6FDA-Based Polyimide Membranes with Different Chemical Structures for Gas Separation

Six derivatives of (4,4'-hexafluoroisopropylidene) diphthalic anhydride, i.e. 6FDA, based polyimides, have been studied to clarify crosslinking benefits due to selectivity, permeability tradeoff for systematically varied structures.  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.