Synthesis of Partially Carbonized Polyimide Membranes with High Resistance to Moisture

Abstract

Partially carbonized polyimide (CPI) membranes were prepared on a porous alumina support from carbonization of polyimide membranes at 460–500°C. The permeance of O₂ through the CPI membrane increased with the increasing carbonization temperature, although the separation factors (O₂/N₂) reduced from 11 to 4. The highly permeable CPI membranes carbonized at 500°C were silylated with 1,1,1,3,3,3-hexamethyl disilazane (HMDS) at 90°C, 150°C and 180°C before or after carbonization to solve an adverse humidity effect on gas permeation. The molar ratios of C/Si in the cross section of the silylated CPI membranes were analyzed by SEM-EDX measurements. The separation of dry air through the nonsilylated and silylated CPI membranes was performed before and after a steam treatment. Continuous air permeation tests under saturated steam were also performed with prolonged time. The CPI membranes silylated at 180°C had deposition of organosilicate on the outer surface of polyamide layer, resulting in the reduction of the O₂ flux. On the other hand, the flux of O₂ and the separation factor for the CPI membrane silylated at 150°C before carbonization did not reduce drastically under saturated steam. The results of chemical composition analysis for polyimide and carbonized polyimide suggests that water-resistant CPI membranes with high O₂ permeation flux were synthesized by taking advantage of reactive carboxyl groups in the membrane before carbonization.