Synthesis And Gas Separation Membrane Of Fluorinated Polyimide With Bulky Phenyl Side Groups

Gas separation membrane performs high-efficiency separation based on the difference in gas permeation rate.Membrane material is crucial to gas separation membrane,whose properties play a key role in separation performance.Among multiple materials,polyimide has attracted much attention in the preparation of gas separation membranes due to the advantages of excellent comprehensive properties and designability of molecular structure.However,there are few studies on membrane separation of fluorine-containing system.In this thesis,commercial polyimide was firstly used to study the feasibility of membrane separation of fluorine-containing gas.Targeting the problems of low permeability and easy plasticization of the material,a series of homo-,co-and crosslinked FPI membrane materials with bulky phenyl side groups and high free volume were designed and synthesized.Then the gas separation performance was studied.Firstly,resistance composite membrane was prepared using commercial Matrimid 5218 polyimide.Aiming at the problem of current low recovery ratio of tetratluoroethylone(TFH)polymerization tail gas.a permeate gas quasi-steady-state membrane separation system was designed,and the efficient separation of H2/TFE was achieved for the first time.Compared with conventional membrane separation process,TFE recovery ratio increases from 48.8%to 90%.For the 5.0 kt/a TFE polymerization device.the recovery system can achieve a gross profit of 21.368 million yuan with a growth rate of 346.5%.However,based on the serious fact that Matrimid 5218 polyimide membrane has low flux of fluorine-containing gas and is banned abroad,independent research and development of high-flux polyimide membrane material is necessary.Furthermore,the diamine monoler ?,?-di(4-amino-3,5-dinethylphenyl)-1-(4'-trifluoronethylphenyl)-lethane)(BATFMM)and ?,?-di(4-amino-3,5-dimethylphenyl)-l-(3',5'-di(trifluoromethyl-phenyl))methane(BABTFMM)with bulky phenyl side groups and fluorine-containing groups was designed from the view point of molecular structure.Molecular simulation software was used to establish the simulation process ot polyimide structural property paraleters,adsorption and diffusion.Besides,the density,chain spacing and free volume of two kinds of 4,4'-(hexafluoroisoylene)diphthaIic anhydride(6FDA)type polyimide(6FDA-BATFMM and 6FDA-BABTFMM)were calculated to predict the gas separation performance.In diamine structure,the-CF3 group structure increases the rigidity of the segment by strong electron-with drawing effect,while the bulky phenyl side groups reduce the bulk density by increasing the steric hindrance,enlarging the tree volume and improving the gas permeability of the membrane.Compared with Matrimid 5218,the perneabilities of CO2 were improved by 18.4 and 20.4 times in 6FDA-BATFMM and 6FDA-BABTFMM polyimide membranes-respectively.At the same time,the values of simulated solubility parameter predicted that all polyimide materials showed good solubility in NMP,DMAc,DMF and THF,thus,the,embranes could be prepared by solution casting and phase inversion methods.According to the results of the molecular simulation,diamine monomers with bulky phenyl side groups ?,?-di(4-amino-3.5-dimethylphenyl)-l-phenylmethane(BAPM),BATFMM and BABTFMM were prepared.Based on the idea of industrialization,the synthesis process,purification and characterization methods were established from 2,6-dimethylaniline to large-side phenylenediamine and from hexafluoropropylene oxide(HFPO)to 6FDA.The optimum conditions of the reaction were obtained by orthogonal experiment which improved the reaction yield.The crystallization conditions were optimized to obtain high purity monomers.Noticeably,fluorinated diamine monomers(BATFMM and BABTFMM)were new structures,whose purity could reach 99.9%and 99.6%,respectively.while the 6FDA monoler was higyher than 99.5%.The purity of the obtained products met the requirements for the preparation of high molecular weight polyimide membrane materials.A series of new structure fluorine-containing polyimide materials with bulky phenyl side groups were prepared by above synthesized monomers.The materials showed excellent heat resistance,the decomposition temperatures for 5%weight loss were above 491?,and the glass transition temperatures were higher than 330?.The incorporation of bulky side groups and fluorine-containing groups increased the free volume and then improved the gas permeation,which solved the low gas permeability of polyimide material.Meanwhile,6FDA-BABTFMM polyimide showed the largest permeability coefficient of fluorinated and conventional gases.Compared with Matrimid 5218.the permeability coefficients of TFE.trifluorochloroethylene(CTFE)and 1,1,1,2-tetrafluorothane(R134a)were increased by 182.7.177.9 and 96.5 times,respectively.However,when the test pressure was below 0.6 MPa,difluoromethane(R32)and 1,1-difluoroethane(R152a)had significant plasticization effects on the above materials.Targeting the plasticization problem of fluorine-containing polyimide with bulky phenyl side groups,copolymerized and crosslinked polyimides were prepared by introducing a carboxyl group-containing monomer.3?5-diaminobenzoic acid(DABA),to improve the anti-plasticizing ability.The crosslinked polyimide had a significant improvement in plasticizing resistance.When the test pressure was below 0.6 MPa.crosslinked 6FDA-BATFMM/DABA(90:10)and 6FDA-BABTFMM/DABA(90:10)inhibited the plasticization effect of R152a,while crosslinked 6FDA-BATFMM/DABA(75:25)inhibited the plasticization effect of R32.Furthermore,the CO2 permeability of 6FDA-BAPM/DABA(75:25)was 73.78 Barrer,while the CO2/CH4 selectivity was 47.91.The permeability of CO2 was 111.02 Barrer,the selectivity was 39.94 for CO2/CH4 in 6FDA-BAPM/DABA(90:10).The permeability coefficient of CO2 was 227.79 Barrer,and the selectivity of CO2/CH4 reached 29.89 of 6FDA-BATFMM/DABA(90:10).