Diamino modification represents a potential approach for enhancing the gas transport properties of polyimide membranes. For effective membrane modification, several critical intervening factors are in play, namely electrophilicity and free volume of the polyimide, and nucleophilicity and molecular dimensions of the diamines. The extent of the crosslinking reactions between various polyimides and diamines were investigated both in solution and membrane forms. The reaction of a specific diamine with Matrimid/CH2Cl2 and 6FDA-durene/CH2Cl2 solutions revealed that the appearance of a cross-linked network was more rapid in the former. This is attributed to the higher electrophilicity of Matrimid compared to 6FDA-durene. A series of aliphatic and cyclic diamines were used to effect the cross-linking reaction in Matrimid and 6FDA-durene solutions. The cross-linking rate was highly dependent on the nucleophilicity of the diamines.

Solid state diamino modification of polyimide membranes was performed. In addition to the reactivity of the diamines and polyimides, the degree of cross-linking was related to the molecular dimensions of the diamines which in turn influence the diffusivity of the diamines within the polymeric matrix. In our study, polyimides with considerably different free volume were employed i.e. free volume of 6FDA-durene > 6FDA-ODA/NDA > Matrimid. Polyimides with high free volume favor the access of diamine molecules to the reactive sites, thereby giving better crosslinking effects. Hence, for the same modification reagent, the greatest decline in gas permeability was observed in 6FDA-durene membranes. Similar extent of cross-linking can be achieved at shorter modification duration by using polyimides with greater free volume. Two other side reactions occur during diamino modification, namely chemical grafting and etching (polyimide main chain scission). The varying extent of each reaction occurring is dependent on the nucleophilicity and molecular dimensions of the diamines. It has been proven that the diamino modification approach is useful for enhancing the H2/CO2 separation performance of polyimide membranes. For 6FDA-durene membranes, H2/CO2 selectivity increased from an intrinsic value of ~1 to 100 after modification with 1,3-diaminopropane (PDA) for 5 mins. Similarly, an enhancement in H2/CO2 selectivity from 2 to 64 was obtained by modifying 6FDA-ODA/NDA membranes with PDA for 90 mins. Therefore, the appropriate selections of polyimide-diamine pairs and modification condition can successfully alter the intrinsic gas separation performance of polyimide membranes.