PCU)with recyclability were prepared by the two-step pre-polymerization-chain expansion method using polyhexamethylene carbonate diol(PHMC diol-2000),isophorone diisocyanate(IPDI),and glycerol(GLY)as raw materials.The cross-linked PCU had excellent mechanical properties.And the tensile strength of PCU-30% sample reached 59.2 MPa and elongation at break reached 674.8%.The complex shape editing of cross-linked PCU was enabled through the synergistic effect of solid-state plasticity and one-way shape memory effect.However,the reprocessability of crosslinked PCU was slightly poor,with the maximum recovery of 88.0% in tensile strength and 105.1% in elongation at break.The effects of hard section content and reprocessing temperature on reprocessing efficiency were also investigated.It was shown that the higher the hard segment content,the worse the reprocessing efficiency;too high or too low temperature would lead to poor reprocessing performance.(2)Structure and properties of recyclable crosslinked polythiourethaneIn order to improve the reprocessability of cross-linked PCU,the cross-linker pentaerythritol tetrakis(3-mercaptopropionate)(PTME)was selected instead of GLY.And multiple dynamic covalent bonds(ester,urethane,and thiourethane bonds)were introduced into molecular chains to develop a series of cross-linked polythiourethane(PCTU)with colorless transparency,excellent mechanical properties,solid-state plasticity,recyclability,and shape memory properties.Compared with PCU,the reprocessing efficiency of PCTU was greatly improved.The tensile strength of the PCTU-25% sample was recovered by 144.4% and the elongation at break was recovered by 160.1%.In addition to the reprocessing recovery,PCTU could be recovered by solvent at room temperature.The efficient recovery of carbon fiber cloth(CFs)was achieved under mild conditions by solvent recovery from polythiourethane/carbon fiber cloth(PCTU/CFs)composites,which provided a new way for the recovery of composites.(3)Structure and properties of recyclable crosslinked polythiourethane/CNTs compositesThe modified carbon nanotubes(CNTs)were introduced into PCTU by in-situ polymerization to develop the polythiourethane/carbon nanotube(PCTU/CNTs)composites with dual photothermal responsiveness.It was found that the addition of CNTs had the strengthening and toughening effect on PCTU and imparted excellent photothermal properties to the composites.When carbon nanotubes were added at 5wt%,the tensile strength and elongation at break of the composites were 117.2% and193.5% of pure PCTU,respectively.The composites still exhibited good solid-state plasticity,but the reprocessability of composites were slightly inferior compared with pure PCTU.The excellent photothermal properties and photoplasticity of the composites enabled the precise regulation of local deformation.