Transport properties of cross-linked polyimide membranes induced by different generations of diaminobutane (DAB) dendrimers

We have discovered a series of novel multi-functional cross-linking agents, namely polypropylenimine tetraamine (DAB-AM-4; G1), polypropylenimine octaamine (DAB-AM-8; G2), and polypropylenimine octaamine (DAB-AM-16; G3) dendrimers, which can cross-link 6FDA-durene films at room temperature and enhance its separation performance. The pure gas tests show that the maximum selectivity increases by about 400, 300 and 265% for the gas pairs of He/N₂, H₂/N₂ and H₂/CO₂, respectively, after 60 min of cross-linking with G1 dendrimers. For the gas pair of CO ₂/CH₄, the maximum increment is about 74% after 20 min of G1 cross-linking. The change in gas transport properties is mainly attributed to the effects of cross-linking on diffusion coefficient. Gel content, FTIR and XPS spectra confirm the existence of cross-linking modifications and show that, at the same conditions, G1 has the highest degree of cross-lnking, followed by G2 and then by G3. When comparing with different generations of diaminobutane (DAB) dendrimers, the gas permeability decreases in the order of G1>G2>G3, which is consistent with the increasing order of the degree of cross-linking. Dendrimer-induced cross-linked 6FDA-durene membranes also showed superior gas separation performance to the traditional trade-off line of permselectivity versus permeability relationship.