Preparation,Structural Optimization And Performance Of Cross-linked Polyimide Membrane For Gas Separation

Networked polyimide（Polyimide,abbreviated PI）membranes have received significant attention in the field of gas separation because of their excellent thermal stability and resistance to plasticization.Networked PI materials can be constructed by in-situ crosslinking and post crosslinking.In our previous research work,we prepared a series of in-situ cross-linked network-type PI materials by means of low-temperature retarded gelation.The highly interconnected covalent cross-linked structure resulted in a high cross-link density of the constructed membrane materials,which led to the problem of low gas permeability and limited their practical applications.In this paper,we regulate the crosslink density of network PI membranes through strategies such as extending the crosslink center,extending the crosslink arm and amine capping,and combine the rigid structure to increase the membrane microporous structure and free volume（FFV）to optimize their gas separation performance and improve their industrial implementable value.Three kinds of network polyimide membranes were synthesized by low temperature polycondensation polymerization of Tris（4-aminophenyl）amine（TAPA）and three commercially available dianhydride monomers（3,3’,4,4’-tetracarboxylic dia-nhydride（BTDA）,homophthalic dianhydride（PMDA）,and 4,4’-（hexafluoroisopropyl）diphthalic anhydride（6FDA））.The relationship between the structure and properties of network polyimide membranes was systematically studied,and the effect of heat treatment on the physical and separation properties of the membranes was discussed.Among them,the CO₂,O₂and H₂permeation coefficients of TAPA-6FDA membranes reached 37.4 Barrer,6.41 Barrer and 54.54 Barrer,respectively,with a selectivity of32.8 for CO₂/N₂;the CO₂permeation coefficient for a gas mixture with simulated flue gas content CO₂/N₂（15/85,v/v）was 28.67 Barrer The selectivity of CO₂/N₂reached32.9,which is very close to the pure gas separation data,indicating that the membrane has good application prospects.After heat treatment at 600℃,the gas permeability was significantly improved,in which the permeability coefficient of TAPA-6FDA-600membrane for CO₂（4131.85 Barrer）was increased 110 times and the selectivity for CO₂/N₂（19.1）was maintained at a high level.The FDA-TAPA network type copolymerized PI membrane was prepared by introducing fluorenylamine-containing monomer 9,9-bis（4-aminophenyl）fluorene（FDA）into the polyimide material using a copolymerization method.When the molar content ratio of FDA to TAPA was 4.5:1,the best gas separation performance of the network-type PI membrane was achieved,and the permeation coefficients of CO₂and O₂were increased by 50%（55.5 Barrer）and 160%（15.5 Barrer）,respectively,while maintaining a high gas selectivity（16.8 for CO₂/N₂）.This is mainly due to the lower crosslink density and increased free volume FFV of the network polyimide through the extension of the crosslink arm,which improves the gas permeability of the membrane.In addition,the FDA contains a bulk twist group fluorene structure,which enhances the rigidity of the polyimide backbone and improves the microporous structure within the membrane,further improving the gas separation performance of the PI membrane.To further improve the separation performance of the membrane,2,4,6-trimethy-lisophthalamide（DAM）was selected as the copolymer monomer and the crosslinking arm was extended to prepare DAM-TAPA network type PI membrane.Since DAM contains three methyl groups,it can significantly increase the potential resis-tance effect of diamine,which is beneficial to the formation of torsional structure and improve the gas permeability coefficient of the membrane.The prepared membrane6FDA-DAM/TAPA（5:1）obtained the maximum O₂and CO₂permeation coefficients of 36.53 Barrer and 158.43 Barrer,which improved by 469.9%and 323.6%.Respec-tively,The selectivity for O₂/N₂and CO₂/N₂also remained high at 4.7 and 20.4.The N,N-（1,4-Phenylene）bis（N’-（4-aminophenyl）benzene-1,4-diamine）TAPPE,diamine DAM and 6FDA were selected to copolymerize and extend the crosslinking center to prepare the DAM-TAPPE network-type PI film with four crosslinking arms.The PI membrane formed by 6FDA-DAM/TAPPE（5:1）has permeability coefficients of 40.78 Barrer and 190.2 Barrer for O₂and CO₂,respectively,and selectivity of 19.51for CO₂/N₂.Compared with the original membrane 6FDA-TAPA,the permeability coefficient of CO₂is increased by 409%.The PI structure of the four cross-linked arms changed the stacking of the polyimide material,expanded the micropore size as well as the FFV of the gas separation membrane,and substantially improved the gas permeation performance of the membrane.Finally,the method of monoanhydride Ac₂O capping was explored to partially acetylate the amine group of the triamine monomer TAPA to prepare acetyl network type PI membrane 6FDA-TAPA/Ac in a simple and convenient way,and the regula-tion of the cross-linked arm structure was achieved by the amount of Ac₂O.The capped PI membrane exhibited the best permeability and gas selectivity when the molar ratio of TAPA/Ac₂O was 5:1.Compared with the uncapped PI membrane,the permeability coefficient to CO₂（66.32 Barrer）was improved by 77.3%,and the gas selectivity to CO₂/N₂was also improved from 32.8 to 48.4.The results showed that partial capping of the amine group of the triamine monomer TAPA using acetic anhydride to derive diamines,extend the cross-link arm,and reduce the cross-link density,which provided the opportunity to improve the network-type polyimide membrane permeability performance provides a convenient way.