The gas separation effects of annealing polyimide–organosilicate hybrid membranes

Polyimide–organosilicate hybrid membranes were subjected to annealing to enhance gas separation performance. These membranes consisted of organosilicate domains covalently bonded to a 6FDA–6FpDA–DABA polyimide using partially hydrolyzed tetramethoxysilane (TMOS), methyltrimethoxysilane (MTMOS) or phenyltrimethoxysilane (PTMOS). The transport properties of the hybrid membranes were evaluated using pure gases (He, O2, N2, CH4, CO2) at 35 °C and a feed pressure of 4 atm. The permeability for most of the membranes increased 200–500% after the annealing process while the permselectivity dropped anywhere from 0 to 50%. The exceptions were the 6FDA–6FpDA–DABA-25 22.5% TMOS and MTMOS hybrid membranes, both of which exhibited increases in the CO2 permeability and CO2–CH4 permselectivity. The transport data was compared to Robeson’s 1991 “upper bound” and exceeded the boundary in some cases. The increase in permeation was attributed to changes in the free volume distribution and enhanced local segmental mobility of the chain ends resulting from the removal of sol–gel condensation and polymer degradation byproducts.