Preparation And Gas Separation Performance Of Heat-resistant And Soluble Morpholine-based Polyimides

The separation technology of polymer membrane is widely used in the field of gas separation due to its advantages of green environmental protection,simple process,and low energy consumption.The development of polymer membrane materials with high heat resistance,easy formation,and high separation efficiency has become a research hotspot.Therefore,by the molecular structure design,this thesis synthesized two diamine monomers,and prepared a series of homopolyimide and copolyimide through"one-step"high-temperature polycondensation respectively.We systematically studied their various performance and gas separation efficiency,and observed their free volume distribution and gas adsorption state through molecular simulation.1、The diamine monomer 4-（4-（1-morpholino）phenyl）-2,6-bis（4-aminophenyl）pyridine（MPAP）was obtained by a"two-step"reaction from 4-（4-morpholino）benzaldehyde and 4-nitroacetophenone.Homopolyimides（ho-MPPIs）were derived from MPAP and three dianhydrides by polymerization.The test results show that ho-MPPIs have good thermal performance,optical properties,solubility,and hydrophobic properties.Meanwhile,ho-MPPI-1（MPAP-6FDA）exhibits high gas permeability（P（CO₂）=18.94 Barrer,P（He）=26.03 Barrer,P（O₂）=3.85 Barrer）,and molecular simulation shows that it has a maximum fractional free volume（FFV）which is 0.239,and the highest gas solubility coefficient（S（CO₂）=28.99,S（O₂）=4.04,S（N₂）=2.92）.2、A series（MPAP:BAFL=2:1,1:1,1:2,1:3,1:4）of copolyimides（co-MPPIs）were synthesized by polymerization using MPAP,commercial diamine 9,9-bis（4-aminophenyl）fluorene（BAFL）and commercial dianhydride 6FDA as raw materials.Compared to homopolymers,co-MPPIs have excellent thermal performance,optical properties,solubility,and hydrophobic properties.In addition,compared to ho-MPPI-1,co-MPPI membranes exhibit improved gas selectivity,especially the O₂/N₂,CO₂/N₂and He/N₂ selectivity of co-MPPI-5 increased to 183%,186%,and 211%,respectively.3、Through molecular design,the diamine monomer 4-（4-2,6-bis（4-（4-aminophenoxy）phenyl）pyridyl）phenylmorpholine（AOPPM）was synthesized from 4-hydroxyacetophenone,1-chloro-4-nitrobenzene and 4-（4-morpholino）benzaldehyde by"three-step method",which introduced an oxygen ether bond in the design concept of MPAP.Novel homopolyimides（ho-mop PIs）were derived by polymerization of AOPPM with four dianhydrides,respectively.The test results show that ho-mop PIs have improved optical,solubility,and mechanical properties compared to ho-MPPIs,and the gas selectivity of ho-mop PIs has been improved,especially with the increase in He/N₂ of ho-mop PI-1（AOPPM-6FDA）to 182%.4、AOPPM,BAFL,and 6FDA were polymerized to obtain a series（AOPPM:BAFL=3:1,2:1,1:1,1:2,1:3）of copolyimides（co-mop PIs）.Studies have shown that co-mop PIs have excellent optical,solubility,and mechanical properties,while retaining the good thermal and hydrophobic properties of MPPIs.Molecular simulation results indicate that co-mop PIs also have large free volume（FFV=0.218-0.275）for gas diffusion,while gas adsorption results indicate that co-mop PIs have an increasing effect on the dissolution and adsorption of CO₂.Therefore,compared to ho-mop PI-1,co-mop PI-5 membrane has significantly improved gas permeability and selectivity,with CO₂ and O₂ permeability increasing to 285%and 234%respectively,and,O₂/N₂,CO₂/N₂ and He/N₂ selectivity increasing to 195%,238%,and 160%respectively,approaching the Robeson upper limit in 1991 and showing a trend of overlap.