Preparation And Performance Study Of Polyimide-polyurea Copolymers And Their Composite Films

Polyimide（PI）is widely used in aerospace,microelectronics,mechanical industry and other fields for its excellent performance.The traditional polyimide preparation leads to redundant steps,high energy consumption and low production efficiency because of high chain rigidity,which is against the concept of green and low-carbon development,however,the process is difficult when low rigidity aliphatic monomers are used,and it causes loss of thermal and mechanical properties,therefore,it is important to investigate how to reduce the thermal imidization temperature of PI without losing its thermal and mechanical properties.Polyurea（PU）contains strong polar urea groups that induce the formation of hydrogen bonds,which improve inter-chain interactions,resulting in excellent overall performance of polyurea.Based on this,this thesis prepares a series of polyimide-polyurea copolymer membranes and their composite membranes by introducing flexible PU chain segments into PI and adding nano-material composite based on theoretical analysis and combining relevant domestic and international research,starting from three aspects:monomer selection,copolymer modification and nano-composite,while effectively reducing the thermal imidization temperature of PI,the thermal and mechanical properties are better maintained.The main research contents and results are as follows:（1）PI was synthesized with 4,4-diaminodiphenyl ether（ODA）,9,9-bis（4-aminophenyl）fluorene（BAFL）and homophthalic tetracarboxylic anhydride（PMDA）,respectively,and the effect of diamine structure on the film-forming properties of PI and its performance was investigated,the results show that:the BAFL structure is rigid and has strong site-resistance effect,and the synthesized PI has poor film-forming properties,while ODA contains ether bonds and has high symmetry,and the synthesized PI has good film-forming properties and excellent thermal（decomposition temperature>540℃）and force（breaking strength 90.36 MPa）chemical properties.（2）Based on PI synthesis,ODA and polyetheramine D-400 were compounded,then with diisocyanate（MDI-50）to synthesize PU,and the effect of ODA content on the structure and properties of PU was investigated.The results show that:the synthesized PU molecular chain is flexible（T_g≈17～28℃）,but the introduction of ODA will weaken the hydrogen bonding leading to The thermal stability is greatly reduced（Td<120℃）,with the increase of ODA content,the PU fracture strength first decreases and then increases,up to more than 6 MPa,and the elongation at break gradually decreases from 134%.（3）Synthesize terminal anhydride based polyamide acid by ODA and PMDA,and terminal isocyanate based PU by polyetheramine D-400 and MDI-50,Then two were mixed and copolymerized and thermally imidized to prepare polyimide-polyurea copolymer films（PUI）with different PU contents.The effects of PU on the molecular structure and chain aggregation morphology of PUI were investigated,the mechanism of PU promoting the completion of PUI thermal imidization was analyzed,and the relationship between PU content and the thermal and mechanical properties of PUI was elucidated.The results show that PUI has the characteristic peaks of both PI and PU;the flexibility of PU reduces the rigidity of PUI molecular chains,so that PUI can be thermally imidized at about 160℃;the interaction between PUI chains is enhanced by the hydrogen bonding of PU,and the higher the PU content,the more orderly the PUI molecular chains are arranged;the thermal stability of PU is lower compared with PI,resulting in the gradual decrease of PUI thermal stability with the increase of PU content,and the thermal The temperature T₅ and T₁₀ of 5%and 10%weight loss are between 288～259℃and 412～283℃,respectively;the fracture strength of PUI is highest at 30 mol%PU content,reaching 63.03 MPa and 18.47%elongation at break.（4）DCNTs were prepared by functionalized modification of carbon nanotubes（MWCNTs）with polydopamine（PDA）,and PUI30@DCNTs composite films were prepared by in situ polymerization by introducing them into the PUI30 system.The effects of DCNTs on the molecular structure and chain aggregation state of PUI30 were investigated,the enhancement mechanism of DCNTs on the composite film was analyzed,and the relationship between the content of DCNTs and the thermal and mechanical properties of the composite film was elucidated.The results show that DCNTs participate in the reaction of the system and play a cross-linking role,hindering the movement of molecular chains,leading to weakened hydrogen bonding and reduced molecular chain orderliness,but not destroying the molecular structure of PUI30;the thermal stability of the composite film increases with the increase of DCNTs content,and the temperature T₅ and T₁₀ of 5%and 10%thermal weight loss are between268～282℃and 304～323℃,respectively;The highest fracture strength of 81.73 MPa and elongation at break of 16.08%were achieved for the composite film with DCNTs content of 3 wt%,SEM shows that high content of DCNTs causes pores,which in turn weakens the mechanical properties.