(244a) Connecting Physical and Mathematical Models of Carbon Molecular Sieve Membranes

Carbon molecular sieve (CMS) membranes combine outstanding separation performance for many different important applications. The special combination of ability to discriminate between similarly sized penetrants, coupled with scalable manufacturing possibilities makes CMS membranes attractive for next generation membranes. Properties of polyimide-derived CMS membranes, whose transformation from flexible polymer coils into rigid molecular sieves during pyrolysis will be discussed within a mathematical framework. During pyrolysis, entangled random coil polymer chains experience stress, causing them to aromatize and fragment into shorter rigid strands. Similar to isotropic-nematic entropically-driven phase transitions occurring in liquid crystals, the aromatic strands are believed to favor arrangement into plate-like structures. Ultramicroporous gaps between the individual aromatic strands provide ideal molecular sieving sites. Imperfect packing of such plates creates larger microporous volumes for sorption, characterized by a Langmuir sorption model. This presentation will focus upon the connection between the formation process and functional separation properties. Experimental results suggest that a small but important disordered continuous phase characterized by an effective Henry’s Law model applies, resulting in dual modes of sorption in local equilibrium. To complement this dual mode sorption model, a dual mode transport mode is also discussed in the context of the CMS formation process.