Molecular Design Of Isomerization Polyimide And Molecular Dynamics Simulation Of Performance

Human civilization is closely associated with the development of materials.Compared with the experimental and theoretical research,molecular simulation,as the third experimental method,greatly shortens the development time and resource consumption of materials,and provides help for the development and design of materials,and thus accelerates the social civilization.Due to many excellent properties,polyimide materials are used in defense,military,microelectronics and other fields.The polyimide materials are rigid chain polymer due to the backbone contains lots of aromatic heterocycles.Because of its flexibility of structural design,the molecular chain structure and performance can be adjusted easily.What's more,the modulus is closely related to the molecular chain structure.This study aimed to establish a method to compare the modulus of different polyimide systems without a lot of experiments,and to investigate the stretching and orientation behavior of the molecular chain when the stretching behavior occurs.In this study,based on two pairs of isomeric monomers,3,4'-ODA and 4,4'-ODA,m-PDA and p-PDA,a series of conformational isomerized polyimides were designed.Besides,combined with molecular dynamics simulations,the effects of changes in multilevel conformation on the multilevel structure and properties of polyimide were explored.Based on BPDA and PMDA,a series of polyimide films were developed by changing the ratios of 3,4'-ODA and 4,4'-ODA.The effects of these two diamines on the modulus,thermal stability and glass transition temperature of the films were studied.The results showed that with the increase of 3,4'-ODA monomer content,the thermal stability and glass transition temperature of the polyimide film changed slightly,but the modulus of the film increased gradually,and thus polyimide film with high modulus,high thermal stability and high glass transition temperature was developed.The amorphous and tensile models of four polyimides systems(PMDA/4,4'-ODA,PMDA/3,4'ODA,BPDA/4,4'-ODA,BPDA/3,4'-ODA)were established by molecular dynamics simulation.The deformation resistance of each system was analyzed in the repeating unit scale and the molecular chain cluster scale.The effects of bond length,bond angle and internal rotation angle on the modulus were analyzed at the micro-level.The results showed that the internal rotation of the molecular chain was the key factor affecting the modulus of polyimide.BPDA was used as dianhydride to prepare series of polyimide films with different proportions of diamine by condensation polymerization with pPDA and m-PDA.The modulus,thermal stability,and glass transition temperature were characterized.The results showed that the mechanical properties of polyimide decreased with the increase of m-PDA monomer content.Two amorphous and tensile models(BPDA/p-PDA,BPDA/m-PDA)were established by molecular dynamics simulation.The anti-deformation ability of each system was analyzed in the repeat unit scale and the molecular chain cluster scale.The relation between anti-deformation ability at different scales and modulus were analyzed,which again verified the importance of rotation within the molecular chain on the modulus of the film.In conclusion,our study established and verified an intuitive method to observe the changes of molecular chain and repeating unit conformation under strain within the range of elastic deformation and to qualitatively analyze the modulus of polyimide.The mechanism for high modulus of polyimide was analyzed and the factors affecting the modulus of polyimide were also analyzed at the micro-level by molecular dynamics simulation.Our results provide theoretical support and foundation for the future development of polyimide products with high modulus and high strength.