Reconfigurable Shape Memory Resins Based On Dynamic Covalent Bonds

Shape memory polymers,as a unique class of stimulus-responsive polymers,can recover from temporary shapes to permanent ones under external stimulus conditions spontaneously;at the same time,the advantages of light weight,easy processing,large deformation and easy adjustment of transition temperature endow them with great application prospects in the fields such as aerospace self-deploying structural parts,biomedical materials,intelligent robots and oil drilling,etc.Among them,thermal-triggered chemical cross-linked shape memory resins show unique huge potential in the cutting-edge field of structural/functional integration materials due to excellent mechanical properties,outstanding thermal stability and heat resistance.However,on one hand,chemical cross-linked resins are so brittle that could not allow big deformation of temporary shape,making the resins easily fail in thermomechanical cycles,and thus limits its application for large deformation structures.On the other hand,once the chemical cross-linked resins are formed,they cannot be reshaped or reprocessed,so shape memory resins cannot be able to change the permanent shape,limiting their application for complex deformable devices.Recently,researchers have found that cross-linked resins with dynamic covalent bonds show attractive properties that are different from traditional thermoplastic and thermosetting polymers.Besides,the resins with dynamic covalent bonds,called as the third type of polymers,also show self-healing,reshapeable and reprocessable functions.However,there are two major shortcomings of shape memory resins containing dynamic covalent bonds,one is that the temporary shape is limited with single functionality,the other is that the thermal and mechanical properties cannot meet the requirements for application in harsh environments.To overcome these two disadvantages,our project reported in this thesis aims at designing reconfigurable shape memory resins with new types of cross-linked networks containing dynamic covalent bonds to achieve one-way dual shape memory effects(need to be reprogrammed,fixed one temporary shape),complex one-way triple shape memory effect(need to be re-programmed,fixed two temporary shape)and smarter reversible shape memory effect(no need to be reprogrammed),successively,for high performance resins with outstanding mechanical properties and thermal resistance.The specific research contents are as follows:Firstly,a diamine-extended bismaleimide oligomer(CBMI)was introduced into anhydride cured epoxy resin(EP)system to design a complex cross-linked network with both dynamic ester bonds and permanent cross-linked points,and thus a self-healing reconfigurable dual shape memory resin(EP/CBMI)with high heat resistance and high mechanical strength was prepared.A series of dual shape memory resins with adjustable properties were obtained by adjusting the content of CBMI.The effects of the content of CBMI on dynamic thermomechanical properties,thermal stability,mechanical properties,shape memory properties and self-healing properties of EP/CBMI resins were studied systematically.Results show that high glass transition temperature(Tg)and initial thermal decomposition temperature(Tdi)for the res:in with 30 wt%CBMI(EP/0.3CBMI)reach 125℃ and 434℃,and the flexural strength and modulus are 129±1.5 MPa and 3.23±0.01 MPa,respectively.All EP/CBMI resins show excellent shape memory performance and shape memory cycle stability.The shape fixation ratio(Rf>92.9%)and shape recovery ratio(Rr>99.6%)are both at a high level.EP/CBMI resins can fix complex temporary shapes(bend,twist)and be reconfigurable to new permanent shapes("O”,"S","Z","U" shapes).Owing to the thermally-triggered dynamic exchange reaction of hydroxyl-ester bonds,EP/CBMI resins exhibit excellent self-healing performance(healing efficiency up to 80%).These outstanding comprehensive properties show that EP/CBMI can be used as a high-performance shape memory resin for fabricating hinge-driven deployable structures,oil drilling fasteners and space deployable truss structures.Secondly,a reconfigurable dual shape memory epoxy resin(EPSi)with high heat resistance and good mechanical strength was designed by using EP,2-aminoethanol,4,4’-diaminodiphenylmethane and 3-isocyanatepropyltrimethoxysilane based on silyl ether dynamic covalent linkages.A series of dual shape memory resins