Effects of Polyimide Chemical Structure On Membrane's Gas Separation Performance

To clarify crosslinking benefits due to selectivity and permeability tradeoff in gas separation systematically, several (4,4'-hexafluoroisopropylidene) diphthalic anhydride, i.e. 6FDA, based polyimides, which have related chemical structures were synthesized. The gas permeability, selectivity, and plasticization response were measured using both pure gases (He, O₂, N₂, CH₄, and CO₂) and mixed gas (CO₂/CH₄ with 10% CO₂ and 50% CO₂, respectively).   Special focus has been placed on plasticization response in aggressive high pressure and high CO₂ content feed stream.

Pure gases (He, O₂, N₂, CH₄, and CO₂) and mixed gas (CO₂/CH₄ with 10% CO₂ and 50% CO₂, respectively) were tested.  All uncrosslinked membranes show a selectivity reduction and permeability increase reflecting plasticization response at a critical CO₂ partial pressure, and the initial plasticization partial pressure depends on the chemical structure of the polyimides.  For the same gas and under the same feed pressures, membranes show the following order in permeabilities: 6FDA-DAM » 6FDA-DAM:mPDA(3:2) > 6FDA-DAM:DABA(3:2) » 6FDA-mPDA > 6FDA-DABA > 6FDA-mPDA:DABA(3:2).  The selectivities of gas pairs comprising He/CH₄, He/N₂, O₂/N₂, O₂/CH₄, CO₂/N₂, N₂/CH₄, and CO₂/CH₄ are also influenced by backbone structures.   

The DABA-containing polyimides, such as 6FDA-DAM:DABA and 6FDA-mPDA:DABA,  were found can be cross-linked through decarboxylation at a temperature even below its glass transition temperature, which can avoid the collapse of substructure and transition layers of the asymmetric hollow fiber, thereby enabling eventual extension to asymmetric structures. The cross-linked membranes gained good plasticization resistance, even up to 700 psia for pure CO₂ gas or 1000 psia for 50 % CO₂ mixed gas. Furthermore, the crosslinking yields the membranes enhanced permeabilities, with satisfactory selectivity.