Effect of End Group Modification on Gas Transport Properties of 6FDA-TAPOB Hyperbranched Polyimide-Silica Hybrid Membranes

Physical and gas transport properties of end group-modified 6FDA-TAPOB hyperbranched polyimide (HBPI)-silica hybrid membranes were investigated. Hyperbranched polyamic acids as precursors were synthesized by polycondensation of triamine, 1,3,5-tris(4-aminophenoxy)benzene (TAPOB), and dianhydride, 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), and the molecular end groups were subsequently allowed to react with 3-aminopropyltrimethoxysilane (APTrMOS) and fluorine compound, 3,5-bis(trifluoromethyl)aniline (6FMA) and 1H,1H-heptadecafluorononylamine (17FN). The HBPI-silica hybrids were prepared by sol-gel reaction using the polyamic acids, water, and tetram-ethoxysilane (TMOS). The 5% weight-loss and glass transition temperatures of the hybrids considerably increased with increasing silica content, indicating effective crosslinking at polymer-silica interface mediated by APTrMOS moiety. The CO2, O2, N2, and CH4 permeability coefficients of the hybrids increased with increasing silica content. In particular, 6FMA-modified and 17FN-modified 6FDA-TAPOB HBPI-silica hybrids showed high gas permeability, arising from their high fractional free volumes. The CO2/CH4 selectivity of the hybrids increased remarkably with increasing silica content, whereas their O2/N2 selectivity remained almost constant against silica content. It was concluded that the HBPI-silica hybrids have high thermal stability, high gas permeability, and excellent CO2/CH4 selectivity, and are expected to apply to high-performance gas separation membranes.