Microstructure Regulation Of Polyimide And Their Gas Separation Characteristics

Due to the massive exploitation and combustion of fossil resources such as coal and oil to meet the needs of transportation,heating and power supply,the emission of greenhouse gases（such as carbon dioxide）has increased sharply,which has a serious impact on the climate and environment.Therefore,greenhouse gas emission reduction has become a difficult problem for countries all over the world.China strives to reach the peak of carbon dioxide（CO₂）emission by 2030 and achieve"carbon neutralization"by 2060.The development of new and efficient CO₂ capture technology and low energy consumption and environment-friendly CO₂ separation technology are closely related to environmental protection.They are also important means to achieve the goals of"carbon peak"and"carbon neutralization".Gas separation membrane is widely used in CO₂ separation in chemical industry,petroleum and other fields because it can efficiently separate single gas from mixed gas and has many advantages such as low cost,no pollution and simple use.Gas separation membrane is mainly divided into polymer membrane and inorganic membrane.Among them,inorganic membrane has good gas separation performance,but the membrane is very fragile,which is difficult to meet the harsh requirements of industrial gas separation;The polymer membrane has low manufacturing cost and easy processing.Among many polymer membranes,polyimide（PI）membrane materials have attracted extensive attention because of their excellent heat resistance,mechanical properties and high gas separation performance.However,due to the trade-off relationship between gas permeability and selectivity,the industrial application of polyimide membrane is limited.The permeability of polyimide membrane is usually closely related to the free volume and molecular chain accumulation of the membrane,and copolymerization is one of the effective methods to adjust and improve the microstructure and gas permeability of polyimide membrane.Based on this,we introduced flexible polypropylene oxide（PPO,M_n～2000）into Kapton PI molecular chain by copolymerization modification.Polyimide films with controllable PPO content（0～65 wt%）were prepared by introducing the third monomer PPO with pyromellitic dianhydride（PMDA）and 4,4’-diaminodiphenyl ether（ODA）as the main body.The molecular structure and PPO content were confirmed by infrared and thermogravimetric analysis.Kapton has strong structural rigidity,small free volume of membrane and poor gas permeability.On the one hand,the introduction of PPO segment is conducive to increase its free volume,on the other hand,the copolymer membrane shows microphase separation structure.Under the action of dual factors,the permeability coefficient of CO₂ gas in the membrane is increased by 22 times;Based on the solubility selection,the selectivity coefficient for CO₂/N₂ gas is increased from 18.77 to 30.12.Then,we further explored the effect of PPO on fluorine-containing PI gas separation membrane.Different from Kapton structure,PI with fluorine structure has large free volume and good gas permeability,but poor selectivity due to the existence of fluorine atom trifluoromethyl.A series of polyimide films with different PPO content（0～38 wt%）were prepared by copolymerization of hexafluorodicarboxylic anhydride（6FDA）,4,4’-diamino-2,2’-bistrifluoromethylbiphenyl（TFMB）and PPO.PPO chain is introduced into 6F/TF system,and the soft and hard segments"penetrate each other".The soft segment affects the original pores,which reduces the free volume of the film and the gas permeability,but increases the gas selectivity.When the PPO content was 38 wt%,the CO₂/N₂ selectivity coefficient increased from 21.61 to 23.62.In order to further improve the gas permeability of 6F/TF system,amino modified hyperbranched polysiloxane NH₂-HBPSi was synthesized by hydrolytic co condensation,and introduced into 6F/TF PI matrix by in-situ polymerization to prepare composite membrane.The composite membrane has a dense structure and no obvious agglomeration of particles.The gas permeability of the composite membrane was significantly improved with the increase of NH₂-HBPSi content,and the permeability coefficient increased first and then decreased with the increase of nano particle content.When the NH₂-HBPSi content was 15 wt%,the CO₂ permeability coefficient of the membrane increased from 30.85 Barrer to 71.13 Barrer.