Study Of Interlocked Covalent Adaptable Networks Based On Aromatic Disulfide And Imine Bonds

Thermosets have the advantages of high strength,light weight and dimensional stability,and are widely used in aerospace,automotive and other fields.However,the internal covalent bonds of thermosets are permanently cross-linked,which makes it difficult to degrade and recycle thermosets.The waste generated has a serious impact on the ecological environment.Covalent adaptive networks(CANs)based on dynamic covalent bond cross-linking effectively combine the advantages of thermoplastics and thermosets.Through the exchange reaction between dynamic covalent bonds,the on-demand degradation and recovery of thermosets are realized.However,the inherent reversibility of dynamic bonds makes materials in practical applications.When they are unexpectedly faced with common stimuli such as water,acid /alkali,and temperature,the dynamic reaction will be accidentally activated to cause material degradation and failure,affecting the safe use of materials.Therefore,how to ensure that CANs are recyclable and biodegradable while having stronger stability is the current research difficulty.In order to improve the stability of CANs,an interlocked covalent adaptive network(ICANs)was designed in this paper.It was prepared by one-pot reaction of two amine curing agents containing disulfide bonds or imine bonds with epoxy monomers.It has excellent basic properties and solvent resistance.The tensile strength is 78.7 MPa,the storage modulus is 1.71 GPa,and the glass transition temperature(Tg)is 150℃.Due to the parallel cross-linking of the imine and disulfide bonds in ICANs,the network is interlocked,so that ICANs remain crosslinked even after breaking a dynamic bond under a single stimulus(HCl or-SH),and there is still excellent mechanical strength,thereby improving the stability of CANs.ICANs can be completely degraded only when two stimuli are applied to break the disulfide and imine bonds at the same time.In addition,carbon fiber reinforced polymer(CFRP)composites were prepared with ICANs as the matrix.It exhibits excellent mechanical properties(321 MPa)and stability,and achieves non-destructive recycling of carbon fibers.In order to recombine the broken dynamic bond energy of the interlocked network under a single stimulus,and to make the interlocked structure suitable for other thermosetting frameworks,a polyurea-based interpenetrating and interlocked covalent adaptive networks(IP-ICANs)was prepared by one-pot reaction of two amine curing agents containing disulfide bonds or imine bonds with polyisocyanates.The material has high glass transition temperature(100℃),ultra-high strength(43 MPa)and solvent resistance.The interpenetrating and interlocked structure enhances the stability of IP-ICANs,so that it still exhibits excellent strength and modulus under single stimulus conditions(HCl or-SH).Importantly,the dynamic covalent bonds broken by stimulation can also be reorganized to relock the network,and the strength and stability can be restored,and the strength can be restored to more than 92 %.This interlocking-unlocking-relocking characteristic further enhances the stability of IP-ICANs.At the same time,IP-ICANs can be completely degraded after the disulfide bond and imine bond are completely broken by applying two stimuli at the same time.Since the oligomers after complete degradation of IP-ICANs have some economic value,closed-loop chemical recovery can be achieved by extracting the oligomers after chemical degradation of IP-ICANs to crosslink again.The IP-ICANs fragments can be molded by hot pressing,and their strength is comparable to that of the original IP-ICANs,achieving closedloop physical recovery.In addition,IP-ICANs can also be applied to coatings with high transmittance,obvious ultraviolet shielding in the ultraviolet range,rapid repair of surface scratches,and seawater corrosion resistance.