Polyimide precursors for carbon molecular sieve membranes.

摘要

Carbon molecular sieve (CMS) membranes were produced for gas separations by pyrolyzing an asymmetric hollow fiber composed of a polyimide. Using an industrially obtained precursor fiber, the role of pyrolysis conditions on CMS membrane performance was examined. In order to examine the role of precursor morphology and composition on carbon membrane performance, fibers composed of Matrimid{dollar}spcircler{dollar} 5218 were spun and pyrolyzed.; Using vacuum pyrolysis at 550{dollar}spcirc{dollar}C, a CMS membrane was produced that exhibited permselective performance that was above the "upper bound" for air separations. In order to create "robustness" in the pyrolysis process for potentially "nonideal" fiber morphologies, several processing variables were investigated--pyrolysis in a vacuum versus an inert purge gas, the purge gas flow rate, the type of purge gas, residual oxygen content in the purge gas, and the pyrolysis temperature. The role of the support material was examined and a potential model for precursor pyrolysis was constructed.; Utilizing previously studied techniques, asymmetric hollow fibers were spun to create a variety of morphologies and compositions. While some of the fibers could be considered "good" polyimide membranes, some of the fibers would be considered "nonideal". For the precursor morphology and composition studies, certain characteristics were chosen for investigation. Hollow fibers having different inner and outer diameters were produced. The thickness and integrity of the skin layer as well as the overall fiber density were also controlled. The morphology of the fiber substructure was varied. The composition of the fibers was a mixture of the polyimide Matrimid{dollar}spcircler{dollar} 5218 and the epoxy resin F-2300, which have "high" and "low" carbon yields, respectively. Having produced and characterized a variety of precursor fibers, carbon membranes were produced, primarily using a vacuum pyrolysis protocol at 550{dollar}spcirc{dollar}C. The effects of different precursor fibers compositions, densities, and dimensions on carbon membrane performance were studied. The role of precursor substructure and skin layer morphologies was also examined.