Preparation And Properties Of Polyimide Films Based On Metal Coordination Bond

High performance polymer materials,such as polyarylether ketone,polybenzimidazole and polyimide,are widely used in high-tech fields such as aerospace,military industry,medical devices and microelectronics because of their excellent comprehensive properties.Polyimide has variability in synthesis due to the flexibility of molecular structure design,which is not possessed by other polymers.However,most polyimides are insoluble and infusible,which brings great inconvenience to processing and molding,and limits its application.Soluble polyimide has attracted much attention because of its excellent processability.However,their heat resistance,mechanical properties and solvent corrosion resistance are insufficient,which limits their wide application.Therefore,it has become one of the research focuses in this field to improve the comprehensive properties of soluble polyimide while taking into account its processability.In order to further expand the application of soluble polyimide and make it meet the needs of special environment in some high cutting-edge fields,researchers continue to strengthen and modify it.Crosslinking is an effective measure to enhance,and hydrogen bonding enhancement is a common measure.However,the low hydrogen bond energy leads to poor thermal stability and easy failure at high temperature;For covalent crosslinking,it generally needs to be initiated by light,heat and other conditions,resulting in greater energy loss,and some polyimides are unstable under light and heat conditions.Therefore,it is necessary to explore new crosslinking methods to improve the comprehensive properties of soluble polyimides.Coordination bond not only has the stability of covalent bond,but also has reversibility like non covalent bond.The comprehensive properties of soluble polyimide materials can be improved by metal ligand coordination.In addition,the materials can be endowed with certain functionality.Based on this,polyimide containing end carboxyl group and side carboxyl group was designed and synthesized in this paper.Polyimide films based on end group and side group metal coordination were prepared through the coordination of carboxyl group and Cu²⁺,and the changes of structure and properties before and after coordination were systematically studied.The specific research contents are as follows:（1）Hexafluorodicarboxylic anhydride（6FDA）and 4,4’-diaminodiphenyl ether（ODA）were used as monomers to synthesize anhydride terminated Polyimides with different polymerization degrees at the molar ratio of n+1:n.The polymer was precipitated in deionized water and its end group was hydrolyzed to carboxyl group,and then coordinated with Cu²⁺to prepare end group coordinated polyimide.Polyimide films were prepared by tape casting method.The film-forming properties of polyimide with different polymerization degrees were compared.The results showed that the film-forming properties were better when the polymerization degree n=50.The effect of precipitation mode of solution on its aggregation structure was studied.The results showed that the crystallinity of polyimide prepared by slow cooling precipitation was higher than that obtained by rapid cooling precipitation.The improvement of crystallinity was beneficial to improve the comprehensive properties of the film.After coordination,the comprehensive properties of the films were improved to some extent.After coordination,the linear thermal expansion coefficient decreased from 68.05 ppm/K to 52.47 ppm/K,the fastest thermogravimetric temperature increased from 568 ^oC to 572 ^oC,the tensile strength increased from 55.3 MPa to 63.3 MPa,an increase of about 14%,and the dielectric constant was also improved.（2）Because the coordination crosslinking density of polyimide based on end group coordination is not high,the improvement of comprehensive properties is not obvious.In order to further enhance the comprehensive properties of soluble polyimides,in this chapter,carboxyl containing monomers 3,5-diaminobenzoic acid（DABA）,4,4’-diphenyl ether tetraacid dianhydride（ODPA）and 4,4’-diamino-2,2’-bis trifluoromethyl biphenyl（TFMB）are copolymerized into polyimides with high molecular weight carboxyl groups on the side groups according to the molar ratio of diamine and dianhydride.By adding different amounts of copper chloride,the side coordination polyimides with different cross-linking degrees were prepared.When a certain amount of copper chloride was added to form enough crosslinking point,gel would be generated.The polyimide film based on side group coordination crosslinking was soaked in DMF,DMAC and other strong polar solvents for 48 hours,and the mass residue rate was up to 80%;After coordination,the heat resistance was improved.With the increase of Cu²⁺content,the crosslinking degree increased,T_gincreased from 320 ^oC to 362 ^oC,and the temperature of 5%weight loss increased from 426 ^oC to 466 ^oC;The tensile strength increased from 93 MPa to 128 MPa,an increase of about 37%,and the improvement effect was significantly better than that of end group coordinated polyimide.（3）In order to further enrich the functionality of polyimide.Polyimide was synthesized by copolymerization of flexible polyether amine（PPO）with ODPA and DABA.In addition,the carboxyl group on the side group of polyimide is complexed with Cu²⁺,and the carboxyl group can form hydrogen bond with ether bond and carbonyl group,so as to prepare double dynamic reversible polyimide based on hydrogen bond and metal ligand coordination.The introduction of PPO can weaken the rigidity of polyimide molecular chain and have a certain impact on its formability.When the content of PPO exceeds 68 wt%,it can not be formed without the introduction of Cu²⁺,but it can be promoted after the introduction of Cu²⁺for coordination to form sufficient cross-linking points.At the same time,the introduction of PPO improves the flexibility of polyimide film.The hydrogen bond between polyimide molecules was characterized by in-situ heating infrared.It was proved that there was coordination between polyimide molecules by XPS and NMR.The introduction of coordination enhanced its mechanical properties and dimensional stability.The introduction of PPO not only significantly improves the movement ability of polyimide molecular chain,but also endows the material with certain self repair performance.When the content of PPO is more than 68 wt%,the scratches on the film surface can be repaired after heating at 80 ^oC for 23 hours;When the membrane is cut,the section is closely pasted and heated for 72 hours,the repair efficiency can reach about 80%.