with adjustable cross-linked network density were obtained by adjusting the molar ratio of-OMe/-OH.At the same time,the permanent shapes can be reconfigured through triggering the dynamic exchange between hydroxyl groups and silyl ether bonds in the cross-linked network by heating,and the reconfigurable shape memory behavior of EPSi resins can be observed.The cross-linked structure and comprehensive performance of EPSi resins were systematically studied.Results show that EPSi resins not only exhibit high Tg(adjustable at 118.1-156.4℃),38.1-76.4℃ higher than the Tg values of reconfigurable shape memory resins reported in the literatures,and but also show higher tensile strength(59.31-82.37MPa).EPSi resins show excellent reconfigurable shape memory behavior(Rf:97.1-98.9%,Rr:95.6-99.8%,plastic retention ratio:80.5-86.3%).The outstanding comprehensive properties of EPSi resins are attributed to good thermal stability and flexibility of dynamic silyl ether bond as well as the stable cross-linked network structure.EPSi resins show good application prospects in high-performance three-dimensional shape self-folding/self-expanding deformation structure devices.Thirdly,a reconfigurable triple shape memory resin(ESBD)containing dynamic ester bonds with broad thermal transition temperature and high bending strength was designed through forming an interpenetrating network based on copolymeric action of EP,sebacic acid(SA),bismaleimide(BDM)and diallyl bisphenol A(DBA).A series of triple shape memory resins with adjustable performance were obtained by adjusting the ratio between EP/SA network and BDM/DBA network.ESBD resin exhibits wide glass transition temperature range(up to 180℃)owing to its unique interpenetrating network.The curing reaction behavior,the structure of interpenetrating cross-linked network,dynamic thermomechanical properties,thermal stability,bending mechanical properties and shape memory properties of ESBD resins were investigated systematically.Results show that the dynamic transesterification endows ESBD resins with a reconfigurable function,while the BDM/DBA network provides a second temporary shape fixation and recovery.When the content of BDM/DBA network reaches 53 wt%,the resulting resin(ESBD-4)shows two highest thermal transition temperatures(Tg1=75.9℃,Tg2=296.1℃),which are the highest value of reported shape memory resins with double Tg.Meanwhile,ESBD-4 also exhibits the highest bending strength(136.6±6.3 MPa).The Rf and Rr of ESBD-4 are close to 100%in the bending-unbending shape memory tests.The unique interpenetrating network of ESBD resins,which combines the advantages of EP/SA network and BDM/DBA network,not only improves the comprehensive performance of ESBD resins,but also provides a new strategy for the design and preparation of a new type of triple shape memory resins.ESBD resins show good application prospects in the field of preparing intelligent structural parts with step-deformation function.Fourthly,a reconfigurable reversible shape memory epoxy resin(LCEP)with large deformable and high mechanical strength was prepared from biphenyl liquid crystal epoxy monomer(BPEP),4,4’-dithiodibutyric acid(DTBA)and SA.By adjusting the ratio of DTBA and SA,a series of reversible shape memory resins with adjustable properties were obtained.The effects of the contents of disulfide bonds on the dynamic thermomechanical properties,thermal stability,tensile mechanical properties and reversible shape memory properties of LCEP resins were explored.Results show that because of the dynamic exchange characteristics of disulfide bonds in the cross-linked network,the reconfiguration of the cross-linked network can be triggered under the conditions of external forces and thermal stimulation,thereby the function of reconfigure its original shapes is obtained.When the content of disulfide bond reaches 19.8 wt%,the obtained resin(LCEP-2)shows the highest tensile strength(17.44±1.55 MPa)and the highest elongation at break(221±8.6%),and also exhibits the highest reversible driving strain(61%)under the small stress condition of 0.05 MPa.The outstanding performance of both high toughness and high reversible actuating strain is derived from a liquid crystal element with long flexible spacers and a cross-linked network containing disulfide dynamic exchange bonds.The excellent performance of LCEP resins shows that it has great potential in the field of high-performance pull-type actuators.