Study On The Enhanced CO₂/CH₄ Separation Performance Of The Polyimide Membranes Modified By Hyperbranched Method

Aromatic polyimide is a new type membrane material due to rigid chain structure and high steric hindrance.Compared with traditional glass polymer,polyimide membranes have better permeability and selectivity,so they are widely used as gas separation membranes.Hyperbranched polyimide is often used to improve gas separation performance because of its unique branched structure.At present,hyperbranched polyimides based on triamine monomers have been extensively studied.Hyperbranched polyimides based on tetraamine monomers have higher degree of branching and more excellent gas separation performance.But the reaction involving tetraamine can more smoothly promote an insoluble network-like structure.Thus,co-HBPIs with tetraamine monomers are highly difficult to synthesize.For this reason,in this paper,multifunctional tetraamine monomer was selected to participate in the polymerization reaction and prepared copolyimides and the hyperbranched polyimide.Moreover,the different membrane preparation methods（chemical imidization and thermal imidization method）were explored.And the CO₂/CH₄ gas separation performance of the hyperbranched copolyimide membranes and the blend（linear/hyperbranched）polyimide membranes was studied at 35 ^oC and 2 bar.In addition,the effects of different temperatures,pressures and casting solvents on CO₂/CH₄ gas separation performance were also studied.The main content is as follows:（1）N,N,N’,N’-tetra（p-aminophenyl）p-phenylenediamine（NTPDA）,4,4-hexafluoroisopropyl phthalic anhydride（6FDA）and 4,4’-Diaminodiphenyl ether（ODA）were used as the reactive monomers.A series of hyperbranched copolyimide membranes were prepared by chemical imidization after adjusting the solid content of the reaction solution and the dropping rate of the tetraamine monomer.And CO₂/CH₄ gas separation performance was tested by permeameter at 35 ^oC and 2 bar.The results showed that the membrane with a tetraamine monomer content of 7‰（6FDA-ODA-7‰）had a higher ideal selectivity of CO₂/CH₄ and the value was 85.78 which was 73%higher than the pure membrane.（2）N,N,N’,N’-tetra（p-aminophenyl）p-phenylenediamine（NTPDA）,4,4-hexafluoroisopropyl phthalic anhydride（6FDA）and 4,4’-Diaminodiphenyl ether（ODA）were used as reactive monomers.The hyperbranched copolyamic acid membranes（co-HBPAAs）were synthesized at a low solid content.A series of hyperbranched copolyimide membranes（co-HBPIs）were prepared by thermal imidization.And CO₂/CH₄gas separation performance was studied at 35 ^oC and 2 bar.The results showed that its gas separation performance broke the trade-off effect.Among them,the membrane with a tetraamine monomer content of 5‰（T-6FDA-ODA-5‰）has a higher CO₂ permeability and an ideal selectivity for CO₂/CH₄.The values were 93.111 barrer and 37.61 respectively,which were 117.32%and 78.5%higher than pure membrane.（3）N,N,N’,N’-tetra（p-aminophenyl）p-phenylenediamine（NTPDA）and4,4-hexafluoroisopropyl phthalic anhydride（6FDA）were used as reactive monomers to prepare the hyperbranched polyimide by the chemical imidization method and it was blended with linear polyimide（6FDA-ODA）in different ratios to prepare a series of blend membranes.Furthermore,CO₂/CH₄ gas separation performance was tested by permeameter at 35 ^oC and 2bar.The results showed that the blend membrane with the hyperbranched polyimide content of 4%（6FDA-ODA/4%）had a higher CO₂ permeability.And the value was 50.547 barrer which was 115.42%higher than the pure membrane.