Study On Polyimine Electrospun Nanofibers And Their Self-Reinforcing Composites

Electrospun nanofibers are widely used in the fields of intelligent materials,environmental protection and governance,energy conversion and storage,biomedicine and catalysis,etc.They have remarkable characteristics such as high surface-to-volume ratios,high draw ratio,well-controlled structure and morphology,excellent continuity,excellent mechanical properties,easy assembly and processing in applications.The high specific surface area of electrospun nanofibers and the high porosity of fiber mats contribute to integrating and amplifying the intelligent properties of polymers.The development of intelligent nanofibers with high performance and multiple functions has become a research hotspot.Dynamic covalent polymers have attracted much attention due to their unique dynamic properties such as stimulus responsiveness,intrinsic selfhealing,shape memory and complete recyclability.In recent years,researchers have focused on the design and synthesis of dynamic covalent polymers.However,there are few reports on dynamic covalent polymer nanofibers.At present,the processes of fabricating nanofibers based on dynamic covalent bonds by self-assembly method and solute thermal method are complex and time-consuming.Continuous nanofibers cannot be produced on a large scale by these two methods,which limit the application of nanofibers based on dynamic covalent bonds.In addition,due to the low bond energy of reversible covalent bonds,the mechanical properties of dynamic covalent polymers are weak.How to effectively improve the mechanical properties of dynamic covalent polymers with retaining the dynamic properties is a main challenge for expanding their application.In this thesis,long and continuous dynamic polyimine（PIm）nanofibers were produced by electrospinning technology,and the formation mechanism of dynamic PIm nanofibers was studied.The self-reinforcing nanofiber/PIm composites were produced by using the dynamic PIm nanofiber as the reinforcing phase.The mechanical and selfhealing properties,light transmittance and thermal stability of the self-reinforcing nanofiber/PIm composites were inveatigated.The main research results of this thesis are as follows:（1）Terephthalaldehyde and four kinds of diamine with different structure were used as the monomer.The electrospinning technique was used to produce four kinds of continuous,uniform dynamic polyimide fibers successfully.The electrospun nanofibers of PIm-1 prepared from condensation of terephthalaldehyde and diethylenetriamine were crystalline,as well as the electrospun nanofibers of PIm-4 prepared from condensation of terephthalaldehyde and dihexylenetriamine.Compared with PIm-4 nanofibers,PIm-1nanofibers were more easily to form crystal nucleation.The crystallinity of PIm-2nanofibers prepared from condensation of terephthalaldehyde and N,N-bis（3-aminopropyl）methylamine are weak.The nanofibers of PIm-3 prepared from condensation of terephthalaldehyde and polyether amine are amorphous.The four kinds of PIm nanofibers have good thermal stability.Their thermal decomposition temperature is higher than 250 ℃.（2）The formation mechanism of the electrospinning nanofibers of PIm spinning solution formed by condensation of terephthalaldehyde and diamine was studied.The results implied that enough intermolecular interaction between PIm oligomer provided similar cohesion in electrospinning jet as entanglements from high molecular weight polymers,which included aromatic stacking,hydrogen bonding interaction,polar interaction,as well as formation of enough molecular aggregation.As a result,the PIm oligomer nanofibers were fabricated by electrospinning.（3）PIm-1 nanofibers were used as the reinforcement phase.Self-reinforcing nanofiber/PIm composite films were produced by impregnation method.The mechanical properties of the self-reinforcing nanofiber/PIm-1 composite membrane are significantly improved.The tensile strength,tensile modulus and fracture elongation of PIm-1composite films increased by 81%,47% and 58%,respectively.Self-reinforcing nanofiber/PIm-1 composite films have excellent self-healing properties.After three cycles of self-healing,the mechanical strength of the 10 wt% nanofiber self-reinforcing PIm-1 composite film was restored by 97 %.Self-reinforcing nanofibers/PIm-1composite films also show good transparency and good thermal stability.This thesis shows that long and continuous dynamic PIm nanofibers can be produced by electrospinning technology,via using the dynamic imine oligomer solution generated by monomer condensation at room temperature as spinning solution directly.This method is feasible to perform and environmentally friendly.It provides a reference strategy for the production of other dynamic polymer nanofibers.The self-reinforcing nanofiber/PIm composite materials are light,transparency and easy to recycle with excellent mechanical properties,self-healing properties.They have great potential application in intelligent materials with high performances and multiple functions.Further studies will provide insight into the production pathways and formation mechanisms of dynamic polymer nanofibers,expanding the applications of both electrospinning nanofibers and dynamic polymers in more innovative fields.