Synthesis and Gas Transport Properties of Hyperbranched Polyimide-Silica Hybrid/Composite Membranes

Hyperbranched polyimide-silica hybrids (HBPI-silica HBDs) and hyperbranched polyimide-silica composites (HBPI-silica CPTs) were prepared, and their general and gas transport properties were investigated to clarify the effect of silica sources and preparation methods. HBPI-silica HBDs and HBPI-silica CPTs were synthesized by two-step polymerization of A(2) + B-3 monomer system via polyamic acid as precursor, followed by hybridizing or blending silica sources. Silica components were incorporated by the sol-gel reaction with tetramethoxysilane (TMOS) or the addition of colloidal silica. In HBPI-silica HBDs, the aggregation of silica components is controlled because of the high affinity of HBPI and silica caused by the formation of covalent bonds between HBPI and silica. Consequently, HBPI-silica HBDs had good film formability, transparency, and mechanical properties compared with HBPI-silica CPTs. HBPI-silica HBD and CPT membranes prepared via the sol-gel reaction with TMOS showed specific gas permeabilities and permselectivities for CO2/CH4 separation, that is, both CO2 permeability and CO2/CH4 selectivity increased with increasing silica content. This result suggests that gas transport can occur through a molecular sieving effect of the porous silica network derived from the sol-gel reaction and/or through the narrow interfacial region between the silica networks and the organic matrix.