Effects of brominating matrimid polyimide on the physical and gas transport properties of derived carbon membranes

Bromination modification was initially carried out on Matrimid polyimide before undergoing carbonation to produce carbon membranes. Compared with unmodified Matrimid, brominated Matrimid shows lower chain flexibility, which is demonstrated by increased glass transition temperatures and molecular simulation results. Additionally, an increase in space between polymer chains was supported by fractional free volume (FFV) and d-spacing measurements. The improvement of chain rigidity of polyimide precursors serves to strengthen the membrane morphology during the production of carbon membranes. Thermal gravimetric analysis indicates that the thermal stability of polyimide decreases after bromination. The lower thermal stability and higher FFV value of brominated Matrimid result in higher gas permeability of carbon membranes pyrolyzed at a low pyrolysis temperature, while the selectivity remained competitive to those pyrolyzed from the original Matrimid precursor under the same conditions. However, the gas permeabilities of carbon membranes derived from modified Matrimid decrease significantly and become lower than those of carbon membranes from the original Matrimid, when the pyrolysis temperature is raised to 800 degrees C. This is due to the formation of more graphitic-like structure in carbon membranes from brominated polyimide, observed by the wide-angle X-ray diffraction. Therefore, it is concluded that bromination of Matrimid polyimide has significantly affected the pyrolysis behavior and the structure of the resulting carbon membranes. At a low pyrolysis temperature, carbon membranes derived from brominated precursors show attractive and superior gas separation performance.