Reversible diamine cross-linking of polyimide membranes

The reversible chemical cross-linking of polyimide membranes with diamines has been investigated. This investigation has been carried out on model compounds, polymers and flat sheet polymeric membranes. Reaction between an imide containing model compound and an amine causes each imide ring to be converted into two amide functional groups. Upon heating under vacuum, this reaction can be reversed. The reaction between an amine and the polyimide 6FDA–durene and the subsequent thermal reversal has been investigated. Upon thermal reconversion of the amide linkages back to imide rings, the molecular weight of the polymer is significantly reduced. It is proposed that the polymer undergoes backbone scission with incorporation of the amine as an end group. Polymeric gas separation membranes were formed incorporating the cross-linker in the casting process. The process in which these membranes were constructed ensured that there was a low degree of cross-linking. While significant changes in the physical properties of these membranes are observed upon cross-linking, no insoluble gels are formed. These membranes show a decrease in carbon dioxide permeability and an increase in plasticization resistance and selectivity upon reaction with a diamine. Compared with some literature examples of diamine cross-linked membranes, the reduction in permeability in the membranes presented here is considerably lower but the plasticization resistance is less extensive.