The effects of 1,3-cyclohexanebis(methylamine) modification on gas transport and plasticization resistance of polyimide membranes

We have identified that 1,3-cyclohexanebis(methylamine) (CHBA) can be effectively used as a new cross-linking agent for the chemical modification of polyimide membranes. We have also observed that the combined effects of diamino cross-linking and thermal annealing would significantly change the chemical compositions, micro-structure, gas sorption, gas transport properties and plasticization resistance of polyimide membranes. The membraneʹs physicochemical changes after diamino cross-linking and thermal annealing were characterized by FTIR-ATR, XPS, gel content, UV, SEM-EDX and TGA and the possible reaction mechanisms during chemical modification and annealing have been proposed. Interestingly, it is found that the chemical reactions between diamino and polyimides are reversible during thermal annealing and the membrane micro-structures are modified and charge transfer complexes (CTCs) are formed during the post-treatments. The gas sorption and gas transport properties of membranes before and after modifications are reported and discussed. The gas sorption concentration continuously decreases with an increase in the degree of cross-linking and the subsequent thermal annealing. Experimental results illustrate that thermal annealing not only improves CO2/CH4 selectivity of the cross-linked membranes but also greatly enhances the plasticization resistance by the formation of CTCs. The critical plasticization pressure is significantly improved from about 300 psia of the original samples to more than 720 psia of the cross-linked and 200 °C thermal treated samples. Based on the experimental results, a novel approach to enhance plasticization resistance of polyimide membranes by means of diamino cross-linking and followed by thermal annealing is elucidated.