Design,Synthesis And Performance Regulation Of Polyurethane Covalent Adaptive Networks

Due to the excellent dimensional stability,solvent resistance and mechanical properties of thermosets,thermosets have been widely used in various fields,including structural composite materials,adhesives,coatings and electrical insulation.However,conventional thermosets cannot be remolded,reprocessed,or recycled because they cannot melt or flow at high temperatures.On the contrary,because the thermoplastic resin can be molded at high temperature and can return to a solid state after cooling,the thermoplastic resin has good processability and recyclability under heat stimulation.It is worth noting that if you want a polymer to have the advantages of both a thermoplastic resin and a thermosetting resin,the most effective chemical strategy is to introduce dynamic covalent bonds in the cross-linked polymer networks to form covalent adaptive networks.Reversible covalent bonds can participate in reversible rupture and recombination under specific conditions or stimuli,rearranging the network and giving it dynamic properties(such as recyclability,shape memory,or self-healing).Among the thermosetting resins,the market share of thermoset polyurethanes is about 31%.Therefore,introducing dynamic covalent bonds in polyurethane,giving it the characteristics of remolding,processing and recycling,is of great significance to the recycling and sustainable development of thermoset polyurethanes.At present,there have been many research reports on the polyurethane covalent adaptive networks based on dynamic covalent bonds,but there are still two main problems to be solved.One is that the polyurethanes based on dynamic covalent bonds have basically only studied polyurethane elastomers and adhesives,and no researchers have made relevant reports on dynamic covalently crosslinked polyurethane foams;the second is that the current dynamic covalent bonds introduced into the polyurethane networks are mainly disulfide bonds and imine bonds,lacking the introduction of some new dynamic covalent bonds.Based on the above problems,this article first introduced the disulfide bonds into the polyurethane foams to prepare a recyclable polyurethane foams;then systematically studied the dynamic properties of the new urea bonds(piperazine-urea bonds);finally,the piperazine-urea bonds were introduced into castor oil-based polyurethanes to prepare a recyclable castor-based high-performance polyurethane-ureas.The main achievements of this article are as follows:(1)The research is based on the structure and properties of recyclable polyurethane foams based on dynamic disulfide bonds.This research is the first time that dynamic covalent bonds have been incorporated into polyurethane foams.The disulfide exchange reactions under heating allows the polyurethane foam to carry out network topology rearrangement,and make the material malleability and processability.The physical properties of polyurethane foams are studied by resilience and apparent density.The results showed that the physical properties of polyurethane foams(PUSFs)containing disulfide bonds are similar to those of ordinary ones.It is worth noting that PUSFs are prepared by conventional foaming process without any modification,which has important guiding significance for large-scale production of recyclable foams.In addition,the PUSFs can be easily recycled into PU films through thermal compression molding.The tensile test results showed that RPUSFs have excellent mechanical properties,with elongation at break in the range of 110-230%,tensile strength in the range of 12-21 MPa,and Young’s modulus in the range of 22-61 MPa.Which depends on the composition of the original malleable polyurethane foams.Furthermore,the PU films recycled from PU foams with a well-designed composition can be further reprocessed several times without obvious loss in mechanical proprieties and change in chemical structures.This investigation provides a novel methodology to recycle and reuse PU foams and is expected to promote the sustainable development of the PU foams industry.(2)A new type of urea bonds(piperazine-urea bonds)were discovered,and the properties of piperazine-urea bonds(Pa UBs)as dynamic covalent bonds were systematically studied.First,a small molecule model was constructed,and the dynamic properties of Pa UBs in small molecule compounds were studied by using proton nuclear magnetic resonance spectroscopy and liquid chromatography-mass spectrometry(LC-MS).The results showed that the exchange reactions between Pa UBs can take place at a certain temperature.At the same time,the room temperature stability was also studied by proton nuclear magnetic resonance spectroscopy,and the results showed that Pa UBs were stable at room temperature without bond exchange reactions.Next,the piperazine-urea bonds were introduced into the linear polymers,and the change in the molecular weight of the polymer was characterized by gel permeation chromatography(GPC).The results showed that Pa UBs also showed obvious dynamic properties in linear polymers.Specifically,the molecular weights of the polymers had changed significantly.Finally,the formation of urea bonds was studied by molecular simulation,and the dynamic properties and stability of Pa UBs were studied by calculating resonance energy,so as to compare with other urea bonds.The order of the resonance energy of the three is:hindered urea<piperazine-urea<normal urea.This showed that Pa UBs have a certain degree of stability and dynamics at the same time.This study provides a solid foundation for the introduction of piperazine-urea bonds into cross-linked polymers.(3)Pa UBs were introduced into castor oil-based polyurethanes,and polyurethane covalent adaptive networks containing Pa UBs were obtained.Since petroleum resources are non-renewable resources and are not conducive to sustainable development,a renewable resource-castor oils is introduced as the matrix in this study.Compared with the mechanical properties of the control sample(without dynamic urea bonds),the polyurethane-ureas containing Pa UBs showed very excellent mechanical properties.And simply change the feed ratio of flexible castor oils and rigid N-(2-hydroxyethyl)piperazine,you can adjust the polyurethane-urea networks stiffness and crosslinking density,which allows us to easily adjust the mechanical properties of the materials over a wide range of flexibility and stiffness.Through tensile tests and dynamic thermomechanical analysis,the castor oil-based polyurethane-ureas reprocessability,reconfigurability and shape memory behavior were studied.In terms of mechanical properties,HPU-1,which has the least dynamic covalent bonds content,has a recovery rate of more than 87%after four repeated processing.At the same time,after several circles of shape memory and reconstruction,the R_f and R_r of this materials are both above92%.These results indicated that castor oil-based polyurethane-ureas based on Pa UBs have excellent malleability,reprocessability,shape memory and reconfigurability.Therefore,it has great potential to replace traditional thermoset polyurethanes with castor oil-based polyurethanes based on Pa UBs.This strategy provides a novel method to prepare thermoset materials with excellent mechanical and dynamic properties,and to endow these materials with high-value functions.In summary,this article reasonably designed,synthesized and systematically characterized a series of different types of polyurethane covalent adaptive networks.The results showed that the prepared polyurethanes all have excellent mechanical and thermal properties;at the same time,after these polyurethanes have been processed several times,their chemical structure and cross-linking structure are almost unchanged.In addition,their mechanical properties can also get a higher recovery rate after several times of processing.This series of phenomena all showed that the polyurethane covalent adaptive networks prepared in this article have excellent reprocessability or recyclability.Firstly,from the perspective of polyurethane foams,this article introduces dynamic covalent bonds into polyurethane foams for the first time,and successfully obtains recyclable polyurethane foams,which has important guiding significance for the sustainable development of polyurethane foams.Secondly,from the perspective of dynamic covalent bonds,this article discovered a new type of dynamic urea bonds,and studied the dynamic properties of the urea bonds;at the same time,the urea bonds were introduced into the castor oil-based polyurethanes to successfully prepare the recyclable and high-performance polyurethane-ureas.In short,this article provides effective solutions to the two main problems of thermoset polyurethanes at present.Moreover,these solutions also have profound guiding significance for the recycling of other thermoset materials.It is believed that the effective strategies proposed in this article will help promote the development of sustainable polymers and reversible polymers.