Effects of Brominating Matrimid Polyimide on the Physical and Gas Transport Properties of Derived Carbon Membranes Youchang Xiao, † Ying Dai, ‡ Tai-Shung Chung,* ,† and Michael D. Guiver ‡ Department of Chemical & Biomolecular Engineering, National University of Singapore, Singapore 117602, and Institute for Chemical Process and Environmental Technology, National Research Council of Canada, Ottawa, Ontario K1A 0R6, Canada Received June 25, 2005; Revised Manuscript Received September 12, 2005 ABSTRACT: 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 °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. 1. Introduction Membrane technology offers an alternative to tradi- tional separation processes for gas separations such as air separation, natural gas production, olefin/paraffin separation, and hydrogen recovery, 1-4 due to its lower cost, smaller size, higher energy savings, and better environmental benefits. Among the various membrane materials, carbon membranes show numerous advan- tages 5,6 such as (1) consisting of ultra-micropores with similar sizes to the dimensions of gas molecules and showing excellent gas separation performance, (2) ex- hibiting high thermal stability, mechanical strength, and chemical resistance, and (3) having gas permeation properties less influenced by the feed pressures and also time independent. Carbon membranes are typically prepared by the pyrolysis of thermosetting polymeric precursors, which do not fuse and retain their membrane shape during carbonization. The gas separation performance of carbon membranes largely depends on the chemical structures of polymeric precursors, membrane formation methods, pyrolysis conditions, and posttreatment methods. Al- though great efforts have been made to understand the processes involved in developing high-performance car- bon membranes, most work still relies on empirical methods to prepare carbon membranes because of the complexity of the structure formation during carboniza- tion of the polymers. One of the most important factors determining the separation performance of carbon membranes is the choice of polymeric precursors. The polymeric precursors reported include poly(furfuryl alcohol) (PFA), 7 poly(vinylidene chloride) (PVDC), 8,9 cellulose, 10,11 phenolic resins, 12,13 polyacrylonitrile (PAN), 14 poly(ether imide)s, 15,16 and polyimides. 17-27 Among these polymers, aromatic polyimides are the most frequently utilized for the preparation of carbon membranes with- out support because this class of polymers has a rigid structure, with high glass transition temperatures (T g ) and excellent thermal stability. Koros et al. reported hollow fiber carbon membranes derived from a copoly- imide (BPDA/6FDA-TrMPD), which exhibited an O 2 permeance of 15-40 GPU and a separation factor of 11- 14. 22 Kusuki et al. developed a manufacturing method to continuously prepare hollow fiber carbon membranes using polyimide as the precursor. 23 Okamoto et al. reported excellent performance for olefin/paraffin sepa- ration with carbonized hollow fiber membranes derived from BPDA-DDBT/BADA copolyimide. 24 After a com- parison with a series of carbon membranes pyrolyzed from four polyimides (DAI/6FDA, DAI/BTDA, DAI/ BPDA, and DAI/ODPA) synthesized from different dianhydrides, Xiao et al. concluded that higher FFV and lower thermal stability of polyimide precursors led to higher gas permeability of carbon membranes at a mild pyrolysis temperature (550 °C). When polyimides were carbonized at a high temperature (800 °C), better chain flatness and in-plane orientation of polyimides contrib- ute to higher gas selectivity in carbon membranes, due to the formation of more graphitic-like structure during carbonization. 27 However, most of the polyimides for carbon mem- brane studies are only synthesized on the laboratory scale. From a practical point of view, the high cost of synthesized polyimides is a key factor that limits their utilization in the preparation of carbon membranes. † National University of Singapore. ‡ National Research Council of Canada. * Corresponding author: e-mail chencts@nus.edu.sg, Fax (65)- 67791936. 10042 Macromolecules 2005, 38, 10042-10049 10.1021/ma051354j CCC: $30.25 © 2005 American Chemical Society Published on Web 11/05/2005