Polyimide modification by a linear aliphatic diamine to enhance transport performance and plasticization resistance

A linear aliphatic diamine, ethylenediamine (EDA), for the first time, was used to cross-link polyimides and the EDA modified polyimides were further thermally treated at different conditions to enhance anti-plasticization characteristics. The physicochemical properties of unmodified and modified polyimides were characterized by XPS, FTIR-ATR, DSC, UV and XRD. The results of XPS and FTIR-ATR show that EDA can effectively cross-link polyimides. DSC data indicate that EDA cross-linked polyimides experience significant changes during thermal treatment. Prolonging the cross-linking time decreases d-space between polymer chains. The coupling effects of EDA induced cross-linking and thermal annealing accelerates charge transfer complexes (CTCs) formation, which densifies polyimide membrane structure. As a result, the gas transport properties of modified polyimides generally show reduced permeability and enhanced selectivity during the cross-linking and thermal annealing. CO2 plasticization tests on the resultant membranes indicate that the coupling effects of EDA cross-linking and thermal annealing improve the plasticization resistance from around 300 psia to more than 720 psia, which may be attributed to the effective formation of CTCs. Mixed gas tests demonstrate that the CO2/CH4 selectivity of the EDA cross-linked polyimides was higher in mixed gas tests than that in pure gas tests because of the strong attractions between CO2 and secondary amines. This elucidates why the modified polyimides have better performance in actual separation situations.