Preparation And Gas Separation Performance Of Fluorinated Polyimides With Bulky Benzene Side Groups

Compared with the traditional energy-intensive gas separation technology,polymer membrane gas separation technology shows great advantages such as light weight,economy,high process flexibility and large demand space.Aromatic polyimide（PI）with excellent comprehensive properties is an important polymer membrane material in the field of gas separation.However,PI shows obvious disadvantages such as poor organic solubility,poor processability and low gas permeability due to the rigidity of molecular chains.Therefore,this paper introduces the bulky benzene side group containing fluorine group into PI through molecular structure design and structural modification,the main research content is as follows:（1）Through molecular structure design,a new diamine monomer 2,6-bis（4-aminophenyl）-4-（3-（1,1,2,2-tetrafluoroethoxy）phenyl）pyridine（AFEPP）was synthesized by two-step reaction using 4-nitroacetophenone and 3-（1,1,2,2-tetrafluoroethoxy）benzaldehyde as raw materials.Using AFEPP and four commercially available dianhydride monomers as raw materials,four homopolymer PIs were prepared by“one-step”polycondensation reaction.The experimental results indicated that the prepared PIs have outstanding thermal stability,excellent organic solubility,good light transmittance,hydrophobicity and mechanical property.The gas permeability of PI films are improved over Kapton film.In addition,the molecular simulation results showed that the addition of large volume side group tetrafluoroethoxyphenyl to the polymer can effectively increase the free volume of the polymer and improve the gas permeability.（2）Similarly,diamine monomer 4-（3-methoxy-4-（difluoromethoxy）phenyl）-2,6-bis（4-aminophenyl）pyridine（MFPAP）was synthesized by two-step organic reaction using 4-（difluoromethoxy）-3-methoxybenzaldehyde and 4-nitroacetophenone as raw materials.Using MFPAP and three commercially dianhydride monomers as raw materials,three homopolymer PIs were prepared by“one-step”polycondensation reaction.The experimental results indicated that the three PIs exhibit good comprehensive properties,such as thermal stability,hydrophobicity,organic solubility,optical property and mechanical property.The gas permeability test results showed that the PI films have good gas transport property,indicating that the introduction of large volume side group 4-（difluoromethoxy）-3-methoxyphenyl effectively increases the d-spacing and free volume of the PIs.（3）Copolymerization modification is an effective means to improve gas separation performance.In order to further improve the gas transport performance of PI,the copolyimides 6FDA-MFPAP/BAMF（2:1,1:1,1:2 and 1:3）with different diamine monomer ratios were prepared by using MFPAP,commercially available 9,9-bis（4-amino-3-methylphenyl）fluorene（BAMF）and 6FDA as raw materials.The results showed that compared with the homopolymer,the comprehensive properties of the prepared copolyimides were greatly improved,especially the gas transport performance.The molecular simulation results of copolymer further confirmed this conclusion.Among them,6FDA-MFPAP/BAMF copolymers（1:1 and 2:1）are close to the 1991Robeson upper bound of O₂/N₂ and He/CO₂,respectively,indicating that these polymers are expected to be applied in high temperature and high humidity gas separation environments.