Abstract
Polyimides are one of the attractive materials for gas separation due to their high thermal stability and chemical tolerance. Modification of polyimides is a feasible approach to enhance the separation performance of polyimides. In this work, the effect of small molecule releases in membrane formation processes on the gas separation performance of polyimide membranes has been investigated. First, a reversible ring-opening reaction of polyimide membrane with furfurylamine (FFA) is carried out by means of thermal processes. Portion of the polyimide chain undergoes the amidation reaction with the amine groups of FFA to form the product of PIFFA. Upon heat, the sample process a reimidization reaction with release of FFA molecules and result in the sample of PIFFA250. Gas separation experiments results shows the PIFFA film declines only slightly in gas permeation, but maintains the efficiency in gas separation. The PIFFA250 film, which is obtained by 250 oC heat treatment to PIFFA film, shows increases in hydrogen gas (H2) permeability and selectivity for a certain extent. Compared with the original polyimide films, the permeability of H2 increased from 23.1 to 26.7 barrer, and the H2/N2 and H2/CO2 selectivity increased from 55.2 to 71.6 and 1.8 to 2.3, respectively. The second part of this study employs Meldrum’s acid (MA) derivatives as modifiers for PIs. Under heat treatment, MA groups decompose accompanied with releases of acetone and carbon dioxide. The thermally-treated MA-modified polyimide membranes render better permselectivity results of H2. Compared with original polyimide films, the H2/N2 and H2/CO2 selectivity of the modified PI (possessing 20 wt% MA derivative compounds) increase by two or three times. The gas separation properties of the modified PIs has been correlated to the changes in the free volumes caused by the releases of small molecules in the membrane formation processes at high temperatures.